Holding the Line: Controlling Unwanted Moisture in Historic Buildings

The Old House Web

This story has been adapted from Preservation Brief #39, Controlling Unwanted Moisture in Historic Buildings, originally published in 1996 by the Technical Preservation Services office of the U.S. National Park Service.

Sharon C. Park, AIA

Sections of this story: Remedial Actions ~~ How and Where to Look for DamagingMoisture ~~ Looking for Signs ~~ Uncovering and Analyzing MoistureProblems ~~ Transport or Movement ofMoisture ~~ Surveyingand Diagnosing Moisture Damage ~~ Selecting an Appropriate Level ofTreatment ~~ Ongoing Care ~~ Conclusion~~ Reading List ~~ Glossary

Uncontrolled moisture is the most prevalent cause of deterioration inolder and historic buildings.

It leads to erosion, corrosion, rot, and ultimately the destruction of materials,finishes, and eventually structural components. Ever-present in our environment, moisturecan be controlled to provide the differing levels of moisture necessary forhuman comfort as well as the longevity of historic building materials, furnishings, andmuseum collections.

The challenge to building owners and preservation professionals alike is to understandthe patterns of moisture movement in order to better manage it -- not to try to eliminateit. There is never a single answer to a moisture problem. Diagnosis and treatment willalways differ depending on where the building is located, climatic and soil conditions,ground water effects, and local traditions in building construction.

Remedial Actions within an Historic Preservation Context

In this Brief, advice about controlling the sources of unwanted moisture is providedwithin a preservation context based on philosophical principles contained in the Secretaryof the Interior's Standards for the Treatment of Historic Properties. Following theStandards means significant materials and features that contribute to the historiccharacter of the building should be preserved, not damaged during remedial treatment.

It also means that physical treatments should be reversible, whenever possible. Themajority of treatments for moisture management in this Brief stress preservationmaintenance for materials, effective drainage of troublesome ground moisture, and improvedinterior ventilation.


Applying a waterproof coating to an above-ground masonry wall can trap moisture underneath, causing further damage to the historic material. Photo: NPS files.

The Brief encourages a systematic approach for evaluating moisture problems which, insome cases, can be undertaken by a building owner. Because the source of moisture can beelusive, it may be necessary to consult with historic preservation professionals prior tostarting work that would affect historic materials. Architects, engineers, conservators,preservation contractors, and staff of State Historic Preservation Offices (SHPOs) canprovide such advice. Regardless of who does the work, however, these are the principlesthat should guide treatment decisions:

  • Avoid remedial treatments without prior careful diagnosis.
  • Undertake treatments that protect the historical significance of the resource.
  • Address issues of ground-related moisture and rain run-off thoroughly.
  • Manage existing moisture conditions before introducing humidified/dehumidified mechanical systems.
  • Implement a program of ongoing monitoring and maintenance once moisture is controlled or managed.
  • Be aware of significant landscape and archeological resources in areas to be excavated.

Finally, mitigating the effects of catastrophic moisture, such as floods, requires adifferent approach and will not be addressed in this Brief.

Howand Where to Look for Damaging Moisture

Finding, treating, and managing the sources of damaging moisture requires a systematicapproach that takes time, patience, and a thorough examination of all aspects of theproblem-including a series of variable conditions.Moisture problems may be a direct resultof one of these factors or may be attributable to a combination of interdependentvariables.

Factors Contributing to Moisture Problems

A variety of simultaneously existing conditions contribute to moisture problems in oldbuildings. For recurring moisture problems, it may be necessary for the owner orpreservation professional to address many, if not all, of the following variables:

  • Types of building materials and construction systems
  • Type and condition of roof and site drainage systems and their rates of discharge
  • Type of soil, moisture content, and surface /subsurface water flow adjacent to building
  • Building usage and moisture generated by occupancy

    clogged gutter
    Debris will impede the normal flow of water from the roof's gutter and downspout system to the ground and result in moisture problems. Photo: NPS files.

  • Condition and absorption rates of materials
  • Type, operation, and condition of heating, ventilating, cooling, humidification/ dehumidification, and plumbing systems
  • Daily and seasonal changes in sun, prevailing winds, rain, temperature, and relative humidity (inside and outside), as well as seasonal or tidal variations in groundwater levels
  • Unusual site conditions or irregularities of construction
  • Conditions in affected wall cavities, temperature and relative humidity, and dewpoints
  • Amount of air infiltration present in a building
  • Adjacent landscape and planting materials

Diagnosing and treating the cause of moisture problems requires looking at both thelocalized decay, as well as understanding the performance of the entire building and site.Moisture is notorious for traveling far from the source, and moisture movement withinconcealed areas of the building construction make accurate diagnosis of the source andpath difficult. Obvious deficiencies, such as broken pipes, clogged gutters, or crackedwalls that contribute to moisture damage, should always be corrected promptly.

For more complicated problems, it may take several months or up to four seasons ofmonitoring and evaluation to complete a full diagnosis. Rushing to a solution withoutadequate documentation can often result in the unnecessary removal of historic materials-- and worse -- the creation of long-term problems associated with an increase, ratherthan a decrease, in the unwanted moisture.

Looking for Signs

Identifying the type of moisture damage and discovering its source or sources usuallyinvolves the human senses of sight, smell, hearing, touch, and taste combined withintuition. Some of the more common signs of visible as well as hidden moisture damage,include:

  • Presence of standing water, mold, fungus, or mildew
  • Wet stains, eroding surfaces, or efflorescence (salt deposits) on interior and exterior surfaces
  • Flaking paint and plaster, peeling wallpaper, or moisture blisters on finished surfaces
  • Dank, musty smells in areas of high humidity or poorly ventilated spaces
  • Rust and corrosion stains on metal elements, such as anchorage systems and protruding roof nails in the attic
  • Cupped, warped, cracked, or rotted wood
  • Spalled, cracked masonry or eroded mortar joints
  • Faulty roofs and gutters including missing roofing slates, tiles, or shingles and poor condition of flashing or gutters
  • Condensation on window and wall surfaces
  • Ice dams in gutters, on roofs, or moisture in attics

Uncoveringand Analyzing Moisture Problems

Moisture comes from a variety of external sources. Most problems begin as a result ofthe weather in the form of rain or snow, from high ambient relative humidity, or from highwater tables. But some of the most troublesome moisture damage in older buildings may befrom internal sources, such as leaking plumbing pipes, components of heating, cooling, andclimate control systems, as well as sources related to use or occupancy of the building.

In some cases, moisture damage may be the result of poorly designed original details,such as projecting outriggers in rustic structures that are vulnerable to rotting, and mayrequire special treatment. The five most common sources of unwanted moisture include:

  • Above grade exterior moisture entering the building
  • Below grade ground moisture entering the building
  • Leaking plumbing pipes and mechanical equipment
  • Interior moisture from household use and climate control systems
  • Water used in maintenance and construction materials.

Above grade exterior moisture generally results from weather related moistureentering through deteriorating materials as a result of deferred maintenance, structuralsettlement cracks, or damage from high winds or storms.

Damp interior plaster around windows generally indicates moisture has entered from the outside. Photo: NPS files.

Such sources as faulty roofs, cracks in walls, and open joints around window and dooropenings can be corrected through either repair or limited replacement. Due to their age,historic buildings are notoriously "drafty," allowing rain, wind, and damp airto enter through missing mortar joints; around cracks in windows, doors, and wood siding;and into uninsulated attics.

In some cases, excessively absorbent materials, such as soft sandstone, becomesaturated from rain or gutter overflows, and can allow moisture to dampen interiorsurfaces. Vines or other vegetative materials allowed to grow directly on buildingmaterials without trellis or other framework can cause damage from roots eroding mortarjoints and foundations as well as dampness being held against surfaces.

In most cases, keeping vegetation off buildings, repairing damaged materials, replacingflashings, rehanging gutters, repairing downspouts, repointing mortar, caulking perimeterjoints around windows and doors, and repainting surfaces can alleviate most sources ofunwanted exterior moisture from entering a building above grade.

Below grade ground moisture is a major source of unwanted moisture for historicand older buildings. Proper handling of surface rain run-off is one of the mostimportant measures of controlling unwanted ground moisture. Rain water is oftenreferred to as "bulk moisture" in areas that receive significant annualrainfalls or infrequent, but heavy, precipitation. For example, a heavy rain of 2"per hour can produce 200 gallons of water from downspout discharge alone for a houseduring a one hour period.

When soil is saturated at the base of the building, the moisture will wet footings andcrawl spaces or find its way through cracks in foundation walls and enter into basements.Moisture in saturated basement or foundation walls-also exacerbated by high watertables-will generally rise up within a wall and eventually cause deterioration of themasonry and adjacent wooden structural elements.

A clogged or broken downspout causes the water to pour directly into the ground. NPS files. Photo: NPS files.

Builders traditionally left a working area, known as a builder's trench, around theexterior of a foundation wall. These trenches have been known to increase moistureproblems if the infill soil is less than fully compacted or includes rubble backfill,which, in some cases, may act as a reservoir holding damp materials against masonry walls.Broken subsurface pipes or downspout drainage can leak into the builder's trench anddampen walls some distance from the source. Any subsurface penetration of the foundationwall for sewer, water, or other piping also can act as a direct conduit of ground moistureunless these holes are well sealed. A frequently unsuspected, but serious, modern sourceof ground moisture is a landscape irrigation system set too close to the building.Incorrect placement of sprinkler heads can add a tremendous amount of moisture at thefoundation level and on wall surfaces.

The ground, and subsequently the building, will stay much drier by:

  1. re-directing rain water away from the foundation through sloping grades,
  2. capturing and disposing downspout water well away from the building,
  3. developing a controlled ground gutter or effective drainage for buildings historically without gutters and downspouts, and
  4. reducing splash-back of moisture onto foundation walls.

The excavation of foundations and the use of dampproof coatings and footing drainsshould only be used after the measures of reducing ground moisture listed above have beenimplemented.

Leaking plumbing pipes and mechanical equipment can cause immediate or long-termdamage to historic building interiors. Routine maintenance, repair, or, if necessary,replacement of older plumbing and mechanical equipment are common solutions. Older waterand sewer pipes are subject to corrosion over time.

Slow leaks at plumbing joints hidden within walls and ceilings can ultimately rot floorboards, stain ceiling plaster, and lead to decay of structural members. Frozen pipes thatcrack can damage interior finishes. In addition to leaking plumbing pipes, old radiatorsin some historic buildings have been replaced with water-supplied fan coil units whichtend to leak. These heating and cooling units, as well as central air equipment, haveoverflow and condensation pans that require cyclical maintenance to avoid mold and mildewgrowth and corrosion blockage of drainage channels.

Uninsulated forced-air sheet metal ductwork and cold water pipes in walls and ceilingsoften allow condensation to form on the cold metal, which then drips and causes bubblingplaster and peeling paint. Careful design and vigilant maintenance, as well as repair andinsulating pipes or ductwork, will generally rid the building of these common sources ofmoisture.

Interior moisture from building use and modern humidified heating and coolingsystems can create serious problems. In northern U.S. climates, heated buildings will havewinter-time relative humidity levels ranging from 10%-35% Relative Humidity (RH). A housewith four occupants generates between 10 and 16 pounds of water a day (approximately 1 1/2-2 gallons) from human residents. Moisture from food preparation, showering, or laundry usewill produce condensation on windows in winter climates.

If adequate ventilation is installed, damage to interior walls such as this can be prevented. Photo: NPS files.

When one area or floor of a building is air-conditioned and another area is not, thereis the chance for condensation to occur between the two areas. Most periodic condensationdoes not create a long-term problem.

Humidified climate control systems are generally a major problem in museums housedwithin historic buildings. They produce between 35%-55% RH on average which, as a vapor,will seek to dissipate and equalize with adjacent spaces. Moisture can form onsingle-glazed windows in winter with exterior temperatures below 30F and interiortemperatures at 70F with as little as 35% RH. Frequent condensation on interior windowsurfaces is an indication that moisture is migrating into exterior walls, which can causelong-term damage to historic materials.

Materials and wall systems around climate controlled areas may need to be made ofmoisture resistant finishes in order to handle the additional moisture in the air. Moistinterior conditions in hot and humid climates will generate mold and fungal growth.Unvented mechanical equipment, such as gas stoves, driers, and kerosene heaters, generatelarge quantities of moisture. It is important to provide adequate ventilation and find abalance between interior temperature, relative humidity, and airflow to avoid interiormoisture that can damage historic buildings.

Moisture from maintenance and construction materials can cause damage toadjacent historic materials. Careless use of liquids to wash floors can lead to waterseepage through cracks and dislodge adhesives or cup and curl materials. High-pressurepower washing of exterior walls and roofing materials can force water into constructionjoints where it can dislodge mortar, lift roofing tiles, and saturate frame walls andmasonry. Replastered or newly plastered interior walls or the construction of newadditions attached to historic buildings may hold moisture for months; new plaster,mortar, or concrete should be fully cured before they are painted or finished. The use ofmaterials in projects that have been damaged by moisture prior to installation orhave too high a moisture content may cause concealed damage.

Transportor Movement of Moisture

Knowing the five most common sources of moisture that cause damage to buildingmaterials is the first step in diagnosing moisture problems. But it is also important tounderstand the basic mechanisms that affect moisture movement in buildings.

Moisture transport, or movement, occurs in two states: liquid and vapor. It is directlyrelated to pressure differentials. For example, water in a gaseous or vapor state, as warmmoist air, will move from its high pressure area to a lower pressure area where the air iscooler and drier. Liquid water will move as a result of differences in hydrostaticpressure or wind pressure. It is the pressure differentials that drive the rate ofmoisture migration in either state. Because the building materials themselves resistthis moisture movement, the rate of movement will depend on two factors: the permeabilityof the materials when affected by vapor and the absorption rates of materials in contactwith liquid.

The dynamic forces that move air and moistue through a building are important to understand, particularly when selecting a treatment to correct a moisture problem. This drawing shows how moisture can invade "inward" from the exterior; "upward" from the ground; and be generated from "within" the interior. All have damaging effects. Drawing: NPS files.

The mechanics, or physics, of moisture movement is complex, but if the driving force isdifference in pressure, then an approach to reducing moisture movement and its damage isto reduce the difference in pressure, not to increase it. That is why the treatmentsdiscussed in this Brief will look at managing moisture by draining bulk moisture andventilating vapor moisture before setting up new barriers with impermeable coatings orover-pressurized new climate control systems that threaten aging building materials andarchaic construction systems.

Three forms of moisture transport are particularly important to understand in regardsto historic buildings-infiltration, capillary action, and vapor diffusion--remembering,at the same time, that the subject is infinitely complex and, thus, one of continuingscientific study. Buildings were traditionally designed to deal with the movement of air.For example, cupolas and roof lanterns allowed hot air to rise and provided a naturaldraft to pull air through buildings. Cavity walls in both frame and masonry buildings wereconstructed to allow moisture to dissipate in the air space between external and internalwalls. Radiators were placed in front of windows to keep cold surfaces warm, therebyreducing condensation on these surfaces. Many of these features, however, have beenaltered over time in an effort to modernize appearances, improve energy efficiency, oraccommodate changes in use.

The change in use will also affect moisture movement, particularly in commercial andindustrial buildings with modern mechanical systems. Therefore, the way a building handlesair and moisture today may be different from that intended by the original builder orarchitect, and poorly conceived changes may be partially responsible for chronic moistureconditions.

Moisture moves into and through materials as both a visible liquid (capillary action)and as a gaseous vapor (infiltration and vapor diffusion). Moisture from leaks,saturation, rising damp, and condensation can lead to the deterioration of materials andcause an unhealthy environment. Moisture in its solid form, ice, can also cause damagefrom frozen, cracked water pipes, or split gutter seams or spalled masonry fromfreeze-thaw action. Moisture from melting ice dams, leaks, and condensation often cantravel great distances down walls and along construction surfaces, pipes, or conduits. Theamount of moisture and how it deteriorates materials is dependent upon complex forces andvariables that must be considered for each situation.

Determining the way moisture is handled by the building is further complicated becauseeach building and site is unique. Water damage from blocked gutters and downspouts cansaturate materials on the outside, and high levels of interior moisture can saturateinterior materials. Difficult cases may call for technical evaluation by consultantsspecializing in moisture monitoring and diagnostic evaluation. In other words, it may takea team to effectively evaluate a situation and determine a proper approach to controllingmoisture damage in old buildings.

Infiltration is created by wind, temperature gradients (hot air rising),ventilation fan action, and the stack or chimney effect that draws air up into tallvertical spaces. Infiltration as a dynamic force does not actually move liquid water, butis the vehicle by which dampness, as a component of air, finds its way into buildingmaterials. Older buildings have a natural air exchange, generally from 1 to 4 changes perhour, which, in turn, may help control moisture by diluting moisture within a building.The tighter the building construction, however, the lower will be the infiltration rateand the natural circulation of air. In the process of infiltration, however, moisture thathas entered the building and saturated materials can be drawn in and out of materials,thereby adding to the dampness in the air. Inadequate air circulation where there isexcessive moisture (i.e., in a damp basement), accelerates the deterioration of historicmaterials. To reduce the unwanted moisture that accompanies infiltration, it is best toincorporate maintenance and repair treatments to close joints and weatherstrip windows,while providing controlled air exchanges elsewhere. The worst approach is to seal thebuilding so completely, while limiting fresh air intake, that the building cannot breathe.

Capillary action occurs when moisture in saturated porous building materials,such as masonry, wicks up or travels vertically as it evaporates to the surface. Incapillary attraction, liquid in the material is attracted to the solid surface of the porestructure causing it to rise vertically; thus, it is often called "rising damp,"particularly when found in conjunction with ground moisture. It should not, however, beconfused with moisture that laterally penetrates a foundation wall through cracks andsettles in the basement. Not easily controlled, most rising damp comes from high watertables or a constant source under the footing.

In cases of damp masonry walls with capillary action, there is usually a whitish stainor horizontal tide mark of efflorescence that seasonally fluctuates about 1- 3 feet abovegrade where the excess moisture evaporates from the wall. This tide mark is full of saltcrystals, that have been drawn from the ground and building materials along with thewater, making the masonry even more sensitive to additional moisture absorption from thesurrounding air. Capillary migration of moisture may occur in any material with a porestructure where there is a constant or recurring source of moisture.

The best approach for dealing with capillary rise in building materials is to reducethe amount of water in contact with historic materials. If that is not possible due tochronically high water tables, it may be necessary to introduce a horizontal damp-proofbarrier, such as slate course or a lead or plastic sheet, to stop the vertical rise ofmoisture. Moisture should not be sealed into the wall with a waterproof coating, such ascement parging or vinyl wall coverings, applied to the inside of damp walls. This willonly increase the pressure differential as a vertical barrier and force the capillaryaction, and its destruction of materials, higher up the wall.

Vapor diffusion is the natural movement of pressurized moisture vapor throughporous materials. It is most readily apparent as humidified interior air moves out throughwalls to a cooler exterior. In a hot and humid climate, the reverse will happen as moisthot air moves into cooler, dryer, air-conditioned, interiors. The movement of the moisturevapor is not a serious problem until the dewpoint temperature is reached and the vaporchanges into liquid moisture known as condensation. This can occur within a wall oron interior surfaces.

Vapor diffusion will be more of a problem for a frame structure with several layers ofinfill materials within the frame cavity than a dense masonry structure. Condensation as aresult of vapor migration usually takes place on a surface or film, such as paint, wherethere is a change in permeability.

The installation of climate control systems in historic buildings (mostly museums) thathave not been properly designed or regulated and that force pressurized damp air todiffuse into perimeter walls is an ongoing concern. These newer systems take constantmonitoring and back-up warning systems to avoid moisture damage.

Long-term and undetected condensation or high moisture content can cause seriousstructural damage as well as an unhealthy environment, heavy with mold and mildew spores.Reducing the interior/exterior pressure differential and the difference between interiorand exterior temperature and relative humidity helps control unwanted vapor diffusion.This can sometimes be achieved by reducing interior relative humidity. In some instances,using vapor barriers, such as heavy plastic sheeting laid over damp crawl spaces, can haveremarkable success in stopping vapor diffusion from damp ground into buildings. Yet,knowledgeable experts in the field differ regarding the appropriateness of vapor barriersand when and where to use them, as well as the best way to handle natural diffusion ininsulated walls.

Adding insulation to historic buildings, particularly in walls of wooden framestructures, has been a standard modern weatherization treatment, but it can have adisastrous effect on historic buildings. The process of installing the insulation destroyshistoric siding or plaster, and it is very difficult to establish a tight vapor barrier.While insulation has the benefit of increasing the efficiency of heating and cooling bycontaining temperature controlled air, it does not eliminate surfaces on which damagingmoisture can condense. For insulated residential frame structures, the most obvious signof a moisture diffusion problem is peeling paint on wooden siding, even after carefulsurface preparation and repainting. Vapor impermeable barriers such as plastic sheeting,or more accurately, vapor retarders, in cold and moderate climates generally helpslow vapor diffusion where it is not wanted.

In regions where humidified climate control systems are installed into insulatedframe buildings, it is important to stop interstitial, or in-wall, dewpointcondensation. This is very difficult because humidified air can penetrate breaches in thevapor barrier, particularly around electrical outlets. Improperly or incompletelyinstalled retrofit vapor barriers will cause extensive damage to the building, just in theinstallation process, and will allow trapped condensation to wet the insulation andsheathing boards, corrode metal elements such as wiring cables and metal anchors, andblister paint finishes.

Providing a tight wall vapor barrier, as well as a ventilated cavity behind woodenclapboards or siding appears to help insulated frame walls, if the interior relativehumidity can be adjusted or monitored to avoid condensation. Correct placement of vaporretarders within building construction will vary by region, building construction, andtype of climate control system.

Surveying andDiagnosing Moisture Damage: Key Questions to Ask

It is important for the building to be surveyed first and the evidence and location ofsuspected moisture damage systematically recorded before undertaking any major work tocorrect the problem. This will give a baseline from which relative changes in conditioncan be noted.

When materials become wet, there are specific physical changes that can be detected andnoted in a record book or on survey sheets. Every time there is a heavy rain, snow storm,water in the basement, or mechanical systems failure, the owner or consultant should noteand record the way moisture is moving, its appearance, and what variables might contributeto the cause

Standing outside to observe a building in the rain may answermany questions and help trace the movement of water into the building.

Evidence of deteriorating materials that cover more serious moisture damage should alsobe noted, even if it is not immediately clear what is causing the damage. ( For example,water stains on the ceiling may be from leaking pipes, blocked fan coil drainage pansabove, or from moisture which has penetrated around a poorly sloped window sill above.)Don't jump to conclusions, but use a systematic approach to help establish an educatedtheory-or hypothesis-of what is causing the moisture problem or what areas need furtherinvestigation.

Surveying moisture damage must be systematic so that relativechanges can be noted.

Tools for investigating can be as simple as a notebook, sketch plans, binoculars,camera, aluminum foil, smoke pencil, and flashlight. The systematic approach involveslooking at buildings from the top down and from the outside to the inside. Photographs,floor plans, site plan, and exterior elevations-even roughly sketched-should be used toindicate all evidence of damp or damaged materials, with notations for musty or poorlyventilated areas. Information might be needed on the absorption and permeabilitycharacteristics of the building materials and soils. Exterior drainage patterns should benoted and these base plans referred to on a regular basis in different seasons and indiffering types of weather. It is best to start with one method of periodic documentationand to use this same method each time.

Because moisture is affected by gravity, many surveys start with the roof and gutteringsystems and work down through the exterior walls. Any obvious areas of water penetration,damaged surfaces, or staining should be noted. Any recurring damp or stain patterns, bothexterior and interior, should also be noted with a commentary on the temperature, weather,and any other facts that may be relevant (driving rains, saturated soil, high interiorhumidity, recent washing of the building, presence of a lawn watering system, etc.).

The interior should be recorded as well, beginning with the attic and working down tothe basement and crawl space. It may be necessary to remove damaged materials selectivelyin order to trace the path of moisture or to pinpoint a source, such as a leaking pipe inthe ceiling. The use of a basic resistance moisture meter, available in many hardwarestores, can identify moisture contents of materials and show, over time, if wall surfacesare drying or becoming damper. A smoke pencil can chart air infiltration around windows ordraft patterns in interior spaces. For a quick test to determine if a damp basement iscaused by saturated walls or is a result of condensation, tape a piece of foil onto amasonry surface and check it after a day or two; if moisture has developed behind thefoil, then it is coming from the masonry. If condensation is on the surface of the foil,then moisture is from the air.

Comparing current conditions with previous conditions, historic drawings, photographs,or known alterations may also assist in the final diagnosis. A chronological record,showing improvement or deterioration, should be backed up with photographs or notations asto the changing size, condition, or features of the deterioration and how these changeshave been affected by variables of temperature and rainfall. If a condition can be relatedin time to a particular event, such as efflorescence developing on a chimney after thebuilding is no longer heated, it may be possible to isolate a cause, develop a hypothesis,and then test the hypothesis (by adding some temporary heat), before applying a remedialtreatment. If the owner or consultant has access to moisture survey and monitoringequipment such as resistance moisture meters, dewpoint indicators, salt detectors,infrared thermography systems, psychrometer, fiber-optic boroscopes, and miniaturizedvideo cameras, additional quantified data can be incorporated into the survey.

If it is necessary to track the wetting and drying of walls over a period of time, deepprobes set into walls and in the soil with connector cables to computerized data loggersor the use of long-term recording of hygrothermographs may require a trained specialist.Miniaturized fiber-optic video cameras can record the condition of subsurface drain lineswithout excavation. It should be noted, however, that instrumentation, while extremelyuseful, cannot take the place of careful personal observation and analysis. Relying oninstrumentation alone rarely will give the owner the information needed to fully diagnosea moisture problem. To avoid jumping to a quick-potentially erroneous-conclusion, a seriesof questions should be asked first. This will help establish a theory or hypothesis thatcan be tested to increase the chances that a remedial treatment will control or manageexisting moisture.

How is water draining around building and site? What is the effectivenessof gutters and downspouts? Are the slopes or grading around foundations adequate? What arethe locations of subsurface features such as wells, cisterns, or drainage fields? Arethere subsurface drainage pipes (or drainage boots) attached to the downspouts and arethey in good working condition? Does the soil retain moisture or allow it to drain freely?Where is the water table? Are there window wells holding rain water? What is the flow rateof area drains around the site (can be tested with a hose for several minutes)? Is thestorm piping out to the street sufficient for heavy rains, or does water chronically backup on the site? Has adjacent new construction affected site drainage or water tablelevels?

How does water/moisture appear to be entering the building? Have all fiveprimary sources of moisture been evaluated? What is the condition of constructionmaterials and are there any obvious areas of deterioration? Did this building have abuilder's trench around the foundation that could be holding water against the exteriorwalls? Are the interior bearing walls as well as the exterior walls showing evidence ofrising damp? Is there evidence of hydrostatic pressure under the basement floor such aswater percolating up through cracks? Has there been moisture damage from an ice dam in thelast several months? Is damage localized, on one side of the building only, or over alarge area?

What are the principal moisture dynamics? Is the moisture condition fromliquid or vapor sources? Is the attic moisture a result of vapor diffusion as damp aircomes up through the cavity walls from the crawl space or is it from a leaking roof? Isthe exterior wall moisture from rising damp with a tide mark or are there uneven spots ofdampness from foundation splash back, or other ground moisture conditions? Is thereadequate air exchange in the building, particularly in damp areas, such as the basement?Has the height of the water table been established by inserting a long pipe into theground in order to record the water levels?

How is the interior climate handling moisture? Are there areas in thebuilding that do not appear to be ventilating well and where mold is growing? Are therehistoric features that once helped the building control air and moisture that can bereactivated, such as operable skylights or windows? Could dewpoint condensation beoccurring behind surfaces, since there is often condensation on the windows? Does thebuilding feel unusually damp or smell in an unusual way that suggest the need for furtherstudy? Is there evidence of termites, carpenter ants, or other pests attracted to moistconditions? Is a dehumidifier keeping the air dry or is it, in fact, creating a cyclewhere it is actually drawing moisture through the foundation wall?

The owner used long black extender pipes to test a theory that it was faulty roof drainage causing the problem. Photo: NPS files.

Does the moisture problem appear to be intermittent, chronic, or tied to specificevents? Are damp conditions occurring within two hours of a heavy rain or is therea delayed reaction? Does rust on most nail heads in the attic indicate a condensationproblem? What are the wet patterns that appear on a building wall during and after a rainstorm? Is it localized or in large areas? Can these rain patterns be tied to gutterover-flows, faulty flashing, or saturation of absorbent materials? Is a repaired areaholding up well over time or is there evidence that moisture is returning? Do moisturemeter readings of wall cavities indicate they are wet, suggesting leaks or condensation inthe wall?

Once a hypothesis of the source or sources of the moisture has been developed fromobservation and recording of data, it is often useful to prove or disprove this hypothesiswith interim treatments, and, if necessary, the additional use of instrumentation toverify conditions. For damp basements, test solutions can help determine the cause. Forexample, surface moisture in low spots should be redirected away from the foundation wallwith regrading to determine if basement dampness improves.

If there is still a problem, determine if subsurface downspout collection pipes or castiron boots are not functioning properly. The above grade downspouts can be disconnectedand attached to long, flexible extender pipes and redirected away from the foundation. If,after a heavy rain or a simulation using a hose, there is no improvement, look foradditional ground moisture sources such as high water tables, hidden cisterns, or leakingwater service lines as a cause of moisture in the basement. New data will lead to a newhypothesis that should be tested and verified. The process of elimination can befrustrating, but is required if a systematic method of diagnosis is to be successful.

Selectingan Appropriate Level of Treatment

The treatments that follow this section in chart format are divided into levels basedon the degree of moisture problems.

  • Level I covers preservation maintenance;
  • Level II focuses on repair using historically compatible materials and essentially mitigating damaging moisture conditions; and
  • Level III discusses replacement and alteration of materials that permit continued use in a chronically moist environment.

It is important to begin with Level I and work through to a manageable treatment aspart of the control of moisture problems. Buildings in serious decay will requiretreatments in Level II, and difficult or unusual site conditions may require moreaggressive treatments in Level III. Caution should always be exercised when selecting atreatment. The treatments listed are a guide and not intended to be recommendations forspecific projects as the key is always proper diagnosis.

Start with the repair of any obvious deficiencies using sound preservation maintenance.If moisture cannot be managed by maintenance alone, it is important to reduce it bymitigating problems before deteriorated historic materials are replaced. Treatmentsshould not remove materials that can be preserved; should not involve extensive excavationunless there is a documented need; and should not include coating buildings withwaterproof sealers that can exacerbate an existing problem. Some alteration to historicmaterials, structural systems, mechanical systems, windows, or finishes may be needed whenexcessive site moisture cannot be controlled by drainage systems, or in areas prone tofloods. These changes, however, should, be sensitive to preserving those materials,features, and finishes that convey the historic character of the building and site.

Level IPreservation Maintenance

Exterior: Apply cyclical maintenance procedures to eliminate rain and moistureinfiltration.

Installing ventilating fans can improve damp conditions or reduce cooling loads. Photo: NPS files.

Roofing/ guttering: Make weather-tight and operational; inspect and cleangutters as necessary depending on number of nearby trees, but at least twice a year;inspect roofing at least once a year, preferably spring; replace missing or damagedroofing shingles, slates, or tiles; repair flashing; repair or replace cracked downspouts.

Walls: Repair damaged surface materials; repoint masonry with appropriatelyformulated mortar; prime and repaint wooden, metal, or masonry elements or surfaces;remove efflorescence from masonry with non-metallic bristle brushes.

Window and door openings: Eliminate cracks or open joints; caulk or repointaround openings or steps; repair or reset weatherstripping; check flashing; repaint, asnecessary.

Ground: Apply regular maintenance procedures to eliminate standing water andvegetative threats to building/site.

Grade: Eliminate low spots around building foundations; clean out existingdownspout boots twice a year or add extension to leaders to carry moisture away fromfoundation; do a hose test to verify that surface drains are functioning; reduce moistureused to clean steps and walks; eliminate the use of chlorides to melt ice which canincrease freeze/thaw spalling of masonry; check operation of irrigation systems, hose bibleaks, and clearance of air conditioning condensate drain outlets.

Crawl space: Check crawl space for animal infestation, termites, pondingmoisture, or high moisture content; check foundation grilles for adequate ventilation;seasonally close grilles when appropriate-in winter, if not needed, or in summer if hothumid air is diffusing into air conditioned space.

Foliage: Keep foliage and vines off buildings; trim overhanging trees to keepdebris from gutters and limbs from rubbing against building; remove moisture retainingelements, such as firewood, from foundations.

A vent may be added if there is none. Close grilles in the summer, if hot humid air is getting into air conditioned spaces. Photo: NPS files.

Basements and foundations: Increase ventilation and maintain surfaces to avoidmoisture.

Equipment: Check dehumidifiers, sump pump, vent fans, and water detection oralarm systems for proper maintenance as required; check battery back-up twice a year.

Piping/ductwork: Check for condensation on pipes and insulate/seal joints, ifnecessary.

Interior: Maintain equipment to reduce leaks and interior moisture.

Plumbing pipes: Add insulation to plumbing or radiator pipes located in areassubject to freezing, such as along outside walls, in attics, or in unheated basements.

Mechanical equipment: Check condensation pans and drain lines to keep clear;insulate and seal joints in exposed metal ductwork to avoid drawing in moist air.

Cleaning: Routinely dust and clean surfaces to reduce the amount of water ormoist chemicals used to clean building; caulk around tile floor and wall connections; andmaintain floor grouts in good condition.

Ventilation: Reduce household-produced moisture, if a problem, by increasingventilation; vent clothes driers to the outside; install and always use exhaustfans in restrooms, bathrooms, showers, and kitchens, when in use.

Level II Repair and CorrectiveAction

Exterior: Repair features that have been damaged. Replace an extensivelydeteriorated feature with a new feature that matches in design, color, texture, and wherepossible, materials.

New drainage systems for roof run-off may be installed in order to remove moisture from the base of the building. Photo: NPS files.

Roofing: Repair roofing, parapets and overhangs that have allowed moisture toenter; add ice and water shield membrane to lower 3 to 4 feet or roofing in cold climatesto limit damage from ice dams; increase attic ventilation, if heat and humidity build-upis a problem. Make gutters slope at 1/8" to the foot. Use professional handbooks tosize gutters and reposition, if necessary and appropriate to historic architecture. Addventilated chimney caps to unused chimneys that collect rain water.

Walls: Repair spalled masonry, terra cotta, etc. by selectively installing newmasonry units to match; replace rotted clapboards too close to grade and adjust grade orclapboards to achieve adequate clearance; protect or cover open window wells.

Ground: Correct serious ground water problems; capture and dispose of downspoutwater away from foundation; and control vapor diffusion of crawlspace moisture.

Grade: Re-establish positive sloping of grade; try to obtain 6" of fall inthe first 10' surrounding building foundation; for buildings without gutter systems,regrade and install a positive subsurface collection system with gravel, or waterproofsheeting and perimeter drains; adjust pitch or slope of eave line grade drains or Frenchdrains to reduce splash back onto foundation walls; add subsurface drainage boots orextension pipes to take existing downspout water away from building foundation to thegreatest extent feasible.

Crawl space: Add polyethylene vapor barrier (heavy construction grade or Mylar )to exposed dirt in crawlspace if monitoring indicates it is needed and there is no risingdamp; add ventilation grilles for additional cross ventilation, if determined advisable.

Foundations and Basements: Correct existing high moisture levels, if other means ofcontrolling ground moisture are inadequate.

Mechanical devices: Add interior perimeter drains and sump pump; adddehumidifiers for seasonal control of humidity in confined, unventilated space ( but don'tcreate a problem with pulling dampness out of walls); add ventilator fans to improve airflow, but don't use both the dehumidifier and ventilator fan at the same time.

Walls: Remove commentates coatings, if holding rising damp in walls; coat wallswith vapor permeable lime based rendering plaster, if damp walls need a sacrificialcoating to protect mortar from erosion; add termite shields, if evidence of termites anddampness cannot be controlled.

Framing: Reinforce existing floor framing weakened by moisture by addinglolly column support and reinforcing joist ends with sistered or parallel supports. Add avapor impermeable shield, preferably non-ferrous metal, under wood joists coming intocontact with moist masonry.

Interior: Eliminate areas where moisture is leaking or causing a problem

Plumbing: Replace older pipes and fixtures subject to leaking or overflowing;insulate water pipes subject to condensation.

Ventilation: Add exhaust fans and whole house fans to increase air flow throughbuildings, if areas are damp or need more ventilation to control mold and mildew.

Climate: Adjust temperature and relative humidity to manage interior humidity;Correct areas of improperly balanced pressure for HVAC systems that may be causing amoisture problem.

LevelIII Replacement / Alterations For Chronically Damp Conditions 

Exterior: Undertake exterior rehabilitation work that follows professional repairpractices-i.e., replace a deteriorated feature with a new feature to match the existing indesign, color, texture, and when possible, materials. In some limited situations,non-historic materials may be necessary in unusually wet areas

Roofs: Add ventilator fans to exhaust roofs but avoid large projecting featureswhose designs might negatively affect the appearance of the historic roof. When replacingroofs, correct conditions that have caused moisture problems, but keep the overallappearance of the roof; for example, ventilate under wooden shingles, or detail standingseams to avoid buckling and cracking. Be attentive to provide extra protection forinternal or built-in gutters by using the best quality materials, flashing, and vaporimpermeable connection details.

Walls: If insulation and vapor barriers are added to frame walls, considermaintaining a ventilation channel behind the exterior cladding to avoid peeling andblistering paint occurrences.

Windows: Consider removable exterior storm windows, but allow operation ofwindows for periodic ventilation of cavity between exterior storm and historic sash. Forstained glass windows using protective glazing, use only ventilated storms to avoidcondensation as well as heat build-up.

Ground: Control excessive ground moisture. This may require extensive excavations,new drainage systems, and the use of substitute materials. These may include concrete ornew sustainable recycled materials for wood in damp areas when they do not impact thehistoric appearance of the building.

Grade: Excavate and install water collection systems to assist with positiverun-off of low lying or difficult areas of moisture drainage; use drainage mats and underfinished grade to improve run-off control; consider the use of column plinth blocks orbases that are ventilated or constructed of non-absorbent substitute materials inchronically damp areas. Replace improperly sloped walks; repair non-functioning catchbasins and site drains; repair settled areas around steps and other features at grade.

Foundations: Improve performance of foundation walls with damp-proof treatments tostop infiltration or damp course layers to stop rising damp. Some substitute materials mayneed to be selectively integrated into new features.

Walls: excavate, repoint masonry walls, add footing drains, and waterproofexterior subsurface walls; replace wood sill plates and deteriorated structuralfoundations with new materials, such as pressure treated wood, to withstand chronicmoisture conditions; materials may change, but overall appearance should remain similar.Add dampcourse layer to stop rising damp; avoid chemical injections as these are rarelytotally effective, are not reversible, and are often visually intrusive.

Interior: Control the amount of moisture and condensation on the interiors ofhistoric buildings. Most designs for new HVAC systems will be undertaken by mechanicalengineers, but systems should be selected that are appropriate to the resource andintended use.

Windows, skylights: Add double and triple glazing, where necessary to controlcondensation. Avoid new metal sashes or use thermal breaks where prone to heavycondensation.

Mechanical systems: Design new systems to reduce stress on building exterior.This might require insulating and tightening up the building exterior, but provisions mustbe made for adequate air flow. A new zoned system, with appropriate transition insulation,may be effective in areas with differing climatic needs.

Control devices/Interior spaces: If new climate control systems are added,design back-up controls and monitoring systems to protect from interior moisture damage.

Walls: If partition walls sit on floors that periodically flood, considerspacers or isolation membranes behind baseboards to stop moisture from wicking up throughabsorbent materials.

Ongoing Care

Once the building has been repaired and the larger moisture issues addressed, it isimportant to keep a record of additional evidence of moisture problems and to protectthe historic or old building through proper cyclical maintenance. In some cases,particularly in museum environments, it is critical to monitor areas vulnerable tomoisture damage. In a number of historic buildings, in-wall moisture monitors are used toensure that the moisture purposely generated to keep relative humidity at rangesappropriate to a museum collection does not migrate into walls and cause deterioration.The potential problem with all systems is the failure of controls, valves, and panels overtime. Back-up systems, warning devices, properly trained staff and an emergency plan willhelp control damage if there is a system failure.

Ongoing maintenance and vigilance to situations that could potentially cause moisturedamage must become a routine part of the everyday life of a building. The owner or staffresponsible for the upkeep of the building should inspect the property weekly and note anyleaks, mustiness, or blocked drains. Again, observing the building during a rain will testwhether ground and gutter drainage are working well.

For some buildings a back-up power system may be necessary to keep sump pumps workingduring storms when electrical power may be lost. For mechanical equipment rooms,condensation pans, basement floors, and laundry areas where early detection of water isimportant, there are alarms that sound when their sensors come into contact with moisture.


Moisture in old and historic buildings, though difficult to evaluate, can besystematically studied and the appropriate protective measures taken. Much of thedocumentation and evaluation is based on common sense combined with an understanding ofhistoric building materials, construction technology, and the basics of moisture and airmovement. Variables can be evaluated step by step and situations creating direct orsecondary moisture damage can generally be corrected.

The majority of moisture problems can be mitigated with maintenance, repair, control ofground and roof moisture, and improved ventilation. For more complex situations, however,a thorough diagnosis and an understanding of how the building handles moisture atpresent, can lead to a treatment that solves the problem without damaging the historicresource.

It is usually advantageous to eliminate one potential source of moisture at a time.Simultaneous treatments may set up a new dynamic in the building with its own set ofmoisture problems. Implementing changes sequentially will allow the owner or preservationprofessional to track the success of each treatment.

Moisture problems can be intimidating to a building owner who has diligently tried tocontrol them. Keeping a record of evidence of moisture damage, results of diagnostictests, and remedial treatments, is beneficial to a building's long-term care. The morecomplete a survey and evaluation, the greater the success in controlling unwanted moisturenow and in the future.

Holding the line on unwanted moisture in buildings will be successful if:

  • there is constant concern for signs of problems and
  • there is ongoing physical care provided by those who understand the building, site, mechanical systems, and the previous efforts to deal with moisture.

For properties with major or difficult-to-diagnose problems, a team approach is oftenmost effective. The owner working with properly trained contractors and consultants canmonitor, select, and implement treatments within a preservation context in order to managemoisture and to protect the historic resource.

Reading List

  • Conrad, Ernest A., P.E. "The Dews and Don'ts of Insulating." Old-House Journal, May/June, 1996.
  • Cumberland, Don, Jr. "Museum Collection Storage in an Historic Building Using a Prefabricated Structure."
  • Preservation Tech Notes. Washington, DC: National Park Service, issue PTN-14. September, 1985.
  • Jessup, Wendy Claire, Ed. Conservation in Context: Finding a Balance for the Historic House Museum. Washington, DC: National Trust for Historic Preservation (Symposium Proceedings March 7-8, 1994).
  • Labine, Clem. "Managing Moisture in Historic Buildings" Special Report and Moisture Monitoring Source List. Traditional Building, Vol 9, No.2, May-June 1996.
  • Leeke, John. "Detecting Moisture; Methods and Tools for Evaluating Water in Old Houses." Old House Journal, May/June, 1996.
  • Moisture Control in Buildings. Heinz R. Trechsel, Editor. Philadelphia: American Society for Testing and Materials (ASTM manual series: MNL 18), 1993.
  • Mus

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