Insulation: The Facts Part 4

The Old House Web

Confused about R-values, rolled or blown-ininsulation, vapor barriers and other insulation terms? This series of stories,adapted from the U.S. Department of Energy "InsulationFact Sheet" explains the ins and outs of home insulation -- and may helpyou save significantly on your heating and cooling costs.

Topicscovered in Part 4: Typical areas of airleaks * Othersources of leaks and how to fix them * Controllingmoisture * The importance ofventilation *

Typical areas of air leaks
house insulationSometimes joints between walls and floors allow open passage of air betweenthe heated part of the house and the attic area or outdoors.

Look for suchjoints in your attic or in the space over a porch ceiling. This air leakage pathis commonly found in Cape Cod-type houses, or if attic space has been convertedto living space (see 2D in figure at right).

A similar arrangement occurs when the second floor of a two-story houseis larger than the ground floor and has an overhang over the outdoors (see 4D).

Another major source of air leakage can be the joint between a porchroof and a side wall. If you can reach these areas, you can stop the leaks bycarefully covering the openings with plywood. If the areas are more difficult toreach, you can greatly reduce the air leakage by blowing high-density insulationor injecting plastic foam insulation into these joints, thus reducing theseenergy-gobbling air paths.

Other sources of air leaks and how to fix them

Air leaks into houses through small openings around doors and window frames and through fireplaces andchimneys. Air also enters the living space from other unheated parts of thehouse, such as attics, basements, or crawl spaces. The air travels through anyopenings in your walls, floors, or ceilings, such as cracks where two wallsmeet, where the wall meets the ceiling, or near interior door frames. Otheropenings may also be found, such as gaps around electrical outlets and switchboxes, recessed fixtures, recessed cabinets, pull-down stairs, furred or falseceilings such as kitchen or bathroom soffits, behind bath tubs and shower stallunits, floor cavities of finished attics adjacent to unconditioned attic spaces,and plumbing connections.

These leaks between the living space and other partsof the house are often much greater than the obvious leaks around windows anddoors. Since many of these leakage paths are driven by the tendency for warm airto rise and cool air to fall, the attic is often the best place to stop them.

It'simportant to stop these leaks before adding attic insulation because theinsulation may hide them and make them less accessible. Usually, the atticinsulation itself will not stop these leaks and you won't save as much as youexpect because of the air flowing through the insulation. Sometimes these leaklocations are visible because the existing insulation has been stained by dustcarried by the air flow. Some of the openings to look for include:

Source of Leak Solution
Top openings of interior partition wall cavities Staple a plastic sheet over the opening and seal it around the edges with a high quality caulking material.
Around the chimney Pack gaps around an insulated chimney with unfaced rock wool or unfaced fiberglass insulation. Do not insulate bare, hot flue pipes. Do not use any combustible products, such as cellulose insulation or plastic foams here.
Around the attic trap door or entry door Weatherstrip the edges.
Areas above staircase ceilings and dropped ceilings Staple a plastic sheet over the opening and seal it around the edges with a high quality caulking material.
Around pipes (look under your sinks and behind your toilets) and ducts penetrating a wall or attic floor Pack insulation tightly into the gap. You can also fill the area around them with spray polyurethane foam.

Controlling moisture

Moisture control is a major concern associated with installing thermalinsulation. The warm air inside your house contains water vapor. If this vaporpasses into the insulation and condenses, it can cause significant loss ofinsulating value.

If moisture becomes deposited in the building structure, itcan cause mold growth, peeling paint, and eventual rotting of structural wood.To guard against moisture problems, use vapor retarders and provide adequateventilation for the house. If you have a crawl space you should place a vaporretarder on the ground surface.

Vapor retarders are special materials including treated papers, plasticsheets, and metallic foils that reduce the passage of water vapor. Vaporretarders should be used in most parts of the country. In colder climates, placethe vapor retarder on the warm side--the lived-in side--of the space to beinsulated. This location prevents the moisture in the warm indoor air fromreaching the insulation. If you live in an area where the climate ispredominantly hot and humid, check with a local builder to determine the correctplacement or need for a vapor retarder.

Batts and blankets can be purchased with a vapor retarder attached. However,if new material is being added to insulation already in place, use batts orblankets that do not have an attached vapor retarder. If this type is notavailable, be sure to remove the vapor retarder facing (or slash it with a sharpknife) between layers of insulation to allow any moisture which does get intothe insulation to pass through.

For loose-fill insulation or for batts and blankets not having an attachedvapor retarder, heavy-weight polyethylene plastic sheets are available in rollsof various widths for use as vapor retarders. In places where vapor retardantmaterials cannot be placed, such as in finished wall cavities being filled withblown-in insulation, the interior surface of the wall can be madevapor-resistant with a low-permeability paint, or with wall paper that has aplastic layer.

The importance of ventilation

Adequate ventilation in your house is important for two reasons:

  • Moisture control. Ventilation will prevent elevated moisture levels within the conditioned space during the heating season. These elevated levels can lead to condensation on window surfaces and give rise to surface mold and mildew, as well as concealed condensation within walls and roof spaces.
  • Avoiding indoor air pollution. When natural ventilation has been sharply reduced, as in super-energy-efficient houses, it may be necessary to provide fresh air ventilation to avoid build-up of stale air and indoor air pollutants. Special air exchange units with heat-saving features are available for this purpose.

A well-insulated attic should be adequately ventilated to prevent moistureaccumulation. Attics may be ventilated with a combination of soffit vents ateaves and continuous ridge vents. Attic vents may also be installed in gablefaces.

Many codes and standards require one square foot of unobstructedventilation opening for each 300 square feet of attic floor area if a vaporretarder is included in the top floor ceiling. Twice as much ventilation isrecommended if there is no vapor retarder.

The net free area of a vent issmaller than its overall dimension because part of the vent opening is blockedby meshes or louvers. The openings should be equally distributed between thesoffit and ridge vents or between each gable face. Never cover or block ventswith insulation. Take care to prevent loose-fill insulation from clogging ventsby using baffles.

There is some controversy on whether or not to ventilate a crawl space. Most building codesrequire installation of ventsto provide ventilation with outside air, but a recent symposium on crawl spacedesign organized by the American Society of Heating, Refrigerating and AirConditioning Engineers concluded that there is no compelling technical basis forcrawl space ventilation requirements.

However, if the crawl space is notventilated, it is crucial that all of the crawl space ground area be coveredwith a durable vapor retarder, such as heavy-weight polyethylene film.

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What kind of insulation should you buy?

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