Insulating walls

The Old House Web

Sections of this story: Overview: Essential Knowledge ~~ Techniques, materials and tools ~~ Taking it further

Overview: Essential Knowledge

insulating attic
Installing batt insulation in an attic
Photo: Karen Doherty

Insulation is one element in a tightly knit construction system intended toimprove indoor comfort and reduce energy consumption. In rehab work, installinginsulation or improving existing insulation levels will be critical in providingcomfort.

Insulation should never be applied without considering its effect on otheraspects of construction. Some factors to consider when evaluating wallinsulation are density and compressibility, air leakage, moisture control, firesafety, and wall construction in existing homes.


Each type of insulation has a density at which its R-value per inch isgreatest, but reaching this density is not always cost-effective.

For 3 1 /2-inch thick fiberglass batts, an R-13 batt contains 40percent more material. An R-15 batt contains 180 percent more material, than anR-11 batt (Fig. 1).



To achieve a desired overall R-value for dry blown-in insulation, and toprevent settlement, the installed density must be above a recommended minimum.For convenience in comparing estimates or monitoring the installation, have thebidder or installer calculate the number of bags required to achieve therequired density.

Unless insulation completely fills all the wall cavities, air leakage canbypass the insulation and create a risk of condensation. Reducing air leakage isan inseparable part of insulating: you should not do one without doing theother. Typical locations for air leakage through walls are at the sill, the wallplates, vertical corners, around openings, and at electrical devices.

Before filling stud cavities of older homes with blown-in or foamed-in-placeretrofit insulation, explore the construction. Stud cavities are ofteninterrupted by blocking half-way up the wall, or in girt-frame construction, byfull-depth diagonal corner braces. The outside walls may be"back-plastered," where a hidden layer of plaster creates two parallelcavities within each stud space, neither deep enough to receive loose-fillinsulation.

After insulating, an infrared camera scan of the wall will show cavities thathave not been fully insulated. Avoid deliberately ventilating walls, since anyconvective airflow within an outside wall risks condensation within the wall andcompromises its R-value. Ventilation passages behind the exterior finish arecalled for when extreme interior humidity is expected and no vapor retarder canbe applied, or where wood siding is applied directly over exterior foaminsulation.

Return to Top of Page

Techniques, materials and tools

Of the innumerable possible combinations of insulating materials and wallconfiguration, the following list covers those in common use and uncommonsystems that are recommended.



batt insulation

Fiberglass insulation is available in batt form, typically sized 93 incheslong to fit within the stud-space of an 8-foot wall, or in continuous rolls.Both forms are here referred to as "batt insulation." It is availablein many thicknesses, densities and in widths to fit framing at 16-inch and24-inch centers.

Unfaced batts can easily be cut to fit into odd-sized spaces, and arepreferred where a continuous membrane vapor retarder is installed. Residentialbatts are available faced with kraftpaper and aluminum foil, and commercialbatts with a flame-resistant foil facing are available.

All have extended tabs on the facings to secure them in place. When properlyapplied, the facings create a partial vapor retarder. Only unfaced orfire-retardant-faced batts can be left exposed in attics or occupied spaces. Ifnot accurately cut around wiring and other obstacles, faced batts create largeair cavities that compromise their effectiveness. Tabs can be "insetstapled" to the sides of the studs, or "face stapled" to theinner face (Fig. 2).

Face stapling is preferred because it creates a better vapor retarder andavoids the air cavity left between the facing and the wall finish when insetstapling. Unless this cavity is carefully sealed at the top and bottom, it cancompromise the wall's air-tightness and R-value.

Staples into stud faces must be fully set to avoid interfering with drywallinstallation.

In a three-sided wall cavity, friction-fit unfaced batts, covered with aseparate vapor retarder, will typically result in a more effective installationthan will stapled faced batts. In an open wall, the facings are usuallynecessary for attachment.

  • ADVANTAGES: An economical, flexible, and well-known product. Provides a dependable thickness of uniform density and does not settle, if properly installed. Faced batts can insulate an open stud wall.
  • DISADVANTAGES: Effectiveness requires careful installation to avoid gaps and consequent convective losses. Glass fibers can be irritating if touched or inhaled.


Fiberglass insulation is available in rolls or batts, encapsulated with kraftpaper or plastic to reduce mechanical irritation to installers. These can beused in any installation where batts are appropriate. One face is extended toform attachment tabs. The faces on sound control batts do not have a vaporretarder; and some exterior wall batts have a polyethylene vapor retarder on theflange side.

Some encapsulated batts have a Class A fire rating and can be left exposed ifallowed by local codes. Owens Corning markets Miraflex, made from loose,virtually itch-free glass fibers with no binder, contained in a plastic sleeve.Cutting encapsulated batts around obstructions is possible, but exposes thefiberglass. Cutting Miraflex releases the fibers; the product is is thereforeintended primarily as attic floor insulation.

  • ADVANTAGES: Ideal for rehab contractors. Can be left exposed (check local codes). Flange-attached with or without a vapor retarder on some products.
  • DISADVANTAGES: More costly than regular batt insulation. Cutting encapsulated batts exposes the fiberglass. Cutting Miraflex releases the fibers.


Loose-fill insulation (fiberglass, cellulose, or mineral wool) can be blowninto closed stud spaces through openings formed by drilling holes through theinterior or exterior finish, or by removing strips of interior or exteriorfinish, at the top and bottom of each rafter space.

Careful installation is required because material can bridge over electriclines and other obstructions, causing voids and later settlement. Beware ofblocking; blow into cavities above and below it. At least a moderate amount ofpressure is required to produce sufficient density to inhibit settlement.

Fiberglass blown into a 2 by 4 stud cavity at a density of about 1.5 pcfproduces R-13 without excess pressure on finishes. In a "dense-pack"installation of cellulose, dry material is applied at high velocity through anarrow tube inserted through a single hole at the top and extended to the bottomof the cavity. The tube is gradually withdrawn, compacting the material to adensity of 3 to 3 1 /2 pcf.

"Stabilized" cellulose includes an adhesive, and "fiberized"cellulose is made in strands instead of chunks; both processes are claimed toinhibit or prevent settlement. Insulation packed into and filling wall cavitiessuppresses air movement within the cavity, does not create a vapor retarder, butmay substantially improve fire safety.

  • ADVANTAGES: If the cavities are completely filled under sufficient pressure, provides superior insulating performance without settlement, greatly reducing air circulation within the walls. May improve fire safety.
  • DISADVANTAGES: Some types of installation may leave voids and/or settle after installation. Blown-in materials do not form a vapor retarder, and form only a partial air barrier.


Figure 3: Installing Ark-Seal's BIBS system

An inexpensive material can be applied as a membrane over open studs to form"see-through" cavities, within which any blown-in insulation can beapplied under moderate pressure. In Ark-Seal's "Blown-in-Blanket(BIBS)" system, fiberglass mixed with some water and adhesive is blownthrough slits cut in a tightly stretched nylon netting. In Par-Pac's "Dry-PacWall System," dry cellulose is blown at 3 pcf density into a cavity closedby a reinforced polyethylene vapor retarder (Fig. 3).

The material is installed from the bottom up to minimize voids andsettlement. All such installations will cause the membrane to bulge out; makesure this bellying does not interfere with drywall installation. Various typesof polyurethane-and polyisocyanurate-based insulations, such as Icynene, canalso be spray-applied into stud cavities. A thin layer of such material can forman air-barrier skin, over which cheaper material can be placed.

Excess sprayed on insulation must be scraped off.
Photo: Paul Torcellini

Excess material must be scraped off, and windows and electrical devicesprotected or cleaned. Water is mixed with cellulose in a "wet-spray"application. The water combines with the starch in the cellulose to form anatural adhesive, which holds the material in place. Excess material must bescraped off, but can be reused. The material must dry out before a finish isapplied; excessive water can prevent drying and generate rot or mildew.

High-density insulation filling cavities may improve fire safety.

  • ADVANTAGES: Fills the cavities without settling. Visual inspection is possible to ensure filled cavities. Greatly reduces air circulation within walls. May improve fire safety.
  • DISADVANTAGES: Bellying of the interior membrane may interfere with drywall installation. Sprayed-on foam products are more expensive than batt or loose-fill installations, and are messy processes, requiring cleanup and protection. Excess water in wet-spray applications may lead to rot and mildew.


A 3 /4-inch to 1-inch layer of rigid insulation, typically polyisocyanurate(ISO), molded expanded polystyrene (EPS), or extruded polystyrene (EPS), is awidely used adjunct to cavity insulation. Where cavity insulation cannot beinstalled, rigid foam may be the only way to insulate a wall.

It is preferably applied on the outside of the framing, keeping the framingwarm enough in cold weather to prevent condensation within the walls, andinhibiting thermal short-circuits through the studs. It is also useful on theoutside as a base for cement stucco or exterior insulation and finish systems (EIFS),although the latter should be part of an engineered system that providesinterior drainage.

A layer of foam is essential in conjunction with conventional steel framingto prevent surface condensation. In Gulf-coast climates, a layer of foam behindthe interior finish is preferred over an exterior layer or a layer on both sidesof the studs, especially with steel framing.

Since foam cannot be relied upon to resist racking, it must be applied overstructural sheathing or in parallel with a system of wall shear bracing.Structural sheathing separated from the framing by an outside layer of foam maynot meet code racking requirements (consult with a structural engineer). Celotexmakes a structural polyisocyanurate foam sheathing that, when glued and nailedto the framing, acts as racking bracing. Wood siding should not be applieddirectly to foam insulation.

  • ADVANTAGES: Isolates framing to minimize or eliminate internal condensation and reduce cold bridging through framing. Can add more than its rated R-value to a wall assembly.
  • DISADVANTAGES: More costly per R than fiberglass insulation. If substituted for exterior sheathing, requires other measures to create racking resistance in the structure. Should not be used with EIFS except as part of an engineered system with interior drainage. EPS cannot support one-coat cement stucco over more than 16" stud spacing.


While radiant barriers and coatings are commonplace in high-temperatureindustrial applications (typically 500 F or more), they are marginallyeffective at ordinary temperatures. To be cost-effective in buildingapplications, they must have a very low incremental cost (from 2 to 10 cents/squarefoot, depending on the application).

Clean, shiny aluminum foil facing a 3 /4 inch or deeper air space can createa radiant barrier. Foil-faced insulation held back and inset-stapled createsonly a marginally effective radiant barrier because the insulation bulges intothe air space, and because the cavity can create heat loss through convection. Itis always more effective to fill the cavity with insulation.

If an air space is present for other reasons (for example, the cavity betweensheathing and brick veneer), facing the air space with foil-faced sheathing willadd thermal resistance if the material remains clean. A new form of radiantbarrier is Radiance paint, which contains aluminum dust.

  • ADVANTAGES: An easy way to add insulating value at brick cavity walls.
  • DISADVANTAGES: Marginally cost-effective; unlikely to be effective if exposed to dirt or condensation. Not cost-effective in cold climates, except possibly Radiance paint. Seldom or never cost-effective if the air space is deliberately "stolen" from conventional insulation. Foil may create a vapor barrier where one is not desired.


Rigid foam insulation adhered to structural skins can create a structuralinsulated panel (SIP). SIPs provide a combination of structure and insulation.Depending upon the design, the panels may be self-supporting or may be anon-load-bearing exterior skin applied over a post and beam frame.

Very tight joints are crucial, since a small amount of air leakage through ajoint is guaranteed to create destructive condensation at the most criticalstructural point. Panels are typically fabricated to order and delivered to thesite for quick erection. Acoustical tightness is readily noticeable.

  • ADVANTAGES: A high-R wall that can be load bearing and resists racking. Provides excellent acoustical resistance. Allows a high level of prefabrication and fast on-site erection.
  • DISADVANTAGES: Not yet in common use, and therefore more expensive than ordinary framing. Requires careful air-sealing at all joints.

Return to Top of Page

Taking it further: Reading list & Supplier information


  • ASHRAE, 1997 Handbook of Fundamentals, Inch-Pound Edition, Chapter 22: Thermal and Moisture Control in Insulated Assemblies--Fundamentals; Chapter 23: Thermal and Moisture Control in Insulated Assemblies-- Applications; Chapter 24: Thermal and Water Vapor Transmission Data, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc., Atlanta, GA; 404-636-8400; www.ashrae.org.
  • "Fiberglass vs. Cellulose: Making the Choice," Ted Cushman, Journal of Light Construction, Sept. 1995, pp 27-31.
  • Energy Source Catalog; Iris Communications, Inc.; 800-346-0104.
  • Energy Star Fact Sheets; Builder Guides; U. S. Environmental Protection Agency (EPA), Atmospheric Pollution Prevention Division; 888-STAR-YES.
  • Home Energy's Consumer Guide to Insulation; www.homeenergy.org/
  • Residential Energy Efficiency Database (REED);
  • Sawnee Energy Library; www.energydepot.com.
  • U. S. Department of Energy (DOE) Energy Efficiency and Renewable Energy Clearing House (EREC); 800-363-3732; e-mail: doe.erec@nclinc.com.
  • "Insulation Fact Sheet," August 1997, Document DOE/CE-0180.
  • "Loose-Fill Insulations," May, 1995, Document DOE/GO-10095-060.
  • "Insulation Materials: Environmental Comparisons," Alex Wilson, Environmental Building News, Volume 4, Number 1, January/ February 1995.
  • "Insulation Comes of Age," Alex Wilson, Fine Homebuilding, February/ March 1996, No. 100.

Return to Top of Page


  • Cellulose Insulation Manufacturers Association, 136 South Keowee Street, Dayton, OH 45402; 937-222-2462; www.cellulose.org.
  • American Rockwool, Inc., P. O. Box 880, Spring Hope, NC 27882; 919-478-5111.
  • Ark-Seal International, 2190 South Kalamath, Denver, CO 80223; 800-525-8992.
  • Building Products Division, The Celotex Corporation, P. O. Box 31602, Tampa, FL 33631; 813-873-4000.
  • CertainTeed Corporation, 750 East Swedesford Road, Valley Forge, PA 19482; 800-523-7844.
  • GreenStone Industries Inc., 6500 Rock Spring Drive, Suite 400, Bethesda, MD 20817; 888-592-7684.
  • Icynene Inc., 376 Watline Avenue, Mississauga, ON L4Z 1X2, Canada; 800-946-7325.
  • International Cellulose Corp., 12315 Robin Boulevard, Houston, TX 77245-0006; 800-444-1252.
  • Johns Manville Corporation (formerly Schuller International Inc.), 717 17th Street, Denver, CO 80202; or P. O. Box 5108, Denver, CO 80217-5108; 800-654-3103.
  • Knauf Fiber Glass, 240 Elizabeth Street, Shelbyville, IN 46176; 800-200-0802.
  • Owens Corning, One Owens Corning Parkway, Toledo, OH 43659; 800-354-PINK or 800-GET-PINK.
  • Par-Pac, 4545 East 52nd Avenue, Commerce City, CO 80022; 800-850-8505.
  • Rock Wool Manufacturing Co., 203 North Seventh Street, Leeds, AL 35094; 205-699-6121.
  • Sloss Industries Corporation, 3500 35th Avenue North, Birmingham, AL 35207; 205-808-7803.
  • U. S. Fiber, Inc., 905 East Martin Luther King Dr., Suite 400, Tarpon Springs, FL 34689; 800-666-4824.
  • Western Fiberglass Group, 6955 Union Park Center, Suite 580, Midvale, UT 84047; 801-562-9558.


  • Structural Insulated Panel Association, 1331 H Street NW, Suite 1000, Washington D. C. 20005; 202-347-7800; e-mail: sipadc@aol.com.


  • Expanded Polystyrene Molders Association (EPSMA), 1926 Waukegan Road, Suite 1, Glenview, IL 60025-1770; 800-607-3772.
  • Polyisocyanurate Insulation Manufacturer's Association (PIMA), 1001 Pennsylvania Avenue, N. W., 5th Floor, Washington, DC 20004; 202-624-2709; www.pima.org.
  • Celotex Building Products, P. O. Box 31602, Tampa, FL 33631-3602; 813-873-4230.
  • Dow Chemical Company, Styrofoam Brand Products, 2020 Willard H. Dow Center, Midland, MI 48674; 800-258-2436.
  • Johns Manville Corporation (formerly Schuller International Inc.), 717 17th Street, Denver, CO 80202; or P. O. Box 5108, Denver, CO 80217-5108; 800-654-3103.
  • Owens Corning, One Owens Corning Parkway, Toledo, OH 43659; 800-354-PINK or 800-GET-PINK.
  • Tenneco Building Products, 2907 Log Cabin Dr., Smyrna, GA 30080; 800-241-4402.


  • Energy-Brace reflective sheathing, Fiber-Lam, Inc., P. O. Box 2002, Doswell, VA 23047; 804-876-3135.
  • Radiance Low-e interior paint, ChemRex, 889 Valley Park Drive, Shakopee, MN 55379; 800-433-9517.
  • Thermo-ply reflective sheathing, Simplex Products Division, P. O. Box 10, Adrian, MI 49221; 517-263-8881.

Return to Top of Page

This story is excerpted from TheRehab Guide: Roofs , one in a series of guidebooks produced by the U.S.Department of Housing and Urban Development (HUD) to keep the design andconstruction industry abreast of innovations and state-of-the-art materials andpractices in home rehabilitation.

About the Author
The Old House Web

Search Improvement Project