Exterior Walls and Insulation: The Most Complex Systems in a Building

An effective wall should have the following characteristics:

• Airtight – all air leaks should be sealed in the wall during construction and before the insulation is installed in the wall;

• Control moisture – an exterior rain drainage system, an air barrier, and a vapor diffusion retarder should be installed on the wall on the correct side of the wall; and

• Complete insulation coverage – the wall framing should provide enough room for complete, maximum insulation coverage and reduce thermal bridging, with no open gaps or compressed insulation, and continuous insulated sheathing.

Air sealing reduces heat flow from air movement (convection). Air sealing prevents water vapor in the air from entering the wall assembly. In a 100-square-foot wall, one cup of water can diffuse through drywall without a vapor diffusion retarder in a year. 30 quarts of water can enter through a 1-inch hole in a 4x8 sheet of drywall. That’s a lot of water through a small hole.

Air sealing is 10 to 100 times as important as installing a vapor diffusion retarder.

Control Moisture:

It is incorrect to think that a vapor diffusion retarder is the most important step for controlling moisture in a wall. Vapor diffusion retarders only retard moisture due to diffusion, while most moisture enters a wall either through fluid capillary action or as water vapor through air leaks. A drainage plane in a wall system makes an easy pathway for water to drain away from the house. Rain leaks cause a lot of problems for walls. Water penetration into a wall can come from improper installation of the siding, poor flashing installation, bad weather stripping, poor caulking around joints at the building exterior, and wind-driven rain into and through the exterior siding material. A drainage plane in the wall assembly protects against water intrusion. Houses in all climates require the following to control moisture:

• A polyethylene sheet should cover the exposed dirt of houses with crawlspaces;

• The grading around the house should slope away from the foundation;

• A continuous vapor diffusion retarder with a perm rating less than 1.0 should be installed;

• A termite shield, sill gasket, or other vapor-impermeable membrane on the top of the foundation wall should be installed to prevent moisture from wicking into the wood framing components by capillary action.

OVE Techniques:

The U.S. Forestry Products Association has come up with “optimum value engineering” (OVE) framing techniques that reduce unnecessary lumber use and improves the R-value of the wall by reducing thermal bridging and maximizing the wall area that is insulated. Those OVE techniques include:

• Eliminating unnecessary framing at intersections using two-stud rather than 3-stud corners;

• Interior-bracing (or let-in bracing) to allow the use of insulated sheathing in corners;

• Framing based upon a 24-inch instead of a 16-inch on-center measurement;

• Building a house using compact designs, simpler shapes, and volumes;

• Aligning windows and doors with existing stud spacing; and

• Using insulated headers.

Wall Sheathing:

Many builders use the common -inch wood sheathing, which has an R-value of 0.6. They may use an asphalt-impregnated sheathing called blackboard that has an R-value of 1.3.

If you are inspecting this type of insulated wall sheathing during construction, the sheathing should completely cover the wall and should be sealed to the top plate and band joist at the floor. Once installed, the sheathing should be sealed around all holes and penetrations. The seams should be sealed with caulk or housewrap tape according to the manufacturer’s recommendations.

There is way to inspect the insulation that exists inside a wall cavity, but it is very dangerous you could hurt yourself or have a fatal accident. You might consider hiring an electrician to accompany you on your inspection.

To inspect the insulation inside an exterior wall:

• Find an electrical outlet located at that wall;

• Turn off the power to the electrical wall outlet;

• Remove the outlet cover and shine a flashlight into the crack around the outlet box. You should be able to see if there is insulation in the wall and possibly how thick it is;

• Pull out a small amount of insulation if needed to help determine the type of insulation. A wooden skewer stick might be helpful in this task;

• Check outlets on the first and upper floors, if any, and in old and new parts of a house. Just because you find insulation in one wall doesn't mean that it's everywhere in the house.

Inspect and measure the thickness (inches) of any insulation in unfinished basement ceilings and walls, or above crawlspaces:

If the crawlspace isn't ventilated, it may have insulation installed at the perimeter walls. If the house is relatively new, it may have been built with insulation outside the basement or foundation walls. If so, the insulation in these spaces won't be visible. The homeowner might be able to tell you about the insulation.

If the house has newer siding, there may have been some new thermal insulation installed prior to the siding being installed. Sometimes insulation is blown into a wall cavity, and you’ll see drilled circular holes (1  to 2  inches in diameter) in the siding outside. The holes were the place through which the new insulation was blown into the wall.

The first-floor band joist may be readily accessible from the basement or crawlspace. It should be properly insulated.

If Batt or Rigid Insulation is Installed:

If batt or rigid insulation is installed to insulate the inside of concrete basement foundation walls, it might be necessary to have installed furring strips to the walls by nailing or bonding. Or you may find that an interior stud wall on which the insulation and interior wall finish is attached was installed around the basement perimeter.

The kraft paper or standard foil vapor diffusion retarder facings on the blanket insulation should be covered with gypsum or interior paneling because of the fire hazard. Some types of blanket insulation have a special flame resistant facing (labeled FS25 - flame spread index 25). There are some blanket insulation products for basement wall installations that can be left exposed, and they have a flame-resistant facing and are labeled with ASTM C 665, Type II, Class A.

Look at the underside of any floor that is over an unheated space like a crawlspace, basement, or garage. Check and measure the insulation that is installed there. It will likely be batt insulation made from fiberglass. Multiply the thickness of the fiberglass matt insulation by 3.2 to figure out the general R-value. The insulation might be identified with the R-value printed on it. If the insulation is a foam board or a sprayed foam application and you can’t find any labeling, then multiply the thickness by 5 to get a good estimate of the R-value.