Insulating Unvented Attics With Spray Foam

Closed-cell polyurethane foam provides the insulation, air barrier, and vapor retarder necessary for an unvented attic assembly

Code Provisions for Unvented Attics

Every state except California and Hawaii has adopted some version of the IRC. And California is expected to adopt it in 2008.

Until recently, the IRC required all attics and enclosed rafter spaces to be vented. But the latest version allows unvented attic assemblies if certain conditions are met.

According to Section R806.4 of the 2006 IRC, unvented assemblies are allowed if "no interior vapor retarders are installed on the ceiling side (attic floor) of the unvented attic assembly" and if "air-impermeable insulation is applied in direct contact with the underside/interior of the structural roof deck."

Why Install Roof Venting?
Traditionally, venting has been used to deal with problems that occur when heat or moisture escapes into the attic (Figure 2).

Figure 2. While attic ventilation can mitigate problems caused by ineffective insulation or leaky air or vapor retarders, a better approach is to build the attic as an unvented assembly. The foam insulation used for unvented attics stops air movement and with it the transport of moisture. Any hvac equipment located in the attic is within the conditioned shell of the house, which also cuts energy losses.

In cold climates, the escaping heat can cause ice dams by melting the snow on the roof. Venting the space above the insulation helps keep the roof cool by carrying this heat away. If moisture enters the attic through the ceiling (usually as an air leak), the vents are supposed to allow it to exit before it condenses on something cold.

However, ventilating above fiber insulation comes with an energy penalty. Fiber insulation is designed to be enclosed in an airtight cavity. When air flows over and through fiber insulation, there is a substantial loss of thermal performance.

Also, most hvac ducts and air handlers leak to some degree, so when these are installed in vented attics, conditioned air is lost to the exterior. And because vented attics are subject to extreme high and low temperatures, additional energy is lost through the thin insulation on the hvac equipment.

In cooling climates, venting the attic can bring humid outdoor air into contact with attic ductwork. If the ducts are not properly insulated, they can be cold enough to cause condensation.

Venting and shingle temperature. It's a common misconception that code-required venting significantly lowers the summer temperature of the roof surface. In fact, tests have shown that it lowers the surface temperature of asphalt shingles by at most about 5°F.

For many years, roofing manufacturers required that shingles be installed over vented substrates, but today, several companies - including Elk and CertainTeed - will guarantee shingles installed over properly constructed unvented roofs.

How Unvented Assemblies Work
A properly constructed unvented attic is immune to the moisture problems that occur in vented assemblies and is much more likely to be energy-efficient.

In an unvented assembly, anything below the insulation - including an attic - is considered conditioned space. Turning the attic into conditioned space saves energy; if heat or air escapes from the hvac equipment, it remains within the conditioned space (Figure 3).

Figure 3. The ducts visible in this unvented attic will be concealed after drywall is installed. But because they are in conditioned space, they won't be subject to the extremes of temperature typical of attics.

If enough energy is saved in this manner, the hvac system can actually be downsized, reducing installation and operating costs.

A number of insulation materials can be used in an unvented assembly, but the one with the greatest applicabil-
ity across the country is SPF. It's an extremely effective insulation and air barrier all in one, and since it's spray-applied, it conforms to irregular shapes that otherwise might be difficult to insulate and seal (Figure 4).

Figure 4. This barrel ceiling (top) would be difficult to insulate and seal with traditional materials. It's an ideal candidate for spray foam, which conforms to its irregular surfaces (bottom).
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Closed-cell foam is denser and less permeable than open-cell material. The typical closed-cell foam weighs 2.0 pounds per cubic foot and provides R-6.6 per inch of thickness.

In our climate zone, it's important to avoid excessive vapor diffusion, and we think the best way to do this is to use closed-cell foam. One of the great benefits of closed-cell foam is that if you install it to a thickness of at least 2 to 2 1/2 inches, it will have a permeance of 1.0 perm or less.

This means that in addition to being an air barrier, closed-cell foam is a vapor retarder. It's actually a vapor retarder from both sides, so it ends the debate about which side of the insulation to put the vapor retarder on in climates where interiors are both heated and cooled.

Some companies that make both open-cell and closed-cell foam advise insulation contractors not to use the open-cell material in unvented assemblies - or to do it only in certain climates where vapor diffusion will not be a problem. Open-cell foam is an air barrier but not a vapor retarder

Contractors often ask about the air space below the foam; most were taught that it's bad to leave an air space below insulation. This is true of fiber insulations because convection currents can form in gaps and degrade the insulation's thermal performance. But it is not true of foam, which can't be infiltrated and is relatively unaffected by surrounding air currents.

Any space left below the foam is considered conditioned space (Figure 6).

Figure 6. Fiberglass and cellulose insulation are usually installed in contact with the back of the drywall; the concern is that leaving a space there allows convective air currents to degrade the insulation's thermal performance. Because closed-cell foam is unaffected by air movement, the space between it and the drywall is not a problem.

Fire Resistance
When the unvented assembly is a cathedral ceiling, the foam will be covered with drywall, which is a code-approved thermal barrier. In an attic, though, the rafter bays are not normally covered by drywall, so the issue of fire-resistance comes into play

If the attic area is not accessible or is "accessed only for the service of utilities," it is possible to leave the SPF exposed. Providing access through a ceiling hatch is okay but not necessary; venting to the room below is prohibited by the fire code.

Written by James Moreshead
American Services Company