As a general contractor, I was taught that attic and cathedral ceiling assemblies should always be vented. Since then, however, studies have shown that properly designed and installed unvented attic assemblies outperform vented assemblies. They reduce energy loss and protect against rot and mold by preventing moisture from passing through the insulation and condensing on cold surfaces. Although many builders — and even some building inspectors — are unfamiliar with them, unvented assemblies are already part of the 2006 IRC and will soon be allowed by most building codes.
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."
There is an exception that allows air-permeable insulation (fiberglass and cellulose) to be used in unvented assemblies in certain parts of the South (climate zones 2B and 3B).
It has long been possible to get an unvented assembly approved by the inspector as an "alternate construction method." But once states update their codes to the 2006 IRC, it will no longer be necessary to get special approval for unvented assemblies.
In the meantime, the fact that the 2006 IRC allows unvented assemblies should make it easier to get special approval in states that have adopted earlier versions of the code.
Do not build an unvented attic assembly without first talking to the local building inspector. Unvented assemblies are new in the IRC, and your state might be using an older version of the code. Also, the committee that wrote this section is still working on it, so more changes may be on the way.
I work for a company in Northern California that installs spray polyurethane foam (SPF) insulation, and we are frequently asked to insulate unvented assemblies. Sometimes the building has a flat roof or a cathedral ceiling that would be difficult or impossible to ventilate. In other cases, the existing framing cavities are too shallow to accommodate a sufficient amount of insulation plus a vent space. And occasionally customers request unvented attics because they make the building more comfortable and energy-efficient.
Spray foam is a good choice for roofs that are difficult to vent, like a turret with converging rafters.
A flat roof with its rafters hung between flush beams.
Why Install Roof Venting?
Traditionally, venting has been used to deal with problems that occur when heat or moisture escapes into the attic.
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.