Unvented Roof Problems

In unvented roof assemblies the thermal moisture and air control boundaries are moved to the roof deck so that the attic space is at the same condition as the living space and air movement between the two does not contribute to energy waste occupant discomfort or contaminant problems.

Unvented roof problems.

Like a wall that is severely out of plumb an unvented roof has insulation packed into the framing cavities aligned with the sheathing. An air leak in the building envelope which transports water vapor into the roof cavity. However if moisture is able to enter the unvented attic space it may lead to roofing panel issues. Over the past five years several unvented roof insulation systems have been developed by the building science consortium for use primarily in hot dry and hot humid climate zones.

It s too late for that. At these low levels the moisture has a negligible effect on the strength and structural integrity of the wood panels. You can install rigid insulation above the sheathing to keep it warm. Heat is thus less likely to escape into an unvented attic from hvac equipment and if it does it will remain within the conditioned space.

There are several ways to address this problem. Going back to the moisture which develops under the underlayment if this moisture has nothing to dry it out air movement under the roof deck you have a problem and having an insulation directly under it exacerbates the problem because any moisture will expand in high heat conditions. The problem is that your roof sheathing has no insulation above it so it will be very cold in winter. So why not replace 150 degree air dark roof sunny climate with 80 90 degree air only when there s a benefit via two small fans one in one out to eliminate any concerns about negative pressure.

Because the attic space is now inside the thermal envelope blowing a little heat up there is not like throwing twenty dollar bills out the window. In the early 1990s construction of unvented attics became popular especially for structures in warmer climates. Wood roofing panels endorsed by the apa usually contain an in service water content of between 4 and 16. An unvented attic is conditioned space and won t be subject to the extremes of temperature common to vented attics.

Unvented attics do not experience such problems. Roof leaks which may be caused by ice dams flashing problems roofing failure and wind driven rain and snow. Most of these attics stayed dry. In cold climates insulating sheathing must be added exterior to the roof sheathing to prevent condensation on the underside of the roof sheathing.

The typical construction scenario involved the use of low density open cell spray foam insulation for the thermal barrier at roof deck and duct work installed in the unvented attic space. From inside the building via water vapor diffusing through the interior sheathing. Cold sheathing is at risk for moisture accumulation and rot.