Unvented Roof Problems

An unvented attic is conditioned space and won t be subject to the extremes of temperature common to vented attics.

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. In cold climates insulating sheathing must be added exterior to the roof sheathing to prevent condensation on the underside of the roof sheathing. There are several ways to address this problem. 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.

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. Wood roofing panels endorsed by the apa usually contain an in service water content of between 4 and 16. An air leak in the building envelope which transports water vapor into the roof cavity. Unvented attics do not experience such problems.

In the early 1990s construction of unvented attics became popular especially for structures in warmer climates. However if moisture is able to enter the unvented attic space it may lead to roofing panel issues. 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. Roof leaks which may be caused by ice dams flashing problems roofing failure and wind driven rain and snow.

Now the problem has been isolated to reducing summer attic air temperatures without reducing free heat in winter or creating negative pressure. At these low levels the moisture has a negligible effect on the strength and structural integrity of the wood panels. The energy savings touted by unvented attics can be as high as 20. Cold sheathing is at risk for moisture accumulation and rot.

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. Unvented roof systems can be safely used in many different climates. 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.

Most of these attics stayed dry. You can install rigid insulation above the sheathing to keep it warm. The problem is that your roof sheathing has no insulation above it so it will be very cold in winter.