Unvented Roof Problems

Cold sheathing is at risk for moisture accumulation and rot.

Unvented roof problems.

Now the problem has been isolated to reducing summer attic air temperatures without reducing free heat in winter or creating negative pressure. It s too late for that. Roof leaks which may be caused by ice dams flashing problems roofing failure and wind driven rain and snow. Unvented roof systems can be safely used in many different climates.

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. 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. There are several ways to address this problem. The energy savings touted by unvented attics can be as high as 20.

In the early 1990s construction of unvented attics became popular especially for structures in warmer climates. From inside the building via water vapor diffusing through the interior sheathing. Like a wall that is severely out of plumb an unvented roof has insulation packed into the framing cavities aligned with the sheathing. An unvented attic is conditioned space and won t be subject to the extremes of temperature common to vented attics.

At these low levels the moisture has a negligible effect on the strength and structural integrity of the wood panels. 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. You can install rigid insulation above the sheathing to keep it warm. 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.

Wood roofing panels endorsed by the apa usually contain an in service water content of between 4 and 16. Most of these attics stayed dry. 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. 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.

An air leak in the building envelope which transports water vapor into the roof cavity. 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. The problem is that your roof sheathing has no insulation above it so it will be very cold in winter.