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.

Unvented roof problems.

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. 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. 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.

Now the problem has been isolated to reducing summer attic air temperatures without reducing free heat in winter or creating negative pressure. In the early 1990s construction of unvented attics became popular especially for structures in warmer climates. 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. At these low levels the moisture has a negligible effect on the strength and structural integrity of the wood panels.

An unvented attic is conditioned space and won t be subject to the extremes of temperature common to vented attics. An air leak in the building envelope which transports water vapor into the roof cavity. There are several ways to address this problem. However if moisture is able to enter the unvented attic space it may lead to roofing panel issues.

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

Unvented roof systems can be safely used in many different climates. From inside the building via water vapor diffusing through the interior sheathing. Unvented attics do not experience such problems. It s too late for that.

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. Roof leaks which may be caused by ice dams flashing problems roofing failure and wind driven rain and snow. The energy savings touted by unvented attics can be as high as 20.