Every decision made is evaluated considering functional use, environmental contribution and, of course, cost. Like everyone undertaking this process, I started out with an idealistic vision. My design vision is loaded with “what ifs.” I have renovated and built many homes since 1970, including passive solar structures, not only as an architect but as a builder, and I never get tired of improving the quality and design expectation. That leads me back to this article because this is the first home that I am constructing which will use an unvented attic space by encapsulating the attic with air-impermeable foam. Stay with me as I share some of the facts that are being discovered in the process of making this evaluation.

For my definition for unvented attics I will use the Joseph Lstiburek , Ph.D., P.E explanation. You can find it in an article entitled, “The Top Ten Dumb Things To Do In The South.” An unvented attic is one where there are no vents and where attic insulation follows the roof slope thus including the attic in the conditioned building enclosure. These roof assemblies eliminate ventilation openings and locate the thermal, moisture and air control boundaries to the plane of the roof deck. Sounds simple enough, doesn’t it? Well, remember that there is a lot of preparation work to do this if you are renovating before you insulate. We will save that discussion for another time.

Building codes typically do require ventilation in order to flush out heat and/or in cold climates to prevent ice damming. Since we live in the Southeast in a mixed humid climate, we are more concerned about solar heat gain and how to reduce the roof cooling load for part of the season and heating for the remainder. I read that in a typical home, the contribution of the roof cooling load to the total roof load is around 10 percent. I also read that the number is of questionable validity. So I had to dig for more facts.

Recent studies are now indicating that unvented attic spaces result in energy savings of between 5 percent and 19 percent during cooling seasons and closer to 50 percent during heating seasons. One of the reasons is that placing ductwork into a conditioned attic negates the effect of duct leakage which is between 10 to 20 percent of the rated air flow. Another reason is that the HVAC system is operating at much less stressful temperature, around 3 percent to 5 percent of the indoor set point, instead of the typical attic temperatures.

Moisture is another concern. Where we live, the outside air is hot and has humidity. Bringing this air into the attic where it can migrate toward the ceilings of the air conditioned rooms can create moisture and cause mold. The same can happen with ductwork insulated with R-6 wrapping. That would just lead to disaster. Moisture moves toward the cold surfaces. Much of the insulation removed from our previously vented attic showed serious signs of deterioration from animal feces, improper installation such as bunching, and mold.

In unvented attics, when you move the thermal and air-barrier to the roof deck, the attic needs to be constructed to be air-sealed along the perimeter, at soffits and roof intersections and where it is exposed to locations such as garages or open porches. I have seen a recommendation that suggests that when we construct unventilated attics we should be subject to a blower door test with the attic access door open. This would test the air tightness of the entire building shell, not just living spaces. That is exactly what I plan to do. I also plan to provide controlled mechanical ventilation as required by the mechanical design. This method of insulating the roof deck is allowed by the North Carolina Residential Building Code in section R 806.4 as long as no vapor barrier is placed between the living space and the attic.

After all of this consideration, what do I plan to do? Continue to proceed with the unventilated roof construction while continuing to research and consult with local practitioners and experts as I move ahead.

So far, my tally sheet shows the following:

•    Control air flow by utilizing closed cell medium-density spray polyurethane insulation against the roof deck. Choosing the proper product provides a higher thermal resistance and lower vapor permeance than open cell spray foam, which is the alternative.

•    Control rain by installing an appropriate underlayment, flashings and roof finish system.


•    Control heat flow by installing enough insulation to meet or exceed the minimum amount required by code or the calculated thermal resistance of the project. Medium-density spray polyurethane foam provides a higher thermal resistance value per inch than either low- density open cell spray foam, brown cellulose, or fiberglass batt, although it is more expensive.

•    Control vapor diffusion by consulting the insulation manufacturer’s application and technical data sheets for maximum allowable thickness and minimal thickness required to satisfy the requirements of a suitable vapor retarder.


•    Control fire by verifying that the exposed face of the insulation is an appropriate material to serve as a thermal and ignition barrier. Consult with local code and manufacturers before proceeding.

•    Control cost by securing at least three bids and references, in addition to being supplied with their manufacturer’s technical data sheets including their ASTM tests.


As this project progresses, we will continue to collect data and post the results either here or on my blog http://renovatetogreen.blogspot.com.

This e-mail address is being protected from spambots. You need JavaScript enabled to view it is the Department Chair for Construction Management Technologies (a part of A-B Tech Construction Science) at Asheville Buncombe Technical College.

Find out more about Ken.

Interested in our program? Fill out our Enrollment Questionnaire.

Return to Green Building Articles.

For further information:

Green Builder, February 2009, Out Front, “Unvented Attics Revisited”.

The following articles can all be found on the www.buildingscience.com website:
Building Science Digest 149, “Unvented Roof Assemblies for All Climates”, by Chris Schumacher @ 2007 Building Science Press.
Insight -015, Insight “The top Ten Dumb Things To Do In The South”, by Joseph Lstiburek, Ph.D.,P.Eng.