A Moment With Unvented Conditioned Attics
Our eyes are amazing organs. They filter something we can’t otherwise perceive and allow us to perceive the worlds which we see. But, what happens when we pay for a service that addresses what we can’t see with our eyes? How do we know we are getting what we pay for? Or, if we’re the contractor, how do we really know what we’re doing is effective?
We test.
In our middle TN region, we don’t have many homes that are built with unvented conditioned attics, or are converted to unvented conditioned attics. To my knowledge, we also don’t have a hardy education for trades professionals regarding the building science behind vented unconditioned attics vs. unvented conditioned attics. So, our line of ignorance places us in a precarious position as home owners and builders.
In our region, what I see mostly is low density open cell spray foam installed along the roof line and eaves. In theory, this closes off the attic from the exterior environment extending the interior enclosure from the finished ceiling to the now-foam-filled roof framing. In theory.
In actuality, because our trades professionals aren’t regulated in relation to the depth of the installed low density open cell spray foam, often there are irregularities, voids, and gaps. These, individually, are no big deal. But taken as one larger whole (get it…?), they are a very big deal. Thermal imaging helps us see this clearly.
Further Complication
In addition to no real standardization of installation and resulting thermal intrusion and potentially moisture issues (for our immediate market), we would naturally need to worry about moisture issues anyhow. The biggest potential issue for unvented conditioned attics is moisture. Per this article by Joseph Lstiburek and Building Science Corporation, moisture is the most pernicious concern for unvented conditioned attics. Why is this so complicated?
Low density open cell spray foam in an unvented conditioned attic
Installation is key — the bright yellow indicates thermal inefficiencies which indicate questionable installation
Well, if it’s a new home, then there is moisture from the building materials. If it is an existing home, there is moisture (in our market) from deficiencies related to poor water management and indoor air quality. And then there’s our subtropical environment that tends to be hot, wet, and humid. This is why some building science folks argue for use of high density closed cell spray foam only — it blocks moisture getting to the roof sheathing/decking a bit better (when properly installed). But, it carries it’s own pitfalls.
So What’s To Say
In the end, I put my money where Lstiburek puts his decades of experience and knowledge — the man is a compendium of building science experience and testing. So, if you ask me I will tell you low density open cell spray foam is acceptable when properly installed. The problem, as this photo from a recent new build shows, is installation. Now, this particular home has an in-line dehumidifier in the basement which should help. I recommended further monitoring via hygrometer installation in the attic as close to the ridge line as possible. If the moisture loads are noted to be beyond ideal percentages, then the client can manage the unvented conditioned attic space in one of several ways. Lstiburek’s recommendation for balanced ventilation would be sweet to see, but in our region I suspect it might be too complicated for many professionals to get right without causing more harm. FYI, that article also has great information on walling and basement configurations, too. If that kind of thing tickles your fancy.
