Unvented Conditioned Attics, Take II

A wrinkle to consider, if you read the last post.

I’m in an attic that has been sealed with our foamy plastic stuff that is both amazing and potentially torturous for the planet. I’m staring at the far gable wall of the attic over the garage and it’s spray foamed. So is the roof framing. This garage is shaped like a rectangle until it turns 90 degrees and opens into a giant square which is the main structure living space (so, think an “L” shape). My brain is screaming at me and I can’t interpret fast enough. So, I stand there.

The garage attic is spray foamed. The garage attic attaches to the main attic. The main attic is spray foamed. There is no partition between the two spaces. Okay. Thank you brain.

The garage is an exterior enclosure. This means every time the overhead doors are opened, or left open, untreated humid air (summer) and cold air (winter) will enter into the garage.

The main home, and now attic, are an interior enclosure. The main home, and now attic, are conditioned via the HVAC systems and commercial grade dehumidifier. The garage is conditioned via the outside. Ah-ha!

My brain is screaming at me because our garage is going to introduce an entirely different moisture load into the attic space than the main structure. This moisture load, particularly in more humid times of the year (so, like 6-8 months per year), is going to stress the unvented conditioned attic. Couple with this the inefficiencies of foam installation we chatted about last time, and we have all the variables we need for potential moisture issues developing over time to the roof framing (likely, near the ridge if my educated brain recalls properly — see people smarter than me).

Where Do We Land?

The fix for this interesting configuration is to insulate and air seal the ceiling of the garage. Basically, block off the garage from being able to mix with the unvented conditioned crawlspace. If done properly, ta-da! No additional stress or strain to the home’s attic space. Now, the attic likely merits monitoring to make sure the size of the attic does not present issues with relative humidity over time (the volume of the total unvented conditioned attic space will be greater than the perimeter footprint of the interior enclosure’s main floor). If RH is found to be consistently elevated (say, greater than 55%), then perhaps a balanced moisture management system would be beneficial, or a vapor diffusion port as written in Green Building Advisor by Martin Holladay back in 2018 (yeah, we’re not reinventing the wheel here — other peeps have done that for us — we just have to seek it out and read and digest). All we need do is think as a system. If that fails, reach out to these industry giants that came before us and everyday make this stuff look more like paint drying on a wall than performing complicated physics equations.

G.

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.