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.
