Dehumidifying Conditioned Attic Air and Exhausting it into Living Space Below
New here and learning a lot:
We are replacing our roof to include adjusting a flat roof to match the existing 5/12 pitch; so we are taking the opportunity to seal the attic using a peel and stick (SA) type underlayment membrane. Our intention is to seal the attic space from the outside by overlapping the underlayment over the sides.
Additionally we run a whole house dehumidifier and would move all the air intakes into the attic space and exhaust dry air into the living area to create a circulation affect, constantly moving the moistest air away from the attic.
My questions is would I need to use spray foam on the interior of the attic space or could I simply use mineral wool insulation up against the sheathing, since the attic space will be sealed from the outside and I will be dehumidifying the home anyways?
Kind of want to avoid the fire issue… Also, the crawl space has already been sealed off with closed cell foam by prior owner, but will encapsulate at a later date and remove foam.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Dale,
Are you saying the plan is to pull air from the attic and exhaust it into the living space?
Basically yes. In the SEALED AND CONDITIONED ATTIC, the dehumidifier will take the more humid air from the attic space which is due to everyday living, filter and dehumidify it, then discharge into the living space, where the same air will increase in humidity due to living and being in a 1940 brick home where walls/windows air leakage is common, then this moist air will rise where it will be filtered and dehumidified again...
I’m not sure I understand the objective? Pulling from a sealed attic space and exhausting into the living space is going to create negative pressure in the attic and positive pressure in the living space.
I’d just dehumidify the living space. As long as the living space does not have excess humidity, the attic space should be fine. No matter how well sealed it is (if sectioned off from the living space) you’re bound to have some infiltration in the summer through stack effect.
If you create a "conditioned" attic with insulation and air sealing, you need to actually condition it with some airflow. But it doesn't take many CFM.
The insulation needs to be done to code - see IRC 806.
Very helpful, IRC 806. It basically answered my initial question. Thank you Austin and Jon!
I'll leave the dehumidification the same, drop a small HVAC vent into the attic space and in lieu of insulating the outside of my roof, I'll apply closed cell spray foam up against the underside of the structural sheathing to address condensation and attain an R 26, then add additional 2" mineral wool to get us over R30 and double as a secondary ignition barrier in our area.