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Feb 7, 2016
Spray foam attic ceiling to make it a conditioned space
I was debating open cell spray foam in my colonial style attic to make it a conditioned space. I'm worried about a couple items. I have a gas furnace in the attic, but was told by the install company a six inch duct to the outside would be installed to bring air into the attic (wouldn't this make the space un-sealed). Also, I was told that the present inadequate 4-6 inch fiberglass blown in insulation would be vacuumed at about an 6-8 foot perimeter around attic ceiling. Wouldn't you need to vacuum all the insulation to allow air transfer, i.e. humidity, back into the lower levels of the house. Last question, I live in Virginia, and my roof assembly is plywood, membrane, then 26 gauge standing seam metal roof. Would there be a moisture problem, if the attic is sealed to a conditioned space where the plywood is concerned in the roof assembly using 6 inches of Demilec open cell on the attic ceiling.
Feb 8, 2016

We've done this application numerous times in southeastern PA with one subtle change, we use closed cell foam insulation directly against the underside of the roof sheathing. Open cell will continue to let air/humidity through, closed cell will not. Since air can't move through the closed cell foam there isn't any need for air flow under the roof sheathing because there won't be any humidity there to potentially condensate. Air already moves through the fiberglass insulation and that won't change when the Attic becomes conditioned space. I believe it would be best to remove ALL of the existing fiberglass ceiling insulation so that the Attic space becomes 'one' with the rest of the House. Fresh air intake to the fossil fuel furnace is necessary. It should be set up with a power damper that opens when the furnace turns on to allow combustion air directly to the heat chamber. When the furnace isn't running the damper is closed to keep unconditioned air out of the Attic.

Dennis D. Gehman, CR, MSA, CLC, CKBR, GAC, CAPS


Gehman Design Remodeling

NARI = National Association of the Remodeling Industry

CR = NARI Certified Remodeler

MSA = CertainTeed Master Shingle Applicator

CLC = NARI Certifled Lead Carpenter

CKBR = NARI Certified Kitchen & Bath Remodeler

GAC = Green Advantage Certified

CAPS = Certified Aging in Place Specialist

Tom Schiebout of Tomco Company PRO answered:

Feb 9, 2016

Christofer, There is a lot written about your particular condition. What I would do would be to install a spray polyurethane insulation (closed cell insulation) into the attic ceiling and down to and including the eaves. this installed over a 1/4" plywood that would be "cleated" down at a minimum of 1" below the bottom of the roof sheathing, thus eliminating the "hot roof" situation. As far as moisture... if you understand that warm air rises and carries the moisture up (another reason i would use a closed cell product) then it makes sense to foam any walls or kneewalls as well. The venting on the top side would not be to eliminate any interior moisture, but to make your shingles last longer (keep them from overheating). Quite a few asphalt companies have disclaimers on warrantees because of this issue. You would need to check with your steel roof manufacturer how they respond to a hot roof application. 

Ok so you don't really want all that moisture collecting on your windows and or finished wall substrates in the attic. So an ERV/HRV or even the furnace fan run continuously would help distrubute or evacute the excess moisture if this is your main house furnace (air handler). FYI- change filters monthly in these systems to help mitigate possible mold issues. I recommend the 6" combustion air (bringing in dry outside air) code required in IRC. I am not sure if this furnace unit in your attic is the primary furnace (air handler) for the home. If so that would help distribute attic moisture to the lower parts of the home. If separate attic unit only, I'm not thinking it will ever distribute moist warm air to the lower level like you suggest.

Remember this... once you start changing the function of your home's ventilation/conditioned or unconditioning of a space you will be changing the whole dynamics of how the house used to function. Henceforth do your homework. All this said...I run my company business in the extreme temperature zone of Minnesota. Your local heating guys should have a better handle on providing a safe living environment for your family. Mold can be a serious issue.  

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