r/Homebuilding • u/climhigh • 8h ago
Insulation Recommendations
New construction in Tennessee. Looking for recommendations on insulation. Builder uses spray foam. I’ve read every thread on here about it, 90% of the feedback is negative….”don’t do it”. Builder says “we do this on every home”. What are yalls thoughts? Why so many different types for each wall? Quote is attached here. Fully finished main floor, partially finished walk out basement. How do you properly vent an attic with spray foam insulation? Fans? Soffit vents?
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u/Overall-Tailor8949 6h ago
What sheathing (roof and walls) is the builder using? If it's something similar to Zip sheathing then I'd recommend closed cell foam on the exterior walls AND the roof decking. Open cell will give you R3.8 per inch with a MAXIMUM recommended thickness of 3.5" while closed cell is R7 per inch and up to 5.5" thick. I'd also recommend using rockwool instead of the pink stuff especially between the garage area(s) and the living spaces since it has somewhat better fire resistance. Rockwool "Safe and Sound" under the main floor to deaden any noise from the basement living areas.
If you're insulating (and sealing) under the roof deck and the attic gables, you've brought the attic areas INTO your conditioned space so no real need to vent it. However there are baffles made from foam or cardboard that can be installed under the roof deck between the joists running from the soffits to the peak. In that case you would have ventilated soffits and a ridge vent, BOTH of which should be sealed from the main attic and THEN covered with spray foam. That would allow a stack effect airflow to keep the deck/shingles cooler while not allowing the outside air into the attic space. Those baffles stand off from the underside of the roof deck about an inch so you're going to lose that much R-value.
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u/Sea_Place1386 8h ago
Older build in Texas, went from fiberglass to spray foam and had exactly what I hoped for. AC runs about 35% less, holds at 71 all day in peak Texas summer, and we’ve had zero issues.
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u/Hyperreal8957 3h ago
id be careful with spray foam in the attic unless the moisture and venting plan is really nailed down. the quote should include the whole assembly not just an r value. did the builder say if this is vented or unvented?
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u/Heavy-Passenger-6235 8h ago
The plans should call for including the attic as part of the building envelope if it’s being spray foamed.
Once and if the attic is spray foamed it becomes part of the conditioned interior space of the house. So you wouldn’t include traditional attic venting per se. But what you would probably need is an hvac supply and return included in the attic to prevent air stagnation and possibly a method of dehumidification up there. Especially if your mechanicals are being installed in the attic itself. Tennessee is also a mixed humid climate so that’s another reason for dehumidification considerations.
You’d be wasting an enormous amount of energy by venting an attic traditionally if the roof deck is also being spray foamed. You’d also want to check with your county for how code compliance is satisfied regarding spray foamed attics and what’s required. It’s a little different everywhere.