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Two Common HVAC Pitfalls In Spray Foam Insulated Attics

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Earlier this year I got a question about a home that had spray foam insulation in the attic. Nothing unusual about that. A lot of builders and homeowners are going with spray foam insulation because of the airtightness benefits. But then the questioner mentioned that the spray foam contractor had intentionally left big holes to the outside by not sealing the gable vents.

The 2 pitfalls

The attic you see in the photo here is a different home, but it does have one feature in common besides the spray foam. If you look closely, you’ll see two exhaust flues on the left side. There’s the first pitfall: The installer sprayed foam directly on the metal flues. This obviously does not meet the requirement for proper clearance between B-vent and combustible materials.

spray-foam-insulation-roofline-atmospheric-combustion copy

The other pitfall is that the house my friend asked about has atmospheric combustion appliances in the spray foam insulated attic. The spray foam contractor on that job saw the two furnaces in the attic and knew they needed combustion air from the space around them. If he had sealed up the attic completely, where would the furnaces get combustion air? That’s why they didn’t seal the gable vents. Now when those furnaces run, they can pull in the combustion air they need through those holes.

But wait. If the homeowners are getting spray foam on their roofline to make the home more airtight, why would they leave big holes to the outside?


A house is a system

It’s nice that the spray foam insulation contractor recognized the need for combustion air in that case. In many, they don’t get that far. Whether you’re building a new home or retrofitting an existing home, though, you’ve got to remember that the hip bone’s connected to the thigh bone; or, as we say in building science, a house is a system.

Newer homes have to meet building codes that help prevent this problem. If there’s atmospheric combustion inside the building enclosure, the HVAC contractor is required to install vents that are supposed to bring combustion air into the room where the appliances are located. If the atmospheric combustion appliances start off in a vented attic, the building code doesn’t require those vents because the appliances are already connected to the outside and should get plenty of air.

Whether the home is old or new, however, anyone considering spray foam insulation would be wise to ask this question:

What impact will encapsulating the attic have on the HVAC system(s)?

This question applies to encapsulated crawl spaces as well.

If a home currently has atmospheric combustion, you’ve got two options: Don’t spray foam in the attic or replace the old equipment with sealed combustion, power-vented, or direct vent appliances. If the house isn’t built yet—or at least doesn’t have the HVAC equipment installed yet—make sure there’s no atmospheric combustion equipment going inside the building enclosure.

Remember: A house is a system.


My friend David Richardson, an instructor with the National Comfort Institute, likes to point out that the high-low vents required by code don’t always do what they’re told, as air will move in response to pressure differences and look for the paths of least resistance.

Allison Bailes, III, PhD
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Posted In: Building Performance, Residential Buildings

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