The #1 Question to Ask before Putting Spray Foam in Your Attic
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.
A case of incompatibility
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. (You'll also see another safety hazard and code violation: The installer sprayed foam directly on the metal flues. You should NEVER do this!) Those metal pipes indicate to the trained eye that this house has atmospheric combustion appliances.
In the photo here, the furnaces were down in the conditioned space, but in the house my friend asked about, the two furnaces were in the attic. They also were of the atmospheric combustion type.
See where I'm going with this yet? The spray foam contractor on that job saw the two atmospheric combustion 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?
Aha! 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 you have atmospheric combustion in the home now, it might be best to wait and do the spray foam after you've replaced 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. Perhaps he'll elaborate in the comments.