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The #1 Question to Ask before Putting Spray Foam in Your Attic

Using Indoor Air For Combustion Appliances In A Spray Foam Insulated 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 at the left side of that attic, you’ll see two exhaust flues. (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 on the second floor, 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.


Allison A. Bailes III, PhD is a speaker, writer, building science consultant, and the founder of Energy Vanguard in Decatur, Georgia. He has a doctorate in physics and writes the Energy Vanguard Blog. He also has a book on building science coming out in the fall of 2022. You can follow him on Twitter at @EnergyVanguard.


Related Articles

Are Spray Foam Contractors Putting Themselves at Risk?

3 Problems with Atmospheric Combustion Inside the Building Envelope

Does It Make Sense to Move the Building Envelope to the Roofline?



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.

This Post Has 10 Comments

  1. My thoughts from looking at
    My thoughts from looking at this picture are first, if you put the exhaust and fresh are ducts close together the fresh air will suck in the exhaust. Putting the exhaust higher than the intake helps, but the more space between the exhaust and intake the better. What is going on with vaulted ceiling? It does not look sealed to the outside at the ridge. There is a major difference in spray foam insulation and sealing with spray foam insulation. 

  2. I get the message there is a
    I get the message there is a news blackout regarding some lawsuits over spray foam in the hot-humid Houston area. Have heard reports of failures, but way too little detail about them… many times a settled lawsuit will include a clause to never discuss terms, and I think it has happened here. If anyone could point me to a post-mortem on the Building America experiment in Cinco Ranch (Katy) area, that would be invaluable to me. If mistakes were made, then there are lessons to be learned, and it is maddening to not be able to learn about this.

  3. This illustrates a couple
    This illustrates a couple reasons why I shy away from recommending spray foam 
    1) Many contractors don’t know enough about building science or the products they use. Spray foam is considered a high end insulation because of the cost and because it is seen as a fix-all solution for insulation or air sealing. Unfortunately it’s a labour intensive process during a time where good labour is hard to find. Many manufacturers are trying to offer products that remove labour from the equation.  
    2) Now that attic is considered conditioned space (or at least semi-conditioned). I can’t help but think that the extra cost of heating this space will hurt the ROI of the sprayfoam.

  4. Gene W.: I
    Gene W.: I believe the house in the photo tested out pretty well, so I think what you’re seeing may just be in the photo. 
    M. Johnson: I don’t know anything about a Building America experiment in Katy, but Treehugger has been aseries on spray foam problems, including info about lawsuits. 
    Lee: Spray foam can work well, but you’re right: Contractors really have to know what they’re doing. It’s good to have a building science pro involved, too. A lot of people look at the extra volume in an encapsulated attic, but the heating and cooling loads on a house are based mostly on the surface areas.  
    Dennis B.: The standard joke about that, which works better when you’re saying it out loud, is that an HVAC contractor who doesn’t do the V is a HAC. The good news is that more and more HVAC contractors are learning building science and how to do ventilation right. (Of course, there’s plenty of debate among the experts about what ‘right’ actually is, but that’s another matter.) 

  5. The easy way to check an
    The easy way to check an attic seal is to start the blower door with the attic access closed, then open it. If it is sealed, the blower door will not fluctuate.  
    Testing pretty well and sealed is like pregnancy. It either is or it isn’t. There is no in between. I know it is not sealed as I can see the open cell spray foam and I see a lot of thermal bridging without my camera. You would be amazed at the leakage at the vaulted ceiling. I’m not a betting man and I love blueberry doughnuts, but I’d bet a blueberry doughnut on that one. 
    M. Johnson, I started my spray foam division in 2008. My general liability insurance company told me this year they will insure my whole general contracting firm except my spray foam. They say it has become a multi billion dollar lawsuit problem. The industry is killing itself with ignorance and bravado. 
    The lack of general building experience is one of the problems and the ignorance of the trigger man in building or building science, and his understanding of the product itself is the root of the problem. 
    I would hate to see the lawsuits from body shops that tried to employ foam sprayers. 90 degrees to substrate is as important to foam application as it is to spray painting. 
    Take a can of spray paint and try to get anything but runs and errors spraying off 90 degrees. 
    The myth that open cell is more economical is bullshit. Open cell foam weighs 1/2 pound per cubic foot and closed cell weighs 2 lbs per cubic foot.  
    There is 300% difference there, but the cost they give you is about a third difference and open cell foam at 12 inches will not give you the performance of 2 inches of closed. 
    I have open cell in my house since 2004 and I would not use it in a dogs out house. 
    I have been living in a foam laboratory since 2004.  
    After 4 years of listening to the industry BS, I started my own operation. 
    It is a great product, but much like firearms in the hands of the ignorant, it can kill.  

  6. The example picture left me
    The example picture left me cringing as soon as I saw the 2 single walled flue pipes with spray foam directly on them.  
    As combustion air, that is a nightmare… 
    Leaving the gable vents open… what a nightmare… 
    I know not every spray foam contractor can have BPI certifications or auditing equipment… BUT it is easy enough to partner up with an HVAC, or auditing company.  
    A blower door is needed on every job EVERY job. I sell a lot of ERV’s and whisper Green Panasonic bathroom fans. 
    Unfortunately this just shows why the SPFA has started certifications for installers, etc. in the industry.

  7. I am sorry some of you have a
    I am sorry some of you have a bad taste in your mouth as far as spray foam is concerned. As far as poor labor or companies, that is where the contractor or auditor is at fault just as well. 
    Align yourselves with a good company, sure you will pay more but your paying to be mistake free, and for the knowledge the professionals have. 
    In NJ we have many HVAC contractors who are trail blazing for the HP industry. Our company was number 4 in the nation in Home Performance with Energy Star job completions, we were and still number 1 in the state for QC, and I believe the top 3 in the nation were all NJ HVAC contractors…. 
    I happen to be a Home Performance Contractor, 3 generations ago we were a HVAC company. Now we do HVAC, Spray Foam, Insulation, Weatherization, Audits, Plumbing…. 
    It is all about your knowledge and intensity… to many people trying to cash in without the proper education.

  8. More quality content from AB.
    More quality content from AB.  
    As a reader it’s reassuring to know that if I make the trip, the meal will usually be satisfying and sometimes stellar.  
    Wish some other would be bloggers offered the same consistency (ahem, ahe-MR. Honestly, just clearing my throat!)

  9. Great post Allison! You are
    Great post Allison! You are dead on with passive combustion air. It’s easy to forget that the opening is dependent on a pressure difference for flow to occur. The issue is it’s a two way street for airflow. It’s common to see combustion air ducts and grilles functioning in direct opposition to their original design intent.

  10. New ICF construction home in
    New ICF construction home in Florida, tile roof, need great insulation. All gas appliances are inside house, but galvanized gas pipes run over the ceiling in attic. Is foam safe, do I need vents? Should I add a radiant barrier as well?

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