GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Ventilation or no ventilation in spray foam insulated building?

[email protected] | Posted in General Questions on

I am planning to use open cell spray foam insulation in my metal building and then build out the interior. I have read two articles by Allision Bailes that seem to contradict one another concerning whether or not a home (or building) that has SPF should be ventilated or not. One article was entitled “Spray Foam Insulated Homes Need Ventilation” Posted on Oct 22 2014 by Allison A. Bailes III, PhD, GBA Advisor on The other article was posted on and was entitled “Beware of Roofers in Homes with Spray Foam Insulation” (Posted by Allison Bailes on Mon, Nov 18, 2013). We are planning to have LP heat and were told by the HVAC contractor that we would need a gable vent, but after reading these two articles, I am more confused than ever about the correct approach.

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. mpg9999 | | #1

    You are confusing two different things: Attic ventilation and whole house ventilation. You should include mechanical whole house ventilation. Whether or not you ventilate your attic depends on whether or not you have a conditioned attic. You have a conditioned attic if you place your insulation at the roofline.

  2. GBA Editor
    Martin Holladay | | #2

    Michael is right.

    If you are aiming to build a house with a low level of air leakage (that is, a tight house) -- and that's exactly what you should be aiming to do -- then your house needs a mechanical ventilation system to provide fresh air for occupants. For more information on mechanical ventilation systems, see Designing a Good Ventilation System.

    If you house has an attic, the attic can either be vented or unvented. Either approach can work, as long as the attic is well designed and well detailed. If you plan to have any HVAC equipment or ducts in the attic, the best approach is usually to make an unvented conditioned attic. For more information on this issue, see Creating a Conditioned Attic.

  3. [email protected] | | #3

    Michael and Martin, Thank you for pointing me in the right direction. I understand now. This raises a couple more questions. I am asking these questions with the assumption that I am using open cell foam to create an airtight space. First, if my attic area is conditioned and therefore unventilated, is it safe to use gas or LP appliances provided they have sealed combustion and proper exhaust? Secondly, am I understanding correctly that if the attic area is conditioned and unventilated that I need to introduce some method of whole home ventilation?

  4. GBA Editor
    Martin Holladay | | #4

    Q. "If my attic area is conditioned and therefore unventilated, is it safe to use gas or LP appliances provided they have sealed combustion and proper exhaust?"

    A. Yes.

    Q. "Am I understanding correctly that if the attic area is conditioned and unventilated that I need to introduce some method of whole home ventilation?"

    A. The need for a mechanical ventilation system -- either an exhaust-only ventilation system, a balanced ventilation system, or a central-fan-integrated supply ventilation system -- has nothing to do with whether or not you decide to build a conditioned attic.

    In general, many leaky homes work OK without a mechanical ventilation system. All tight homes (whether they have a conditioned attic or not) need a mechanical ventilation system.

    If you are building a new house, you want (a) to strive to make your home's building envelope as tight as possible, and (b) to include a mechanical ventilation system to ensure that the occupants get enough fresh air.

    Whether or not you want a conditioned attic is an independent decision.

  5. [email protected] | | #5

    Martin, Thank you for so thoroughly answering my questions. I apologize if many of them seem simple minded, but having no experience with SPF and the tight seal I hope to achieve by using it, I want to be certain that I don't make mistakes that could lead to major issues down the road. One other question concerning appliances - Is it safe to use a gas stove in relatively air tight space since it does not has sealed combustion?

  6. GBA Editor
    Martin Holladay | | #6

    When used for cooking, gas kitchen ranges emit pollutants that can injure human health. It is always advisable to use a range hood exhausted to the exterior of your home when using a gas kitchen range.

    For more information on this topic, see The Hazards of Cooking With Gas.

  7. Dco1 | | #7


    I have a similar question as the OP . I am a handy but not an expert . Located in zone 6 Toronto

    Wondering if someone can answer for me. I have raised my ceiling in my living room bungalow and in the process we have CC SPF the roof deck. We more than meet the min R value. I have added baffles and a ridge vent prior to spray foaming the deck to try and eliminate the hot roof. I have been battling My city inspector and now building code examiner as they are claiming I need to a) add more blow in insulation + poly vapor barrier or b) ventilate the attic in my living room because it’s “ a unconditioned space” .. Who is wrong here ? I am weary of adding poly in the unnvented space .

    Do I need to vent this small attic ?
    Height of the space between new ceiling and roof pitch is about 3 ft...

    Any help is much appreciated

  8. BrianPontolilo | | #8


    If you have installed spray foam in the rafter bays, which it sounds like you did, and that insulation is continuous with the insulation in the walls, meaning that it was installed from the top plate of your walls up to the ridge and back down to the other tops plate (assuming a simple roof), then the attic is now part of the conditioned space of the home. The inspector can ask for more R-value and a vapor barrier if those are the codes in your area, but to ventilate the attic space would be to negate the insulation that you installed under the roof deck.

    I hope I am understanding your question right. Here are some articles that should be helpful.

  9. Expert Member
    BILL WICHERS | | #9

    To add to what Brian said, closed cell spray foam (ccSPF), applied directly to the underside of the roof sheathing, would seal a ridge vent shut. There is no need for a ridge vent in this application since the ccSPF seals the roof. You don’t need soffit or gable vents either since the attic is now conditioned space. I’m not sure what the point of a vapor barrier would be since the ccSPF serves that function as well.

    It’s possible that we don’t fully understand your situation. Can you provide some pictures or a drawing? Its important to know how the spray foam was installed.


  10. walta100 | | #10

    I do not think we understand what is going on with your building a few photos may be worth thousands of words. Please post a few to help us understand how you have vents with spray foam.


  11. Jon_R | | #11

    I think that they are legitimately concerned about your mini-attic being insulated (above) and air sealed (ie, set up to be conditioned) and yet not actually conditioned. You should add a supply duct to fix.

  12. Dco1 | | #12

    Thank you for the replies,

    Unfortunately my phone is wiped but I will do best to articulate and illustrate what’s going on

    The rafter bays have baffles installed from soffit to the ridge vent and is sealed upwith spray foam 5.5” . I meet the min r value (32). Suggestion from my engineer to eliminate the “hot roof.” Made sense we then spray foamed in between the rafters over top of the plastic baffles

    I have no issues with adding in ventilation to move the air in and out around the space above from below but I do have concerns venting outward to the roof side .

    I am also more concerned with adding in a another vapour barrier since the ccSPF is supposedly not needing one

    Verbatim from the inspector “ the attic is unconditioned space” . I am confused when everyone suggested it is. It’s just not venting air currently

    I have the drawings and managed to get one photo salvaged

    Also looking for solutions on mechanically venting

    Thanks again for your help all and links. I will try to sway the inspectors

  13. walta100 | | #13

    Thanks for the images.

    Ask the inspector “if you add supply and return register to the attic will it still be unconditioned”? Be careful how you handle the inspector they can make your life difficult if they decide to.


    1. Jon_R | | #14

      I agree, perhaps something like "I don't have an attic, I have a dropped ceiling".

  14. Expert Member
    BILL WICHERS | | #15

    First, have you had an engineer evaluate your removal of collar ties and the rest? I don’t see anything on those print excerpts marking something like “insert new ridge beam”. If you remove the collar ties and rafter ties, and you have only a ridge board like is shown on that drawing (which is VERY different from a ridge BEAM), you can have structural failure of your roof. This is a BIG DEAL. BE SURE it’s done correctly.

    Regarding the insulation, it looks like you have a vaulted/cathedral ceiling? If that’s the case, the drawing shows a normal vented roof assembly with spray foam over baffles to create a vent space. There is nothing wrong with this, provided it’s done correctly and the ridge and soffit vents are connected to the vent space between the roof sheathing and spray foam.

    If you have a partial attic at the end of this area of vaulted ceiling, but the roof assembly is insulated the same way in that area, then the attic is INSIDE of the building envelope and is considered to be conditioned space. It’s possible the inspector just wants some air circulation in that closed area as others have said. That shouldn’t be a problem, provided the vented part of the roof is not connected to the attic space anywhere.

    The other possibility is the inspector doesn’t know what he’s talking about. This is a surprisingly common situation in construction, unfortunately, especially when complex modern building assemblies are involved that the inspector may be unfamiliar with. If you’re working with an architect and/or engineer on this project it may be worth having them talk to the inspector for you. As others have mentioned, be careful what you say to the inspector since they can make things hard for you. The classic “inspector is mad at you” issue is that they start nitpicking EVERYTHING else on your project.


    1. Dco1 | | #16

      Thank you all for yours words

      I will take caution and ask the questions for air circulation. Though I don’t know what the concern would be , Stale air?

      @Bill, thank you for taking the time. You’re correct I have a partial attic that’s unvented and sealed. I will ask if adding in mechanical vents to circulate air will be fine.

      I have an architect and structural engineer working with me on this. The plans were stamped and approved. The attic insulation question came up on inspection and I tried to explain as did my SPF installer that the attic is considered “conditioned space” because of the ccSPF. Now I am trying to look for ways to educate or sway them. I have done everything they have wanted including submitting revisions.

      To answer your q, the existing ridge board is in play and engineer suggested to sister existing rafters with larger ones to achieve the depth we need to apply the ccSPF.

      Safety is my number one concern for me and my family . I will try to have a talk with the engineer.

      I have no issues adding in ventilation to circulate air, I do have issues adding blown in insulation and another vapour barrier which is what the inspector suggested .

      At this point I feel I may have to file a grievance with the city.

      1. Expert Member
        BILL WICHERS | | #17

        The problem with an unsupported ridge is that the downward force created by the weight of the roof causes spreading and tends to push the two walls the rafters are connected to apart. Sistering the rafters doesn’t help here, you need either support at the peak (a ridge beam), or something else to prevent the walls from spreading out. Your engineer should have thought about this, and likely did, so if it’s all good then some other aspect of the structure is resisting that spreading force.

        Regarding the attic space being conditioned space, it’s not the ccSPF that makes it conditioned space, at least not by itself. What makes the attic conditioned space is the LOCATION of the foam, in the roof. If the ccSPF was installed above the ceiling, then the attic would not be conditioned space. It all has to do with where the building envelope is, and in this case it’s the location of the insulation that defines the building envelope.

        You might want to look through articles on GBA, and the website of building science corporation. Both sites have articles explaining this that may be of help convincing your inspector.

        You can also call up your state inspectors and see if they will tell the local inspector it’s ok. Your engineer will know what to do if you have to go that route.


  15. GBA Editor
    Martin Holladay | | #18

    I'm not sure whether this information is useful, but here goes. To clarify: Your roof assembly is a vented assembly. However, the attic space below the insulated roof assembly is an unvented conditioned attic.

    It's quite possible to have an unvented conditioned attic with a vented insulated roof assembly above the attic. That's what you have.

    Of course you are right about the vapor barrier. Closed-cell spray foam is already a vapor barrier, so you don't need an additional interior vapor barrier.

  16. BrianPontolilo | | #19


    Hopefully you have a good idea of what is going on with your insulation/venting/vapor barrier situation now, but let us know if it is still unclear.

    This will help you understand what Bill is talking about when it comes to the roof framing:

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |