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Community and Q&A

Vented versus unvented attic retrofit

GBA Editor | Posted in Interior Design on

I am considering having the fiberglass insulation removed from the attic floor and having open-cell foam sprayed in its place. Most foam installers are pushing the idea of sealing my ridge and soffit vents, spraying the underside of the roof and creating a so-called conditioned attic. This is a 2,100 square foot all brick ranch with a 6-12 roof built in 1967. We want to keep the insulation within the 2 X 6 framing as we have much of the attic floor covered with osb. We use it for some light storage. We’ve done tons of updating– new windows, doors, new high effeciency furnace/AC units which are in the basement. No ductwork is in the attic. I am getting a lot of conflicting information ( example: Demilac says to spray a vapor retardant paint on the foam; their installers say that they have never heard of this! One source says to tie the attic space into the heating/cooling system. Most say NOT to do this. One source says ice dams will be prevented by plenty of insulation AND ventilation. Another says unvented attics have no ice dams! )
Then there are health concerns over off-gassing, etc. Obviously, a vented attic would mitigate this somewhat. We are in Climate Zone 5 near Pittsburgh, PA.
Any help will be much appreciated. Thanks in advance! Rgn 2-27-10

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  1. Riversong | | #1

    You have no ductwork in the attic and use it only for storage, so there is no need or advantage to sealing/insulating the roof and there are disadvantages.

    Open cell foam is vapor permeable and, if there is a potential for humid air exfiltrating into the attic, then a sprayed roof should be sealed. Code also requires that it have an ignition barrier of at least 1/4" plywood or intumescent coating. And even an R-38 hot roof can experience ice dams with more than 9" of snow on the roof (snow is an insulator and moves the thermal gradient outward).

    You should certainly take up the existing fiberglass in order to air seal all penetrations from the conditioned space below, including at outer wall plates. The most sensible and cost-effective improvement would be to increase the insulation value of the attic floor. That's a challenge with 2x6 joists.

    You could spray closed-cell foam into the joist bays at a price, but that would also air seal the ceiling below. Or you could blow cellulose into the bays and cover the joists with 2" of EPS or XPS foam board, then reinstall the OSB decking. That would also air seal the floor, particularly if you used two layers of 1" foam board with offset and taped joints, as well as eliminating the thermal bridging through the framing. The latter would be my best advise.

  2. Riversong | | #2

    P.S. And keep the attic vented. That's the best way to keep roof framing and sheathing dry, minimize summer heat gain and control ice dams.

  3. Russell | | #3

    Thank you and response to Robert B.-- Funny that you should mention rigid boards. Before I started looking into spray foam, I considered taking up the osb and laying Owens-Corning Foamular 2"=R10. I am much more of an " idea " guy than I am the " applicator " guy, but I did think that I might be creating a second vapor barrier. Numerous e-mails and phone calls yielded about 6 different answers on whether this was appropriate. I priced boards and realized that I'd have quite a bit of cost. By chance, one of my customers is building a new house and it will be insulated with foam. That got me to thinking. Are you in favor of putting open-cell in the joist bays and covering the whole floor with 7/16 osb? We have 7/16 osb on more than half of the area now.
    Thank you! RgN

  4. Riversong | | #4

    Open cell foam will provide an air seal and some modest insulation value, but it'll give you only R-18 including drywall and OSB if you completely fill the 2x6 joist bays because of the thermal bridging through the framing. The IECC calls for R-38 ceilings in climate zone 5.

    I stand by my recommendation. EPS is quite vapor permeable and XPS is modestly so. If the fiberglass is in decent condition, simply adding the rigid foam board would be the most cost-effective approach.

  5. Russell | | #5

    Response to Robert R. Thanks again.

  6. Russell | | #6

    Response to Robert R. Thanks again.

  7. Russell | | #7


    You confirmed my suspicions about spraying the roof deck and eliminating the vents. I plan to reinstall the osb flooring no matter what we add in terms of insulation. If we have the floor sprayed with open cell foam, must I extend the osb over the ENTIRE floor as a fire precaution? Would it make sense to remove the fiberglass, spot seal around penetrations, and apply cellulose instead of open-cell foam?
    The fiberglass is not in bad shape. I just think that there are some gaps, etc. Would I be "air-sealed " enough if I leave the fiberglass and simply add two inches of foam board + the osb?
    I got a price of around $6,000 to spray foam on the floor. That includes taking up the osb, removing the fiberglass, and reinstalling the osb. Two inch rigid boards are aroung $25 each and I am looking at around 2,000 sq. ft. A downside to the rigid boards is that I would lose a couple of inches of headroom in the attic.
    We are also considering adding two small dormers with operable windows. We extended the length of the house in 1993 and have always felt that we should have broken up the roofline for appearance purposes. These would simply be open to the attic and I might open the windows from time to time for more ventilation. Do you see that having any affect on the insulation as long as we insulate the floor as opposed to the roof?

    Thanks again,

  8. Riversong | | #8

    I believe only walls and ceiling/roof have to be fire-protected with spray foam, but you should check with your fire marshall. Open-cell foam will air seal well but won't offer any more R-value than fibrous insulation and the closed cell won't eliminate the thermal bridging at the joists. So you'd be better off with either the existing fiberglass or cellulose (but why throw away the batts if they're in good condition?) and the rigid foam thermal break. If the attic is just for storage then the headroom shouldn't be an issue. You would need the 2" of rigid foam to even approach a decently insulated ceiling.

    Make sure all penetrations into the attic are sealed, including the stairs or hatch. Adding dormers and windows to an unconditioned attic seems extremely wasteful, will create a weather entry when the windows are open and will short-circuit the roof ventilation as well. It will also create more potential for roof leaks and ice dams.

  9. Jonathan Reitkopp | | #9

    I am a little confused by the open and closed cell foam insulating solutions. I have a similar situation to Russell. I am adding on to a 1940's masonry house with no insulation in the walls except the 3/4" in between the furring strips and the brick... in some areas. 2x6 attic floor and hip rafters with old fiberglass r-13. My question is 2 part. I want to add a 2nd HVAC system in the new attic. Can I insulate the roof and seal the vents only in that area, and seal it off from the rest of the attic? The rest of the attic would be vented and insulated on the ceiling. I assume I would use closed cell foam against the underside of the plwd there? 2. Over the existing house I would like to seal the ceiling with spray foam and then add batt or blow in cellulose in the ceiling bays. I may also cover the joists with rigid foam and plwd in some areas for storage. Do I want a vapor permeable or non permeable barrier against the plaster/drywall ceiling. If I seal the ceiling with open cell foam and then fill the joist cavities with batt or cellulose, than humidity would condense on the cold surface of the EPS or XPS foam applied to the tops of the joists (in a vented attic). EPS & XPS are somewhat vapor permeable but wouldn't it dampen the insulation below, potentially causing mildew problems? Or is it better to go with closed cell foam against the ceiling, and then it wouldn't matter what I did on top? Also, does it make a difference if the ceilings are drywall or plaster? Thanks Jon

  10. User avatar GBA Editor
    Martin Holladay | | #10

    Q. "I want to add a 2nd HVAC system in the new attic. Can I insulate the roof and seal the vents only in that area, and seal it off from the rest of the attic?"

    A. Yes.

    Q. "I assume I would use closed-cell foam against the underside of the plywood there?"

    A. You can do that if you want.

    Q. "Do I want a vapor permeable or non permeable barrier against the plaster/drywall ceiling?"

    A. In this case, the main purpose of the spray foam installed against your ceiling is to seal air leaks. The vapor permeance of the foam doesn't matter; what matters is whether or not the foam creates an effective air barrier. For a thin application of foam, closed-cell foam is a much more effective air barrier than open-cell foam -- so you want closed-cell foam.

    Q. "If I seal the ceiling with open-cell foam and then fill the joist cavities with batt or cellulose, than humidity would condense on the cold surface of the EPS or XPS foam applied to the tops of the joists (in a vented attic)."

    A. Even if you chose the wrong type of spray foam (open-cell), your condensation scenario is unlikely. Vapor diffusion rarely causes moisture problems in attics; almost all moisture problems in attics are caused by air leakage, not vapor diffusion. But why would you want to install EPS or XPS? That's a complicated sandwich -- spray foam, then cellulose, then XPS. Just use closed-cell spray foam to seal air leaks, and cellulose as your insulation. Forget the EPS and XPS.

    Q. "EPS & XPS are somewhat vapor permeable but wouldn't it dampen the insulation below, potentially causing mildew problems?"

    A. No.

    Q. "Or is it better to go with closed cell foam against the ceiling, and then it wouldn't matter what I did on top?"

    A. As I said, closed-cell foam provides a better air seal in thin layers.

    Q. "Does it make a difference if the ceilings are drywall or plaster?"

    A. No.

  11. Jonathan Reitkopp | | #11

    Thanks Martin! I was thinking to put the XPS foam over the joists for a thermal break. Since I wanted to be able to walk up there, I can't blow in 12" of cellulose over 6" ceiling rafters. But you are right, it gets way too complicated.
    Back to the spray foam, I also read that if I spray the underside of the roof deck, that it makes sense to pan the rafters (with something other than foam pans for durability), in case I have to do a roof repair or something. This would also create a ventilation pathway. In my case I don't have any eaves and therefore no ventilation, but doesn't the foam have to be applied in direct contact with the underside of the roof sheathing to prevent condensation problems? If that were true, is there anything to prevent the foam from bonding to the plywood but keep it in contact with the plywood?

  12. User avatar GBA Editor
    Martin Holladay | | #12

    It's fussy work, but you can create ventilation channels in each rafter bay (for example, using 2x2 sticks in the corners and thin plywood), whether or not you have soffit vents at the eaves. These channels would make it easier to repair the roof sheathing in the future if that become necessary.

  13. User avatar
    Michael Schreiber | | #13

    Sorry to re-dog up this 2011 thread.

    I have a similar situation, where our secondary HVAC was placed in our unconditioned attic. With outside temperatures of 25°F presently, our gas heat coming from the vents is lukewarm at best.

    I’m considering creating a partially enclosed area for the furnace and main ducts with rigid foam and covering the branches with blown insulation.

    The furnace is a 98% efficiency unit, so it has its own external fresh air supply.

    Please let me know if this works, or if there are alternative recommendations.

    I know closed-cell spray is great, but I’m also trying to maximize bang for the buck. And fully conditioning the attic I feel has too much risk/cost.

    1. User avatar GBA Editor
      Martin Holladay | | #14

      Here is a link to an article that directly addresses your question: "Should I Build a Mechanical Room in My Attic?"

      1. User avatar
        Michael Schreiber | | #15

        Thanks, Martin!

        Luckily a lot of the issues raised in that article are already addressed as local code required me to build a platform with walkway and lighting in our attic. Obviously an air barrier becomes an issue. I am hoping to enclose the furnace and drop-down stairs in the same "room". Yes, this will probably cost around the $1-2k range to do properly. And while the ducts themselves will technically still be in unconditioned space, they will be covered with R-30+ insulation.

        I think I may be "lucky" that the combustion, platform, lighting, and access area were already addressed and do not need to be included in the additional costs.

        What do you think?

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