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Making ceiling plane airtight, and attic insulation

fyHwJnSTuV | Posted in Energy Efficiency and Durability on

Just watched the video regarding vented attics and insulation.

We are re-purposing a 40 year old mill building. (Norfolk, Ct 06058, the icebox of CT). Radiant heat in finished concrete floors is in, German Shucco windows installed. We have a new standing metal seam roof, done. There are sofit vents on both sides, all along the gutter line, ridge vent entire length of roof, as well as 2 gable vents. We have already sprayed soy foam in the walls, 5.5 inches (closed cell). The HVAC trunks are installed just above the proposed drop ceiling.

My question is, how do we make the ceiling plane airtight? Will paint be enough?
And should we spray open cell foam on top of the trunks, and recessed cans (IC and sealed)? roughly 10″-12″. We could opt for closed cell above the ceiling also.

I would like to save a few bucks on the open cell application, if you think its smart money.

Please advise on how to create an airtight ceiling plane, and help me choose the right insulation
for our ceiling. (Building inspector is making noises about spraying directly over sheet rock. he said the moisture from the foam would ruin the sheetrock–kindly comment on this, too)

Thank you in advance

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  1. GBA Editor
    Martin Holladay | | #1


    Q. "My question is, how do we make the ceiling plane airtight? Will paint be enough?"

    A. No.

    If you have never performed any air sealing work, you might want to hire a home-performance contractor to do the work for you.

    Here are some resources at GBA to get you started:

    Video: Blower-door-directed air sealing

    10-part video series: How to Air-Seal an Attic

    Questions and Answers About Air Barriers

    Pinpointing Leaks With a Fog Machine

  2. Foamer | | #2


    I am not sure I understand your description correctly. Are you installing a metal grid drop ceiling as the only barrier between the conditioned space and the vented attic? If that is your plan, all I can say is "don't". You need a proper air barrier and a drop ceiling leaks like a sieve. Install a drywall ceiling and use sprayfoam to seal over all leak spots such as electrical penetrations and walls. Don't forget the top plates of the exterior walls - the drywall never seals to the framing. Build boxes around your can lights and foam over the boxes. An air space is necessary even with IC rated cans. Once you have air sealed, you can insulate with cellulose to reduce your cost or you foam if you prefer. Use open or closed cell for air sealing. Either will work.

    This is the method we use and we achieve good air tightness values. The most recent house we tested came in at 1.08 ACH50. This was standard, run of the mill construction insulated with spray foam and carefully caulked. No additional air barrier techniques were used.

    Your inspector is wrong. There is no reason why you can't spray foam on drywall. We do it all the time.

  3. fyHwJnSTuV | | #3

    Yes, we have a metal armstrong grid that the sheet-rock will be attached to.
    just above the grid, sits the ac trunk and duct work. a few cans (IC and sealed)
    will be in the sheet-rock ceiling. we're using mostly track lighting.
    (Tech low wattage halogen) So, the plan is now to use 5.5-6" closed cell biobased foam
    OVER all the cans, electrical, duct-work, and hppefuly create an airtight seal, as well as high R value
    above us-
    We will leave the attic space uninsulated and vented
    from soffit the length of the building on both sides, with ridge vent, and 2 gable vents.
    would you recommend a fan installed with temp/switch to blow out the hot summer air???
    thank you so much for your reply.

  4. GBA Editor
    Martin Holladay | | #4

    Q. "Would you recommend a fan installed with temp/switch to blow out the hot summer air?"

    A. No. Such a fan would simply waste energy.

    Are you worried about heat transfer from the attic to the indoors during the summer? Instead of adding to your monthly electric bill, why not take the money you would have spent on a fan (and all the money it would take to run it) and invest in thicker insulation?

  5. fyHwJnSTuV | | #5

    the 5.5-6" closed cell foam will be almost R-40
    won't that be enough?
    and would this negate the need for the fan?
    hoping to install a PV system.
    Ct has just funded the program, again
    and we do have an excellent site for it.
    thank you for your reply

  6. Foamer | | #6


    You have adequate ventilation without the fan.

    R-40 closed cell will be enough. A word of caution - the foam will find any gaps between the drywall and the grid and you will get foam coming out on your ceiling. It may be worth it to tape over the joints before you spray.

  7. fyHwJnSTuV | | #7

    thank you for your reply and pointing that out.
    the sheet-rocker wanted to wait and tape after the foam.
    i need to finish electrical, then sheet-rock, then spray the foam.
    because the attic is all vented, the place won't hold heat until the ceiling is foamed and the radiant
    turned on for the first time...
    I should have him tape and maybe not sand until after we're tight and heated???
    thank you for your reply

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