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Dow Expanding foam

GBA Editor | Posted in General Questions on

I have read several posts regarding off gassing off 2 part foams both closed and open cell….However my question specifically relates to using the Great Stuff Dow Expanding foam around HVAC supply vents. My concern is any off gassing that may occur around these supply vents if I use this foam. I have some minor gaps that I was going to seal to enhance the air barrier. But have concerns regarding any off gassing AFTER the foam has completely cured.

Anyone have any insights into this.

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  1. GBA Editor
    Martin Holladay | | #1

    I'm not sure what you mean by "supply vents." Do you mean supply ductwork? Supply registers? Register boots? An air intake grille on the outside of your house? (The word "vent" is fairly meaningless.)

    Don't use spray foam to seal leaky duct seams or register boots. Instead, use duct mastic. For wide cracks, use duct mastic and fiberglass mesh tape.

  2. Matt | | #2

    Yes. supply ductwork. Specifically when you remove the supply register/grille from the boot register There is typically a small gap between the sheet rock and the actual boot register.

    Yes caulk is an option.......But in lieu of the article I believe you written about off gassing etc of foam....I was curious what your opinion was using foam to seal around ductwork supplys and rtn's. Here your awful close to the airstream and potentially dispersing any off gassing throughout the house especially on rtn's

  3. GBA Editor
    Martin Holladay | | #3

    Personally, I don't worry about outgassing from canned spray foam. But if you have chemical sensitivities or other medical issues, I suppose you could make an argument in favor of a foam-free house.

    You could also use one of the green caulks that promise no VOCs or outgassing problems.

  4. Riversong | | #4

    Canned Great Stuff is hardly great stuff: over-expanding, difficult to control and wasteful.

    If you have any amount of air sealing to do, buy an inexpensive foam gun and use low-expansion canister foam, like PurFill.

    However, all spray foams are toxic. PurFill is among the best, but it comes with these warnings:

    • Wear disposable chemical resistant plastic or rubber gloves and wear goggles or a face mask while foaming.
    • In case of eye contact, wash thoroughly with water and seek medical advice immediately.
    • In case of skin contact, wipe with a dry cloth and wash immediately with soap and water.
    • Product contains isocyanate and flammable components.
    • Foam in a well ventilated area—fumes are flammable.
    • Do not smoke while foaming.
    • Keep away from open fl ame or any source of flame.
    • If you feel unwell, seek immediate medical advice.

    Container is pressurized. Protect from sunlight and temperatures above 50° C. Do not pierce
    or burn even after use. Do not spray against flames or incandescent objects.

    Harmful when inhaled. Irritates eyes, respiratory organs and skin. Can result in sensitization
    when inhaled. Can produce highly flammable vapor/air mixture during use.

    It contains: polol prepolymer, monomeric diphenylmethane disocyanate (MDI), tetrafluoroethane, dimethyl ether, butane/propane

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