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Advice on insulating Colorado garage/storage addition (to foam or not to foam)

EBGCO | Posted in General Questions on

I’m adding a third garage bay with storage above it to my SW Colorado home (~7,000 hcd days per year, Climate Zone 5).  The plan for the roof calls for 4″ of closed-cell foam and 5 1/2″ batts between the rafters (9 1/2″ I-Joists); the plan for the exterior walls calls for 5″ of closed-cell foam (2×6″ walls).  Both the walls and ceiling will be covered with 5/8″ sheetrock and painted.

I met with the insulation contractor last week and he suggests using open-cell foam rather than closed-cell foam to avoid the HFC propellant in closed-cell foam.  He also likes the idea of a wall that can breathe to the interior.

I have two questions: 1) Is foam a good material for this project and location?  If so, which of the foam options is best and why.  2) If foam is not a good option here, what would a better option be?


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  1. Expert Member
    Dana Dorsett | | #1

    Garages are inherently leaky, and have indoor dew points similar to the outdoor air, whereas living spaces have higher humidity. In this situation open cell foam will usually be moisture safe. If it were later converted to tighter, occupied living space there would be some risk to the roof deck.

    Using HFO-blown closed cell foam would have far lower environmental impact than HFC blown closed cell foam, but not as low as open cell.

    It only takes an inch of closed cell foam to protect the roof deck even if it were fully conditioned living space, which would leave you 8.5" of space for fiber insulation. At 2" there is still a reasonable drying path for the roof deck, which would leave you at 7.25", which is perfectly sized for rock wool R30. Using HFO blown foam the foam layer would be R14 @ 2", and with R30 cavity fill the foam would be ~32% of the ~R44 total. While that is below teh ~40% IRC prescriptive for fully conditioned living space, it would be completely moisture safe for a garage. If converting it to living space later a smart vapor retarder or vapor barrier latex primer on the ceiling would ensure that the fiber insulation didn't accumulate enough moisture to be a problem.

  2. joshdurston | | #2

    Is there any concern about the moisture brought in during the winter when you park a wet slushy car in the insulated garage?

    1. Expert Member
      Dana Dorsett | | #3

      Not really. Unless the slab is heated the rate of moisture transfer from the puddle on the floor to the garage air will be small relative to the typical air leakage rates of garages.

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