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Insulation for shop in zone 6

troubleshooter | Posted in General Questions on

I have a shop in Zone 6 with 2×6 walls covered with OSB and then metal siding (no building wrap that I can see or tell).

I want to insulate the walls. I am looking at different options. The shop will not be heated full time so I don’t want to spend a fortune but when in it want it comfortable.

I am looking at just 2″ of closed cell foam by itself. (The garage in our new house has 2″ in the wall and 6″ in the roof and is working well.)
I think I can do 2″ of closed cell foam and then R13 batts which should be correct for the proper ratio of foam/fiberglass for moisture control but this would be expensive for a shop.


I am more interested in just using R19 batts. Will they work if I place a vapor barrier on the inside of the OSB and then place the batts on top of that?

Which of these would work for moisture control? The batts would be the cheapest.

The attic is unvented so my plan there is just 4″ of closed cell foam sprayed on the bottom of the roof decking. ThenĀ  later I will add a drywall on theĀ  ceiling and walls


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  1. user-2310254 | | #1

    I'll give your post a bump. Is the OSB the sheathing (on the exterior)? Where are you located?

  2. troubleshooter | | #2

    Westcliffe CO- about 3 hours south of Denver.

    The osb is on the exterior underneath the steel siding.

  3. user-2310254 | | #3

    I'll post a response, but I'm not an expert. Maybe bumping this thread back to the top of the list will garner a few more opinions.

    You can spray 2 inches of closed cell (HFO blowing agents are better for the environment) in the walls, which would help to air seal the interior. But I also think you would want to install fiberglass batts in the rest of the cavity to keep the foam's inner surface warm enough during the winter. Otherwise, the OSB might accumulate too much moisture.

    If you wanted to similarly "flash and batt" the roof, you would need to install R-25 of closed cell against the sheathing and then make up the balance with fiberglass or another air permeable insulation.

    See this article for more info: Note that Martin emphasizes that flash and batt is riskier in colder climates such as yours. I would venture that it is probably extra risky if you are only doing the flash part.

    That said, I'm more concerned about the sub-code levels of insulation in your house. Are there any areas of the wall or ceiling where you can insert a moisture probe in the sheathing--just to be on the safe side? I would be particularly interested in assessing conditions at the roof ridge and on the north side of the home.

  4. creativedestruction | | #4

    For an unvented roof the closed cell foam is probably the safest route but the drywall is a good idea to protect it right away. Codes require a thermal or ignition barrier to cover almost all types of foam plastic and drywall is the cheapest way to go.

    Consider what moisture sources will be present (or not present) within the shop. Your walls would be okay with just batts and an interior air barrier, assuming you're not putting a swimming pool or a steam sauna in this shop. Air barrier could be sealed poly but personally I would suggest plywood or osb with taped seams for more drying capability in spring and summer.

    1. creativedestruction | | #5

      Also the noted lack of building paper or other weather resistive barrier is concerning. Adding insulation is very risky if bulk water isn't properly managed first. Insulation drastically reduces drying capability of the exterior sheathing... It's worth temporarily removing siding to add it if it saves your sheathing from rot.

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