We live in Milwaukee WI so it gets fairly cold in the winter, I'd like to use my garage as a gym over the winter and am getting started with insulating it. I used Owens-Corning R15 batts for the walls, which has made an improvement, but there are still openings (soffits?) to the outside above the wall, and the roof is completely uninsulated.

I'm wondering how to best/most easily insulate the ceiling and deal with the open soffits.

The ceiling has exposed rafters, so it seems easiest to me would be to find the right sized batts and install them in the ceiling the same as I did with the walls. But, is OK then to just cover up the soffits with insulation? I've read this can cause moisture issues, as the soffits allow the garage to "breathe".

Pictures of the soffits, roof, and garage walls included. Thanks,

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  • You want the soffits open to ventilate the roof deck. You want to insulate to space between the walls on the ceiling. You want a cold roof but warm ceiling. Your location should be warmer so R-20 will be okay, but double that for the ceiling, if you want.
    – crip
    Nov 4, 2023 at 21:36
  • Thing is, you don't have a ceiling. You could install one and insulate above it to get what you want.
    – gnicko
    Nov 5, 2023 at 0:10

1 Answer 1


What is that, USDA Zone 5? With just the soffit vents, the code requires 1 ft² of vent area per 150 ft² of floor area. The code required them, so there they are.

The code provides for enclosed rafters differently. The spaces need "cross ventilation," meaning an opening at each end of the cavities. The intent is that air will move through the soffit vents instead of blowing into effectively a closed bottle.

If you happen to have a metal roof, I would look at how it was built. If the metal roof panels were elevated on furring strips, then the roof can dry pretty well from above and I would be tempted to obstruct the soffit vents.

Alternatively I noticed some collar ties along your ridge beam. You can achieve "cross ventilation" between the soffits at both sides by enclosing this space and installing insulation below it instead of above it:

  • Take some 2x2s and run 2 lines below the collar ties with the rafter spacing between their outer edges.
  • Install 1/8" plywood strips to the outside of these 2x2s hanging down enough to contain the same fiberglass batts that you're using in the rafter cavities.
  • Staple the same insulation from the rafter cavities inside this new cavity.
  • Run your insulation up the rafter cavities, but end them by butting into the plywood strips instead of the ridge beam.

You can't fill the whole rafter cavity. You need to leave a 1" space between the roof sheathing and the insulation. See IRC R806.3.

  • Thanks for the detailed response. If I were to enclose the space directly under the ridge beam (essentially making an inverted \ _/ shape on the roof apex, rather than \/), do I need to produce a channel from this air chamber to the soffits?
    – doppz
    Nov 4, 2023 at 22:26
  • @doppz, the air chamber linking the soffits is the point. Without it your rafter cavities would have no cross ventilation. R806.3 is ambiguous as to whether a 1" minimum channel depth needs to be maintained over the entire cross-ventilation path or just in the neighborhood of soffit vents. As long as you're going through the trouble to do it right, it seems silly to potentially steal defeat from the jaws of victory.
    – popham
    Nov 4, 2023 at 22:34
  • @doppz, I imagined that you would butt the end of the bats at the ridge end against the new plywood pieces. Without the rafters, there are probably air gaps in the transition region. By-the-book me would cut insulation strips to fill the rafter gap and then adhesive a class II vapor barrier below to keep everything properly positioned.
    – popham
    Nov 4, 2023 at 22:38
  • I think I understand, let me check if I'm getting this right. Block off the ridge beam, and insulate the region underneath it. But, on the slanted rooflines leave an air space for air flow from the soffits to the larger air pocket at the apex. What's the best way to create this air gap? The rafters are 2x6s, could I just use insulation ~4-5 in deep? (I think the R15 I used for the walls was roughly this deep). Or is it better to create some type of more rigid barrier between the ceiling wood and the insulation
    – doppz
    Nov 4, 2023 at 22:42
  • @doppz, I'd use batts with 4.5" thickness, but the standard thicknesses certainly leap from 3.5" (R15) to 5.5" (R21) thickness. I once had a tight cavity where I used rigid barriers so I could stuff the space full never mind inefficiency (I used a table saw to make little 1" blocks and wood glued them to the surface of 1/8" plywood). Maybe as a compromise, you could shield the top inch above wall top plates and then trust the sag in the insulation to provide the necessary air gap. The problem is that somebody later can ignorantly sheath the ceiling and destroy any air gap.
    – popham
    Nov 4, 2023 at 22:57

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