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Currently the detached garage walls are insulated with batts of insulation, however the ceiling is not, so here in Illinois, the garage is pretty much unusable most days in the winter. I use this garage as a woodworking shop.

The obvious approach would be to drywall the ceiling and just add more batts between the joists. However, the joists are spaced at 32", so Id have to install more joists if I wanted to drywall as well moving electrical that has been installed on the surface of the joists, so that option is last.

Another approach that would be cheap is to just staple down rigid foam boards on top of the joists.

Additionally, the roof does have 2 vents.

All in all, is there a better approach that wouldn't require drywalling?

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  • Is there a heat source (other than the woodworker's body) in the space?
    – bib
    Jan 10, 2019 at 21:31
  • @bib Not yet. I want to get the ceiling insulated before trying to add a source of heat. That'll be another fun problem to solve since I only have a 30A sub-panel to that back garage, and no natural gas hook-up.
    – seggles
    Jan 10, 2019 at 21:42
  • Propane space heater - and CO detector.
    – CoAstroGeek
    Jan 10, 2019 at 22:16

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A quick & dirty approach is to simply staple plastic sheeting on the underside of the garage rafters, and cut holes for the electrical fixtures. It isn't pretty, isn't nearly as warm as actually drywalling & insulating the space. But it's surprising how much warmer it is then just the open joists.

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  • What about stapling down polystyrene foam boards on top of the joists? I think it'd offer a bit more insulation than plastic.
    – seggles
    Jan 10, 2019 at 22:35
  • Sure - but an order of magnitude more expensive! But seriously, I think that would work also. But I think the biggest improvement will come from reducing the heated volume and eliminating the convection up to the roof (and through the vents). I think the reduction in conduction that you'll also get with the foam will be small relative to that. But in absolute terms, the foam is cheap as well. Get the foil backed stuff.
    – CoAstroGeek
    Jan 10, 2019 at 22:58
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Prorib steel siding screwed up to your rafters. They could carry that span no problem. Put plastic up first before the steel for a vapor barrier and blow in insulation.

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