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I’m building a small (10 x 12 ft) shed in my backyard that I plan to use occasionally as a workshop. I live in Vermont, and wouldn’t use it during the dead of winter, but want to be able to make it semi-comfortable for a few hours at a time in late fall and early spring on a day when outside temps are in the 40s.

I’m wondering if there are any roof underlayment products that provide some amount of insulation for holding in heat. My thinking is that heat rises, so if I can add a little insulation to the roof, the space may trap a little more heat and stay more comfortable on cool days. The shed will have a steel roof.

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    Will there be rafters directly attached to the roof? E.g. bays for batt insulation
    – TylerH
    Oct 6 '21 at 13:40
  • When temps are in the 40s you'd want to catch as much solar energy as possible. That little box would heat up on its own. Insulating it forces you to pay for heat and produce pollution as a result.
    – isherwood
    Oct 6 '21 at 13:45
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    When I think of "roof underlayment" I think of a fairly thin material for waterproofing purposes. What you'd probably want is some sort of actual foam insulation board with the roof underlyament on top of that, then the external roofing. Are you planning on some sort of heat? If so, you'll need to ensure that there's no chance of melting/burning any exposed foam.
    – FreeMan
    Oct 6 '21 at 13:46
  • Also, heat doesn't rise--warm air rises. Why do you think you'll have warm air down low to begin with? My guess is that the solar gain through the roof will provide most of the heat.
    – isherwood
    Oct 6 '21 at 13:46
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    I guess what I'm getting at is that any tiny heater will be more than adequate for that volume in that weather. However, both you and the heater will produce moisture, so you can't really seal up your box or you'll sweat and then suffocate. On that note, you haven't said whether you plan to heat it or just hold heat.
    – isherwood
    Oct 6 '21 at 13:48
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You insulate a shed like any other building when you opt to insulate it at all. You have the same issues, trade-offs, and constraints. And a well insulated roof with uninsulated walls/floor will be of comparatively little benefit, though it does not even sound like you are looking at well-insulated for the roof, if I get what you mean.

If running an electric resistance space heater at typical Vermont power prices, insulation done well will pay back quite quickly...

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  • Thanks Ecnerwal. I’m not certain how much I will use it outside of cold months. Helpful to have it confirmed that insulating just the roof doesn’t have much value. I think I will see how I use it this year, the possibly insulate next year. I may also explore what I can do with passive solar as someone above mentioned.
    – 4restg
    Oct 6 '21 at 20:02
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You can insulate walls & ceiling with XPS, at least 1 in thick, and plywood. The XPS is easy to handle, can be cut to fit between joists, won't house rodents, and works well in moist/wet conditions.

A thin underlayment for just the roof is not really going to accomplish much to retain heat and save money.

You could also just heat without insulation if it's only for few hours here and there and it's at least 40F outside. This saves work and cost, and handles water ingress and moisture quite well. As a heater you could use a (parabolic) infra-red type heater for directed heat.

Many tasks that require chemicals, like glueing, painting, filling, sealing etc... need 50F-60F or so but not much more. If the outside is already 40F that's not a large gap to overcome.

The rest of the comfort gap can be bridged with a warm sweater and appropriate beverages. Cheers!

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    Thanks. Yes, I think I am leaning in the don’t insulate direction given my planned use. By the way, the warm beverage of choice around here for cold weather is hot cider with a dash of whiskey. Between one of those and my space heater, I bet I’ll do just fine!
    – 4restg
    Oct 7 '21 at 16:31
  • Oooh @4restg I am beginning to like Vermont...
    – P2000
    Oct 7 '21 at 18:14

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