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I have a metal garage, which I want to insulate to a level of a well insulated house (if that's possible at all). I live in the are where the coldest it gets is about -15 C (5 F) for a couple of weeks, and about +30 C (86 F) for a couple of weeks in summer.

At the moment the garage is insulated with an inch of spray foam from the inside - both walls and ceilings.

Could you advice good but affordable material to insulate it with (walls, ceilings and the floor) from the inside? How thick should that be for such climate?

The garage looks looks as such, the floor is made of concrete:

enter image description here

EDIT: in case anyone will need this in the future, I decided to go for 10cm EPS80 polystyrene foam on the floor, 15cm of rock wool on the walls and roof. The garage will be heated by a 6kW wood burning stove, which should be enough during the cold winter months. If you are interested how this installation would cope with the winter please contact me directly or in the comments.

  • An inch or two of spray foam approaches what is put into homes. Is it an inch, or is it two? What's its R-value? – isherwood Oct 31 '19 at 14:26
  • I don't know the R value or exact thickness of spray foam - this is what the garage came with. What about the ceilings? I'd assume they need a bit thicker insulation layer? – Nigi Tanka Oct 31 '19 at 14:50
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    Poke a nail in it and measure. It's hard to offer good advice not knowing what we're starting with. You haven't said anything about the ceiling, so I wouldn't know where to begin there. Maybe update your post with more detail--construction type, etc. Help us help you. – isherwood Oct 31 '19 at 14:56
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    In any case adding insulation inside is the way to insulate a metal building. Since there is a layer of spray foam I would suggest adding more. As far as loosing space we don’t know the type of construction so we can only guess. Their are many different types of metal buildings. – Ed Beal Oct 31 '19 at 15:02
  • I updated the post. What do you think would be a reasonable thickness of spray foam for the ceiling and the floor in a climate I described? – Nigi Tanka Oct 31 '19 at 15:05
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I don't think you can do this. Even 1" of spray foam will stop most of the heat going in or out of the garage by conduction through the walls.

What it won't stop is heat being transferred by cold or hot air coming in around the door. In a well insulated house, it is vitally important to control draughts; highly insulated houses have heat-recovery units to bring fresh air into the house without losing the heat. The problem is that garage doors are not designed to be draught proof - so you will have massive heat loss/gain around the door.

  • I plan completely seal the existing doors (please see the image attached in the original post) and I plan to cut a hole in them to install new, more house-like doors. – Nigi Tanka Oct 31 '19 at 16:08

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