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Insulating Barn Floor With Rigid Foam

Alex9999999 | Posted in General Questions on

Hi all,

I am ripping out, leveling and replacing the floor of my barn, which I am slowly transitioning to more of a cabin.

I am gradually adding insulation to make this structure cozier, though by no means will I ever reach code. My question: While I have the floor off and the joists exposed, should I install a continuous layer of rigid foam over the joists, and put the plywood subfloor on top of it? I’m thinking it would be easy to do, and even 1/2” or 1” might make a big difference, particularly because of its air sealing properties.  But I don’t know if it is used in this way.

There is a basement. I don’t imagine ever insulating its dirt floor or field stone walls, but maybe rather than putting foam under the subfloor I’m better off insulating from underneath?

Any advice for my somewhat quick and dirty insulation of this floor would be appreciated.


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  1. charlie_sullivan | | #1

    It's usually more effective, performance wise and cost wise, to insulate the basement rather than the floor, even if you don't particularly want to condition the basement, because the above-grade area is smaller and the below grade portion has a smaller temperature difference across the insulation. And you can solve some moisture/mold problems while you are at it. That said, field-stone walls are hard to deal with so your situation might be one of the rare cases where it's worth considering floor insulation.

    If you want to put foam above the joists, you'll need a subfloor layer under the foam and other above the foam, to distribute the structural load. That gets expensive and eats up vertical space, so it's rarely done. More common is to add fluffy insulation to the space between the joists and/or add foam beneath the joists.

    Taped foam can be a good air barrier, but you can also use a membrane of some kind or tape the subfloor to get a good air barrier, and do the insulation separately.

    For a short term modest improvement, you could use roof decking with a radiant barrier coating on the bottom as the sub-floor. That's one of the few cases in which a radiant barrier actually works. That would only have an effect while the cavity below is open space, but for heat flow down, it would not be that different from 1" of foam: theoretically it would give you as much as the equivalent of R-6.5 for downward heat flow, although it practice I would expect more like R-4 or R-5.

  2. Sv63D3 | | #2

    I’d insulate below, definitely. I wouldn’t want to deal with losing ceiling height in the main space, or with figuring out how to reinforce the rigid foam between the floor and the joists without letting heat through. Rigid foam below is going to be a lot easier to manage.

    One way or another, insulating the floor (above or below) is definitely worth it if it's going to be a living space (as opposed to a barn). In my non-expert experience, floor insulation makes a huge difference in comfort because it evens out the temperatures in the room.

  3. Alex9999999 | | #3

    Thank you for your very informative replies. Charlie I love the idea of using the radiant-barrier roof sheathing as a subfloor. It seems so much easier and probably cheaper than foam and two layers of plywood. I assume the foil should face down? Any idea if it would still work even somewhat once all the nails from the hardwood flooring were poking out the bottom?

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