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Barn renovation — floor insulation dilemma

ccullen | Posted in GBA Pro Help on

I am renovating a historic post an beam barn in VT. We are turning the structure into living space. This has lead us to relay and mortar the stone foundation. Spray foam the outside of the barn and add an additional layer of siding. The space is roughly 1000 sq ft.

I am having issues resolving my floor insulation plan. Currently the plan is to leave the crawl space under the joists cold with a humidity controlled fan and two vents. We were going to use 5″ rigid poly iso (not foil faced)I between the 9″ joists and 3″ xps above the joist. Add .75 advantech subfloor and use the original 1.5 inch wood floors (refinished) to finish.

I am concerned about layer upon layer of rigid foam leading to moisture issues on the beams. I am also concerned that the r15 from the foam above the joists will be inadequate even with 9″ joists if I am only filling the joist gap to a 5″ depth. Am I better off insulating the gaps from the bottom of the joist and leaving an air gap?

Thanks for any advice.


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  1. GBA Editor
    Martin Holladay | | #1

    First of all, you don't want a ventilated crawl space in Vermont -- especially if the building above the crawl space will have any plumbing. Your plan is likely to result in frozen pipes.

    What you want to do is to insulate the crawl space walls, not the floor assembly above the crawl space. This article will explain the required work: Building an Unvented Crawl Space.

    Second, if you can't be dissuaded from your insulation plan, it would probably work -- especially if you managed to do careful air sealing work around each piece of rigid foam installed between the joists, and if you came up with some way to protect your plumbing pipes from freezing. However, the cut-and-cobble approach that you plan to use is fussy, time-consuming work, and I don't really recommend it. For more information on this issue, see Cut-and-Cobble Insulation.

    Multiple layers of rigid foam does not lead to any moisture accumulation concerns.

    For more information on better ways to insulate this type of floor, see How to Insulate a Cold Floor.

  2. charlie_sullivan | | #2

    From an environmental perspective, XPS and closed-cell spray foam are about the worst types of insulation--their bubbles are blown with gasses that are extremely potent greenhouse gases, more than 1000 times worse than CO2.

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