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Wondering about best (cold climate) strategies for insulating a floor system built on a pier foundation

I poured a pier foundation for a wrap-around covered porch for my house in Vermont last year. Due to changing circumstances we've decided to add-on to our living space by enclosing and insulating this space rather than building the porch. I'm wondering about best strategies for insulating the floor system over the piers. Thanks!

Asked by Micah Whitman
Posted Jan 16, 2012 11:03 AM ET


5 Answers

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How much space is there between the floor and the ground? If the piers are relatively tall then this could be more like a soffit over a carport than a crawl space. In that case you could drop 2x3 framing below and parallel to the floor joists, using 3"-4" wide plywood gussets every 3' or so, to hang the 2x3s. This will allow a space as deep as needed for insulation, and you could dense-pack cellulose to R-60 if desired. Clad the bottom with plywood, under the 2x3s. However, this might be a poor idea if the floor is too near the ground.

Answered by TJ Elder
Posted Jan 16, 2012 8:25 PM ET


At a minimum, I would suggest at least doing a "flash" coat of closed cell spray foam to get it air-tight. Are the joist already in? If so, I am guessing pressure treated? In that case, it may be better to put some foam on the underside, caulked and taped for air tightness.

Answered by Aaron Vander Meulen
Posted Jan 16, 2012 8:35 PM ET


All I know is floors like these are cold in my parts. Insulate it way beyond what you are thinking. You may even want to build a foundation under it instead. Heat the floor if hard surface or if carpet go thick for comfort. Good ideas above, see what Martin has to say....

Answered by aj builder, Upstate NY Zone 6a
Posted Jan 16, 2012 9:11 PM ET


I'd like to offer a counter-argument to AVM's comment (#2) about closed-cell foam. When you start with lumber, cut from the trunk of a tree, and then spray polyurethane foam on top, you've doomed this material to end up as garbage. It is inevitable that this structure (not yet built) will be dismantled someday, when old age or new circumstances require that it be dismantled. If there's wood (the remains of a tree) that can be pulled from the debris--without a crusty coating of non-biodegradable plastic--then it may be either reused, safely burned, mulched or composted. The crusty coating of plastic would turn this otherwise natural and useful material into something unsuitable for any new use, and it will end up instead as garbage in a landfill. Food for thought.

Answered by TJ Elder
Posted Jan 17, 2012 12:41 AM ET


Two choices:

1. Spray polyurethane foam.

2. Fluffy insulation between the joists (fiberglass batts or cellulose) with at least 2 inches of rigid foam installed under the joists. This option requires careful attention to air sealing, both at the rim joists and at the perimeter of each sheet of rigid foam. The rigid foam should be protected by a layer of OSB or plywood.

Answered by Martin Holladay
Posted Jan 17, 2012 7:53 AM ET

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