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rohman 12-17-2009 02:08 PM

Insulate Post and Beam Foundation
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Hi, I have been browsing this board and scouring the internet for an answer to my unique insulation problem. Before I purchased, my home was added to/rebuilt and the old part of the home is built on a post and beam foundation (front part of house in picture), and the floors are unbearably cold.

I took a look at the underside from inside the crawlspace of the new part of the house and I can see no insulation under the T&G subfloor. What is the best way of insulating this area? I was thinking of using XPS between the beams (2-4") or unfaced bat in the beam cavity then XPS attached to the bottom of the beams.

My only concern is that I don't have enough room to get under the house (see picture). If that proves to be the case could I dig a trench around the outside of the "foundation" and install 2" XPS to the frost line and up the side of the house a foot or so (sort of like insulating the outside of a slab on grade foundation), or would that be worthless due to the fact that there would be significant airspace between the ground and floor.

My location is Chicago, IL
Thank you in advance for all your help.

Bondo 12-17-2009 06:06 PM

Ayuh,... I'd probably do Both....

Have you thought of underfloor radient heat,..??
Staple it up, Then wad the insulation to the joist bays....

rohman 12-17-2009 09:49 PM

Underfloor radient heat is out of the question as new hardwood was recently put down and im not ripping that up, plus hardwood is a terrible conductor for radient heating.

If im not able to insulate under the house would insulating around add much value?

Gary in WA 12-17-2009 11:13 PM

Under the floor is the best for me. They do make 36" wide batts.
Be safe, Gary

rohman 12-18-2009 08:22 AM

Thanks for the article link, the building science website has alot of good information dealing with insulation, moisture, ect.

Would it be possible/advisable to use extruded polystyrene (XPS) instead of foil faced insulation as they suggest, or is the permeability of XPS too great to act as an effective vapor barrier?

ccarlisle 12-18-2009 08:52 AM

Up here, we would consider 2" of XPS would have a perm rating of 0.5, so a class II vapour retarder. But we're one zone colder than you are, I think.

rohman 12-18-2009 09:52 AM

That is what I was thinking, 2" XPS, fastened to the bottom of the beams (creates complete thermal break between interior and exterior) over unfaced R-19 fiberglass, which will be placed between the beams. That should give me R-29 without consideration of the subfloor R value.

Since I have hardwood laid on top of 15-20lb roofing felt (cant remember which I used) the floor assembly (beams, joists, subfloor) should be able to dry to the inside or outside should it ever become wet correct?

Gary in WA 12-19-2009 12:51 AM

It also thought it odd that he compared f.f. foam to SPRF: "It gets better, the foil facing, if you expose the shiny side (face it down into the crawlspace) almost eliminates radiative coupling and means that the surface of the insulation approaches the temperature of the ventilation air reducing condensation" From Building Science. Yet, he goes on to say three inches of 2# SPRF (.45 perms) is just as good, except for vinyl in mixed and hot-humid climates. The Tuff R foil-faced = .03 per inch, Pink, as cc said, = .55 perms for 2", Blue = .75 for 2" SuperTuff = .03 per inch. All v.b. just that the foil faced perm is so much lower, better than 6mil plastic(.06). I would go with foiled, the only Class I - impermeable vapor barrier.
Be safe, Gary

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