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What type of insulation should I install under the floors of pier foundation house?

I have a 1932 pier and beam house in Austin, TX. The floors are very cold in the winter since there is no insulation underneath and many air vent holes in the side of the house to promote ventilation. I'm confused whether I should use open or closed cell insulation underneath the floors. I've heard that I could use open cell, open cell with a vapor paint barrier or closed cell. Does anyone know what the right answer is?
Thank you!

Asked by Katie Guthery
Posted Dec 1, 2010 2:22 PM ET


10 Answers

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In your climate, you don't want to use open-cell foam, which is vapor-permeable. If you do, your subfloor will accumulate moisture during the summer and may rot.

The easiest solution -- but not the cheapest -- is closed-cell spray polyurethane foam.

Another option is rigid foam (EPS, XPS, or polyiso). You can cut and fit the foam between the joists -- a laborious process -- or, better yet, extend it across the bottom of the joists. In either case, you'll need to protect it from physical damage with plywood or OSB. Pay attention to air sealing! The rigid foam won't do you any good unless you create an air barrier with the foam.

Answered by Martin Holladay
Posted Dec 1, 2010 3:21 PM ET


I personally would never do sheet board style foam on the bottom of joists creating trapped airspace. That airspace could be fine or not. Too risky. Also one tiny hole and all the money spent will be doing no insulating.

Use closed cell spray foam, buy what you can afford. I do think flash and batt would work too (for even higher R), though Riversong would have to chime in for me to be comfortable with it.

Answered by aj builder
Posted Dec 1, 2010 3:48 PM ET


Trapped airspace is how every insulation works. And "one tiny hole" will do almost nothing to reduce the insulating value, nor allow air movement which requires an entrance and an exit.

I would leave the joist bays empty and install foil-faced polyiso board under the joists, to provide insulation, a thermal break and a radiant barrier (as much as R-9 from the lowE air space in addition to the R-value of the foam).

Answered by Riversong
Posted Dec 1, 2010 10:17 PM ET


Robert, everyone that uses foam sheeting finds out a single layer is not reliably leak proof. Two contractors near me say they have to very carefully install sheeting and tape and have to use multiple offset layers. Your install is possible but spray foam is much more doable to me in this application. Also if I did it your way the sheets should be caulked/glued in place to seal in the performance you are suggesting. You also know dusting up of the foil is going to degrade that R gain over time.

And my biggest concern is if moisture does find its way into your assembly it is not getting out, and in Texas... rot heaven.

I will agree to disagree at this point and move on.

Answered by aj builder
Posted Dec 2, 2010 12:37 AM ET


If the seams are taped, there is no need to take any additional measures for air sealing. The improvement will be an order of magnitude. Since Austin is an AC-dominated climate, the vapor barrier will be on the correct side and - unless there is a vinyl or other impermeable flooring above it, the open-cavity floor assembly can dry to the inside. Leaving the framing cavities internally open creates a far more durable assembly than sealing the wood with closed-cell foam while leaving the bottom edges exposed to the ambient humidity.

Answered by Riversong
Posted Dec 2, 2010 12:58 AM ET


Thank you all for your expertise. I'm not sure that I can get foam sheeting boards through the one entry to the area underneath my house. I have a scuttle hole in a closet. There is no outside entry. It sounds like you all are saying that even if I do closed cell foam that I should cover it with sheeting boards. Hmmm, this is sounding very difficult. Is there any batting that you would recommend? Many thanks. I appreciate hearing everyone's thoughts!

Answered by Katie Guthery
Posted Dec 2, 2010 7:54 AM ET


Katie: Since the crawl space is entirely enclosed, gents, is it not wise to knock a hole in the wall of the crawl space, insulate he walls and VB the dirt floor, leaving the floor of the house alone? It seems to me that that is what I read here a lot. j

Answered by John Klingel
Posted Dec 2, 2010 10:39 AM ET


OK, that should have not been addressed to Katie. Give me a break. It is 0540 and I have not been to bed yet. A bit weird in the head.

Answered by John Klingel
Posted Dec 2, 2010 10:41 AM ET


It sounds like a leaky skirt, not a true crawl space foundation with concrete stem walls.

Answered by Martin Holladay
Posted Dec 2, 2010 10:44 AM ET


My house is brick and there are 15 air vent holes surrounding the house to ventilate the area underneath the house. If it is a leaky skirt as you say, what do you recommend to keep the floors warm in winter? Many thanks to all of you!

Answered by Katie Guthery
Posted Dec 2, 2010 10:47 AM ET

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