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

hUgicQcAi8 | Posted in Energy Efficiency and Durability on

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!

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  1. GBA Editor
    Martin Holladay | | #1

    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.

  2. aj builder | | #2

    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.

  3. Riversong | | #3

    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).

  4. aj builder | | #4

    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.

  5. Riversong | | #5

    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.

  6. hUgicQcAi8 | | #6

    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!

  7. John Klingel | | #7

    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

  8. John Klingel | | #8

    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.

  9. GBA Editor
    Martin Holladay | | #9

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

  10. hUgicQcAi8 | | #10

    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!

  11. Tinybuilds | | #11

    Hi, I am building a tiny house on a pier and beam foundation in Arizona. I would like to do an affordable insulation for the floors for winter warmth. Any recommendations for this. Would there be issues with doing batt insulation between the floor joists and attaching a layer of plywood bellow the joists to protect the insulation from rodents. Essentially a plywood sandwich with joists and batt insulation between the subfloor and additional bottom layer of plywood.

    With good airflow and drainage under the house should I be concerned about trapped moisture in the floor insulation? I could use pressure treated plywood for underneath and spray foam or tape the seams? Thoughts?

    Thank you

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