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Community and Q&A

Insulating Under Floor Joists

Paul_C_Louisiana | Posted in General Questions on

I’m in southern Louisiana in CZ2A (humid).  I’ve got a small raised house and want better insulation under the floor.  About 12 years ago, I put fiberglass batts held up with chicken wire to keep animals from it, but occasionally raccoons have robbed a little here and there anyway.  The insulation did help, but I’m sure it’s not fully doing it’s job since it’s exposed to wind.

By raised, I mean it’s floor joists sitting on blocks.  There is no crawl space wall or skirting to insulate the sides.  Pretty much all the insulation advise I see is to insulate the sides that I don’t have.  The joist bottoms are about 24″ above the ground.  Joists and plywood are both treated.

I was wondering if I could add about R10 or so of EPS or XPS foam board below the joists and tape the edges and seams to seal out air.  This would keep all sun exposure off the foam boards.  Can I use foam boards if left exposed like this?  I’m pretty sure I have a vapor barrier under the laminate flooring by the way.  I remember putting a plastic underlayment when installing it.

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  1. Expert Member
    PETER G ENGLE PE | | #1

    IN warm, humid climates, vapor barriers on the interior are a no-no, as A/C keeps the interior surfaces below the dewpoint of exterior air and you can get condensation behind your finishes. So we can hope that the underlayment you used is not a complete vapor barrier type material.

    If not, then using foil-faced polyiso insulation might be your best bet. The foil facing is a vapor barrier (not retarder), and depending on the brand and type, can be relatively rugged. Also, taping the foil facing with foil tape is easy and permanent. The insulation will keep all of the lumber inside at least some of your thermal envelope and that tends to keep it dryer. The taped surfaces will stop the wind washing that you are correctly concerned about. The end result should be more durable and comfortable than what you have now.

  2. Expert Member
    BILL WICHERS | | #2

    I agree with everything Peter said regarding vapor issues.

    If you're having racoon problems though, they can scratch and chew through polyiso or any other type of rigid foam. You'll want to put some plywood sheathing over the foam for physical protection. You don't need it for structure, so you can get by with 3/8", maybe even 1/4" plywood here. The only downside to the thinner material is that it will sag more between supports and will look worse.


  3. tim_dilletante | | #3

    I am having a sun porch insulated, and it's a similar situation. Raised about 24" on piers. My insulation guy came up with a solution, where we'll attach plywood to the bottom of the joists and he will blow cellulose into the joist bays.
    I am in CZ6 so what works here may not work in LA.

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