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Insulating and sealing off the underside of pier built structures?

jonscholl | Posted in Energy Efficiency and Durability on

I am planning a kitchen addition to be built on concrete piers….to eliminate the cost and effort of perimeter footings and crawl space walls. The floor would be built using I-joists and I would install (2) layers of 2″ rigid foam down into the joist bays so that the panels sit on the lip of the I-joist flange. I would also seal the panels with spray foam around their entire perimeter within each joist bay. Is this insulation concept acceptable? Does some other material/barrier need to be installed on the bottom of the joists to keep out pests or animals and/or moisture? I do not think so but I am looking for comments. Thanks!

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  1. BobHr | | #1

    What climate zone are you located in? How is the plumbing going to be run and serviced.

    You may want to consider rigid insulation on the bottom side of the floor joists. This would eliminate thermal bridging.

  2. s6bR8Dr9qA | | #2

    soffit materal yes

  3. user-659915 | | #3

    You'd get a better result for less money and effort by filling the joist space with blown cellulose. Unless you're in an extremely cold climate I wouldn't worry too much about thermal bridging in the normal sense as the web of an I-joist is too slender to transfer much heat, but if you have plumbing/mechanical runs close to the bottom of the joists (keep 'em as high as you can) a layer of foam board beneath the joists will be a good idea.

    Whatever insulation you use a soffit board is essential - plywood, or my preference, cement board. Squirrels and other critters will make their way through that foam into the snug cavity above in seconds flat.

  4. GBA Editor
    Martin Holladay | | #4

    My vote: include any insulation you want between the joists, but be sure to pay attention to air sealing, and include a continuous layer of rigid foam under the joists. Protect the rigid foam with plywood.

    For more information, see How to Insulate a Cold Floor.

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