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Community and Q&A

Best option for insulating walkout basement?

GBA Editor | Posted in Energy Efficiency and Durability on

I live in Knoxville, TN and am looking at insulating my basement, it is a walkout (door/windows face south). The area is about 1000 sf. Presently there is no insulation on the walls or under the floor joists, the walls are 8ft block setting on a slab. Washer, dryer, W/H and HVAC unit are in this space, ducts, plumbing and wiring run in this space as well. But there are no vents in this area. I have thought about spray foaming the rim joists and putting XPS boards on the walls and sealing the slab joints. Can I put the boards directly on the walls or do I have to use some type of furring strips? Also, should I apply some type of water barrier such as dry-lok before attaching boards? I have also thought about putting batts in the floor joists,but if I insulate the walls I should not have to do this, correct? Any ideas would be appreciated. I will most likely do this myself except maybe the foam-too much mess potential. Thanks.

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  1. GBA Editor
    Martin Holladay | | #1

    Check out this document -- it probably answers many of your questions:

  2. scott | | #2

    Thanks Martin, that did give me good info but I am not looking to finish the basement at this point and with the XPS being flammable that may not be a good idea by itself. But that is a bunch of heat loss through the walls. WOW! I need to do something.

  3. Allison A. Bailes III, PhD | | #3

    Martin has already steered you to the best source for answers to the question you asked. They've got another document called "READ THIS: Before You Design, Build or Renovate," which goes into the details. You can find it at:

    You mentioned possibly putting batts in the floor above the basement, but if you do, you're mostly going to waste your time and money. Stick to spray foam on the band joists and foamboard on the walls. If you're going to leave the foam uncovered, use Thermax (a Dow product) because it's got the fire rating to be left exposed.

    Allison A. Bailes III, PhD

  4. Ashley | | #4

    Scott, Check out products like Thermax and Thermax White Finish from Dow. These are products that are used in crawl space and basement applications. ;

  5. scott | | #5

    Thanks for all the info, looks to be just what I needed. This a great forum/website for information, I check it out a couple of times a week and learn something new all the time. Keep it up.

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