Best way to insulate concrete slab

Discussion in 'Bricks, Masonry and Concrete' started by cibula11, Nov 27, 2009.

  1. Nov 27, 2009 #1

    cibula11

    cibula11

    cibula11

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    Our garage will be converted. I plan on placing 2x8 as joists for subfloor.

    Could I add 6mm poly, followed by rigid foam board and then lay the 2x8 on top of this?
     
  2. Dec 2, 2009 #2

    itsreallyconc

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    of course - its your house,,, poly 1st, joists, & foam inside the joists on the floor
     
  3. Dec 8, 2009 #3

    cibula11

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    Wouldn't it be easier to lay the foam and then the joists on top? I didn't know if there was a reason you suggested.
     
  4. Dec 8, 2009 #4

    mudmixer

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    Might be easier, but not as good.

    How level is your existing garage floor?
     
  5. Dec 10, 2009 #5

    cibula11

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    Last edited: Dec 10, 2009
  6. Dec 10, 2009 #6

    inspectorD

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    Always go for the continuos barrier.
    I think your on the right track, from my experience this method has been far superior than installing between joists. The thermal differences are big between the two materials.
    Follow the building science recommendations for your climate.
     
  7. Dec 12, 2009 #7

    cibula11

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    Thanks. Do you have an idea on the size of rigid foam I would need. I'd like to keep it to a minimum to keep ceiling height at a premium.....but I'm guessing I'll need 2" foam.
     
  8. Dec 12, 2009 #8

    inspectorD

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    Two inch, and get the best R-value you can find. Compare your choices available,And you can infill between the joists for a little better floor if you need piece of mind.It will help.
     
  9. Dec 12, 2009 #9

    cibula11

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    Just to be sure, it would be okay to lay the joists directly on the rigid foam?

    I thought of even adding unfaced batt insulation between joist cavities where possible. With a small gap between the top of the insulation and the subfloor
     
  10. Dec 12, 2009 #10

    inspectorD

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    Joists or sleepers?
    Joists are installed upright, sleepers lay flat on the wide side.
    If you are talking about sleepers, forget the insulation under the wood...it will flex with point loads and large people. If you are going to use sleepers use a 2x4 and nail it into the concrete.

    For Joists you need a ledger board at the wall section to carry the wood upright, for those you can install a ledger board (like on a deck)and joist hangers with the foam underneath...and fiberglass if you want in between. I think fiberglass is just a srewup that got perfected....use some other blown in material for better results.

    Hope this helps.
     
  11. Dec 13, 2009 #11

    cibula11

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    I need to use joists because I will be installing plumbing and need the space for drain pipes to be run. So, yes...I'm talking about joists.

    Ideally, I 'd like to run the joists right on the floor and shim in areas where the garage floor slopes down.

    If this is the case, you're saying that the best way would be to lay the 6 mm poly and then joists across (with ledger) and then insulation batt or blow in between the joists (which would essentially fall directly onto the plastic)?? Or with joists could I still use the rigid foam underneath the joists?
     
  12. Dec 13, 2009 #12

    inspectorD

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    Plastic down, foam board, then joists with the ledger...just like the one Joe Listbureck recommends. And attach, seal, the plastic to the bottom plate if it is PT wood to keep the moisture from the concrete out.
    I think your ready, have fun.:)
     
  13. Jan 4, 2010 #13

    cibula11

    cibula11

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    I'm back :)

    I was planning on running a ledger on opposite walls to essentially "hang" the flooring. However, one wall is the extension of the home's foundation and the ledger would need to be attached to concrete block. I'm guessing it's hollow block. Is there a way to do this )the floor will need strength since it will have a bathtub)

    I suppose I could resort to placing the joists directly on the concrete and shimming, but Id like to get the rigid insulation added on the concrete floor and I'm not sure I could do that with this method.
     
    Last edited: Jan 4, 2010
  14. Jan 15, 2010 #14

    GaTomCat

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    be sure to use treated floor joist..
     

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