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Thread: slab insulation

  1. #1
    Join Date
    Feb 2010
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    Default slab insulation

    I calculated the heatloss through the concrete slab (no crawlspace) to be 50% of the total heatloss in my ranch style house. Can I use Tuff-R on top of the concrete, add 1/2 inch plywood and put a wooden floor on top?

    Will the foam lose its rigidity over time and sag?

    thanks for advice!

  2. #2
    Join Date
    Aug 2008
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    10

    Default Re: slab insulation

    Bless you for writig in a question that nearly matches mine. It is so nice to know that someone else is puzzlig over solutions to dumb, stupid, crumby work on the part of a contractor.

    Our home is 20 yrs old. We bought it when it was in the framing stages. We did watch as the house went up and was finished and not informed enough to realize mistakes had been made. We have a mediterranean style w/1 1/2 stories. Everyone in the track noticed in winter that when they came down the stairs and got to the third step from the bottom, we were all stepping down into a pool of very cold air. No one has come up with an answer. We've all had carpeting or somekind of new flooring put in and the flooring/carpeting people said nothing was out of the ordinary.

    I finally this past week, took the time to study building techniques using the cement slab. I now know that the slab is missing a vapor barrier, staggered strips of lumber to nail your choice of sub-floor onto followed by your choice of flooring. I can certainly imagine that we would feel much more comfortable if we had that amount of space between the cold concrete and our feet and legs, not to mention we too are cold and miserable all winter long. We have a new HVAC system that is powerful and efficient but the pool of cold air just nuetralizes the heat. We also have a gas stove insert (a heating stove) that can heat the whole house but it must displace all the cold air prior to filling the area with heat. And yes our heating bills are high, ridiculous and must be brought into control. I too am looking for how best to deal with these floors. I would need to pull up the carpet and padding in large areas. I have a sizeable area of tile that we had no idea would be so cold once put down. We did not think we would ever need the heating mat under the tile. We are in the CA high desert so we go from 17 - 110. I just can't imagine my husband agreeing to tear up the tile. There is no after market solution for tile already down.

    I also have a second problem with the space between the ceiling of the first floor and floor of the second floor. There is no insulation so we freeze upstairs as well. I don't know how we can pay for all this repair and replacement either but I too am looking for guidance. Any chance you live in CA also, it seem as if the state is loaded with homes lacking quality building practices. It has been hard to enjoy the home when I'm constantly finding a fix due to a lazy work ethic.

    eureka

  3. #3
    Join Date
    Feb 2010
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    Eastern MA
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    75

    Default Re: slab insulation

    I would check on the compressive strength of the foam. Dow Blue board extruded polystyrene would probably be better. Attach the pywood to the slab and probably best to use a floating floor for the finish floor.

  4. #4
    Join Date
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    Maryland
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    Default Re: slab insulation

    Quote Originally Posted by paulnederland View Post
    I calculated the heatloss through the concrete slab (no crawlspace) to be 50% of the total heatloss in my ranch style house. Can I use Tuff-R on top of the concrete, add 1/2 inch plywood and put a wooden floor on top?
    Will the foam lose its rigidity over time and sag?
    thanks for advice!
    It seems unlikely that 50% of your heat loss is through the slab. The heat loss to the ground that is at about 55 degrees, except at the edges of the building, is much less than through the roof, walls & windows when the outside temp is 30.
    If you insulate the floors I would do it with a sleeper system below the subfloor and infill with rigid board. Be aware that all the doors will need work including the entry doors & sills. Don't forget toilets and probably a half dozen other things.

  5. #5
    Join Date
    Aug 2007
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    Default Re: slab insulation

    ed21 and jkirk ----- if the slab was below grade ( basement ) then i agree with you guys.
    However - if it's a slab on grade this changes things. The temperature differential will be greater and with no thermal break between the slab and ground , the floor will be colder and could result in higher heat loss ---- though 50% seems high.





    Quote Originally Posted by paulnederland View Post
    I calculated the heatloss through the concrete slab (no crawlspace) to be 50% of the total heatloss in my ranch style house. Can I use Tuff-R on top of the concrete, add 1/2 inch plywood and put a wooden floor on top?

    Will the foam lose its rigidity over time and sag?

    thanks for advice!
    Using rigid foam laid on the slab and covered with plywood is a fairly common system used around these parts in finished basements and on grade slabs.

    Ideally with slab on grade it also goes a long way to insulate along the exterior edges of the slab to provide a thermal break from the exterior.

    While I'm uncertain as to the origin of the method I believe it was adopted from the commercial roofing systems where rigid foam is laid on the decking first as a thermal insulator and covered over with a wood ply material and roofing layer.

    This system for floors is an effective way to provide a comfortable continuous thermal insulated floor as well as providing some softness as opposed to the hard concrete.

    Depending on the type and brand of rigid foam they will have different compressive strengths. The common types found at the home centers will usually be in the 25 - 30 psi range regardless of thickness. Generally --- for recreation areas --- such as a finished basement --- they will work for the floor. For high traffic and for the main living areas it might be advisable to go with the type having 40 psi or more compressive strength.

    While the foam itself is soft and easily punctured having a solid surface like the concrete slab on one side and a plywood surface on the other creates a solid base. Having this type of sandwich with solid material displaces any point loads which doesn't affect the foam. Personally I would recommend 5/8 inch tongue and groove plywood over the 1/2 inch since the thicker plywood provides a better base for any finish floor covering.

    As for putting wood flooring down --- this can be tricky depending on the final temperature of the floor surface regardless of putting the foam down. If the floor surface temperature is cool enough and compared to the temperature and relative humidity of the air in the home could cause condensation under the wood flooring causing cupping or other moisture related issues ----- depends.

    2 cents.
    "" an ounce of perception -- a pound of obscure "
    - Rush

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