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Community and Q&A

Concrete Slab Insulation – Reflective Barrier

whanna27 | Posted in Green Building Techniques on

I have a house (1976) with a concrete slab, that to my knowledge, is uninsulated or very poorly insulated. As you can imagine it makes heating costs very high. The 1st floor is about 1000sqft of which 150sqft is tile, and the rest is carpet over a thick carpet pad. I am tearing up the carpet this summer and installing hardwood floors over the slab. I am planning on a poly or paint on vapor barrier and then 1×4 PT bunks secured to the slab and spaced either 12″ or 16″ OC. Need to do some testing on what will be the best spacing to prevent flexing and bowing of boards. While everything is torn apart I would like to add some sort of reflective barrier or insulation to the slab. Adding several inches of foam or similar is not possible because the ceiling is already under 8FT and I am tall and don’t want to lose any more ceiling height and have to raise a bunch of door frames.

So that quickly ruled out foam board or any sort of faced batt insulation. In my search for reflective barriers I came across this:

Is this actually R-21 as the website says? I’m a mechanical engineer so I understand how heat transfer and R values work and I find it difficult to believe that the radiant heat transfer coefficient is small enough to produce a R value of 21. In my experience radiation heat transfer typically isn’t expressed in R values and the contribution of radiation to overall heat transfer (Q) is typically very small. But after some digging I did find an equation expressing R values of radiation with respect to a “radiation heat transfer coefficient” which I understand to be a film coefficient based on the equation Q = emissivity * (SB constant) * area * (T^4-T^4).

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  1. Expert Member

    No, and if you search, you'll find a ton of posts on this site about it.

    Anything that isn't insulation, doesn't insulate. That's pretty much all there is to it.

    If your cieling heights are under 8 ft already, an option you may want to consider is insulating the perimeter of the foundation. That will stop the heat loss outwards, and probably be more effective depending on your exact location. See this section of the Oak Ridge Foundation Handbook.

    regarding the lower section with the exterior of the thickened edge slab.

    1. whanna27 | | #2

      Okay, that's what I thought. My engineer brain was confused how tin foil with a small air gap could be R21. Well...I'll have a 3/4" air gap with the design I'm going with which amounts to about R3, slightly better than my carpet, carpet pad combo.

      1. Expert Member

        At best your 3/4" air gap is R1, I wouldn't count on it for anything.

        1. whanna27 | | #4

          Where are you getting R-1? R = L/(k*A). k(@60F) = 0.015 BTU/(hr-ft-F)

          => R = (0.75/12)/0.015 = 4.2 (hr-ft2-F)/BTU

          Assuming a much colder air temp k(@45F) = 0.0144
          => R = 4.34

          Assuming a much higher air temp k(@85F) = 0.0154
          => R = 4.06

          Granted any air movement will change this number. But between a hardwood floor and slab there should be little to no drafts or air leakage that would create bulk air movement.

          1. Expert Member
            KYLE WINSTON BENTLEY | | #5

            My friend I think you’re over engineering, otherwise we’d just make wall insulation out of several 3/4” air gaps :).

            Check out this study, which relies heavily on a study performed by ORNL in 1991. Around page 15 there are tables that show the r value of a air cavity, on the floor, by climate zone, mean temp, delta T, and emissivity of the surface. Concrete is certainly 0.5 or higher, and most of those r values max out at 1.6 or lower.


        2. whanna27 | | #6

          Very interesting read. Of course I am over-engineering it, that's what engineers do.

          Main takeaway I got from that is that the emissivity of the surface has a large influence on the R value of the air space. Looking at the table from ASHRAE 90.1 towards the end...agreed that bare concrete, especially fouled and uneven, will have a very high emissivity and therefore the R value would be ~1. However, this then brings me back to my original question. Since emissivity has a huge influence on the R value of an air gap wouldn't it then make sense the encapsulate the air space with a reflective barrier which would lend itself to an R value of 3 (climate zone 5)?

          1. Gshps4all | | #7

            With such a small air gap between materials that are relatively close in temperature, the primary heat transfer process will be conduction with the possibility of some convection loops. Placing a radiation barrier there will be counterproductive because the metal used in the barrier will be highly conductive. A radiant barrier is optimal between two materials at sharply different temperatures with a substantial air gap between them and the ability for convection currents to carry away the air as it is heated, such as during the summer in an attic. In this case, I agree with Kyle that your best option is to try to insulate where the slab is exposed to ambient temperatures, and to fill the 3/4" gap with a good insulation sheet rated for under-floor concrete contact.

  2. Expert Member
    DCcontrarian | | #8

    You could put 3/4" foam between the bunks. The foam would support the floor somewhat so you could probably space the bunks 24", which would allow you to have more foam coverage. So would going to 1x2 instead of 1x4. You could also rip 1" strips out of pressure treated 2x material and get an extra quarter inch of insulation.

    Make sure the paint you use is actually rated as a vapor barrier. Most paints, including waterproof paints and even waterproofing coatings, are actually vapor-open. Things like Drylok or exterior paint. I've only been able to find primers that are true vapor barriers. Benjamin Moore had a vapor barrier primer that's rated for use on concrete, but I think they have discontinued it.

    Depending on your climate you may not lose that much energy through the floor, I'm in climate zone 4 and my Manual J showed an uninsulated floor contributed more cooling in the summer than it cost in the winter. Insulating walls is usually more important. Cold floors are uncomfortable so the insulation is more about comfort than efficiency.

  3. MartinHolladay | | #9

    Home Depot executives should be in jail. More information here:

    "A New Look at Foil-Faced Bubble Wrap"

    "Stay Away From Foil-Faced Bubble Wrap"

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