The Floor Slab

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Our house will be heated by a radiant floor system.  The floor slab must be insulated on all sides to deliver heat to our home, and all plumbing, electrical, and heating penetrations must be made prior to pouring the 4 inch floor slab.

We had hoped to use crushed glass for the floor underlay as well as over drainage tile. The benefits of crushed glass are clear ...  it is a reused waste product. However, once we received the first load of the glass, it became obvious that this would not work.  The glass had been pulverized into sand and retained water.  So, we went with Plan B and used pea gravel to make certain that water drained away from our slab.  The pictures here demonstrate the issue.  Water flows rapidly through the pea gravel and puddles and erodes the glass.  (We'll use the glass elsewhere in the house.)

In addition, because the glass was so fine, the specifications were that we needed to compact the glass which as you can see was like sand.  We did not have to compact the pea gravel.  This is a cost and insurance benefit since we have all sorts of pipes running under the grade.

The stem wall is 6" above the foundation, and we will put a couple of inches of pea gravel, 3 inches of rigid insulation and 4 inches of concrete to form our floor slab. 

The slab is insulated on the bottom and sides ... 2 inches on the sides for an R-10 and 3 inches under the slab for an R-15 insulating value.  The building/energy code allowed us to put only 2 inches under the slab, but we decided to go the extra mile with an additional R-5 1 inch layer.  More along the side of the slab will interfere with the walls; thus, we felt limited to 2 inches of insulation.

(R-Value is a measure of resistance to heat flow. Insulation materials have tiny pockets of trapped air. These pockets resist the transfer of heat through material. The ability of insulation to slow the transfer of heat is measured in R-values. The higher the R-value, the better the insulation material's ability to resist the flow of heat through it.)

The insulation around the slab is Foamular rigid foam insulation from Owens Corning.  It is EnergyStar and GreenGuard Certified which means it works and will not off gas formaldahyde into the home.  According to their website, Foamular contains literally hundreds of millions of densely packed air cells. Since air is nature's most effective insulator, the sheer volume of this compressed trapped air gives Foamular insulation exceptional thermal performance. Plus, the product's built-in rigidity means it can be scored and snapped, cut, or sawed with common tools. Sagging and settling are never a problem.

Foamular is also "hydrophobic," meaning it's practically impossible for it to be penetrated by moisture. Because water is a great heat conductor, moisture penetration can cause a permanent loss of insulating value, and after repeated penetration the insulation can be practically useless.

The slab is underlain with pea gravel, a plastic visquene sheet is put over the gravel as a moisture barrier and 3 inches of rigid foam insulation added before the slab was poured.  You cannot add insulating value later to the floor without totally ripping out the slab, and the added cost was minimal.  It is costing us plenty to have a ground sourced heat pump and it made no sense to lose heat into the ground rather than deliver it where we will be living.

As you can see, we made certain that all of the pipe penetrations were insulated with foam before the slab was poured. 

Of course, we also used fly ash in the concrete in the slab with the huge benefit of waste recycling and greenhouse gas avoidance.

 


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You can email Christine Garst at
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