Natural Spaces Domes Concrete Slab Insulation Test Results

Click here to see monthly radiant floor heating costs.

The test area is in our Bear Creek Dome in the open living room area. Bear Creek Dome is a 3200 sq. ft. two story, super insulated dome home located in North Branch, Minnesota, 45 miles north of Minneapolis / St. Paul.
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The test area is 16’×16′, divided into four 8’×8′ sections, with one of those sections divided in half again. The entire area is now covered with a 4″ thick concrete slab. The tubes are ½” pex which contain water for the radiant floor heating system. We installed 2 temperature probes under each section directly below the insulation lying on top of the sand base. Two additional probes were placed underground in the soil, one at 36″ and one at 84″, both of these approx. 10′ in from the exterior wall.

Section #3 consists of 2″ thick Styrofoam high density insulation (Dow Brand) under 4″ of concrete. This is what was used under the rest of the concrete slab.

Section #1 is divided in half. One 4’×8′ section has a layer of Reflectix Brand “Bubble-wrap” insulation consisting of 2 layers, each 1/8″ thick, of “Bubble-wrap” (3/8″ diam. air bubble pockets) with reflective foil film on the top and bottom. The entire thickness is approx 1/4″ under 4″ of concrete. The other 4’×8′ section has a layer of 3/8″ thick insulation under 4″ of concrete. The black insulation consists of a core component of extruded expanded polystyrene (EPS) 3/8″ thick material with a 3mil black polyethylene film laminated to both sides.

Section #4 has a 4″ thick Styrofoam high density insulation (Dow Brand) under 4″ of concrete.

Section #2 has 1″ of polystyrene high density insulation (“Certifoam” Certanteed Brand) under 4″ of concrete.


What does this chart show? The insulation under the slab is stopping the heat transfer between the warmer mass above (the air or the slab) and the colder mass below the insulation (the soil). Because we are in a heating climate zone (central Minnesota) we want to stop the radiant heat from going down into the soil and keep the heat (or increase the heat) that is radiating upwardly, into the dome home. So, the warmer it is under the various types of insulation, the worse it is, energy wise.

The purpose of the insulation under the radiant floor concrete slab is to block the heat from going into the ground.

Radiant heat from the Pex hot water tubes in the concrete slab is sent out in a 360 degree pattern. The 3/8” thin EPS foam and the Bubble Wrap insulation offer little resistance and allow that portion of the heat that is radiated downward, to quickly transfer into the ground. Because the ground temp is colder as you get lower (see the probe temp at 84″ down), you will be heating the entire Earth forever.

You want to use as much rigid foam insulation as you can afford – with a vapor barrier below the foam to stop ground moisture from going into the foam, diminishing it’s insulation value.

Base your decision on what you think the cost of your heating fuel source is going to be in the next 50+ years.

Blower Door Test

After pressurizing the dome interior, we went around the entire inside with a bottle of “smoke”. We found numerous air leaks where the vapor barrier tape was not tight, at some areas we foamed (but weren’t perfect) and at some sill plates and window corners. Overall, pressurized to 50 pascals, our ACH (Air Changes per Hour) was very low at .06 ACH. . We conducted another blower door test after the sheet rock and dome paneling were completed. We received an Energy Star 5 Star Plus rating and a HERS score of 55. Click here to read about our Energy Star domes.

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