Solar Slab Construction

We resolved our foam insulation problem. This noon we received 15 replacement sheets (at 4” thick they are almost blocks) of insulation. I weighed the first on the pile and one block weighs 21.4 pounds - within specification of 2# per cubic foot. So we were able to finish laying the insulation today, taping the joints, and start laying concrete block atop the insulation. In the photos you can see the black tape on the white insulation.(The white pipe sticking up out of the insulation is meant to suck radon from beneath the insulation and keep it out of the house.)

This is where my heating system differs from most in New Mexico. A large heat reservoir under the floor is not unique - several have been built using large river rock - but this one uses concrete blocks laid on their side. The amount of block and thickness are engineered to match the house insulation and the solar gain of the windows so that the house does not overheat in summer and does not lose more than four degrees Fahrenheit overnight in winter.

On top of the styrofoam insulation we laid 12” concrete masonry units (CMU - blocks) on their sides, aligning the holes to act as air ducts under the floor. There is no mortar between the blocks. At the perimeter of the floor we will have floor registers that let air down into these ducts. In the photos this space is covered with plywood to keep it from filling with concrete when we pour the floor slab. [Boards have been laid across this space to facilitate carrying the 50# blocks from the pallet to the work area inside the foundation forms.]Within each row of blocks is a baffle with carefully calibrated holes (most are 2 3/4” diameter with cross-sectional area of 6 sq in each). You can see the plywood baffle in the photos.

Down the middle of the floor is a blank space that acts as the air duct to return air to the furnace fan that circulates the air. The width of this duct must handle the air collected from the holes in the CMUs, so gets wider as it gets closer to the furnace. This space will be covered before we pour the concrete.Under each interior wall will be reinforced concrete beams created by gaps between the sides of the blocks. These are called ‘thickened slab’ but actually get extra reinforcing steel to support the walls in case the ground settles. The water pipe and electric conduit will rise through one of these beams, as will the radon pipe.

Three of us were able to install about half of the 900 blocks. This includes time to make small adjustments to the position of the blocks — making the ducts slightly wider or narrower. This is because the design did not take into account the lack of mortar between the blocks - they don’t measure exactly 16” long or 8” wide!

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