This weekend work took me to the inside of my slab where I installed insulation and fill in preparation for the cement truck.
Work began with me attaching the outside foam to the forms to prevent them from moving around too much in the cement. Screws or nails will simply pull thru the foam so I needed to improvise a way to securely attach the double foam sheets. I used a nail and some fence wire to create a “T”. Then I drilled a 1/8 inch hole thru the foam and forms. After threading my “T” fastener thru the 1/8 hole I pulled the wire tight and wound it around a screw driven into the form.
The edges of my slab need to be 12 inches wide per IRC. To meet the frost code I also need at least 16 inches buried below the ground and 8 inches above. That makes my slab 24 inches thick which happens to be the dimension of the XPS foam I am using.
The center part of the slab will be 4.5 inches thick so that makes the “channels” on all four sides will be 19.5 inches tall. I took a standard sheet of XPS foam and cut off 4.5 inches for the pieces used inside.
At one point I Was going to use 4 inches of foam for these inner pieces but after some napkin calculations I realized that the difference in heat loss was going to be 4% more with 2 inches versus 4 inches. Frankly, that just was not worth the $140 to me.
I obtained a 16 foot long 2×6 that I will be using as a concrete screed and to help me set the inner forms at 4.5 inches. Next I cut some 12 inch wood blocks to help me set the spacing between the outer and inner foam walls. I also drove in some 3 foot long lengths of rebar to act as a brace for the inner insulation. With all this in place I set the inner foam sheets and braced the bottoms by packing in gravel.
Next the hard work begain….filling the center. I quickly learned that my little BX25 did not have the reach I thought it had. I was barely able to get the fill over the inner form and I ended up moving an awful lot of dirt by hand.
Let me tell you a wheelbarrow of bank run gravel is so heavy that it’s almost too heavy to move. After a lot of hours of sweating I finally had the center mostly full. During the fill I had to install a lot more 12 inch braces to make sure that the foam would not collapse.
The center is very well compacted, but I stayed away from the edges to prevent them from bowing and/or collapsing. I will do a pass around the edge with a small tamper to make the edges really solid.