Question: How much good can I get out of perlite or vermiculite concrete as an insulating material under a slab on ground for my house?

Can you give me an R-value?

Answer: Before giving a specific answer we should point out that it is normally uneconomical to use insulation under the whole slab. As long ago as 1945, the National Bureau of Standards reported in its BMS Report 103 that the average temperature of the slab is only 4 to 6 degrees F below the temperature of the room except within 24 inches of the outside wall.

For this reason it is normally recommended (see Model Code for Energy Conservation, published by the U.S. Department of Energy in December 1977) that insulation be installed only at the perimeter of the slab.If, however, you are building a floor that will contain radiant heating pipes, it would be justifiable to install an insulating concrete below it. The R-value of the insulating concrete depends on its unit weight and thickness. The bag in which the perlite or vermiculite is purchased recommends the mix proportions needed to achieve various unit weights of fresh concrete and oven-dry concrete.

R-values have been given on a slightly different (normally dry) unit weight basis by Stanley E. Goodwin, in "Concrete Energy Conservation Guidelines," Publication EB083.01B of the Portland Cement Association. Data from that publication are given below, together with the oven dry unit weights that are equivalent to the normally dry unit weights.

As can be seen, the normally dry weights run 2.2 to 3 pounds higher than the oven dry weights that are usually given by the manufacturer. The values in this table are reported to be typical or representative. Individual products may have greater insulating effectiveness.