While it's easy to say that insulating concrete forms offer good insulating value, it's more difficult to say how good. The sheet foam systems are straightforward, because of their uniform thickness. For expanded polystyrene, the insulating value depends on density: R-4.2 per in. at 1.5 pounds per cu. ft. or R-4.35 per in. at 2.0 pounds per cu. ft. The R-value of extruded polystyrene also varies, but most products use the 2.0 pounds per cu. ft. with an R-5 per in. Polyisocyanurate's aged insulating value is R-7.2 per in.

When it comes to the blocks, there's much more difference between products. The foam thickness in blocks is far from uniform, making calculation difficult. Ecotope, Inc., an engineering firm in Seattle, Washington, has analyzed the R-values of several block products. They report these results for above-grade installations:

Keeva and Conform both use high density expanded polystyrene. EnerG uses polyisocyanurate and its profile contains more foam than some others, which helps to boost the insulating value.

According to Dave Baylon at Ecotope, Polysteel's insulating value is hampered by a metal spacer. The spacer penetrates both of the expanded polystyrene sides, creating a thermal bridge. Other products avoid this problem by using plastic spacers or by holding spacers back from the surface.

With all the variability among blocks, it's difficult to generalize about insulating value. Ask the manufacturer for supporting data, including independent laboratory tests. Focus on the R-value of the foam itself. Typical concrete is less than R-0.5 per in..

Compared to What?

Advertising for foam forms sometimes cites R-values that are "equivalent" to wood framed walls.

There is legitimate confusion about the insulating value of wood framed structures. For example, when you fill the cavities of a 2x6 wall with high density R-21 fiberglass batts, you may call it an "R-21" wall, but it's not. Taking into account wood framing, which can make up 12 to 20 percent of the wall's area, the actual value drops to somewhere between R-12 and R-17.

A foam form system should give you a more consistent R-value than you will get with a wood framed wall. Whether the overall R-value will be higher depends on the framed wall you're comparing it to. The graph below compares several sample basement walls. These R-values assume an 8 in. reinforced concrete wall. Interior insulation is installed in wood framed walls with studs 24 in. on center and covered with gypsum board. The R-values are calculated for a basement wall with the floor at 3-1/2 ft. below grade.

1. Except where noted the R-values come from "Super Good Cents Heat Loss Reference" Volume II, Bonneville Power Administration, 1988.
2. U-values from various reports by Ecotope, Inc., Seattle, Washington.

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