R-Value Part 3: Fun with Foam

Part Three: Fun with Foam

In the last article, we mentioned that BuildBlock ICFs and most others use about 2.5” of EPS (expanded polystyrene) foam on each side of a concrete core. The concrete core provides the strength and is designed to generally match the construction of standard reinforced concrete wall thicknesses (ACI 318 to be exact).

So that explains the concrete, but why 2.5” foam panels?

Honestly, there are two reasons. First ICF forms must be able to withstand the pressures created when rebar is added and concrete is first poured into the forms. Most ICFs are 16” tall and generally, walls are poured in 48” lifts. A “lift” is when concrete is poured to a certain height when working in a continuous direction around the structure. After the height of your first lift is reached, a second lift is poured following the same path. This process is repeated until the full height of the ICF wall is reached. Pouring concrete in this way keeps the pressure to a minimum and makes it easy to consolidate concrete.

Since the blocks are 16” tall and we’ll be pouring 4’ at a time, a 2.5” panel is the proper thickness to provide the appropriate amount of strength. Remember the foam holds the concrete and the webs keep the foam from distorting. The webs are embedded a ½ inch beneath the foam surface to make sure the foam surface is smooth and the material exposed to finishes such as plaster, stucco, or eifs reacts the same to different temperatures.

BuildBlock goes a step further and places webs every 6 inches. This significantly adds to the strength of the blocks and ensures that they don’t deform even in very tall or thick walls. BuildBlock ICFs take the guesswork out of building a superior defect free wall.

Whew, that was a lot, what’s the second reason?
The 2.5” panels provide 5” of insulation. This averages out to an R-21 wall since EPS foam at 1.5lbs./ft3 density delivers 4.2 Rs per inch. (5 x 4.2 = 21). But hey, I can build an R-21 wall from wood or other things too! Sure, but from the study we looked at in the previous article, a cavity wall loses R-value in the real world while an ICF wall gains R-value.

R-value Vs. Value

It also comes down to value. At what point am I getting the ultimate value for my investment? Does R-value drastically rise if more insulation is added? Just like putting premium gas in your car might increase your mileage slightly, the cost difference typically isn’t worth it. I can’t explain it better than Allison Bales does in her article The Diminishing Returns of Adding More Insulation.

Allison reasons, “If you’re a practical person, I’m sure you realize that life has a tendency to make you aware of certain limits. After the first few drinks in an evening, for example, further libations have less and less of an effect. Well, less and less of a positive effect anyway.”

This is a lot like more insulation on ICF blocks. Let’s compare heat loss reduction at several different R-values. In the graph below you can easily see that the greatest reduction is achieved through R-20. The additional reduction past R-40 is very slight.


From “The Diminishing Returns of More Insulation” by Allison Bailes

Now, this is looking at normal R-values from the materials themselves. We learned earlier that ICFs really deliver an actual R-value of R-21, but what about all that concrete? Does it make a difference?

In the final section, we’ll take a look at how concrete impacts thermal performance, and answer the question “How much is enough?” in regards to ICF panel thickness.

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