Double Wythe ICF
DarthBuilder
| Posted in Energy Efficiency and Durability on
Anyone ever thought through the possibility of a double wythe ICF as an alternative to a concrete sandwich? E.g., two of the Nudura One blocks back to back?
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Darthbuilder,
It makes zero sense. The second concrete wall is entirely redundant. If you want more R-value, add more foam.
So I can’t tell if you have an issue with concrete sandwiches generally or just with my proposed approach. I get that the second wall will not be structural, but that doesn’t mean it does not provide any benefit. Allows you to put the thermal mass on the right side of the enclosure and protects your insulation, and also avoids the thermal bridging issue that the prebuilt concrete sandwich panels can come with. I wasn’t asking if it was the cheapest solution, just if anyone had ever considered it as an alternative.
DarthBuilder,
Let me flesh out my response a bit.
- Doubling the amount of concrete used for the foundation is a bit problematic when it has such a high carbon footprint, is expensive, and provides no benefit.
- The typical North American house already contains enough thermal mass. More might (and that's a big might) make sense if it was part of a passive solar strategy above grade, but in a foundation it does nothing useful. You can model the effect to confirm this. https://www.greenbuildingadvisor.com/article/all-about-thermal-mass
- If you did want the concrete to be entirely inside the thermal enclosure, the way to do that is to add more exterior foam and eliminate any foam on the interior, not sandwich it between two concrete walls.
- If you want more total R-value in the wall assembly, again you can add more foam on the inside or out, or use batt insulation in interior framed walls.
- ICFs have no appreciable thermal bridging you need to cure.
- I don't understand how a second wall protects the insulation? You still would end up with foam on the inside and outside of the foundation.
Thanks. I’m in the Deep South, so no basement and lots of termites. Makes foam on the outside of the concrete a somewhat risky proposition in the long term…
DarthBuilder,
I've had enough bad experiences with carpenter ants here in the PNW that I'm with you about foam. My own preference is as little foam on the inside of concrete foundations as necessary, with the rest being batts.
In that case, I would look at ICCF (insulating composite concrete forms). These don't use foam.
Unless you are in desert area with very high diurnal temperature swing, building heat capacity doesn't matter much.
Also if you are conditioning the interior, the temperature there won't change much more than a couple of F, so that heat storage simply won't work.
"Unless you are in desert area with very high diurnal temperature swing, building heat capacity doesn't matter much."
Under diurnal swing conditions -- or "shoulder" weather -- the heat flow in and out of the building is determined by the relative amounts of insulation and heat capacity. Increasing either by the same proportion has the same effect on heat flow.
When a building is being conditioned all day long -- either heated or cooled -- the heat flow is determined entirely by the insulation level, the heat capacity of the building doesn't matter.
Adding insulation is the 365-day-a-year solution.
Somehow I knew "thermal mass" was going to rear it's ugly head.
There is no energy advantage to increasing the weight of a building. Almost all of what you'll read about "thermal mass" on the Internet is pseudo-science.
I like this article about thermal mass. https://buildingscience.com/documents/building-science-insights/bsi-136-piltdown-man-does-thermal-resistance.
Yes, it can be done in simpler ways than a concrete sandwich, but there can be a benefit to thermal mass.
I love Joe Lstiburek, and I've learned a lot from him, but that snippet in the article is profoundly disappointing. Here's what he says:
After 40 years here is where I am with thermal mass…I am for it. Yup, who knew? But not the way you might think. Go with tile floors and add an extra layer of drywall on interior walls…yup…that is it…distributed coupled thermal mass. These two things have a real impact on the “time constant” of a house…it takes the house longer to cool down when the heating goes out in the winter…it takes longer to heat up when the cooling goes out in the summer.
Setting aside the wisdom of designing for when the heat goes out in the winter or when the AC goes out in the summer, and setting aside the wisdom of increasing the time constant through heat capacity rather than insulation, what really strikes me about that statement is its complete lack of quantification*.
Imagine if he had written, instead of "add a layer of drywall," "add a layer of polyiso foam." People would immediately say, "well, how thick?" and "what if I already have X amount?" Because numbers matter. They matter a lot. We spend a lot of time here talking about the difference between R4 and R5 insulation for example. And I've never heard or read anyone talking about "thermal mass" in anything approaching quantitative terms. I've never seen a serious attempt to calculate the heat capacity of a building. In fact, if you read the attempts at scientific literature on the subject of "thermal mass" there isn't even agreement on what property of matter it is that they purport to be measuring.
And Joe falls right into that trap with a completely non-quantitative statement.
*(along with a complete lack of citation of sources, and use of the non-scientific term "thermal mass.")