MAIL: Compressed Earth

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Compressed earth building blocks
Confusing high R-value wall structures with high thermal-mass wall structures?

I read Josh Denney’s letter about compressed earth buildings in HP155 with interest. I think he’s confusing the high R-value wall structures (what the original article was talking about), with high thermal-mass wall structures. While there are benefits to both, the high-R-value wall has a much wider applicability. A wall with high levels of insulation is able to resist the temperature difference between inside and outside. This is valuable for a high-performance home in nearly all climates, and is really just an extension of the well-understood design of a modern home.

On the other hand, a high-mass wall (masonry, concrete, water, etc.) is completely different. It has very little insulation value, which means that it can’t support a temperature differential across it for very long. What it can do, though, is absorb a lot of energy, very slowly, with a fairly small temperature change. A properly built and designed high-mass wall, in the right climate, will warm up slowly during the day, when the outside temperature is warmer than the interior temperature, and will then radiate that heat in both directions (inside and outside) at night, when the outdoor temperature is cooler than you’d want inside.

In short, high-mass walls provide a temperature-moderating effect. For an exterior high-mass wall to work well, this means that the average outside temperature over the course of the day be close to an acceptable indoor temperature—i.e., your climate must have a fairly wide temperature swing. In some parts of the United States, such as the desert Southwest, this is fairly common.

But if you’re talking about the humid Southeast (like Florida), I don’t think the nighttime temperature gets cold enough in the summer for the average temperature to be acceptable. Similarly, in many parts of northern United States, the average wintertime temperature is far too cold to be acceptable. Even in those regions, though, you could be clever and put a high-mass wall inside a well-insulated building envelope. That way they work together to naturally keep the indoor temperature acceptable. High-mass walls are valuable, but please don’t try to make them be something that they’re not.

Lloyd Brown via homepower.com

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