Check this out! This 24" x 48" x 2" thick piece of rigid stone wool insulation was left out overnight (on purpose) on the High Performance Tiny House project site, to see how it handles a deluge of rain, which we knew was coming. One of the many experiments we're doing (so much more to come, like building floors upside down!). It was laid on a flat surface, so it would absorb as much water as possible during the anticipated rain storm, which turned out to be another Florida FrogStrangler, including a power outage!

Building with Rocks, Rocks
Building with Rocks, Rocks

Building with Rocks, Rocks

In the morning we found that it had absorbed 1/4" of rain water. When we turned it vertical, this is what we saw happen to the rain water.

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Within one hour of standing the piece of insulation on it's end, under the shade of a very large live oak tree in central florida (temperature reached the mid-90s on this day), in the middle of summer (where relative humidity is was about 92% for this experiment), the piece of insulation was completely dry and as new. It didn't expand, contract, deteriorate, crumble, nothing, other than return to it's original state. As good as new, it was. And, only after one minute. One-quarter inch of rain water

Stone wool is hydrophobic, unlike other light=density insulation (fiberglass, etc.). It's water repellent, like water off of a ducks back, because of oils and resins that are in the stone wool. For that, and many other reasons, it can be used below grade, as a continuous insulation on the a building's structure, and most other exterior applications. It's not a new product. In fact, it's been around for at least 50 years, and used outside the US as a preferred insulation. Being made of rock and recycled slag, or stone, it will also not burn below 2000 degrees Fahrenheit.


We use this insulation on all projects as a cavity and continuous insulation on floors walls and roof.

On the Tiny House, we're using 2" ROXUL COMFORTBOARD IS on the exterior of the wood frame floors (yes, outside of floor too, stay tuned), walls and roof, and ROXUL COMFORTBATT in all the cavities.

In our project in North Carolina, where the client specifically asked for a non burnable assembly, we've chosen products made with only rocks or metal. Because the framing is also metal on the project, we've put ALL of the insulation (6" - 8" of ROXUL COMOFORTBOARD AND TOPROCK DD stone wool) on the outside of the structure.

This stuff makes sense for so many reasons. I forgot to mention that ROXUL products, our preferred choice, is NOT as irritating as may mineral wool products are to the installer.

I could go on and on, and I will in another post. For now, I'll leave you with the information above. Building with rocks, rocks!

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