Building with Rocks, Rocks! Check Out It’s Drying Potential.

Building with Rocks, Rocks! Check Out Its Drying Potential.

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 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.

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!

Thanks for stopping by, and stay in touch by following along on our social media sites or with this blog. We love your questions, so ask away!!!

10 Responses so far.

  1. Milan Jurich says:

    Hi Chris … interesting piece. Looks like a super product. How about insulation performance cost per square foot or sheet as compared to EPS, XPS or Polyiso? Or even the Huber Zip-R product?

  2. David Eakin says:

    Chris – rock wool products’ water-shedding and fire resistant capabilities are well known, but the crux of discussion really should be about its insulating abilities. I was really interested in this type of product until recent posts about decreasing R-values (when used as an exterior insulation) due to wind washing – which makes sense since it is a woven product (like that other “rock” product made from melted sand). I cannot conceive of how to install this product on the exterior of framing and then install a WRB on top to eliminate wind washing that would be economical or easy for trades. Maybe its true forte should only be for cavity installations – especially in situations that could encounter flood waters or plumbing leaks.

  3. Richard Beyer says:

    Great post. Of course there will be those who support the chemical form of insulation (‘FOAM’), I’m glad you took it a step further to show the benefit of mineral wool. Not only does it shed water, it will not combust or off gas toxins in the event of a house fire which gives occupants and their children more time to escape. That should be our main priority when assembling homes.

  4. Hello David,

    I would like to react on your comment about wind washing effects on mineral wool but before I do so, I want to say that I work for the Energy Design Center at ROXUL Inc.

    Fibrous insulation is more air permeable than other types of insulation. However, past studies have shown that well-installed semi or high density mineral wool was minimally affected by wind washing.

    RDH Building Science Laboratories has done and published research on this topic and you could find very interesting information about it on their website.

    We also conducted research projects on wind washing and we saw negligible impact. If you wish to learn more, you could contact the Energy Design Centre at ROXUL and we could send you additional information.

  5. Carolyn Bailey says:

    I bought Comfortboard to cover the exterior of a new poured concrete basement in Maine. It will be installed from the footings up to the bottom of the house siding, al the way around. I chose it primarily to divert water from the basement wall, and secondarily for insulation. The basement won’t have a finished interior but needs to be dry. It will have a furnace to keep it above freezing in winter and windows for air circulation in other seasons. The basement pour is a month away. I’ll write again after I’ve seen some results with the Comfortboard.

    Patrick — some questions:

    1. It looks like Roxul has switched from bolting Comfortboard to the basement wall to spot gluing it on. Can it be glued to a fibered liquid foundation coating on the concrete, or only to bare concrete? What kind of glue should I use?

    2. What product should I use to coat the exterior side of the Comfortboard that will be exposed from ground-level up to the bottom of the house siding?

    3. What product should I use to coat the exterior side of the rest of it that will be underground with backfill up against it?

    4. Does it require drainage stone under its bottom edges along the footings, or drainage stone between it and the backfill? Some of Roxul’s illustrations make it look like it does.

    Any other new tips?

  6. Thomas Hackett says:

    Hi Carolyn,
    I am in charge of technical support at ROXUL. Below are my answers to your inquiries. Also, I invite you to check out our COMFORTBOARD installation guide which shows images and details to help illustrate what I am talking about. Link here:

    1) For attaching the COMFORTBOARD on the exterior of the foundation below grade, either mechanical fasteners or a construction grade adhesive can be used.

    2) For finishing the above grade portion (between the ground and the siding), a concrete protection board can be used.

    3) You can backfill directly onto the COMFORTBOARD in the below grade portion. The assembly should go as follows. Foundation wall, waterproofing below grade membrane, COMFORTBOARD, then backfill.

    4) Drainage stone should be used at the base of the footing, under the COMFORTBOARD. This is shown in the installation guide that I mentioned above.

    If you have any further questions, please feel free to call our Technical Support Center at 1-877-823-9790.

    Thank you!

  7. Robert says:

    @ David, so also we’ve found that after sheeting, WRB and adding caps nails adds a small gap for drainage and then mineral wool works best…

  8. Dale Sherman says:

    How did you verify the test sheet was dry? Did you weigh it before the rainstorm and after it drained?

    How good is the crush resistance? Can it be used under floors with load-bearing walls?

    How does Roxul compare to other brands like Knauf?

  9. Richard Beyer says:

    @ Dale Sherman. Those questions are hilarious! It has to be a joke, right?

  10. Mike says:

    If your looking for a solution to fastening Roxul. Check out the Ramset Insulfast system.
    Just recently did a project with 5″ on foundation wall, 4″ on exterior walls behind brick, 6″ on the roof (which was a 14 pitch hip and 8,000 sqft.) and 12″ below basement slab.
    They have fasteners for concrete as well as for wood and metal studs. Also minimal thermal bridge as they are made of plastic.

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