Rock Wool Insulation for Floor of High Performance Tiny House


In August of 2016, we posted a video describing the design and construction of the foundation and floor framing of our High Performance Tiny House in Florida.

Since then, we have made a lot of progress. We’ve also prioritized the design and construction of an off-grid, “zombie proof” homestead, and its permaculture farm, in the mountains of North Carolina, which kept us from sharing our experiences and shedding our knowledge on the blog! Alas, here is an update. Welcome to today’s post!


Whether you’re seeing the video for the first time, or not, you heard me talk about insulating both the cavities and continuously on the outside of the sheathing. What I hope you picked up on, in the video, is that to achieve that continuity with the sheathing, we built the floor in sections, and upside down.

If we had set the floor joists on the masonry foundation wall, first, the sheathing would have been interrupted by the double sill plates on top of the masonry. This detail would have been much more complicated to deal with, but still doable.

By keeping the sheathing continuous, on the outside of the structure, we maintain continuity of our control layers, which gives us a “perfect” enclosure. And, by having continuous insulation, that has the appropriate hygrothermic (drying potential) and thermal properties, on the outside of the sheathing, we’ve protected the structure from exposure to the elements. This helps prevent LOTS of things that cause building failure (e.g. Rot, Mold, Freeze, Thaw, etc.), not to mention keeps us comfortable inside the house (This is a very good thing).

Tiny House Perfect Building Enclosure

Image 1: Perfect Building Enclosure – Continuity is an absolute necessity to maintain a perfect enclosure.


I’ve mentioned before why I like building with rocks. While it’s not practical to use rocks for everything in the structure of the home, rocks could be the best ways to insulate. From the drying potential, as illustrated in the video below, and described in this blog post from July 2016, to the R-Value that increases as the temperature drops, to the fact that rocks don’t burn, there are fewer and fewer reasons to NOT insulate with rocks.

After finishing the floor framing, we were ready for insulation. We started by insulating the cavities with 6″ Batts (R-26.4 max at colder temperatures), and then installed the continuous 2″ rigid (R-8.8 max at colder temperatures) rock wool insulation on the outside of the sheathing. As I mention in the “perfect enclosure” blog post, the walls and the roof were insulated the exact same way. Four (4″) to six (6″) inches of cavity insulation, with two (2) inches of continuous rigid insulation on the outside.

In a later post, I will explain the many reasons why using ONLY continuous exterior rigid insulation, with absolutely NO batt insulation in the cavities, is absolutely the most effective and simple way to insulate a building. HANDS DOWN. For now, we’ll talk about using rock wool insulation batts in the cavities, too.

Here are pics from the installing the rock wool insulation batts and rigid board, before, during and after, at the High Performance Tiny House (Florida).

rock wool insulation

Image 2: 6″ (TH) x 16″ (W) x 48″ (L) batts, with R-Value of 26.4 (at colder temperatures) were used in the floor and roof/ceiling cavities. We installed 4″ batts, with R-Value of 17.6 (at colder temperatures), in the wall cavities.

rock wool insulation

Image 3: Rock wool insulation batts are unfaced, and maintain rigidity well when not handled more than necessary.

rock wool insulation

Image 4: Making sure there are absolutely no gaps or compression when installing insulation batts must be done, without exception, to achieve the maximum thermal resistance potential, and to maintain durability of the assembly. Gaps between the insulation and framing, or between individual batts, can lead to trapping or passage of unwanted moisture, as well as decreased thermal performance. Where the batt meets the sheathing is one of the most challenging, because you can’t see it. But, that doesn’t mean it can’t be done right. Follow instructions!

rock wool insulation

Image 5: Carefully setting each batt in each cavity.

rock wool insulation

Image 6: Installing the batts vertically against the rim joist, will maintain continuity of the thermal layer with the exterior wall that will be built directly above. Continuity is KING! Trimming the insulation is done easily with an insulation knife, or in this case, a hacksaw.

rock wool insulation

Image 7: Leaving 1/4″ to 3/8″ of insulation above the top of the joist will ensure that the insulation will remain in contact with the subfloor sheathing, without reducing R-Value through compression.

rock wool insulation

Image 8: Looking down at the cantilevered corner of the framed floor. The vertical insulation continues around the perimeter of the structure.

rock wool insulation

Image 9: The batts are friction fit in to each cavity, and make a nice “blanket” of woven rocks.

rock wool insulation

Image 10: Because the joists are 2×10, there will be a gap between the top of the insulation and the bottom of the sub-floor. This works because continuous air, vapor and thermal control layers (sheathing and insulation) are outside of the bottom of the joists, instead of on top. This gap was not allowed in earlier building codes, but is now acceptable because it’s understood to perform very well. This gap is within the thermal envelope, and part of the conditioned space.

rock wool insulation

Image 11: Continuous two-inch (2″) rigid rock wool insulation on the outside of the continuous sheathing that’s attached to the bottom of the floor joists, which is an all-in-one air, vapor and water control layer (ZIP System Sheathing). The insulation completes the assembly’s critical control layers, by controlling heat (thermal). On the other side (top) of the continuous sheathing is where the rock wool insulation batts are installed (see Images 2 – 10).

For more on designing and building high performance homes, whether they’re tiny or “zombie proof”, be sure to read more of this blog, and follow along.

Thank you,

Chris Laumer-Giddens

4 Responses so far.

  1. Milan Jurich says:

    Beautiful work!

  2. David Eakin says:

    Very interesting details! Leaving a gap between the top of the cavity insulation and the floor above is how BSC recommends constructing a warm floor. How is the Zip panel against insect invasion? I was under the impression that Southern construction used pressure-treated framing primarily due to insects. Also – did you fasten the Roxul to the outside of the Zip floor panel after assembly (on your back in the crawl)? If so, why not before the assembly was laid down?

  3. Thank you, David.

    RE: Termites –
    1. With a garden sprayer, we sprayed all of the panels with Tim-bor, and let it soak in before flipping it over. With the right mix, Tim-bor also works as an anti-microbial. This stuff works really well.
    2. We have a continuous termite shield at foundation walls
    3. Termites have zero interest in rock wool
    4. The house is elevated 24″ above grade
    5. We treat around the house twice a year

    RE: Fastening ROXUL – We considered attaching it before we flipped it, but the weight of the floor sections, without the ROXUL, was approaching too much for the wench we used. In the end, it was simple enough to attach from below, lying on our backs. Went rather quickly, actually.