Alternative (Non-Foam) Outsulation Strategy Using Rockwool
There’s a lot of talk about the benefits of exterior insulation, but also the downsides of rigid foam materials—that is, the chemicals involved, future disposal issues, wrong-side vapor barriers, susceptibility to fire and pests. It is possible to use rockwool / mineral wool as an alternative, but this is not often discussed or practiced, at least in this country. That may be largely due to the lack of compressive strength of rockwool compared to foam boards, requiring some strategy to support the cladding other than simply driving screws through the insulation.
Here’s a simple solution that seems promising: run 2x2 vertical furring over the rockwool, held in place with long screws, and use squash blocks to avoid compressing the insulation. The squash block is a short segment of wood with an oversize hole at center for the screw. It could be as small as a 2x2 compressing into the end grain, but may be a larger profile (say a 2x4 with the longer axis upright) if greater strength is needed. Use SIP screws (e.g. TruFast SIPTP) long enough to penetrate wall framing by 1-1/2”. Space for squash blocks must be cut from the insulation, but it could be as little as a 1-1/2” square on 24” centers horizontally and vertically.
The vertical 2x2 becomes rainscreen furring. A vent space of 1-1/2” is more than needed but a 2x2 does a good job holding nails for siding. There is limited bearing strength but the assembly should hang off the sheathing and support the siding.
Choose a type of rockwool intended for use in cavity walls, e.g. Roxul CavityRock or Thermafiber RainBarrier. The WRB (say #15 felt) goes under the rockwool against the sheathing. Essentially the outer surface of rockwool is the drainage plane and it works like a superthick housewrap. RainBarrier has a perm rating of 50 but absorbs only 0.03% moisture by volume. Water beads up and sheets off the surface.
With an entry level 2” thick rockwool you get R-8 and need 5-1/2” screws.
3”: R-12, use 6-1/2” screws.
4”: R-16, use 7-1/2” screws. That’s an R-36 wall including R-20 from a 2x6 frame. Total assembly is about 12-3/4".
Adding inches requires minimal extra material, just thicker insulation, longer screws and taller squash blocks. Compared to rigid foam, you get enhanced soundproofing, reduced risk of fire, pest resistance, a vapor open assembly, and probably less long-term environmental impact.
It’s true that rockwool has high embodied energy compared to cellulose, and the system could have one large cellulose-insulated cavity as a truss wall if the sheathing moved to the exterior. But that would eliminate the rainscreen gap and would not provide shear strength. It also adds weight and leverage against the long screws and squash blocks. I have tried to adapt the Riversong truss to my purposes—a snug lot and three stories (topped with a living roof) in seismic zone 3. A rainscreen is also a code requirement. My project requires 2x6 framing and exterior shear ply (both sides in a few places), so a single combined cavity is out. Having exterior sheathing makes air sealing easier. It also seems meaningful that a rockwool/SIP screw/squash block strategy could work for retrofits.
Posted Sun, 10/24/2010 - 01:54
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