Stucco over exterior mineral wool – StoTherm ci Mineral?
I would like to use a stucco finish over mineral wool exterior insulation as an exterior wall. Mainly want stucco for aesthetics, do not have an opinion or knowledge about traditional or acrylic.
The Roxul comfortboard installation guide suggests using traditional metal mesh stucco, affixed to strapping, held to the framing with what looks like fiberglass standoffs. Page 7: https://www.rockwool.com/siteassets/o2-rockwool/documentation/technical-guides/residential/comfortboard80-installationguide.pdf?f=20180718133916
This looks complicated and expensive, also not sure how strong the stucco would be in the gaps between the strapping.
There is a somewhat new system from Sto called StoTherm ci Mineral. https://www.stocorp.com/sto_systems/stotherm-ci-mineral/ It uses special mineral wool from Corning with the stucco applied directly to the insulation. There is theoretically a drainage plane behind the mineral wool boards if they are installed correctly. It is similar to a foam EIFS system but using mineral wool.
1. Has anyone had experience with this system?
2. Has anyone applied stucco over mineral wool in any way, and how were your results?
3. Any opinions on the StoTherm ci Mineral system based just on looking at it / your experience?
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Matt Risinger did a video of stucco over RW.
You could even try dropping a note asking him how the exterior is performing.
Yes, that was a good video. The only one I could find on youtube, definitely something to consider.
Intuitively the coupling between the rigid stucco -> glass mat -> dimple mat -> rockwool seems like it introduces a lot of flexibility that could lead to cracking. But I have not tried it of course.
I also didn't see the assembly in either the rockwool or dimple mat literature so it makes me a little wary.
I have not worked with either system, but I do have a lot of experience with various stucco systems, so take my advice for whatever it's worth.
The Rockwool instructions look pretty robust. It used to be common practice to install stucco over stud walls with no sheathing. The wire lath embedded in the stucco provides more than adequate support and attachment for the stucco, and there is no appreciably weakness between the strapping/studs. There will be plenty of drainage and drying potential in this wall, as there is a cavity between the stucco and Rockwool, and the RW itself allows water to drain. The "thermally broken standoffs" might be a bit fussy, expensive and/or time consuming, so you'll have to do some research. DO NOT USE metal studs for the standoffs. That's done all the time in my area, and the thermal short from the metal reduces the whole-wall R-value substantially. FWIW, if installed well, cement stucco is a very durable cladding system (50+ years). It would be important to install the full thickness (3/4" minimum) of stucco.
The STO system is not all that much different from their standard drainable EIFS. It just substitutes the Rockwool for the EPS in the standard system. This would have several advantages: Rockwool is fireproof (can anyone say "Grenfell Tower?"). It is also self-draining. The finish on this system will be slightly soft to the touch as the whole synthetic stucco skin flexes under pressure. But it should be durable. Lifespan for EIFS is about 20 years. Much more if the joints are regularly resealed and the whole surface is painted or refinished every 10-15 years.
Either system should work fine. I would select based on the slight aesthetic differences, cost, and locally available talent for installation.
+1 Peter that is great info, thanks so much for your advice
Mack - how did this go? Looking to do similar. Cheers.