rockwool as continuous exterior insulation
anonymoususer
| Posted in Green Building Techniques on
For continuous exterior insulation of walls and roof, which is preferable: Rockwool Comfortboard 80 vs the slightly denser 110? I see minor differences in tech specs between the 2 products but am trying to figure out if the higher compressive strength and fire resistance of 11o are worth the extra $. Thanks
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I have not installed Comfortboard myself but I have spec'd it as exterior insulation once or twice and I'm familiar with the important aspects. To me the biggest difference would be the ability to create a flat plane, but I've recently talked with high-quality builders who have installed 80 and say that getting it flat is pretty simple. I imagine 110 would be slightly easier, and if you really need the additional fire resistance, maybe the money would be worth it. But I'm frugal and would go with 80.
Hopefully someone with direct experience with both will chime in.
Like Michael I have not installed Comfortboard personally (I will hopefully be installing it on my own house at some point, but I'm finding it extremely difficult to source in my area without buying an entire semi's worth of it).
But I do spec it on most projects, and the high-end residential contractors I partner with find it easy work with. We use Comfortboard 80, and have never used the denser 110. The first time I spec'ed it, the jobsite super had some concerns about it being too squishy once furring strips were fastened, but once he got it up on the walls found that compression wasn't as issue and he had no trouble getting everything to plane out dead flat. His take was that it was no different than working with any other rigid foam board, and he now prefers it to foam.
Paul,
Both you and Mike seem to be just talking about walls. I wonder if the difference becomes important on roofs?
Great distinction, Malcolm.
Though I spec Rockwool all the time for walls, it never seems to be what winds up going on the roof - and I'm not sure I've ever seen it used on a roof in person. Would snow loads become a factor, maybe? That's the only thing I can think of offhand that might make a difference.
Rockwool is definitely a spendier product, but its fire resistance makes it an easier sell here in wildfire country. For whatever reason, the premium price point always seems to get VE'ed out of the roof assembly. What I see most often is Rockwool on the walls above and below grade, then 5/8" Zip + 1/2" nailbase panels with 1 1/2" polyiso + hi-temp underlayment + metal roof. That always seems to price out lower than a roof with Rockwool, or even a roof with Zip for structural sheathing + Zip R.
density isn't a problem since rockwool has the DD+ product lines. one problem with using rockwool on flat roofs is getting a single source warrantable system, especially one that has been tested. In my market mod-bit roof systems seem to be less favored over tpo or pvc welded systems which rely on poly-iso insulation or EPS in rare instances. i imagine it will just take a little more time for Roxul to get more market penetration.
Rockwool on roof is not the same as rigid. This has come up before, you can't really do an unvented assembly with rockwool.
https://www.greenbuildingadvisor.com/question/split-insulated-roof