Site built nailbase with rockwool comfortboard?
Hello,
What do you think about using exterior Rockwool but then using plywood sheathing instead of strapping? This would create a Rockwool nail base. Innie and outtie windows with bucks seem like a pain and having a flat surface to install windows seems way easier and way more likely to be flashed correctly.
I have a house that will be resided soon. I would like to install exterior Rockwool comfort board 80. The wall sheathing is fiberboard from 1957. Siding will probably be vinyl so no rain screen (from what I have read) is needed. A drainable house wrap will be used instead.
This is what I am proposing for the wall:
– 2×4 studs
– 3/4 fiber board from 1957
– peel and stick WRB mainly for air barrier (blueskin vp100 looks good with 33 perm)
– Rockwool comfortboard 80 (1.5-2.5 inch thick)
– plywood sheathing to make a nailbase
– drainable housewrap (mainly for water mgmt)
– vinyl siding
What do you think?
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Replies
I have spec'd an assembly similar to that on a couple of projects, though with rigid foam instead of mineral wool. But even with foam, I recommend installing 1x4 battens, either vertical or horizontal, as foam is somewhat squishy and mineral wool is even squishier, and plywood alone will likely result in wavy walls. Installing battens first allows you to fine-tune the surface plane to be flat before installing the plywood.
With vinyl siding it's not even important to add a drainable WRB, though it certainly won't hurt anything.
Blueskin VP100 is a good choice for the primary WRB and air control layer. There are other brands that are somewhat more durable but Blueskin is readily available in most places. (Many of the others are readily available via an internet order as well.)
ssdiz,
Wouldn't adding plywood directly to the mineral wool board defeat the vapour-open qualities of the insulation?
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I'll second Malcom and say that putting a class 2/3 vapor retarder (Plywood) over the rockwool makes the vapor open qualities of rockwool moot. Not to mention it would be significantly more materials (and potentially labor) when using plywood vs. battens. I'd be curious why rockwool is attractive in this proposed case as an EPS foam would actually have less of an embodied carbon and one could just buy nailbase pretty affordably. I could see how one would want to avoid plastics with the EPS (microplastics, petro-based materials, etc), but it seems like a lot of effort to get exterior insulation installed. See my recent article in FHB about an install with rockwool and a SA membrane for tips.
Also, I would check the adhesion of the Henry on the wood fiberboard. I have had varying success, mostly dependent on temperature and if the weather is wet when applying it. It may work fine, I've just never done it with the old fiberboard stuff.
J
I think it would be much simpler to build with regular batts.
For example, you could strap out the wall with horizontal 2x lumber on edge. You can get mineral wool AFB in 1.5" and 2.5" thick for 2x2 or 2x3 strapping. These are actual 16" wide so the strapping spacing has to be 17.5 OC. You can go up to 2x4 and use regular R14 batts. The plywood goes over the 2x so pretty much a standard wall from that point.
No need to deal with squishy insulation and the batts are significantly cheaper than the semi rigid board.
In terms of thermal performance, since the extra strapping is perpendicular to the studs, the only thermal bridge is where the studs cross so almost all the thermal bridging is eliminated. Around 95% as effective as continuous insulation.
There are other options such as a Larsen truss that would also work just as well.
Thanks for the responses!
@malcom, Joshua
Wouldn’t the Rockwool then plywood still have better vapor openness compared to eps or xps then plywood like in a standard nailbase? What I was thinking for vapor openness setup starting at most open was Rockwool with battens, Rockwool with plywood and then foam with plywood.
@michael
Wavy siding makes sense. I was thinking that the exterior plywood would uniformly compress the comfortboard but I could see how the compression would not be the same at the studs vs in between them.
@akos
This is an interesting option. Have you seen anyone else do this?
I've done this for over-roof. Was very simple and quick, I can't see why it wouldn't work for a wall.
If you are using larger lumber, for a wall you would want some vertical blocking between the strapping some racking resistance. This assumes light weight cladding. If you are doing something like traditional stucco or adhered stone, I would use something else.