Fastening Exterior Mineral Wool Wall Insulation
I recently asked a question about rockwool sourced from Alibaba. Well after talking to six different factories/exporters I was able to get two samples and it just didn’t seem to be the same material as what we have over here. I decided not to bet my forever home on material of unknown quality.
So back to the drawing board ,and not wanting to plunk down nearly 30k for 5-6 inches of ComfortBoard 80 I’ve been looking at alternatives.
My project is a deep energy retrofit in Chicago. The home was built in the 1880s and 2.5 story balloon frame. The previous owner added dormers but for the most part the roof is pretty simple and the house is a rectangular box.
At this point I have thought about adding Larsen trusses but that seems like it would have a lot of thermal bridging and the local inspector may look at me sideways for screwing floor joists to my walls – not to mention the cost of the trusses. Now I’ve gone down the Cascadia Clip rabbit hole and I need to understand why we can’t just use these with regular Rockwool comfortbatts that we would use in the cavity with these.
The wall as I’m thinking about it now is
Interior
Drywall
2×6 cavity (expanded with furring strips) filled with R23 Mineral Wool
Old school 1 by sheathing
Henry VP100 WRB
5.5 inch Cascadia clips holding R23 mineral wool (same as interior)
Strapping
Siding
Exterior
From what I’ve read this wouldn’t have compression issues like trying to strap directly on to the comfortbatts, the siding would go right from the strapping to the clips to the sheathing so there shouldn’t be an issue with that either. Is there an obvious reason this isn’t done more often as it seems like it saves quite a bit of $$ if you are planning a wall with a lot of exterior insulation?
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[Is there an obvious reason this isn’t done more often as it seems like it saves quite a bit of $$ if you are planning a wall with a lot of exterior insulation?]
Yes,.
Details around windows and doors become much more difficult with thicker insulation. Are you going to build window boxes? How are the deeper windows going to look and how will the boxes impact the amount of light?
Solution: Why not use reclaimed rigid foam on the exterior instead?
Window boxes are being constructed and will push the windows to the new edge of the siding We like this as it functions more or less like a double stud wall with the extra depth in the sills.
This video shows it being done. Unlike wood girts attached directly to the wall, I'd call the system "continuous exterior insulation". How does it compare price wise?
https://www.cascadiawindows.com/products/cascadia-clip#videos/installing-roxul-with-the-cascadia-clip
Various alternatives here:
https://www.rdh.com/wp-content/uploads/2017/07/2018-27-03-RDH-Cladding-Attachment-Solutions-Brochure-V4-Web.pdf
This system with the comfortbatts, even after paying for the clips, will save around $10k compared to using comfortboard for the same R value/thickness. That video shows them using cavity rock, which I think is a bit denser than the comfortbatts but to be honest I've never seen cavity rock in person. But if they are similar products that would ease my FUD around using this externally.
I hadn't considered galvanized clips but according to this report they perform similarly to the fiberglass ones so I'm going to have to take a look at that. Thanks for the link.
CAVITYROCK is 2x-3x denser than COMFORTBATT.
High R value walls are really only worth it if done with inexpensive insulation. An R40+ wall with mineral wool will have an ROI measured in centuries. In zone 5, there is probably not much energy to be saved above an R25 wall assembly.
For example, say you have 2000 sqft of house, that will generally have about the same sqft of exterior walls. You are probably somewhere around 5000 HDD 65. This means that over a season, through an R25 wall you loose (this isn't exact but about close enough):
2000*5000*24/25=96 Therm.
Bump that up to R40, it goes only down to 60 Therm. Even if you are heating with electric resistance heaters, that is not much in operating cost difference.
As for wall assembly with low density mineral wool, I've done over-roof with 2x lumber on edge at 90 deg to the rafters with batts between. As long as you have a solid air/vapor barrier under the structure, this works quite well.
Because the 2x is perpendicular to the structure you get very little thermal bridging, it is almost as effective as continuous insulation.
In your case, I would skip the interior furring out, keep the studs as is and go for exterior insulation only. If you need 4" insulation for the interior, search for Rockwool AFB. Something like horizontal 2x4 on edge with R15 batts on the outside is probably a reasonable ROI. You can install board and batten or metal siding directly over the horizontal 2x4, for board siding add with vertical 1x4s strapping on top.
I would go with a double stud wall and wood fiber board if you can source it (you can check 475 website to see how it's done). I'm not a of fan of thick exterior insulation walls (anything over > 3''). They are just too expensive and hard to build.
I have 3" of exterior ComfortBoard insulation and it was a lot of work and money. Compression is not an issue if you use 2" furring strips and pre-drill the holes with a paddle bit so the HeadLok screws will sit flush as soon as the furring strips starts to tighten against the ComfortBoard.
Jsut an FYI.. The OP wants to use 5.5 inch Cascadia clips instead of furring strips. Basically build a commercial wall but use COMFPORTBATT insulation rather than something denser like CAVITYROCK or COMFORTBOARD 80.
Yes, I saw that. I was giving him my experience just to let him know that compression is not really an issue, which was one of his concerns with ComfortBoard.
+1 on checking the numbers. Compare to something easier like 1.5" of unfaced EPS or GPS.