Can mineral wool replace “foam plastic sheathing” in a continuous insulation assembly that will get furring and engineered wood siding?
Has anyone successfully installed engineered wood siding on furring strips fastened through mineral wool board to the studs while maintaining the warranty?
I just confirmed that the manufacturer does not approve mineral wool board as a substitute in their Furring Over Foam Plastic Sheathing/Rigid Foam Insulation application guidelines.
When I reached out to tech support, they stated:
“Foam plastic sheathing that we state to use should be rigid enough as to not cause waviness in the finished wall covering. Generally, these products are a minimum of 25 psi compressive strength. Mineral wool would not likely meet this requirement.”
I was planning to plumb the furring with a string line, so I had not considered the possibility of overdriving screws as a factor. Now, I see the wisdom in the manufacturer specifying Furring Over Foam Plastic Sheathing/Rigid Foam Insulation. But still, I wish they had stated that continuous insulation substitutions require a minimum of 25 psi compressive strength. Maybe this post will help convince them to add that.
The Problem
We are filling the empty cavities of our 2×4 balloon-frame stud bays with dense-pack fiberglass (Zone 4) and need to maximize drying to the exterior. We worked out an assembly with the highest permeability ratings for each component, but we had not considered how it might impact the warranty of the cladding—until now.
Possible Solutions
Build up the back of the furring by attaching 1.5” rigid foam insulation to it and then fastening through the WRB and sheathing to the studs. To insulate the remainder of the wall, we can insert mineral wool board between the vertical furring strips. This should retain 33 perms across most of the assembly. I like this approach because it maximizes the R-5 rating of the mineral wool board.
Add a rainscreen on top of the WRB, then install rigid foam as continuous insulation across the entire structure. The drainage gap in front of the WRB should allow any vapor drive that passes through the WRB to follow gravity and escape. This seems like it might be faster but also seems like it could compromise the R-value, since we introduce an open drainage gap behind the rigid foam.
Has anyone used either of these approaches or found another way to make engineered wood cladding work with mineral wool insulation while keeping the warranty intact?
Would appreciate any insights!
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Replies
brill.net,
Your first one would work. You could also substitute short 8" pieces of 2"x2" for the foam strips every 2 ft as backing for the furring. Or strap the WRB with 2"x2"s horizontally and install the mineral wool between them. You could also use a Swinburne Truss https://www.greenbuildingadvisor.com/article/alaska-builder-faces-an-insulating-dilemma
If you switch to exterior foam, rather than short circuiting it with a rain-screen to maintain the permeability of the wall, why not use enough that permeability doesn't matter, and the wall no longer needs to dry to the outside? In climate zone 4 that would just be R-3.75.
https://www.greenbuildingadvisor.com/article/calculating-the-minimum-thickness-of-rigid-foam-sheathing
Thank you, Malcolm! It sounds like padding out the furring is at least headed in the right direction. I’m waiting to hear whether the manufacturer’s warranty group will provide a technical note for an exception, but as long as we maintain +25 psi between the furring and the sheathing/stud, I don’t see why they would object.
I think you’re suggesting that we consider padding out the furring with wood as an alternative to using rigid foam – at least every few feet. However, I understood that one of the benefits of furring over continuous insulation is mitigating thermal bridging. If backing the furring with rigid foam works as well as using wood for the same purpose, then why reintroduce a source of thermal bridging into the design? Maybe 1 ½” foam cut to 3 ½” wide isn’t stable enough to pad the furring? My plan was to prep in advance by gluing 4" of EPS to the furring on the ground, then installing it over the WRB in one shot. After that, we’d fill the spaces between the 16” O.C. furring with pre-cut 1/2 size MW boards of 12” x 48”. Obviously, I’d love to save time and money by going another route if there’s a better option!
On your second point, I completely agree. The BSC study cited in this presentation provided all the confirmation we needed https://constructioninstruction.com/pdf/wall-condensation-and-insulation-presentation-by-j-wilson/, so we targeted R-5 for the continuous insulation. I believe that’s also the code requirement for new construction in NYS Zone 4, so even though this is a retrofit, it’s good to know that we’re tracking code. Initially, we considered using insulated OSB panels or WRB-faced EPS, but we changed course after realizing that those (and other rigid foam options, even when outboard of the WRB) would inhibit drying. Given the risk of failure—whether inside or out—we felt it was critical to design the assembly to allow the cavity to dry itself out. The fact that it will be filled with dense-pack fiberglass gave us all the more reason to mitigate any moisture issue.
To be fair, I’m still figuring this out. I’ve pieced the above together from research and discussions with reps and builders, and I’d love to hear from you and others with different points of view. Thanks!
brill.net,
With sufficient continuous exterior rigid foam the wall cavities can be designed to dry to the inside. The question is where any moisture would come from that needed to be removed? In most common wall assemblies the moisture might accumulate due to interior air leakage, but with the foam keeping any interior air below the dew point it won't condense. The other source would be bulk water from the outside. But the wall is now protected by a WRB, foam, and a rain-screen, so that should be unlikely.
Like you I'm temperamentally inclined to design walls with good drying potential to the outside. However these walls with exterior foam seem to perform very well. Well enough Joe L. termed them The perfect Wall.
To be honest, if I had Joe L's perfect crew on my house I could be convinced that The Perfect Wall would be executed perfectly - but with 21 windows - that is a chance I'm not inclined to take. One thing that could save some money for houses like mine is a single CEI panel with high perms, ~25 psi compressive strength, and a built in WRB bonded to it but flipped around on the warm side (I filed the patent today) - seems like there is a market for that as more and more originally uninsulated homes that have been through the asbestos/aluminum/vinyl periods get the TLC they need.
I believe their 25PSI is for installing directly over the foam without strapping. If you are going over strapping, the foam PSI does not matter as the siding is resting on the strapping only. You still have to watch strapping spacing as most require either 12" or 16" OC.
So if the siding can be installed over sheathing with rain screen strapping (pretty much universal nowadays), it can be installed over rigid (doesn't matter type) + strapping.
With rigid mineral wool you have to take a bit of care when setting the screws for the strapping to keep the strapping in plane as the MW is somewhat squishy.
Thanks Akos, but what i've come to understand is that this "squishyness" is exactly what the application instructions are intended to address by specifying that it is furring over "rigid foam insulation"; 25 PSI is their line in the sand on what is considered rigid and what is squishy. I've attached the application instructions below; see page 4. I do wish that they made this more clear so that I didn't have to hunt it down for the past few weeks, but I suppose I can thank them for rounding out my assembly education since I had not considered their point about PSI before hand.
You are doing the install of "FURRING OVER FOAM PLASTIC SHEATHING". The PSI of the insulation under it doesn't matter as the siding does not rest on it.
There is a lot of siding out there installed like this without issues. Make sure you follow the strapping size and spacing for the siding profile you choose.
Thanks but I’m only restating what their tech support said: “Foam plastic sheathing that we state to use should be rigid enough as to not cause waviness in the finished wall covering. Generally, these products are a minimum of 25psi compressive strength. Mineral wool would not likely meet this requirement. As far as warranty, anything done or used outside of the published guidance could impact warranty if that specific practice directly causes a failure.” Note I’m not dismissing the idea that this particular rep was reading between the lines or that I’m missing the point (including that I have nothing to worry about). I’ll continue to seek clarification.
brill.net,
I find a lot of the warranty language we see really annoying, but this one makes some sense to me. Aren't they basically saying that if you choose a substrate that isn't very firm, you may have problems with the siding appearing wavy - and that will be on you?
There will be a blog post coming out tomorrow morning covering these details.
awesome!