Sheep wool insulation – yay or nay?
Hello,
I posted this question in another thread related to a unique situation involving rebuilding exterior walls in a historic home with lathe/plaster removed and NO exterior cladding. I am hoping to gather more feedback on experience with this natural wool insulation product.
We have followed GBA guidance to add an air gap and rigid foam inside the siding. The plan was to fill the remaining cavity with BIBS. Not possible – not enough depth remaining. Then we tried open cell foam. Did not work – peeled off during trimming.
There is varying depth of 1″ – 1.75″ remaining in cavity. I am not as concerned with r value and code as I am with also maximizing sound proofing (house is located in NW FL). Street noise will be an issue (it was very quiet with plaster walls).
Sub says NO blown product will work. I am left with trying to split fiberglass batts or consider a treated sheep wool alternative. I can source wool pre-treated for pests (borate or Thorlan IV). The thorlan-treated batts are more expensive but seem to be a good fit – easily split into plies, thorlan is chemically bonded and stable, safe to work with, good sound and moisture control. And its a VOC sink.
Does anyone have firsthand knowledge or experience? Widely used outside the US as sustainable green building material but very limited adoption in the US.
I have really exhausted other options except standard fiberglass I believe, and I want to see if this may be superior to that for this shallow depth. Photo depicts typical exterior wall.
Thank you!
Robin
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Robin,
If the only issue is sound attenuation I'd suggest stepping back and looking a the wall as a whole rather than just how to fill the small area of the wall cavities that are empty. You may find it makes more sense to add another layer of drywall, a continuous layer of foam board, or resilient channels, or mass-loaded vinyl, or???
In other words look at the whole assembly and compare it to other tested ones. I doubt if adding a small layer of any batts will make an appreciable difference.
Malcolm,
Sound attenuation is not the only consideration but I also realize that no matter what I do, it's going to be somewhat minimal at that depth (and won't achieve r13 code). Still I suppose it's worth adding what I can. My thought process was (for example) r2 fiberglass vs r3 wool but wool has added sound proofing (claimed), then I would pay more for that added benefit.
But, if neither would make much difference, then just doing fiberglass which is cheap is better than nothing. I was hoping for a better solution rather than finish it all out, move in and discover I made a big error in not addressing both r value and sound.
Thanks for your feedback.
Robin
Given the small amount of insulation, I would just go with R8 2.5" fiberglass squished down to 1-1.5".
Matt,
Thanks for the suggestion. That may be what we end up doing.
Robin
Matt,
I agree. That's probably the best bet.
Robin,
If you are set on filling those voids, you might want to look at rolls of UltraTouch denim insulation. The batts come in a variety of thickness, and I believe there are rolls that are about the thickness you are describing. They are available for order from HomeDepot, and possibly less costly than sheep wool.
Aaron,
Thanks - I had read up on denim but I didn't realize it might come in thinner version. I did read that it is somewhat difficult to cut and as you can see, lots of irregular cavities and framing will require that. I also had some concerns about water. The rigid foam is as watertight as we could make it, but if any water makes it past, it would have to dry to the inside. Wool seems to have some benefits there.
I'll check out the UltraTouch again though.
Much appreciated,
Robin
Since all your interior is already off, you could horizontally strap your wall with 2x3 on edge. This would give you around a 4" to 4.5" cavity that you can now blow insulation into. It also has the benefit of significantly reducing thermal bridging.
Assuming R6 foam, that would give you around an R21 assembly VS R11 of the existing setup.
Akos,
Thanks for the suggestion. I'll run it by the GC. But, my cottage is very small and every square inch is crucial! ;-). Our code is r13 and if we approach that, l'll be happy for our somewhat mild climate. Thanks for your input!
Robin
A double layer of 5/8” drywall is going to do more to block sound than anything you use as insulation in that space. If you really want to limit the space used, at least use 5/8” drywall instead of the more commonly used (in residential construction) 1/2” drywall.
For sound blocking, you want mass (heavier drywall, which basically means thicker), and decoupling (resilient channel, double stud walls, etc). The type of insulation you use doesn’t really make a whole lot of difference compared to mass and decoupling.
Note that mineral wool is also available in thinner sizes, but I don’t think you can get it thinner than 2.5-3” or so unless you go with the rigid panel type.
Bill
Bill,
Good info, thanks. We are planning on at least one layer of 5/8" drywall. I have a couple of walls that I also need to put plywood for shear strength plus drywall.
My GC told me today that he has had some success with cutting down the thickness of rockwool, which I didn't really think possible. I'm going to check that out plus an r-8 CertainTeed glass that is 2.5" thick. And maybe extra drywall strategically but at least the 5/8 throughout.
Thank you!
Robin
You can split r15 rockwool batts. The best way is with a long serrated knife and a quick jig consisting of a 2x ripped at 1.75” positioned on each side of the batt. The 2x act as a thickness guide for the knife.
I use this knife but wish it was a bit longer: https://www.amazon.com/Mercer-Culinary-M23114-Millennia-18-Inch/dp/B06XGRZ3ZR
Cutting rockwool like this makes a big mess and part of a batt will disintegrate occasionally. I like rockwool, but it might not be worth it considering the r8 fiberglass.
Similar to Akos suggestion, but lower profile, you can put 1”x1.5” strips of rigid foam over the studs and use long screws for the drywall. You only loose 1” all the way around the way.