Mineral Wool Compression Affect on R-Value
Dayton
| Posted in General Questions on
Roxul comfort board that I can readily purchase is about R4/inch. Roxul comfort batts are about R4.3/inch. Does anyone know how much reduction of R value you would get if you compressed 3.5 inch batt into a 2.5 inch space? If the R value is primarily a factor of the material, not the air space in the material, then R4/inch is possible. Or maybe strand orientation and length also is a factor? I’ve googled, but nothing pops up.
Thanks
Dave
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Replies
>"? If the R value is primarily a factor of the material, not the air space in the material..."
But it's not- it's a combination of factors.
With fiber insulation the R/inch increases with density up to some point, then begins to fall. Batts are at densities that will continue to increase in R/inch with compression, and nowhere near as dense as rigid rock wool.
I'd expect an R15 rock wool batt (R4.3/inch) to perform around R11 when compressed to 2.5" (R4.4/inch) in exactly the same way that R15 fiberglass goes from R4.3/inch at 3.5" to about R4.4/inch when compressed to 2.5", according to the manufacturers' compression charts.
https://s3.amazonaws.com/greenbuildingadvisor.s3.tauntoncloud.com/app/uploads/2018/08/08062722/Compressing%20fiberglass_2-700x310.jpg
Ok thanks for the information. That makes the batts about 3x cheaper for similar R value.
I'd test this before you plan to do it. Mineral wool isn't as compressible as fiberglass, and I'm not sure you'd be able to compress a 3.5" mineral wool batt down to 2.5". Even if you can do it, it might take so much effort as to not be worth doing.
Bill
>"I'm not sure you'd be able to compress a 3.5" mineral wool batt down to 2.5". "
I'm 100% positive that you can stuff R15 rock wool batts into 2.5" deep 2x3 framing.
It shipped compressed to a thickness quite a bit tighter than that, and in 2x4 framing often needs to be tugged & teased-out a bit for loft after tucking in the edges and corners tight to the exterior to make it sufficiently proud of the stud edges for a compression fit to the wall board. It's pretty springy at 2.5" but not steel springs- it won't perceptibly bow out half-inch wallboard with studs are 16" on center, and doesn't require a body-builder's physique to install.
I've never tried squishing it that much so I'll go with your experience.
It might be a bit easier to start with the safe and sound stuff that's a bit thinner to begin with though.
Bill
Yes thanks, testing is always a good idea. I've compressed 3.5" mineral wool over flat 2x blocking using 1/2" plywood, so pretty sure it won't be a problem. But if I do go this route I would like to use 1/4" plywood to compress, so will test that first.
Roxul wool insulation is supposed to be compressed to work properly. I work on new construction projects in the city & we use this stuff all the time to pass a proper inspection it has to be compressed to almost half its thickness
>”we use this stuff all the time to pass a proper inspection it has to be compressed to almost half its thickness”
It most certainly does not, if you’re expecting it to perform as thermal insulation. The specified R values for mineral wool are for the stated thickness as an example, the 3.5” thick batts made for 2x4 stud cavities are rated R15 when they are at the full 3.5” thickness. If you compress them from that rated thickness, you get a reduced overall R value.
If you’re using mineral wool as a fire stop material stuffed around wall penetrations, then it IS necessary to compress it since you want to make it as air impermeable as possible. That’s a common requirement commercially and I personally have stuff many, many wads of mineral wool around conduits and cables back when I was an installer.
These are two very different applications for the same product, each with very different installation methods and purposes.
Bill
@aj84, wrt rock wool insulation compression - where are you that code/inspectors require the insulation to be significantly compressed? I have never heard of this requirement before.
I was wondering about using Rockwool Comfort Batts under a crawlspace rat slab instead of the much more expensive Comfort Board material. Does anyone know how much a 2" thick 25PSF concrete slab would compress the Comfort Batts ? If R30 batts at $1.25/sf compressed down to 4" thick under 25lbs concrete but still had R15 insulation value then it would be a cost effective solution compared to $1.50/sf R8 Comfort Board. I can see not using it under a regular slab foundation because that slab would be heavier and the insulation might spring back to crown the slab in the center some as the concrete cures and loses 10% of its weight. But for a rat slab that isn't locked into the stem walls anyway it wouldn't matter if the slab rose back a fraction of an inch, right ?
Dreamer,
There is a reason you haven't heard of this being tried. I doubt y0u could even get the slab poured and finished without it cracking to pieces, never mind having it stay intact afterwards. What you are trying to do is use the weight of the concrete to make mineral wool boards out of batts. You simply can't do that. It's like trying to pour a driveway directly on a mossy lawn. Concrete relies on a solid substrate. That's a fundamental pre-conditon for pouring any slab.
I would use the analogy of trying to pour a slab on top of a soft mattress. You won’t have the support you need, so you’ll have all kinds of problems with the slab.
Insulation used for under slab applications needs to be rated for compressive strength. Mineral wool batts are not, so they cannot be used in this application. I’d use rigid foam here myself.
Bill
Thanks for the quick replies ! I had thought a tight arrangement of the batts and a continuous heavy vapor membrane above it would allow wet concrete to distribute its weight evenly and once the concrete cured it would move as a unit. I hadn't thought about the cracked driveway on uncompacted dirt or lawn or mattress [:-)] analogy. I thought those cracked because the base was not uniform. People pour structural slabs on the board product and it only has a compressive strength of 400 psf at 10% compression. No inspector would allow you to pour a foundation slab on dirt that soft.
I was thinking about how concrete is sprayed successfully on a relatively soft inflated air form for monolithic domes. I suppose the domed shape helps. On the other hand, I'm not sure cracking in a rat slab would matter all that much with plenty of fiber in the mix to prevent any actual holes.
I won't use plastic foam anywhere for all the usual reasons, so it will have to be board mineral wool or Glasfoam.
With a dome, the dome itself acts as the structure, similar to an arch but in three dimensions. The air support is just a temporary form to support the concrete. Once the concrete has cured, the dome form provides the support. A slab does not have this ability to support itself as relies on the ability of the ground underneath to provide support.
Why do you want to avoid “plastic foam” here? It’s really the best product for this application.
Bill
Well, Bill, it's because:
Bugs tunnel and nest in it. Its manufacture is polluting. It doesn't have a long track record. It expands and contracts, leaving gaps. It releases VOCs over its entire life so I don't trust it isn't leaching something into the ground. Etc. I'm sure you've read all the same articles I have about it. It is also fairly expensive for the R-value you get -- the price of R12 of foam board would get me R30 of Rockwool batts, so it seems worth a little effort to explore whether batts can work. Being retired, I have more time than money.
If they won't, then I may resort to pouring a foot of lightweight cellular concrete to get R15-R20 under a thinner concrete rat slab or no rat slab -- I don't know if gophers can tunnel through compacted gravel and aircrete or not. Aircrete has a compressive strength much higher than rigid insulation board -- 80 psi X 144 in/sf > 11,000psf -- costs less than $1/cf and bugs don't like it. The downside is that you can't always call a ready-mix truck and mixing 2000 cf yourself would be a bit time consuming at 20 cf per batch.
There's also Foamglas, although I have no idea about availability or price.
Any material's manufacturer will be at least somewhat polluting. The three main types of rigid foam, listed from most to least green, would be Polyiso, EPS, and XPS. Polyiso can't be used underground for sub-slab applications, but EPS can (you'd need one of the higher tier EPS types though). XPS is the one with the most environmental issues.
Most plastics are pretty stable and don't leach things out over time. That includes polyethylene and polystyrene. You DO lose blowing agents over time, which is an issue for Polyiso and XPS (much more so for XPS though). EPS doesn't really have this problem, and the blowing agents are reclaimed at the factory for reuse.
The bug issue can be dealt with with mesh. It does add a step though, but a relatively small step.
You could try cellular concrete. I haven't used that product myself, but it sounds interesting for applications where it would be useful. You'd have to check pricing, and you'd probably want an experienced contractor and not just any concrete contractor for installation. I've often found that specialty products installed by contractors unfamiliar with them can result in installation problems, or worse, lifecycle problems that can be very expensive to correct.
Bill
Malcolm,
Yes, that was an interesting article I saw here on GBA about FoamGlas. I looked for price and availability and came up blank, although the article implied it was 2x-3x the price of EPS. I think it also showed workers able to dent it with a thumb, so if a low crush resistance is going to make a slab over it crack it may be no better than mineral wool board that way.