Saturday, October 15, 2011

Mineral Wool - first local install

Last year I started writing about my enthusiasm for Mineral Wool insulation, and now this past week I've seen the first installation of this insulation on a local project.

The mineral wool was used for renovation project, the conversion of an uninsulated 3-season room to full time conditioned space. Because the room has so many existing windows we wanted to achieve a higher insulation level in the roof. The existing framing was only 7.5" deep, not much room for insulation. So we added a 2x furring strip, combined two layers of mineral wool, one R15 3.5" (typically used in 2x4 stud walls) and one R23 5.5" (typically used in 2x6 stud walls), creating R38 in the 9" cavity we had available. We put Certainteed's Membrain over it for the vapor retarder. Its an un-vented roof and we wanted to be sure it would dry to the inside if any moisture got into the roof, so this variable permeability nylon is perfect in this case. You can see it in the photo above awaiting the finished ceiling. End note: The builder did all the cutting of mineral wool with the owner's 18" kitchen bread knife!

The owner emailed me his impressions:

The insulation is truly amazing. We are not far from the flight path into Philly. When standing in the Florida room last night you could barely hear the jets. Last night when I went to take Penny (pet dog) out by the Florida room door the room was amazingly warm and we have yet to turn on the heater this fall! When I opened the door the difference between the outside and the Florida room was remarkable.

This is why I'm advocating for mineral wool in the USA New Wall, because it achieves higher performance in the same amount of space as the status quo fiberglass batts, and the process of installing and working with batts is similar enough to not be intimidating to contractors. The contractor on this project had never worked with mineral wool before. Granted for the first time they did not like the differences from fiberglass, but they did complete the work with the same labor force and in essentially the same amount of time as they were accustomed to working. That is what makes mineral wool superior to blown in cellulose or spray foams when it comes to bringing people along, and coaxing higher performance out of builders who are not versed in higher performance.


  1. Sounds good, I wonder if you could add a comment regarding the price difference compared to fiberglass?(if any) Sounds like labor costs are a wash. Also, was there any benefit to the two layers besides desired thickness and availability of materials?(looks like the furring is running the same direction as the joists)

  2. I've not heard anything about the cost of the insulation here, but it just went in and I've not spoken to the builder about it yet.

    Two layers were used because batts are not available in the same thicknesses as fiberglass. There are 3.5"(R15), 5.5"(R23), and 7.25"(R28). Combinations of these will work in most situations. For instance an attic with 2x6 attic joists would take two layers of 5.5", one between the rafters, one over for a total of R46. This cross layering is more effective than single batts at isolating the tops of the attic joists from the cold side.

    Yes, the furring in this case was right over the joists because the wanted the t&g ceiling to run side to side. If the furring went across the rafters it would compress part of the insulation slightly, but also help retain it till the ceiling was installed.

  3. Fiberglass insulation is readily available in 3 1/2" thick R-15 batts and 5 1/2" thick R-21 batts. The mineral wool combination achieves an R-38, whereas the combination of R-15 and R-21 fiberglass batts would yield an R-36, so there's a small R-2 performance benefit of the mineral wool, but at what additional cost in material and complaining contractors? Are there other benefits to mineral wool insulation? The very slightly higher R-value per inch does not seem to be enough to recommend it over fiberglass. Unlike a full-thickness spray foam or a "flash and batt" spray foam/batt installation, a batt-only system whether fiberglass or mineral wool, still has potential air infiltration problems that can easily overcome the R-2 difference.

  4. Drew,

    R15 in 3.5 fiberglass, and R21 in 5.5 fiberglass are special order items at suppliers in my region, not to mention getting them in unfaced configurations is even more unusual. The reason for getting them unfaced is because we are building up layers and we don't want a kraft vapor retarder in the middle of the insulation, and the second reason is that you can not get a tight air barrier with a kraft paper vapor retarder.

    So the cost difference of the batts is negligible. Second is the composition of fiberglass batts even in these higher R values is a limp blanket, and you are correct that even in a best case installation it can sag, leave gaps, promote convection in the wall, or ceiling in this case, and easily defeat a few R value points. The difference is that mineral wool is not a limp blanket, it is a semi ridgid block, which when cut to fit the cavity space will fill every void, and will not sag or leave gaps that will convect. It is easy to cut precisely around obstructions, and all around a good mineral wool installation is much better than a good fiberglass installation. The sheet vapor retarder ensures there are no air leaks - where it matters - before moist interior air reaches the insulation. Foam + Batt, if you are relying on kraft paper for the interior batt then you are going to invite moist interior air into the ceiling cavity where moisture can condense on the spray foam, and since the foam is air tight it will not dry to the outside. That is a bad system, at least in our climate here. Full spray foam? Knock yourself out if the budget supports it.

    Mineral wool and fiberglass are two different materials, that handle much differeintly, they just both happen to be called Batts. Its easier to do a good job with Mineral wool than fiberglass. That said its always easier to do a bad job with fiberglass! If you need to see the difference, then go back to this blog post and watch the videos.
    Prefab is Dead

    About 3 minutes in to the Swedish factory video they will be installing the mineral wool. I say with complete confidence that it is superior to fiberglass batts. I say that for all the reasons you state that even higher R value fiberglass batts will still have a poor installation and ineffective kraft vapor retarders. The mineral wool will not.

    As far as spray foams, blown ins, all great stuff. If you can get your builder to work with it, and your home owner to pay for it, I say go for it. It will not become mainstream, because if it would, it would have a long time ago. Mineral wool is the natural substitute for fiberglass, contractors already understand how to use it, and builders already understand how to price it. The only gripes the worker's had here is that they could not do the usual sloppy job, because its in the nature of the mineral wool to fit. You can't be lazy and squish it into a space without trimming. Its probably the first good install they've ever done.

    Maybe some day the american fiberglass manufacturers will be willing to bring their higher r-value product here from Canada. By then the mineral wool product here will be bio-soluble, while fiberglass's trajectory is for the carcinogen list. I think its more likely we'll see Owens Corning and Certainteed offering mineral wool to cover their bases.

  5. Greg - What manufacturers of Mineral Wool do you like best in the US?

    Also, have you tried the fiberglass EcoBatts at all that come unfaced? They have a high recycled content and install more rigidly like you have been describing Mineral Wool. We've used them on interior framing only for soundproofing and really like the install.

  6. Chad, Roxul is the only one making a strong effort with product for the residential market.

    My client in Blue Bell used the EcoBatts in his house. They were great, but still R19, still came with a craft vapor retarder, and were still limp blankets. EcoBatt photos here. Once you side staple the kraft you have a 1" void at your drywall. Mineral wool for 5.5" studs is R23, fills the void completely. No staples, fits with friction. Separate vapor retarder sheet. And since the swedes use an interior wiring chase, the drywall can be glued to the furring, so there is no issue with the vapor barrier covering the stud faces, which is why americans like to side staple the kraft batts - so they can glue drywall, and reduce call backs.

    Mineral wool is a "block". Its a volume, it has its own shape. Its more like a block of styrofoam than a fiberglass batt. I realize they call all of this "batts", but its really two different things.

  7. Ahh. That's all helpful info. What about Thermafiber? No good?

  8. I don't know their product well enough to say. There are a number of other brands I've seen in Europe that look the same consistency of the Roxul, Paroc for instance. But I've not looked closely at Thermafiber to know.

  9. Hi Greg!

    I live in Russia and now here there are many disputes about ECO (ecological properties) of mineral wool and fiberglass. Some people and companies (and our Minister of Health also) say that mineral wool allocates phenol because in production mineral used wool phenol-formaldehyde resins that emit hazardous substances, phenol, formaldehyde, acetone, ammonia, etc.

    And now (may be 1 year ago) on our market appeared ECO mineral wool that doesn't allocates dangerous substance, because in production as a binder its used no phenol formaldehyde resin. Instead of it uses acrylic binder or other ECO binders (example ).
    And also how can i protect my living space from micro dust that allocates mineral wool and fiber glass? Should i make maximum hermetically sealed vapor retarder and glue all joints with tape and dont penetrate it via fasteners ?

  10. I've asked Roxul about their phenol based binders, and they have said that the quanitity is very small, and that any off-gassing is complete before the product is shipped. I've not looked into it further to see if there was any testing of this. I think all batt manufacturers will be rapidly moving to non-phenol binders, so I expect with the slow adoption rate that this will have changed before mineral wool becomes popular here.

    As far as particles in the air, you should have mineral wool manufactured in the EU available to you there. The EU formulas are "bio-soluble" meaning they dissolve if ingested, and do not pose the same hazards as suspected with fiberglass. These formulas are not in the US yet, but they are coming.

  11. Oh i see. Also today i was at "Knauf International Symposium on "Sustainable Architecture: Present and Future" ( in Moscow, and i have opportunity to speak with representatives of this company. Here in Russia Knauf is only company that produces ECO mineral wool today. And they saw that there are no phenol-formaldehyde and acrylic binders.

    And in the US, as I understand, fiberglass is often used as insulation material? Here i have no customes that wants insulate house with fiberglase, because everybody thinks that it is very dangerous because of the glass dust..)

  12. Yes, fiberglass is the most popular insulation here in the US. In fact Knauf does not sell their Mineral Wool here, only their fiberglass.

    Fiberglass has been classified as a suspected carcinogen here. It does not look very good for fiberglass. I am predicting a large swing to Mineral Wool in the near future.

  13. Oh wow, i think that Russia is one of the last countries that uses fiberglass and non ECO mineralwool :) because we have big problems with new and innovating materials and technologies :)
    For example: only in this year LVL had appeared on our market in Moscow )

  14. Love that Roxul! Spec'd it on my last two projects and will be using it as much as possible in the future. Goes in well, looks awesome even before drywall goes up, and can be done by anybody using a steak knife / serrated knife for cutting. Awesome product too - have you see their website? Lots of info and videos, etc.

    Thanks for the post!