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Fiberglass or Roxul batts?

ddubx6 | Posted in General Questions on

If you could only choose between Roxul or fibergass batts to fill an 8″ thick wall, would the extra expense of Roxul be worth it?

In my local stores, Roxul is about 50% more in cost. The wall is standard single stud, 24″ O.C., taped and sealed Zip system exterior, MemBrain barrier with airtight drywall interior will be used. 4″ of polyiso will be added to exterior at a later date but it could be a while.

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    Where are you? If zone 5 or above the zip will be iffy until the foam is on, but the membrain and tight drywall will really help. I assume the zip will be exposed for awhile. You may be better off doing the foam and siding first, and the roxul or f.g. later. Nothing to worry about then. Kevin

  2. GBA Editor
    Martin Holladay | | #2

    Either product will work if properly installed. (For more information, see Installing Fiberglass Right.)

    Roxul is denser than fiberglass, and may be worth the upcharge for a variety of reasons. But before you invest in Roxul, find out how much it would cost to insulate your walls with dense-packed cellulose -- a much better choice for an 8-inch-thick wall.

  3. Expert Member
    Dana Dorsett | | #3

    In my neighborhood R15 Roxul costs about the same as high-density "cathedral ceiling" R15 fiberglas batts of equivalent R. The higher density of either makes it more air-retardent, and has real-world performance difference beyond the R13s in spite of the thermal bridging. Between HD fiberglass and rock wool, I'd still go with the rock wool for the higher fire resistance (fiberglass melts at a relevant lower temp, and is somewhat translucent to infra-red radiation.)

    But even low-density damp-sprayed cellulose will beat both on real-world thermal performance, since it eliminates the inevitable thermal-bypass channels of compressions and gaps that occur in even the most meticulous batt installation. Dense-packing is more expensive, but may not be "worth it" if you're adding 4" of foam the exterior, since the foam layer would keep the cellulose at a much more stable temperature & humidity, dramatically reducing the density required to keep it from settling with age. Dense-pack can sometimes be quite expensive, but damp-sprayed should be in the same ball park as HD batts, maybe less.

  4. KeithH | | #4

    I've been retrofitting my 2x6 stick built house with Roxul over the past few months. I had an initial scare with an odor at the same time as the Roxul but it dissipated. Bad bag maybe? It's been great since.

    Anyways, here's my DIY opinion: You could not give me fiberglass batts at this point. The roxul installs very quickly, with less irritation (IMO) than fiberglass, and nice tight bay installations. I like the fireproof nature of it. It seems denser than fb but I don't know if that translates to less subject to air permeability or not. My girlfriend has remarked that the Roxul equipped rooms seem much quieter than before (with 2x6 fb). It could be a placebo effect but I don't think so.

    Incidentally, you might try the pro desk at the big orange. I found special ordering (short delay to get the product) offered the best price for me. In my area in 2013/2014, ~$0.90/sf for 2x6 and maybe $1.15/sf for 2x8.

    I'm sure you know this but there are two widths available in most of the Roxul product lines: true stud widths for steel studs and reduced widths (23.25") for wood studs. Unless you have an odd situation, be sure to get the appropriate one.

    One more 'pro-tip': you can tack the special roxul cutting knife on to that order from the pro deck at the big orange for pretty cheap. You might consider that as alternative to a bread knife, though the bread knife works too. My bread knife is starting to get dull.

    Good luck!

  5. LucyF | | #5

    I really like the Roxul product. In addition to its density and soundproofing ability, I like it's resistance to fire and bugs. I find it very easy to cut. I bought a small saw specifically for the Roxul which works great.

    In terms of irritation, I think the Roxul is pretty aggravating, but not as much as fiberglass.

    We've used dense pack cellulose as well on the house we're building, but my experience is that in my area (upstate SC), they don't really know how to do it. I had to follow them around and make sure that they filled every bay all the way.

    Are you going to be doing the installation? If so, no question - use the Roxul. It is much easier to fill a stud bay because it holds its shape so much better, easier to cut around obstacles.

    Here's a photo of the Roxul on the bathroom walls. We used it on the wall behind the bathtub for insulation, but the walls are insulated with dense pack cellulose held in place by intello plus smart vapor retarder. Since we had the space to add more insulation, we added Roxul. My carpenter insulated that area. The other wall of the bathroom is insulated for soundproofing. I have a few spots I need to fill in.

  6. rshuman | | #6


    Why do you prefer dense-packed cellulose over Roxul? Is this a general preference or does it relate to the thickness of the walls?

  7. Expert Member
    Dana Dorsett | | #7

    Dense packed blown goods are more air retardent than rock wool or high density fiberglass batts, and during installation the blown fiber follows all air leakage paths and plugs them. (Not perfectly, but I'll accept 95%, eh?)

    Cellulose is hygroscopic, and will safely store a large fraction of the wintertime moisture burden, sharing that load with the structural wood, offering significant moisture protection.

    Dense packed fiber also leaves NO compressions or voids- about as perfect a fit as can be had in the insulation biz.

    Cellulose isn't itchy either ! :-)

    If batts it must be, rock wool is the better choice for it's density, air retardency, and fireproof factors. But it's still a step down from blown/sprayed goods for total air tightness & air-retardency.

  8. KeithH | | #8

    I have no idea about dense pack cellulose vs roxul. I can't do dense pack myself (or if I can, I'm not aware of it) and around here insulation contractors are a small fortune. By doing the work myself, I'll be able to do the whole house eventually. My previous quotes for dense pack cellulose and spray foam have been astronomical.

    I'd say that an amateur can achieve a very high quality installation with Roxul quickly and easily. IMHO, you can't say that about fiberglass batts (it's either slow or poor quality, if you are an amateur).

    The one thing about Roxul that could be a deterrent for some people is that I cannot imagine fishing new wire in the future through a stud bay with Roxul batts. Doesn't seem manageable. As such, if future electrical or other wiring work is anticipated, you probably better consider service cavity or some other solution.

  9. rshuman | | #9


    Thanks for your perspective. One benefit I see for the wool is the higher R-value (~4.1 vs 3.8 per inch, depending upon the source). In your opinion, is this benefit worth pursuing if another (efficient) means for air sealing can be found? Things such as taping the sheathing and/or caulking 'intersections' from within?

  10. LucyF | | #10

    I think that we need to have redundant layers of air sealing to make sure it will be effective 10-20 years or more down the road. First you tape the sheathing - pretty much a no brainer when you are interested in energy efficiency. Then you have to decide about air sealing each individual stud bay (which is being done more and more) and then what about a mechanical chase or an air tight layer after that - the air tight drywall approach or a smart membrane below the drywall. I did 3 layers of air sealing on the house we are (STILL) building. I did not do the airtight drywall approach because I knew I would be unable to get the drywall guys to understand the process. We used a smart vapor retarder - Intello Plus instead.

    If you have all three, you've got a very robust, long-lasting air tight system and the resistance to air flow within the insulation becomes less important. You have to decide how far you want to go with the airsealing. I did most of it myself on the house and my labor is free. See the photos below.

    I'm sure you can tell that the regulars here really like cellulose and/or roxul. That preference is based on more than resistance to air flow. Installing fiberglass batts correctly is VERY difficult and is one of the major reasons energy experts resist using fiberglass batts.

    Dense pack cellulose usually costs less than foam (and perhaps not much more than fiberglass), and is more resistant to fire (due the lack of oxygen available for a fire to spread), and helps more with sound attenuation.

    Roxul has similar challenges to fiberglass in that it is a batt insulation, but it is firmer and more easily cut and easier to install correctly.

  11. ddubx6 | | #11

    thanks for all the great info. I went out and bought the roxul. next question/issue... temps are subzero at night and barely 0 daytime, the addition was unheated and formed frost and ice in the empty stud bays. the heat is now on and the ice melted but the best I can get is damp, not dry, can I insulate and finish the the walls and let it dry out in spring or is this too risky and I need to put things on hold until warmer weather???

  12. Expert Member
    Dana Dorsett | | #12

    If you insulate the wall cavities the sheathing will run a lot colder, and soak up scads of water by the time you're able to get the gypsum up and air seal it. It's definitely safer to wait until warmer weather.

    It's safe to put up the exterior foam now though, and even a couple of inches would put the sheathing on the path to drying months earlier.

  13. user-1061844 | | #13

    Darren, you can put the mineral wool or fiberglass insulation in, even in cold weather. but you do have to add the smart vapor retarder, like the INTELLO that Lucy bought from us at 475 High Performance Building Supply" immediately after it is installed and airseal the overlaps with ProClima <a

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