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Community and Q&A

Rock wool vs Wood fiber

ethant | Posted in Green Building Techniques on

I have what I hope is a fairly basic apples to oranges comparison to throw out to the community here.

All else being equal, in a Cross Laminated Timber (CLT) wall with about 8″ of exterior insulation under a rainscreen assembly (see attached detail sketch), what might the relative benefits and drawbacks of rigid rock wool (Rockboard 80) insulation and rigid wood fiber insulation (Homatherm or Gutex).

Rockboard 80 data sheet here:

Homatherm HDP data sheet here:

Get ready to put on your imperial/metric conversion thinking caps!

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  1. Expert Member
    Dana Dorsett | | #1

    The rock wool would be higher R and higher permeance, but lower thermal mass and a PITA to build at 8" of insulation thickness- the stuff is still compressible, and not structural, requiring a bit more analysis & design to be able to support the weight of the siding with that much moment-arm

    From both a design & assembly point of view the fiberboard is hands-down the easier build!

  2. ethant | | #2

    Thanks! So we'll just use the Rockwool subgrade at foundation walls and wood fiber above grade at CLT walls. There is a minor differential between 8" of Rockwool and 200mm of wood fiber but perhaps that can be taken care of where necessary with the compression you describe.

  3. ethant | | #3

    Dana... I guess I should ask... Do your 'PIA' comments apply to below grade application of rock wool as well? To get 8" I'd plan to install two layers of 4". Generally this would be held mechanically by backfill, but I'm not sure how to hold it in place. Roxuls technical manuals are all being upgraded and none shoe 2 layers below grade. Could an adhesive work in this instance?

  4. Expert Member
    Dana Dorsett | | #4

    I'm still somewhat skeptical that below-grade rock wool will go the distance over several decades. My understanding is that in cooler climates freeze/thaw cycling inevitably compresses it. I have no idea how much of a hit 8" of below grade rock wool would take over time, or whether the layers can be bonded with adhesives. That's worth a call to the manufacturer if committed to the approach. It might be better to use something less compressible such as aerated autoclaved concrete blocks for the below grade portion.

  5. ethant | | #5

    Dana thank you. So now I have your freeze thaw concerns to weigh against extensive termite/carpenter ant concerns for exterior foam (esp as the climate warms). An obvious solution is to bring the insulation inside, but this messes with a simple wall connection detail. Another solution is a thermomass solution... but that's expensive.

    Here's my thoughts for a "poor-man's" Thermomass... basically [Concrete] + [EPX/XPS] + [Cement Board]. I wonder if this would need to be mechanically fastened or perhaps it could be glued. Then if it was sealed at the bottom to the footing, perhaps this would create a workable barrier to future termite/ant infestations.

  6. user-2310254 | | #6

    It is too bad that Corning FoamGlass is no longer available in the US market.

  7. ethant | | #7

    But wait! I thought Foamglas had documented freeze/thaw issues. I've never heard of the Rockwool freeze/thaw issues that Dana discussed above.

  8. user-1137156 | | #8

    Pressure treated wood! Wood foundation graded 3/4" plywood over 2x8 staggered stud wall on 2x10 plates 2" foam 7 1/4" mineral wool air sealed inside or use all foam and buy it as a SIP from Canada. Presto an r38 basement! No termite issues, no freeze thaw issues it'll outlive all of us!

  9. ethant | | #9

    Jerry, amazingly enough, I've actually considered wood foundations. This house is cut into a 10 to 15% slope, which gives me pause. I'm just not convinced that the wood foundations are a good idea. Have you seen them used on a slope? Do they still go down below the frost line? Can they tie into a concrete slab? Have you considered going whole hog and eliminating the slab too and doing a PT slab as discussed in JLC recently?

  10. KeithH | | #10


    What's the cost comparison look like? I'd expect the woodfiber, at least until it is manufactured state-side, to be radically more expensive than rockwool board.

    Did you consider another option? Larsen trusses? I've never done them but larsen trusses filled with commodity comfortbatt (off the shelf availability at low price premium at big boxes, at least in Colorado). Avoids the squishy problem, avoids the wood fiber insulation shipped from Europe, allows custom depth, etc. I know there are people out there trying comfortbatt in larsens. If you can do blown cellulose, I don't see why batts would be a problem.

    Where I have used roxul comfortboard as exterior insulation, getting a tight install without differential squish has been tough. I can't say that I'd want to try that with 8" of insulation. It doesn't compress much (i.e. sizing is tough and cuts need to be precise, unlike comfortbatt) but once you screw it down, it sure does compress.

  11. ethant | | #11

    I have taken some time to dig through the various estimates I've gotten and have also taken one extra estimate from an online insulation re-seller for the cavityrock, which may account for its high price.

    For this exercise, I've tried to use what I think of as the gold standards of metrics for insulation cost comparisons, which is $/R/SF.

    Now, what I am presenting here is material cost, not delivered, and not installed. The big difference would be that the wood fiber would go up in one pass, and not have the compression issues faced in installing two layers of rockwool.

    Others I know, such as the Woodstock Passivehouse ( have indeed faced issues with multiple layers of rockwool, as Dana points out in comment #1, and I believe had to create bands of 2x lumber to support the outer layer of rockwool.

    A big remaining concern of mine is termites and wood fiber insulation. It seems like it is an ideal termite food - perhaps this isn't an issue in Europe because they don't have termites (yet)? Anyone care to fan the flames of my termite paranoia as I embark on an all-wood assembly?

    475 has a discussion of this at which states

    Because of GUTEX’s vapor open properties (>40perms/in.), the material by design won’t get sufficiently damp so as to encourage nesting. Its dryness makes it inhospitable to pests..

    Furthermore, GUTEX wood fiberboard is not a food source for pests. As described by GUTEX below:

    “During the production process, the product is heated above 170°C (338°F). Such high temperatures dissolve the natural protein in the wood. Therefore the naturally occurring protein is no longer available as a nutrient for insects, etc. GUTEX has not had any complaints from customers concerning wood pests or rodents settling in our products.”

    This is maybe 33% reassuring...

    I was also able to dig up this abstract of an article which states that with paraffin impregnation (as the woodfiber insulation boards have):

    Termite durability improved from level 4 to level 3 of attack, and higher termite mortality was found in treated wood (52 % against 17 %).

    I have no idea what level 3 and level 4 of termite durability refer to... perhaps someone here knows?

    1. AndyBower | | #26

      Based on the paper you referenced, and this one:

      It appears that Levels 0 through 4 refer to a visual rating in a termite attack test according to EN 117, which is some kind of standard that you must purchase to view. That said, in the above paper, the ratings are:

      0 - no attack
      1 - attempted attack
      2 - slight attack
      3 - average attack
      4 - strong attack

      This suggests that the paraffin treatment is modestly effective, but not foolproof.

  12. user-1137156 | | #12

    PWF foundations are quite routinely used to make "walk out" basements on sloping lots. The PWF literature gives guidance on shear wall requirements and optionally "keying" the slab to the walls There are more limitations on backfill height than with concrete but 10 feet or more can be accommodated with some engineering. Wood foundations can be built with or without concrete footings. Granular (rock sand pea gravel etc) footings are an option. The granular material is considered part of the footing and must go below "frost depth" Personally I'm not a "fan of the no concrete slab options. I'm using a well insulated, with Perlite, concrete slab which will be polished and serve as thermal mass..

  13. ethant | | #13

    Jerry, thanks for explaining... Where can one learn more about PWF? Will my engineer and/or building inspector get behind this? How are you connecting your slab to the PWF?

  14. user-1137156 | | #14

    The WWW is your friend! Start with: log in get a free account and download the design guide It's pretty complete but there are other publications some of the Best are from Canada. I'll use an abundance of nails 16 penny stainless driven about 1" into the studs to " key the slab. My slab is raised from the bottom of my PWF walls by under slab insulation (happens to be Perlite left in the bags) I have a 7 1/4" strip of 1/2" PWF plywood on the inside bottom of my walls. The top of the plywood is the top of the slab, a screed of sorts. Before pouring the slab I will have installed the bottom course of mineral wool leaving the suggested 2" gap at the bottom of the stud cavities. Once the slab is poured I'll add another strip of 1/2" PWF plywood so that an 8 foot sheet of untreated plywood can complete the inner air barrier after the rest of the mineral wool is installed. FWIW my floor system is I joist and they are supported by a 2x4 "ledger" nailed to the PWF studs the ledger is also supported by additional flat wise 2x4 studs to a flat wise PT bottom plate that sits on the slab but is also nailed to the studs this gives a 1 1/2" deep "service cavity inside the air barrier and avoids the expense and thermal ridging of metal hangers. The ledger is positioned so that the top of my sub floor is flush with the second top plate of the PWF which I couldn't do with the hangers. I'm getting excited we close on the lot Friday. As for your engineer hopefully yes, as for the building inspector, he's obliged to live by the code which accepts PWF! I'll soon know at least her in Kentucky.

  15. ethant | | #15

    Keith, I just posted my non-scientific, not to be relied on for anything rockwool/woodfiber cost comparison findings here:

    I found that rockwool and wood fiber are, perhaps, comparable in cost - certainly not orders of magnitude different or anything. They are certainly more in line than reading posts here at GBA would lead you to believe.

    My dense pack numbers are installed costs, but my rockwool and wood fiber costs are material costs, which throws off the numbers a bit - but it doesn't change my findings. It looks to me like a 2x6 framed wall with dense pack cellulose + 5.5" woodfiber (or any thickness woodfiber, really) is a pretty cost competitive wall. I'm not considering exterior foam.

    And by anecdotal evidence, esp. since the wood fiber is tongue in groove and self-supporting (and not itchy) I'm guessing the wood fiber install will be easier/cheaper. I'm open to being dispelled of this assumption- I have heard the wood fiber is more crumbly than desirable, and maybe it is harder to cut/shape than rockwool?

  16. mdb_az | | #16

    HI Ethan, how did you come up with your numbers for rock wool vs wood fiber costs? it doesn't jive with any of the research i've done...for 3" wood fiber @ $3.97/sq ft and 3" cavity rock mineral wool @ $1.83/sq ft. That puts mineral wool at less than half the cost of fiber board.

  17. ethant | | #17

    USER-6878222, you're right! I think I have had some erroneous cost data... but I think wood fiber may save on labor/installation costs...

  18. user-7022518 | | #18


    This passive house used Gutex as exterior insulation and in lieu of sheathing: I'm not sure what the shear strength of Gutex is. In terms of working out a price comparison, eliminating the need for external sheathing seems to matter. Lisa

    1. Expert Member
      Dana Dorsett | | #19

      Lisa- you realize the last post is nearly a year old, right?

      Like almost all other low & mid-density thick fiberboard, Gutex cannot be used as structural sheathing. Note that the truss walls in the video have galvanized steel T-bracing let-in on the diagonals for handling shear loads. Go to 3:14 for a peek at the bracing. Also 5:10-5:15, 6:20

  19. user-7022518 | | #20

    Yes I do! I was hoping to revive the discussion :) I saw the t-bracing for the shear walls but wondered if Gutex was a replacement for plywood or OSB sheathing and you answered my question. There is a similar product called Steico that is being used in some passive homes locally via a company called Blueprint Robotics. They are using it on the exterior of Zip so I was trying to understand the difference between the two wall systems. I also wondered if Ethan learned more about termites and wood fiber. I don't understand why we aren't making this wood fiber insulation in the US and it has to be shipped from Europe... You are very on top of things Dana! Lisa

  20. Expert Member
    Dana Dorsett | | #21

    If it's on the exterior of ZIP (which IS structural sheathing) they don't have to be concerned about the structural capacity of Steico.

    The only comparable product manufactured in N.America I'm aware of is a mid-density fiberboard sheathing called SONOclimat eco4, which is 1.5" thick , runs about R4 and has at least some structural capacity to design with if the fastener spacing and specs are adhered to. Most applications it will need additional structure other than the fiberboard:

    It retails for about CDN$30-34 for a 4x8 sheet where available, which is NOWHERE in the US. CANAC distributes it in Quebec, with many locations within comfortable drive times from the US border in New England. I'm sure there are other distributors handling it in Canada as well. Don't confuse it with SONOclimat R4, which is a foam + fiberboard product (thinner and cheaper.)

  21. Bibbedy | | #22

    So I found this thread when researching for the Termite /Insect resistance of Wood Fiber Insulation panels. Funny, few of the manufacturers seem to mention it! Anyhow, I wanted to add something to the mention by Dana of the SONOclimat eco4 product which is one I looked at and discounted. For me there are a couple of BIG question marks with this product...

    Firstly it is marketed as " The most environmentally friendly insulation panel in Canada " and that's the kind of wide-sweeping (potentially "Greenwashing") statement I always dig deeper into because I'm a sceptical ex Marketing person, so I know how it works - emphasise the positive, conveniently ignore the potentially negative.

    What I discovered is that the composition of SONOclimat eco4 includes a worrying amount of
    Post-consumer recycled content at: 46 %

    The problematic part for me is that they claim a VOC content and Urea Formaldehyde content of: 0 g / L - which may mean ADDED VOC and Urea Formaldehyde content - but as nearly 50% is Post-consumer recycled material - THIS could contain a high level of goodness knows what! (Post consumer being old doors with lead paint on, old kitchen cabinets FULL of Formaldehyde, partially rotten old building timber full of mold spores and the remnants of pressure treatment from back in the day (think noxious chemicals) - basically any old wood type garbage and assorted coatings, embedded fittings, plastics, heavy metals, etc etc etc.

    How do I know? - Because I used to oversee the recycling and production and testing of this type of waste for producing new MDF panels to try and check that the hazardous content stayed beneath acceptable guidelines - and we controlled this by adding other virgin wood fiber to the mix - NOT by removing the cr@p as this was almost impossible to do..

    Also, the declared content on the LEED product sheet only accounts for 80% of the total content - and is at 77% total recycled. Hmmm What accounts for the other 20% of the product I wondered, so I dug deeper...

    What I found is "Our sheathing products are composed by
    interlocking cellulose fibres bonded together
    by heat, pressure and a superior wax

    My other problem being the "Wax Emulsion" part... Wax as in Beeswax? - I doubt it - or as in Paraffin Wax (yeuk) - and "Emulsion" as in Polyurethane emulsion? Plastic again??

    I was sceptical as i said, so I've called & emailed MSL with my direct questions regarding the "missing" (potentially question mark) 20% content, and the testing of the 47% post consumer recycled "waste" for toxins etc - as well as the possible inclusion of flame retardents like Ammonium Polyphosphate... For the moment, they haven't replied (2 weeks now - draw your own conclusions) - but if they do I will update this thread...

    I still haven't decided which route to take on insulation, as I've realised that for certain parts of my house that I want to last at least 100 years to mitigate it's carbon footprint, maybe intrinsically inert recyclable plastic type materials used intelligently might just be the best overall solution. EPS for example is 95% air and only around 5% styrene and is significantly cheaper so I can afford to insulate more for less, and is locally produced.

    1. KauaiBound | | #23

      I am also concerned about termite-resistance of Gutex/wood fiber insulation. And other than a comment above from 2017, still can't find any information about the issue.

  22. Expert Member
    Michael Maines | | #24

    Kauai, I have researched wood fiber insulation a bit and as far as I know there are no insecticides in the rigid sheets. Blown wood fiber, comparable to cellulose insulation, uses the same borate treatment as cellulose to protect against fire, which also happens to deter insects, mold and fungus. Insects may not like the paraffin and other ingredients in wood fiber boards but I would not consider them insect resistant. I don't know of any insect resistant exterior insulation besides polyiso with foil facing. Insects won't eat foam but they love to burrow in it.

  23. TandT | | #25

    Glad this thread is still going because I have an exciting addition (exciting to me anyway). A company in Maine, Golab, has begun the process of manufacturing wood fiber insulation in Maine (first in the U.S. as I am told). They are slated to have blown in as soon as January 2021. I haven't been able to get any info on exterior insulation as of yet, but this could drive the prices down towards that of fiberglass as it is in Europe.

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