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Fastening methods for Roxul ComfortBoard IS

Patrick Walshe | Posted in Green Products and Materials on

I was wondering if anyone has field tested Albert Rooks’ suggestion of using full thread (or broken thread) screws to get consistent screw compression on the strapping and prevent wavy siding. That sounds way simpler than cutting holes in Roxul / inserting sections of plastic pipe. One builder did not know about these options and ripped all the roxul off a side due to this issue. His Comfort Board was 1.5 inches thick I believe. I would like to put untreated strapping over 2″ comfort board then fiber cement siding.

I understand from previous articles that squash blocks are not necessary, but the research focused on deflection due to siding weight rather than the issue of inconsistent compression with fasteners. What are installers’s experience and advice?



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  1. Albert Rooks | | #1


    I know you'd like to hear an independent opinion rather than mine...

    We continue to learn more about the Mineral Wool application:

    < I brought in the nicest full thread screw I could find. It had a sharp tip and even a variable pitch at the head. Sadly it was a waste of money. The ONLY way I am successful with this application is when the tip of the screw has entered the wall before the upper thread has reached the batten. Then the two distances are fixed and can't change their relationship unless either the tip or the thread at the head, break loose.

    < I've got fasteners that work well (in my tests at the shop) for 1 & 1.5" mineral wool in stock. I can order fasteners for 2".

    < The range of 4" & 5" Mineral wool are done by the heavier Heco Therm. They really work well in the shop. We thought we had 6" Mineral wool covered in application, but I think we've found that it is just outside of the thickness range that works (SADLY!). We are working with a builder in the east that is just testing it for 6" mineral wool.

    We made a short video on how and why it works here:

    Note that the MW that we use is not comfort board but is the less expensive RTH60 which has less binders and compresses far easier that comfort board, The RTH 60 would normally be impossible to use in this application. It works only because the distance between the wood wall and the wood batten becomes fixed. Without that, I'd be throwing away lots of mineral wool too.

    Of course... You can always just switch to Expanded Cork Insulation. It has NONE of those application issues. See the very last video on our 2" cork page:

    Best of luck!

  2. Lucas Durand - 7A | | #2

    I like your video.
    Less than a 3/16" variation doesn't sound so bad at all...

    Have you ever tried pre-drilling holes in the battens so that the thread won't catch the batten but the head will?

  3. Lucas Durand - 7A | | #3

    I guess if you drilled a pilot hole, the batten could then be pushed inward against the MW.

  4. Lucas Durand - 7A | | #4

    Just "thinking out loud" here...
    What about something like this?

    You'd have to rip double the number of battens from 3/8" plywood to keep with a 3/4" vent space...
    But having the second batten secured over the first would keep both battens from pushing in on the mineral wool...
    Maybe not so practical, but maybe doable using "off the shelf" hardware...

  5. Mike Eliason | | #5

    or just go the euro way - battens and counter battens? then you can get a nice vertically oriented rainscreen to boot...

  6. Patrick Walshe | | #6

    Thanks for the ideas. Albert, are the 2" screws with the gap in the thread Heco Therm as well? I will take a look and see if I can find some on Vancouver Island. I have not found a Canadian supplier of cork insulation yet.

  7. Albert Rooks | | #7


    My thoughts on: "or just go the euro way - battens and counter battens? then you can get a nice vertically oriented rainscreen to boot..."

    As you will see when you make your mockup, the fact that you place a horizontal counter batten perpendicular and on top of a vertical batten does not "true up" the original vertical batten. That's the issue.

    In all normal screw applications the idea is that the tip is pulling the head into the application. When you have a squishy insulation in-between, the only way to know that the head is truly 2" away from the substrate is to count rotations.

    As your firm: Brute Force Collaborative has requested material to do a mock up on 4" MW, you will get a chance to see what we've been wrestling with. Try one batten with a typical US fastener with a standard thread pattern and the try a HECO THERM. The standard thread will cost HOURS of the builders time having to back the screws in and out to get the siding plane true. - That's why builders have been creatively making "squash blocks".

  8. Albert Rooks | | #8


    The fastener for 2" MW is one that I have to special order from Germany. I's not a stock item in North America. It's no problem ordering it, I just need a little bit of lead time.

    I saw your request for a quote on Cork. Thanks! Mark in our office will get back with you. We are just down the way in Olympia WA. Cork is pretty easy to get up to you.

    I might have to deliver it my self. I know Qualicum Beach and have stayed at the Inn. Any good reason to return is welcome...

  9. Albert Rooks | | #9


    What can I say? There's nothing like a smart guy with an "art department" I'm jealous.

    You've obviously got the theory down. Yes the 6" application is just 3/8" from working prpoerly.

    I seem to be dedicated to developing the fasteners for the application so that as an industry, we have alternatives that are "Less Foamy".

    -I've gotten quotes on a thread modification for the HECO 240mm that would take care of the 6" application.

    -Ive sent drawing to a US manufacturer to make a full range of the fasteners here in the US. These would be in Stainless Steel which would also dramatically reduce what I bet is a pretty good thermal bridge per screw.

    I'm still moving slowly since the inventory dollars are precious and we still only get a small amount of inquires.

    The issue that remains is that these fasteners are expensive and MW is still a real pain to work with. By the time you add the cost of MW + Fasteners + the "itchiness" factor of the application, for me, our Expanded Cork Insulation Board begins to look more attractive.

    We'll make it all work better eventually. Hooray for more insulation on the outside of the building!

  10. Lucy Foxworth | | #10


    Do you have to stagger the seems of mineral wool like you do exterior foam? Do you foam the gaps where one sheet abuts another? And how to you make the stuff stick on the wall before you apply the battens?

    We've got such a heavy insect population here that I would prefer to use mineral wool over poyiso for exterior insulation, but it does sound somewhat challenging.

    Thank you. Lucy
    Upstate SC

  11. Albert Rooks | | #11

    Do you have to stagger the seems of mineral wool like you do exterior foam? Do you foam the gaps where one sheet abuts another? And how to you make the stuff stick on the wall before you apply the battens?"


    My opinion is that since MW is so permeable and does not shrink with age, that you can get a pretty good joint by just butt joining a single layer. If water in any form sneaks past the joint, it can still dry out as it conditions change.

    That said, a double layer of staggard seams would be better. I think they are both OK and that they are a different application than foam. Foam shrinks and can let water through the widening gaps and then perhaps wet the sheathing which can create a problem.

    I like MW over a really good membrane that is applied over taped sheathing. Anything outside of the membrane (in this case the MW) can wet and dry as exterior conditions rise and fall. In my mind it takes a well ventilated rainscreen to maintain venting paths.

    To hang the MW we stock the thermal washer screws in the pic below. If you were to layer 2 @ 1.5' layers you could just use a box nail for the first layer into the framing and then the thermal screws on the next later up.

    You can find the screws here:

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