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Smart ways to fasten mineral wool and spend insulation budget

Ryan Mount | Posted in Green Products and Materials on

Hi All, 

I have been trying to figure out the best way to spend my insulation budget on a new home I’ll be building this spring.  I’m in climate zone 6A. It’s a 2 storey single.  

For the enveloppe it will have 2×6 walls with batts, Blueskin vp100 as the WRB/air barrier, then either 1.5 or 2” or mineral wool followed by strapping and James Hardie lap siding.  

When pricing out materials for the outsulation, it seems to jump from 1.5” to 2” mineral wool I would need to upgrade fasteners for the strapping from a fairly inexpensize (20 cents) 4” construction screw to something more like a 4.5” or 5” headlock screw which goes for about 80 cents each.  Might not seem like much but it adds up to over $1000 for the entire project. Ironically the 2” roxul is actually cheaper around here than the 1.5”. Has anyone done any analysis to see if this $1000 would be better spent on something like blowing in another r30 or more into the attic instead of adding r2 to the walls?  $1000 can buy a lot of cellulose. I do get there are limited returns after you reach a certain point. 

Any thoughts or suggestions on this setup would be much appreciated! 

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Replies

  1. Walter Ahlgrim | | #1

    There are a lot of variables involved in the answer to your question of how much insulation will save enough energy to pay for itself.

    The only way I know to have a real answer is to model your house in a computer program call BEopt watching the training videos and learning to use the program is a 40 hours commitment. The program is funded with your tax dollars so it will not cost you any money. https://beopt.nrel.gov/home

    The program considers
    1 your local weather
    2 The size and shape of your house.
    3 What direction you house faces.
    4 The sizes of your window and door and what direction they face.
    5 The amount of insulation on you wall and its cost per sqf.
    6 Your heat source and current fuel costs.
    7 The interest rate you will pay to finance the home and your prediction of future inflation of fuel costs.

    Since no two homes will have the same answers to these questions there is no one correct R value.

    The biggest factor is how long will you live in this house. Few people have a home built with a plan to sell it in a few years but life happens and plans change.

    Walta

    PS If this post seems familiar it is because I copied my of words from a post I made to another thread.

  2. Expert Member
    Akos | | #2

    Exterior rigid mineral wool is an expensive option. About the only reason I would go for it is if the place in a wild fire area and are looking to build non-combustible. Even then, going with semi rigid mineral wool batts between horizontal strapping is way cheaper. When it comes to continuous exterior insulation, rigid insulation (EPS or polyiso) is a much better deal and way easier install.

    Your assembly R values are:
    -2x6 with 1.5" mineral wool R25
    -2x6 with 2" mineral wool R27

    My guess the 8% decrease in heat loss will take near forever to recover the $1k. Extra attic insulation above code min, saves very little energy again. You are probably better off on spending the extra on better windows or better mechanicals.

    P.S. If you are looking for cheaper assembly, a 2x8 wall 24"OC with R30 batts is around R24. Much simpler build than dealing with exterior mineral wool.

  3. Jan Fillinger | | #3

    You can get screws for cheaper, for example scroll to the bottom of this page: https://www.bestmaterials.com/roofing_screws.aspx These are not the best but they are cheap. HeadLok screws are also not the best because they don't self-drill and must be pounded in to get started.

    Here's a new option, a copy of GRK structural screws, probably made in Taiwan: https://www.homedepot.com/p/BIG-TIMBER-14-x-5-in-Star-Drive-Round-Washer-Head-Construction-Lag-Screws-500-Pack-CTX145/311335253

  4. Jamie B | | #4

    My current build I did 2" of rockwool comfortboard 80.

    My personal opinion: headlok screws are WAY overkill. way beyond what I do, and overkill is my middle name!

    I'm not a structural engineer, so don't take me seriously. At the time, I got sheer and bend strength data from grk on their #10 screws. I modelled how many screws I needed, and then found multiple fastener suppliers, got samples, and conducted a test. I found my local hardware store "noname" #10 - 5" deck screws had a similar bend and shear performance to grks and they were $0.10 Canadian per screw. I used them and the house hasn't collapsed yet.

    However, I'll give some warnings about the process though. Marking and finding the the studs with the screws is a tedious process. Next build I would splurge on 3/4" sheathing so I can screw into that instead of the studs. (Get that ok'd with the structural first). Also, make sure to predrill your strapping and string horizonal lines to avoid overcompressing the rockwool and consider buying comfortboard 110 instead to help that.

  5. Ryan Mount | | #5

    Thank you all for your responses.

    Walter yes there are definitely many variables at play here. I wish I had the time and patience for learning a new software but that's not in my cards anytime soon unfortunately.

    Akos thanks for the envelope suggestions. I am thinking mineral wool would be best in the Canadian climate where I am as it is much more permeable to deal with our humid summers and very cold at times winters. I don't think 1-2" of EPS foam on the outside would meet the criteria for dew point on the sheathing.

    Jan and Jamie: Thank you both for your fastener suggestions and sharing your experience with this setup. I agree the Headlok is definitely overkill. I will check some local spots to see if there are better deals to be had on the fasteners and if not that website had very good pricing for sure. I am dreading trying to find all the studs as well, but upgrading to 3/4" sheathing would cost several thousand extra so I think I will have to deal with it. I wonder if there's a stud finder out there that could work over mineral wool...

    1. Expert Member
      Akos | | #6

      Ryan,

      I'm a just a bit south of you, here it is code (common) to go with 2x6 walls with R5 rigid insulation. With an interior vapour barrier, it works just fine. You can also read here about it here:

      https://www.buildingscience.com/documents/building-science-insights-newsletters/bsi-026-they-all-laughed

      If you are worried about drying, you can always go with one of the permeable rigid insulation products (unfaced EPS or polyiso) which would provide some outward drying. Either one will allow for some drying towards the exterior and would provide more R value for WAY less money than rigid mineral wool panels.

      Having worked with it, I would recommend you try it out before committing. It is a lot of work to get a flat surface for the siding as it is somewhat squishy, you end up having to monkey a lot with the screws to get it flat.

      In terms of budget long screws, try a roofing supply. Roofing deck screws come in many lengths and with decent load rating.

  6. Expert Member
    Rick Evans | | #7

    Ryan,

    Just a couple of thoughts...

    GRK makes a #10 4 3/4" fastener that works well with 1x3 furring strips. This would be perfect for 2" exterior insulation as it would give you 1 1/2" of penetration into the stud. They aren't too expensive...

    Jamie's comment above regarding comfoboard is one that I have read from others: You have to loosen and tighten the furring strips to get a level surface for siding. Sounds like a lot of work! You could consider Foil Faced Polyiso instead of Roxul boards. This would get you around R-11 or more which gets you above dew point in Zone 6a.

    2" Thermax is probably the same price as mineral wool boards but would get you R-13 instead of R-8 and do so without a cold weather r-value degradation. The air gap over the foil face offers another R-1 or so. Polyiso is a significantly "greener" product than mineral wool boards too.

    Finally, I would consider opting for Zip sheathing over a peel and stick WRB. With Zip, you can see the fasteners or tape so lining up the furring strips might be easier. This might save you a few bucks as well.

    1. Ryan Mount | | #9

      Hi Rick,

      Thanks for the advice. I will check out the GRK #10s.

      As for the polyiso, I'm not sure I fully understand how it would work which is why I am leaning towards mineral wool. If I use polyiso, is it best to have sandwiched between sheathing and foam, or on top of foam? I know there are different trains of thought on this. If sandwiched, I can't imagine there is much room if any bulk water if it needs to run down. Couldn't it end up pooling somewhere? If it's on top and you put thousands of holes in it for the siding with the soft foam backing behind it there could potentially be a lot of spots for water to get behind. With the mineral wool it would be fairly easy for the bulk water to either flow down or migrate outwards, and the WRB will have a more solid surface to be attached to (sheathing) without all the fasteners from the siding going through.

      As far as a I know the Zip products are very difficult to get here in Ottawa.

      1. Expert Member
        Rick Evans | | #12

        Hi Ryan,

        If it were me, I would build a 2x6 wall and sheath it with plywood. I would then add Henry Blueskin or any other quality WRB. I would then add the sheets of foil-faced Polyiso (thermal) over the WRB and tape the seams with tuck tape. This creates a drainage plain for water. 1x3 strapping would go over the Polyiso and really hold it to the wall as well as your siding. The siding nails will mainly penetrate the strapping not the Polyiso. Depending on the length of your siding nails, the points of the nails will go through the strapping and penetrate the Polyiso. But I wouldn't worry about that too much. (Water intrusion would be minimum and Polyiso absorbs less water than I think people think. )

  7. BrianVarick | | #8

    I was looking for an alternative to the Headlok screws for a while until I found the big 250 piece buckets on eBay. They come to a little less than .37 cents when you buy a couple buckets for $92 each. Even better if you can buy during one of ebays coupon events!

  8. Alex P | | #10

    For anyone in Canada right now, check the online clearance section of your local RONA. A few stores near me are clearing stock and I picked up a variety of GRK's at 70-90% off. 300x5" 1/8 5/16 for 80 bucks, 270 4' #10 for 22.

    Malcolm Taylor lhas linked this guide previously which I found helpful for guidance on specs for fastening exterior insulation.
    https://www.victoria.ca/assets/Departments/Planning~Development/Permits~Inspections/Example~Plans/Illustrated-Guide-R22-Effective-Walls-In-Wood-Frame-Construction.pdf

  9. Eric Whetzel | | #11

    Ryan, it sounds like you'll be building the home yourself, so it may be worth your time to build a mock wall assembly before you get much further along in your design plans. Even if someone else will be doing the framing and air sealing, it's arguably still worth doing.

    If you build a relatively small wall section (5'-8' wide x 4-6' tall), you can experiment with attaching the various insulation options with furring strips to see which one you think is best for you. If you include a window opening and a penetration through the sheathing with PVC pipe, it will also give you an opportunity to practice air sealing details, allowing you to see before construction begins if there are any areas that might give you problems. Much better to find the trouble spots prior to construction before you're doing everything for real and the pressure is really on. An added benefit is that, once completed, you can show your subs what you expect in terms of penetrations through the sheathing.

    Have you decided yet whether you intend to air/water seal with tapes or liquid membranes?

    We decided to use Rockwool on our project even with the added cost. It's true, the Comfortboard 80 is easy to compress against your sheathing if you're not careful. While not as pronounced with rigid foam, it's still something you'll have to look out for.

    When I spoke with a Fastenmaster engineer about the Headlok screws he told me a full 2" embedded in the framing members is required. Previously I had always read and heard 1.5". We went with 2" to be safe.

    You'll want to get written confirmation from Fastenmaster on what's required for your project based on your construction drawings (in particular your choice of cladding because of the weight), even if your local building department doesn't require it (ours did). If nothing else, it'll give you some peace of mind.

    The Headlok screws are definitely self-drilling, so I'm wondering if Jan is thinking of a different brand of screw (?). As Brian points out, buckets of the screws are available online. We bought ours on Amazon without any issues. They're great screws, and we found them easy to work with.

    Another option is Heco-Topix screws, available from http://www.smallplanetsupply.us
    They're designed to reduce the chances of over compressing the insulation. At the time of our build they were substantially more expensive than the Headlok screws. Maybe this has changed, or the added cost might be worth it to you?

    You can read about the details of our Rockwool install and air/water sealing details here:
    https://kimchiandkraut.net/2019/07/11/siding-1-of-2-continuous-insulation-with-a-rainscreen/

    Again, I can't recommend doing a mock wall assembly enough. It can really help you figure out how to bring all of your wall assembly details together, and raise potential red flags you can deal with before construction begins.

    Good luck with your project!

  10. Ryan Mount | | #13

    Thank you all for your thoughtful comments.

    For fasteners I was able to find packs of 800 of the GRK #10 4 3/4 screws that Rick had mentioned above for a great price. This essentially makes fasteners a non-issue.

    Thanks for the recommendation to build the mock up Eric. I may very well give that a try since there will be a number of new products for me. Yes I will try to build as much of the house as I can myself. I used comfortboard on my current house but had the siding installer install it so I am somewhat familiar with it, but haven't had to deal with all its intricacies. I am intending to use many of the Henry line of products for tapes, vp100 for the WRB, and their sealants although I might also try Soprema products.

    I will do some research into polyiso as that is what my architect had also suggested.

  11. AlexPoi | | #14

    Another option is to use I-Joist as larssen trusses as the europeans do and top it off with a 3 ply membrane like Siga Majvest or Proclima Solitex. You can fill the joist bays with cellulose. Cellulose is so cheap compare to rockwool comfort board and it looks easier to build in my opinion. You'll end up with a much thicker wall though.

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