Tacking Rockwool Outsulation to Sheathing Prior to Battens; glues vs. cap nails
Hi GBA folks,
I’m about to start applying 2 layers of 1.5″ Comfortboard 80 exterior insulation to the Zip sheathing of our house build. All sheathing seams have been liquid flashed, and we’re going for a tight envelope. I’ve seen many posts on GBA were builders are tacking the first layer of rockwool down with cap nails and the second layer with screws and large washers. Is there any compromise of the air/water Zip barrier using nails and screws? I was planning on 1/4″ to 1/2″ of penetration into the sheathing. In the name of air tightness, my builder is suggesting using construction adhesive or foam glue to make the temporary connection before 1×4 screwed battens hold everything down permanently. Has anyone here tried gluing Rockwool Comfortboard to sheathing?
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Replies
montlamothe,
Nails and screws simply don't affect the air-sealing of sheathing. That's why you see them used to attach WRBs, foam, furring and cladding on high performance builds.
The fasteners work by their outer face being tightly held by the materials they penetrate. If there was enough space around them to cause air leakage they would fail. 1/4" to 1/2" penetration won't work. Fasteners in sheathing should protrude at least 1/2" through the opposite face.
Remember that if the sheathing is your WRB, you need to run your flashing at penetrations back to it before applying the exterior insulation.
A related question though--while air sealing isn't an issue, what about extra thermal bridging? It would seem that temporary fasteners would introduce thermal bridging. Glue would solve the thermal bridging but there would need to be some sort of clamping until glue cured.
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pstephens,
You need so few fasteners for the exterior insulation it really isn't an issue. It's chasing insignificant gains, much like worrying about exterior door handles.
All that said, if you really wanted to glue it, something like PL Fast Grab wouldn't need clamping.
Thanks Malcolm,
Based on your response, I think we'll go with cap nails, and screws with large washers. Good to get a reality check on air tightness now and then.
To add to what Malcolm said, think about it like this: When you insert a nail or screw, it forces whatever material you drive it into out of the way, making a tight fit. The material compresses against the nail/screw circumferencially, forming a very tight seal all the way around. The material ends up acting like a gasket compressed around, and sealing to, the surface of the fastener. You won't have a leak. Even if you screw a screw into the bottom of a plastic bucket, you won't make the bucket leak water unless you crack the bucket and cause a complete failure.
As has been said on GBA in the past, in regards to sheathing used as an air barrier, "a hole with a nail in it is not a hole".
Bill
If you can swing the increased cost I think the first product in the video linked below is quite interesting. Attach these "pins" first and then stick on the insulation. If you put one on each stud then you have a clear line to attach the battens too and less of a chance of missing a stud with the ~6inch long screws.
https://www.youtube.com/watch?v=dFMnFjRhMKE&ab_channel=TRUFAST
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You could go with stainless steel screws which conduct only 1/3 the heat as carbon steel screws. Heco Topix is one brand name that comes to mind but they are pricey. There are fiberglass girt systems that hold exterior insulation in place as well and double as support for cladding like Armatherm. I think I saw them in an NS Builders video.