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We install vented rainscreens on buildings to promote drainage and drying, between the wall sheathing/water resistive barrier (WRB) and the siding. There’s some debate on the optimal gap size of the rainscreen, but 3/8 in. (about 10mm) is what many building scientists recommend. Often the material used to create this gap is strips of plywood, wood lath, or a plastic product (such as Cor-A-Vent). When this furring is being installed right over the sheathing and WRB, it can easily be nailed or screwed into framing. When combining a vented rainscreen with continuous exterior insulation (CEI) we may need a more rigid material to create the gap, because it is not only there to create the rainscreen gap but also to hold the CEI on the wall. And it’s the fastening point for whatever siding or cladding you are using. In this case, many builders choose a structural screw for fastening.
This is where things can get tricky. Depending on the thickness and density of the exterior insulation you’re using, there may be challenges in maintaining flatness across the wall. Attaching a 1x product through thick layers of insulation can result in “ins and outs” that require adjusting. Some insulation products are more susceptible to this condition than others; rigid mineral wool insulation, which I used on a recent project, is a good example. With some trial and error, I’ve settled on a fastening system and procedure that works well. I’ll share that here.
Fasteners for rainscreen furring strips: What the code says
What are our fastener options for securing 1x boards…
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22 Comments
Thanks Randy for the timely article--I'm currently working through the final design details for an rain screened wall.
The RDH PDF link is broken though!
edit: googled and found the correct link: https://www.rdh.com/wp-content/uploads/2017/11/CCBST-2017-Structural-Testing-Screws-through-Thick-Exterior-Insulation.pdf
Thank you for catching the link error, it has been corrected.
I’m also finalizing the details of our assembly, and the fasteners are definitely an important area—so this is great info. For what it’s worth, this document that led me down the fastener rabbit hole. https://www.nrel.gov/docs/fy14osti/62422.pdf
Our choice of cladding over CEI raised another concern that may impact others. You mentioned Table R703.15.2 above. As it turns out, one of the largest manufacturers of engineered wood siding uses that same terminology in their application instructions (not a surprise). When I pressed them to confirm whether mineral wool could be considered a substitute for foam plastic sheathing/rigid foam insulation per their application instructions, they explicitly stated that it would not. They said plastic sheathing/rigid foam insulation has a PSI of at least 25 and that this aspect of their instructions must be followed to maintain their warranty—otherwise, if the install becomes wavy, that condition would not be covered.
So, I asked them to approve the use of rigid foam insulation on the back of the furring. I would still use mineral wool board, but it would be cut and cobbled between the vertical furring. It's building yoga, but this seems like the only way we can maintain a thermal bridge and the higher perm rating of the assembly.
I recently posted a question about this on GBA that may be of interest to others.
https://www.greenbuildingadvisor.com/question/continuous-insulation-furring-and-engineered-wood-siding
An interesting solution, and you are right, extra work.
I saw your question pop up on the forum, I have a few friends who work for Rockwell. I sent them the link to your question. My suggestion would be to reach out to them, their R-class program website has the contact info for the area reps who may be able to help.
Awesome - thanks Randy. I’ll post what I find out.
One trick we have found useful in terms of shimming and planing out furring over Rockwool CB is to pre-drill. For example, if we are using a 4” fastener, a 1/2”-5/8” forstner bit works really well as countersink by pre-drilling to a depth just deep enough to set the head of the screw flush works really well. We have found that when furring isn’t pre-drilled, setting the screw head flush ends up drawing the furring in to deep and results in a lot of adjustment.
I am currently building a house with 3” exterior insulation using Rockwool Comfortboard 80 and debated for a long time how to get all the battens in plane, including the shimming screws. What has been working for me is a simple 6” piece of 12ga copper wire with piece of blue tape to mark the depth of the rockwool plus the depth of the batten. Looks like a tiny flag. After loosely applying a batten with Simpson SDWS screws, I then probe the wire though the Rockwool right next to the batten( adjacent to a screw) until it contacts the sheathing, the blue tape flag indicates how much farther the screw needs to be advanced. Lightly pulsing the impact driver it can be dialed in fairly precisely. Works within a 1/16 of an inch and the minuscule puncture made by the wire can be easily rubbed out.
1) At the beginning you say that 3/8" thickness furring is adequate, but then show that the code requires 1x (3/4"). Would you need an exception from the inspector to use 3/8"? The advantage of the smaller cavity would be that it would be less likely for fire to travel up behind the siding.
2) Do you have suggestions for locating where the studs are under the sheathing and CEI so that the screws don't miss?
3) What fasteners did you use for the rockwool? I'm assuming that these are only penetrating the sheathing so that the area for the battons isn't blocked.
Thanks
Adam,
I’m not sure I agree on the optimal depth of a rain-screen. Practically, going under 1/2" makes it more prone to blockage from either the WRB wrinkling, compression of the exterior insulation, or insects. This is what Joe Lstiburek said about the gap and fire risk: "When the air gap is small… ¾ inch or less…the friction from both surfaces bounding the air gap limits the air flow. The boundary layer on both surfaces is an effective fire stop."
The 3/8 rainscreen, or about 10mm is what I use as a rainscreen when not including continuous exterior insulation. I agree with Malcom, a little more is better. With CEI, the building codes require a minimum of 3/4" (20mm), mostly for cladding attachment support. If you are worried about wildfire, there may be some additional protection to put in place
This article from Fine HomeBuilding covers the recent codes and best practices when building in Wildland-Urban Interface (WUI).
https://www.greenbuildingadvisor.com/article/designing-fire-ready-resilient-houses
The piece touches on vented rainscreens.
Randy
Thanks for the responses and article!
TruFast thermal grip washers, and either TruFast grip deck or GRK r4 scews, penetrate about 1" into the stud. We mark stud location on our rockwool with sharpie as we do the externals, as we can see the penetration markings on the zip as we build over.
The X-Bracket system is engineered for holding furring strips over CI with no compression. The Framer can fasten the brackets with a framing nail gun or screws and install the house wrap with furring strips. If you want rockwool your Insulator will be happy to install blown in rockwool or any other type of blown-in insulation or spray foam. No need to worry about compression or marking studs through multiple layers of staggered insulation so you can hit the stud. When I worked as a framing contractor there was never a day I looked forward to working with itchy insulation. I think most Carpentry crews could frame out 2 more homes in less time than it takes to put on a multi layer exterior insulation with long structural screws. Making your framing carpenters do exterior CI is typically a big black hole in the budget and the build schedule. https://www.greenbuildingadvisor.com/article/introducing-the-insofast-x-bracket-system
Where can one purchase the Rothoblaas DRS Timber to Timber Spacer Screw in the US? I don't see a supplier.
I did see that there was regional sales contacts posted on their website. That's going to be my next step when I need to order. https://www.rothoblaas.com/contacts
I reached out to the east coast rep last week and didnt hear back
I have been using Fastenmaster Headlok screws spaced 16" o.c. through 1x3 strapping over 2" mineral wool. I am planning another project like this and was looking for actual specifications on fastener spacing in case the building department asks. And here it is in a code entry. I am shocked by the tight spacing required over foam board, nevermind MW which isn't listed. Building Science Corp has shown virtually no creeping down of fiber cement lap siding on 24" spaced strapping with 16" oc fasteners over Mineral wool. I just read this in another old blog here on GBA. There were studying it and making recommendations for the Deep Energy Retrofit Pilot program in MA around 2010. I've installed deck ledgers with headlok screws through 4" of foam with an engineer's calculations and approval.
So where did 6", 8", 12" spacing of fasteners come from in the code? Why would 6" nails[?] and 1/4" lags have the same capacity through strapping and 4" of foam? In other words, clearly the chart is simplified, but 1/4" lags should logically be safely spaced at 16" or 24" oc. if that spacing were in the chart
I also use a depth gauge made of bent insulation wire. I make it 2.5" for 2" MW with 1x3 strapping to compress the MW a little. Over compress then unscrew until the strap hits the gauge. However, if the wall is wonky, and old walls typically are, this is just a starting point with adjustments required after.
I know anecdotes don't meet code but the BSC studies are legit and I have seen no movement myself.
"I've installed deck ledgers with headlok screws through 4" of foam with an engineer's calculations and approval."
Using Headlocks thru 4" of foam for a deck ledger sounds like a bad idea.
As does it in some jurisdictions no matter what an engineer calculates.
I’m planning a 3” exterior rockwool install over 1/2” ply and blueskin vp100 - can someone recommend a screw (siding will likely be hardie board horizontal and furring strips 1x3) - would #10 4.5” deck screws be sufficient?
and for the roof, do I need put decking (zip) on top of the rockwool for mounting metal roofing or can we mount the metal roofing right onto the purlins if we need to add horizontal furring strips over the verticals? i’m guessing cost wise it’s similar but decking would be better (vent would be under decking)
thoughts?
With regard to fastener length-MW compresses a little so i have chosen to choose to compress it by 1/4” with the strapping. Then i recess the heads of screws about 1/8” into strapping. The math out to in is 5/8” plus 2 3/4” plus 1/2” plus 1-1 1/2” penetration into studs. So ideally 5” screws. I’ve used Fastenmaster Headlok screws, though there are other brands with a washer type head and some may not make that length. You can get regular 5” construction screws but the small heads overdrive too easily in my opinion. I am thankful to hear in this article that 1” framing penetration is enough. I have been worried about poking a wire and the gambling it has been. I bought a Bosch stud finder with electrical sensing technology to mark an upcoming project like this.
I will comment on the metal roofing, but i am
not qualified to answer! I’d like to know people’s experience or knowledge. I’ve seen it done. Seems logical to skip a second, non structural layer of plywood when metal is often installed on strapping anyway. Actually i could use this technique this year! My one caution is that it would be less than ideal to rain onto the insulation while building it and waiting for the roofer. And i’m not sure how a waterproof, vapor permeable membrane applied before the strapping stuff would affect the vapor profile of the assembly. There will be condensation under the metal roofing. Will it drip? Will it run down the slope? Will it matter?
Recently I did a couple "barn look" remodel projects that were to use 1x8 ship lap siding over 2 inches of EPS foam. Rather than hang furring strips on top foam and then attach siding to the furring strips, I decided to attach the new siding back to the original structure with 4 inch screws.
We ran rows of horizontal 2x blocking in the stud cavity about 3 feet apart. (We tried to minimize blocking required by using window sill height, header height, and wall plate heights as fastening points so we didn't need as much blocking)
We installed new WRB, followed by 2 inches of EPS foam. We cut 3/16" "fan fold" XPS into 3 inch strips to be used as a horizontal rain screen spacer and tacked them on with 2 1/2" roofing nails as we installed siding.
Pre-stained 1x8 ship lap siding was then screwed on with 4 inch GRK #10 ACQ compatible screws.
The solid EPS and barn look goal meant we didn't worry about shimming furring or siding.
The 3/16" air space is relatively small, but still much larger than air space create by "drainable" house wrap products. We were limited by the existing roof design in how much thickness we could add to the walls.
The horizontal spacers do block most air flow vertically. (we did gap spacer foam strips every 3 feet). But I felt there is a lot of air circulation horizontally to the exterior via the joints in the siding.
Not ideal, but better than no air space.
The EPS foam was selected because it has a higher perm rating, a lower global warming potential, and it costs less. Although the new WRB on the original house sheathing was now properly detailed, I had hoped to tape all the joints in the EPS as a belt and suspenders approach. However, I could not source any building tape that was effective over EPS's surface.
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