Cleats vs Strapping Roxul Comfortboard 80 – tips wanted
We want to put 2.5″ Comfortboard 80 continous insulation on the exterior walls of our home in the WUI (not adjacent to, but in an alpine forest valley adjacent to a creek). This is an installation question. We are committed to the strong fire resistance qualities of mineral wool.
Fire Context
We are in the highest fire risk zone, in California at 6600 ft, in an alpine forest valley, in an area that has one road, single-lane in each direction, no shoulder, with the nearest fire station 4 miles away.
As a community, we do serious defensible space, and land restoration (clearing out dead trees and fuel from the adjacent forest). This area has not seen a forest fire in 60+ years, although some have come within 10 miles of the mountain ridge behind us in recent years.
Fire risk, safety and insurance are top of mind more than ever before. We’ve been here for 30 years.
Construction
The exterior cladding will be reverse board and batten, using 3/4″ plywood as the “batten” and 1×8 red cedar as the “board” with 1″ gapping. This cladding is obviously not the best choice for fire resistance (Class B), but it does look amazing and aesthetics are winning the day on this.
The cladding weighs about 4 lb/sqf (3/4″ plywood + 3/4″ cedar boards). Not especially heavy, but the 2.5″ thick Comfortboard requires strapping instead of directly attaching the cladding with screws through the Comfortboard.
One contractor we’ve spoken to prefers cleating with a ripped 2×4 (i.e. 1.5″ wide x 2.5″ deep). These cleats are attached to OSB sheathing (with an air barrier) over 2×6 framing with insulation.
The cleats would look like shallow stud bays with the Comfortboard between the cleats. The cleats would be staggered from the 2×6 framing to reduce thermal bridging. Because the Comfortboard is not continous, we would put 5/8″ Densglass over the Comfortboard for a 1-hour fire rated exterior wall. Eaves will also have a layer of 5/8″ Densglass under the eave cladding (1×6 T&G doug fir planks).
Pros of cleating
strong
easy to true and plumb the cladding
quick construction
Cons of cleating
maybe 20% reduction in R-value of Comfortboard from thermal bridging (?)
2.5″ Comfortboard no longer 1-hour fire rated because of the cleats
extra cost of Densglass sheathing for 1-hour fire rating
Some contractors prefer strapping per the Roxul installation guide, but are not keen on strapping in general because it is hard to true and plumb.
Question
Cleating v Strapping. Which way would you go and why? Any installation tips to share for either cleating or strapping?
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Replies
Here's some information on straightening strapping installed over continuous insulation.
Prepping a Vented Rainscreen for Sidinghttps://www.youtube.com/watch?v=UgxgP_cRuwI&ab_channel=ProTradeCraft
And BSC's guidebook on implementing continuous insulation.
BA-1406: Final Measure Guideline: Incorporating Thick Layers of Exterior Rigid Insulation on Walls
https://www.buildingscience.com/documents/bareports/ba-1406-final-measure-guideline-incorporating-thick-layers-exterior-rigid-insulation/view
I would never recommend the "cleating" solution proposed by the contractor, given that it largely defeats the point of continuous exterior insulation.
Additional examples here.
BSI-096: Hot and Wet but Dry
https://www.buildingscience.com/documents/building-science-insights-newsletters/bsi-096-hot-and-wet-dry
Thank you, Kohta. I'm a big fan of BSC, and have several of Lstiburek's books.
The Youtube video you suggested (very good) also highlights the higher labor needed with furring to true and plumb it.
Are there techniques, systems or special screws that allow continuous insulation but with less tedium to get the furring uniformly true and plumb?
I look at the Rockwool videos, and they either stop short of adding the furring, or they use a computer animated video showing picture perfect cladding floating through the air and magically attached to the Comfortboard.
"Are there techniques, systems or special screws that allow continuous insulation but with less tedium to get the furring uniformly true and plumb?"
Randy Williams mentioned these fancy screws that let you adjust furring--very effective, but pricey.
Fastener Choices for Installing Wood Furring Over Continuous Exterior Insulation
The right fasteners can help keep the wall flat with siding installations over a ventilated rainscreen
https://www.greenbuildingadvisor.com/article/fastener-choices-for-installing-wood-furring-over-continuous-exterior-insulation
Also, another completely different direction is to use a thermally broken commercial cladding system, which would let you have continuous insulation and not rely on its compressive strength. Off the top of my head, some product options include Cascadia Clip (https://www.cascadiawindows.com/products/cascadia-clip), Knight Wall MFI (https://knightwallsystems.com/products/knight-mfi-system-rain-screen-attachment/), Armatherm Z girt (https://www.armatherm.com/thermal-break-materials/armatherm-z-girt/, see image below), EcoStud Z furring (https://ecostud.com/category/z-furring), Knight Wall ThermaZee (https://knightwallsystems.com/products/knight-thermazee-cladding-attachment/), GreenGirt (https://greengirt.com/) and others. Unfortunately, these are going to be pricier options than furring and hardware you can get at the lumberyard.
But I'd definitely suggest looking at the InSoFast X-Bracket—installed individually, and furring would be run across them. I do not know how it behaves in terms of combustibility.
InSoFast X-Bracket
https://www.insofast.com/x-bracket-system
Kohta, thank you, I'll check out this short list of cladding systems to look at.
After dredging and sifting through GBA and other resources, I wanted to share what I learned about working with furring and alternatives, and hoping this will help someone new to continuous exterior insulation with mineral wool (e.g. Roxul Comfortboard).
FURRING LEVELING TIPS
If you're trying to get away altogether from strings and levels to true and plumb your furring, it doesn't seem possible.
But it does seem possible to reduce the amount of time you spend with your string and level.
TIP1 - Furr in the center, not at the edges of the CI (Continuous Insulation)
CI edges are more compressible than the center of the boards. Placing furring in the middle of the boards instead of the edges helps reduce the effort to true and plumb the furring since the CI compresses very little, providing more uniform thickness. Still need to use a string and level.
TIP2 - Comfortboard 110 instead of Comfortboard 80
Comfortboard 110 is less compressible. You still need to use a string and level, but with fewer and smaller adjustments. May or may not be worth the extra cost.
TIP3 - Use special screws
I looked at the Heco Topix-plus Therm screws (aka Heco Topix T-Therm). These screws are German. They also seem hard to obtain, possibly expensive. The screw has threads near the head (threads into the furring) and at the end (threads into the sheathing / wall studs). In between the threaded portions is a smooth section that reduces variability of compression of the CI when screwing down the furring. The screw threads into the sheathing/stud and into furring, while the smooth section in the middle is in the mineral wool. The result is allegedly a smaller variation in compression (1/8" was measured in a demonstration). Still need to use a string and level to verify.
FURRING ALTERNATIVES
I was looking for something that was
(a) strong (can withstand high winds / lift)
(b) uniform (dramatically reduce the need for string and level verification)
(c) works with wood framing (residential friendly construction)
(d) simple (less labor)
(e) not metal (reduce thermal bridging)
(f) fire resistant (build a 1-hour rated wall?)
(g) doesn't interrupt the CI (less fabrication and fitting)
Long wishlist, for sure. Nonetheless, I *might* have found something, but I need to do a lot more vetting and get my hands on some samples:
Armatherm Z-girts (fiberglass), also marketed as Armagirt Z-girts:
https://www.youtube.com/watch?v=rmMcP9fTQFQ
Would love to hear from anyone who has tried these with wood framing and Comfortboard.
Item (f), the 1-hour rated wall, would require a fully documented set of tests that are validated by the relevant NFPA et al. In the meantime, a layer of DensGlass or TypeX gypsum will be needed.
zetetic,
I'm not sure how practical tip #1 is. The spacing and location of the strapping is determined by the uniform spacing necessary to support most cladding, and the need to hit the studs. Strapping also occurs at corners, and penetrations like windows and doors where he CI ends. I guess you could try and lay out the CI so the seams didn't match, but CI comes in sizes that conform with the usual 12", 16" or 24" strapping spacing.
Malcolm. True - agreed. If you're going to furr without a cladding system, it seems there will always be string and levels involved, and some tweaking of screws.
Our walls are between 20-feet and 28-feet tall. Most walls are between 16-feet and 30-feet wide. This also is a consideration.
How many HDD are you designing for? 2.5" is a lot of CI, you can simplify the build by dropping that down to 1" or 1.5".
With less MW, you can then buy your contractor a 4" jumbo nailer (they are not cheap but not that expensive and a deal compared to screws) and nail up the strapping with it. Nailed strapping will stay more plumb as it won't compress the MW like overdriving a screw. Also WAY quicker install. You will need better strapping as some of the larger nails can split the wood.
8600 HDD (65F) in a warm winter, 9200 HDD (65F) in a cold winter. We're on the cusp between 6b and 7b. 6600 ft elevation. We're also in a Very High FHSZ (we're in the WUI, not adjacent).
California has adopted IWUIC 2024 as of 2/26/2025. Between the recent wildfires, our location in the WUI, and the fire insurance issues in California, we think it might be wise to build to IWUIC 2024 (our plans were approved before California adopted IWUIC 2024). We are building now - foundation and framing done, windows in, shell and roof are weather resistant, but no CI or cladding yet.
The current build thinking is 1.5" Comfortboard 80 (40-minute fire rating if continuous). If not continuous with cleats, we'll put a layer of 5/8" DensGlass under the cladding (I'm simplifying here), which will give us a 1-hour fire rated exterior.
The two 1-hour fire rated exterior scenarios under consideration are:
1.5" Comfortboard 80 non-continuous with cleats (R5) + 5/8" DensGlass + cladding
2.5" Comfortboard 80 continuous with furring (R10.5) + cladding
Cleats are vertical 1.5"x1.5" made from pressure treated 2x4s that would be ripped, and attached to the underlying OSB (with Blueskin) on 2x6 framing.
The exterior walls are 20-feet tall on average, and as much as 28-feet tall. A little more challenging to furr than a single-story.
Cladding is Class B for fire (Western Red Cedar reverse board and batten).
The trade-off is cost of labor to furr versus cost of cleating and 5/8" DensGlass.
That is real winter, more insulation probably wins.
I would go for one of the clip and rail setups similar to here:
https://www.rockwool.com/siteassets/o2-rockwool/documentation/technical-guides/commercial/cavityrock-and-comfortboard-cladding-attachment-solutions-technical-guide.pdf
Instead of the rail, attach the battens straight to the steel clips (go stainless if budget allows). This gets you a full CI without wood but at a slight R value hit.
B&B is much more forgiving to flatness than clap board siding, so going with screws without fussing too much to get it perfectly flat would also work. Since you are ripping plywood for battens, you can make these a bit wider which will further spread the load reducing un-evenness.