Rain screen furring strips — on or off studs?
With 1×4 studs 5/8″ & CDX sheathing, should we place the 1/2″ PT furring strips on the location of the studs or (roughly) midway between the studs?
Siding will be 1×6 WRC CVG beveled lap siding with a 2″ lap.
The advantage of not putting the furring strips on the location of the studs is that we can place sections of 1/2″ Thermacork
ThermaCork 100% Natural Insulation: 1/2″ Standard Grade
between the furring strips over the location of the studs to mitigate the thermal bridging. (We would leave space on both sides of the Thermacork to allow the airflow in the rainscreen.)
My intuition is that with proper screws attaching the furring strips to the sheathing there will be adequate support for the cedar siding, regardless of whether the furring strips are screwed to studs and we’d get significant “whole wall” thermal performance by placing the Thermocork over the stud thermal bridges.
An alternative would be to place narrower Therma cork sections tight against both sides of the furring strips attached over the studs, since the energy moving across a stud is transferred to the sheathing and moves outward along the sheathing, as well as directly through it.
I would appreciate any insights into this issue.
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Replies
If this were my project, I would install the cork insulation without any breaks, continuous around the house, then install the furring strips over the cork. 1/2" insulation is pretty thin, you shouldn't have any issues with fastening the furring strips through the insulation and into the wall framing.
There is a code reference for cladding attachment to wall sheathing when using continuous insulation, it is found in Chapter 7 of the IRC: https://codes.iccsafe.org/content/IRC2021P1/chapter-7-wall-covering
If you are in Canada, your codes may differ.
Randy, Thanks for the advice. Unfortunately, this tiny, 1946 cottage that we're rebuilding has tiny eaves, barely enough to allow the addition of a 1/2" rainscreen. If we weren't constrained, I'd add continuous Comfortboard. I have a bunch of the Thermocork left over from a prior project but not enough for the entire exterior. I figured I would use what i have to add a small increment of thermal resistance on the bedroom walls.
My personal opinion and just trying to help.
½" of continuous insulation is not worth the cost and effort. Moreover, the plan to break up the insulation between the furring I think significantly lessens it's effectiveness.
It it were my project, I'd rather spend the money and time on good air barrier practices to get better interior thermal performance.
Jamie
User...081,
Our code here in BC allows furring stops to be fastened to the sheathing as long as it is 1/2" or thicker.
Unfortunately the benefits of the 1/2" insulation at the studs are negligible. You will not get an R-2 increase there because of the thermal bypass due to them being just thin strips. By the time you made them wide enough to offer any appreciable difference, the usefulness of having a rain-screen cavity would have disappeared.
Jamie & Malcom, Thank you for the thoughtful advice.
Re cost: This is leftover supply from a previous project, and minimal labor, so essentially "free."
One benefit we found from a previous new house project (with a much different situation) was that having the Thermocork cover a portion of the gap between furring strips provided stability to the 1x6 beveled cedar siding. (I.e., mid-span support from the back side of the siding.)
I'm not dismissing thermal bypass, and I realize the added thermal resistance is small. OTOH, the bypass paths do still provide thermal resistance. I wouldn't bother if I could have constructed a new wall according to best practices, but even small contributions will help, and this contribution would be "free."
Looking at it otherwise, it may do about as much good to just nail the furring strips to studs and use the Thermocork on sheathing between.
Re rain screen benefits: The exterior of the sheathing is virtually "bomb-proof" with SIGA Majvest 500 SA and Wygluv tape. I see the primary value of the rain screen as dissipating air pressure so rain isn't "driven" along narrow gaps behind and among the lap siding and the siding can dry from behind, as well. (Our siding is tight, straight-grain cedar. Primed on five sides, then Duration exterior on five sides before putting up, then a final coat of Duration after installation -- so it's very well protected.) So ... There will remain plenty of channels to dissipate air pressure and allow drying airflow. Thermocork is very "open", so any bulk water that gets to it should (🤞) dry out adequately, at least seasonally in the low humidity summer and fall weather in Oregon's Willamette Valley.
I'll continue to give this more thought and all comments are appreciated.
-- Paul
Paul,
Our code mandates that no more than 8% of the rain-screen gap can be connected by furring. That's an acknowledgement that the most important attribute of a rain-screen is it acting as a capillary break. After that I would put in descending order of importance the functions as being:
- Re-distributing moisture being held in the siding and sheathing.
- Providing a drainage plane
- Vented to allow drying
- Help equalize pressure behind the wall.
Almost all the things on that list rely on having an open cavity unimpeded by other materials that bridge the gap.
The other weakness is that in say a 8 ft long framed wall you will still have 24 ft of un-insulated top and bottom plates, as well as any headers over doors and windows. That's against the 40 ft of insulated studs.
I just don't think the disadvantages of the cork are offset by what are marginal gains at best.