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Rain Screen: Horizontal furring strips with corrugated metal siding installed vertically

I am installing corrugated metal siding vertically over 1" Roxul Comfort Board exterior insulation, thereby necessitating a horizontal furring strip that can bear the weight of the siding. We originally furred the windows out by 2 3/4" with the idea that we would use 1 x something vertical furring strips to allow drainage, then 1 x something horizontal furring strips for the metal. I believe it is a good system, but am now preferring to double my planned exterior insulation to 2 inches and reduce the amt of furring if possible while working within the parameters of the 2 3/4" window pop-outs.

Specifically, I am wondering if the vertical channels in the corrugated metal might be sufficient for drainage if I add the extra 1" roxul in lieu of the vertical furring? I would also consider bevelling the horizontal furring strips toward the exterior to move any bulk water toward the backside of the metal channels. Does this sound like a system that would would work? Any input greatly appreciated.

Other information: 1/2 plywood sheathing, typar warp, planning a bug screen at base of rainscreen, climate is quite wet - interior BC

Asked by Amy Hazeldine
Posted May 28, 2014 6:04 PM ET
Edited May 28, 2014 6:13 PM ET

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7 Answers

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1.

Amy, The vertical channels in the metal provide a good enough drainage plain that the rain screen provisions in the BC code allow for it to be used without strapping. The Comfortboard behind will also act as secondary drainage so I wouldn't bother with bevelling the furring strips. I think you are proposing a very well performing wall. If you wanted to improve it further you might consider venting it at the top too.

Answered by Malcolm Taylor
Posted May 28, 2014 10:00 PM ET

2.

Check Cor-A-Vent, it may work for you: http://www.cor-a-vent.com/siding-vent-sv-3.cfm

Answered by Armando Cobo
Posted May 29, 2014 8:40 AM ET

3.

Amy,
Q. "I am wondering if the vertical channels in the corrugated metal might be sufficient for drainage."

A. You don't need a drainage gap between mineral wool and siding, because the mineral wool itself is free-draining.

Q. "I would also consider bevelling the horizontal furring strips toward the exterior to move any bulk water toward the backside of the metal channels."

A. That's unnecessary. The free-draining nature of the mineral wool, along with evaporation and the normal air exchange caused by the pumping action resulting from daily temperature changes, will easily handle any incidental moisture.

Answered by Martin Holladay
Posted May 29, 2014 8:53 AM ET

4.

Thanks everyone for your quick responses. I'm very glad to get confirmation that this system will work effectively. I'll definitely look into venting the system at the top as well.

Answered by Amy Hazeldine
Posted May 29, 2014 10:57 AM ET

5.

Malcolm and Martin... Am I reading your replies correctly that no strapping is needed with corrugated metal siding over a rockwool substrate? Even 4" of rockwool? My concern would be achieving a flat final product... perhaps the strapping is necessary for a smooth install? I think I'll need at least 1x strapping to take the fasteners from the siding and prevent cupping/water-canning/etc.

Answered by Ethan T ; Climate Zone 5A ; ~6000HDD
Posted Jun 7, 2018 4:49 PM ET
Edited Jun 7, 2018 5:12 PM ET.

6.

Ethan,
The strapping isn't needed to create a rain-screen cavity, as the corrugations in the siding provide that, but as you said, it is necessary for attachment.

Answered by Malcolm Taylor
Posted Jun 7, 2018 8:34 PM ET

7.

Amy,
I agree with Malcolm. Use horizontal furring strips.

Answered by Martin Holladay
Posted Jun 8, 2018 9:26 AM ET

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