Open rain screen questions
stephenr
| Posted in General Questions on
I am part way through building a 1000 square foot PGH in coastal maine, 6A. I have double stud walls and the original plan was to go with a closed rain screen. I have 2 foot overhangs and zip sheathing, partially taped at this point and my windows are in. I milled up a bunch of 1×8 spruce from trees on my site and its dried and ready to go, installing it untreated. I am considering switching to an open rain screen (for air around the boards) with 1/4-3/8 inch gaps and wanted to see if it is possible to switch to an open rain screen at this point in the project. I will either use 3/4 or 1/2 inch battens on 16 inch centers and place my siding horizontally.
Because i have zip and intend to finish taping it (and my windows are already taped to it) the question of how to get UV protection is at this point is my main concern.
1.Could I simply fasten some Fassade S to get the UV protection i want without needing to detail it as a traditional wrb? How about horizontal swaths of black UV paint applied to the zip where the gaps will be as a cost saver?
2.Also, are insects a concern? I have ventilation channels behind my overhangs ledger that allow for air flow into the roof venting. Should i be concerned about insects having access to that? Its a sarking membrane roof assembly.
3. Would the Fassade S be applied over the strapping?
4.Finally, has anyone ever done an open rain screen treatment of a soffit?
Thanks, Stephen
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Replies
stephenr,
It's not primarily the UV protection you need to worry about. Open-cladding lets a large amount of moisture through to the WRB. https://www.rdh.com/wp-content/uploads/2017/10/RDH-BSL-Drainage-Balance-Spray-Rack-Report.pdf
This puts a burden on what our code calls "the second layer of protection" at the back of the rain-screen walls that it was never designed to deal with. These open-cladding WRB's like Fassade S are there to mitigate that new problem, but they definitely need to be detailed correctly to shed the bulk water that gets through.
I would consider open-cladding for the look, but if the intent is to provide more drying to the boards, the technique adds more wetting than drying.
Connecting the wall vent-space with the roof is a bad idea (and is precluded by our code). It allows fire to spread between the two assemblies, and means the damp air you are trying to remove from the walls is instead sent into the roof space.
Thanks Malcolm,
Could you elaborate on how "the technique adds more wetting than drying". It seems that if boards have air flow on all six sides, they are likely to have more drying than say, shiplap, which has more wood to wood connection which creates a moisture exchange.
I hadn't considered the fire question with regards to the wall and roof venting being connected. Thanks for pointing that out. It passed architect, engineer and code review. I suppose I should block the vents at the top of the rainscreen with wood to prevent the wet air from getting into the roof vents. Easily done.
stephenr,
Most profiles of wood siding installed over a rain-screen should only see incidental moisture on the backside - which easily dries into the cavity. An open-cladding means that all six sides get wet. So while you do have better drying potential, much of what it is drying is the result of the open-cladding allowing it (and the rain-screen strapping) to get wet the first place.
Although our code precludes joining the wall and roof venting, Joe Lstiburek says that gaps of 3/4" or less won't allow flames to climb in what is essentially a narrow chimney. However open-cladding would facilitate that, making the risk of fire spread much higher.
Sample of 1.
I built a shed with simple gapped 1x3 horizontal slats about 10 years ago. WRB was 30lb felt, so not anything special.
I recently pulled off one small section to extend it. I was surprised to see minimal wear. That section had small overhangs (~6") with no gutters for most of its life.
Still not a fan of open cladding and not saying you should build a house like but I would be less concerned if built over the correct WRB with proper detailing.
Akos,
I'm sure all this is all pretty climate and site dependent. I had a shed well off the ground in sheltered spot with nothing but painted OSB sheathing on the outside that did fine for over 15 years.
My problem with open-cladding isn't so much whether or not it preserves the siding better, but that it removes one of the fundamental properties we designed rain-screened walls to have - that is a first layer of protection (the cladding) - the purpose of which is to stop bulk water intrusion. We then say it didn't really matter much, we can just do all that with the WRB. I just makes no sense to me.
To me the surprising thing was how little water made it behind the cladding. This area was definitely exposed to rain and yet the backside looked pristine.
So it might not stop bulk water completely but it does reduce it. Not advocating for open cladding, just not as bad as I though it would be.
Akos,
A lot depends on how that bulk water arrives. If it's wind-blown rain, an awful lot gets through. See my link in post #1.