Wood Furring Doesn’t Make Sense
Hey all,
I’ve used Tyvek rainscreen and now looking at Mortair for my home Hardie and Cedear shingle air gap rain screen.
One thing that has bothered me in this as a homeowner DIY person: 1x wood furring for drainage/air gap. If a rain screen is needed, there is an acceptance that SOME bulk water comes through the cladding. By definition, Hardie and the like are not vapor barriers, so drainage is for some level of extra than vapor between sheathing but actual weather/sprinkler driven water.
If ANY water hits a 1x something…isn’t this literally the very worst material to have moisture on? Like, short of cardboard, 1x furring, exterior, with some actual water hitting it. Rot, warp, cracks, etc…That doesn’t make sense but seems like accepted. What am I missing.
I want to be convinced. If convinced, I can return 3 rolls of this expensive 3d mesh 🙂
Thank you
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Replies
I have heard from building science gurus that roughly 1% of the water that hits the face of cladding will get through the cladding. If there is a rainscreen gap, only 1% of the water that gets past the cladding will reach the WRB. Of that water, perhaps 1% will get through to the sheathing.
Wood doesn't mind being wet as long as it can dry before the organisms that cause rot can get a foothold. Very little moisture should get to the rainscreen cavity, and moisture that does get there should dry quickly from the airflow through the gap. That's why it's good for the cavity to be at least 3/8" and preferably 1/2" to 3/4", and why the best rain screens are ventilated at both the top and bottom.
Ryan,
Michael is right. Rainscreen gaps are dry. There is air movement (due to convection and wind) and the siding gets hot when the sun hits it -- so the air gap tends to get baked dry. Not a wet environment at all.
you are forgetting one thing, which is the north facing wall. So you are fine with 3 sides of the house, but we don't know how well the north side will do.
richmass62,
We do know how they hold up because we have been building with rain-screens for decades here and have the evidence. Where I am in the PNW, north facing walls can have real problems with both moisture accumulation, and moss and mold building up on the cladding. With a rain-screen you still get the surface accumulation, but when you open them up, you simply don't see any real difference in the stud bays, or the materials in the cavities.
Letting Joe Lstiburek's previous work speak here. Read the article for the full back story.
PA-1201: Foam Shrinks, and Other Lessons
https://buildingscience.com/documents/published-articles/pa-foam-shrinks/view
Furring strips will not rot
I can’t tell you how many times I have heard, “You’ve got to use pressure treated furring strips,” but it is absolutely untrue. When we took the walls apart, the furring strips looked brand new. Why does this detail work? Because the vent space is designed to dry. That is its whole job.
Why not use pressure-treated furring strips as an added measure of protection? One reason is that it means having to use stainless-steel siding nails, which has proven to be an unjustified extra expense.
Also, the questioner is using "vapor barrier" instead of "water control layer"--definitions of the terminology in the column below.
BSI-024: Vocabulary
https://www.buildingscience.com/documents/insights/bsi-024-vocabulary
If we don’t call things by their right names we don’t really understand how things work.1 If we don’t understand how things work how can we prevent problems from happening? Or how can we fix problems when they do occur? And how can we possibly make things work better?
I am going to attempt to do something completely arbitrary, unilateral, annoying to others and otherwise typical for me because we have to start somewhere to clear up the mess. I am going to take a run at this language thing and try to get everyone to agree. Each one of the terms we typically use or should use needs to be not just defined but defined with a performance metric. This is not easy, but necessary. Here goes.
They are building enclosures—they are not building envelopes. You put letters in an envelope not people. Building enclosures need four principle control layers: a water control layer, an air control layer, a vapor control layer and a thermal control layer. These control layers can be combined in one material or be separate.
"Why not use pressure-treated furring strips as an added measure of protection? One reason is that it means having to use stainless-steel siding nails, which has proven to be an unjustified extra expense."
Or you could use hot-dipped galvanized. Treated wood doesn't require hot-dipped if it is installed dry and will remain dry throughout its life. And if you feel that it isn't really a dry location then you should be using galvanized nails for the untreated wood too.
I do wonder how pine/fir gets a no for exterior trim. It is significantly more exposed, agreed. But the rain screen is never painted from what I see, and at least exterior trim has acrylic paint on it to protect.
I'll accept the reasons as good enough though. I admit it's not bulk water constant or sun exposed. Just don't know where we recommend untreated wood in any exterior application but a fence and pickets are replaced often.
I use untreated pine for exterior finishes often; you just have to allow it to dry. It will discolor over time but it won't rot unless it stays damp. I also use unfinished wood for decking and garden projects, where it will last 5-20 years, depending on the situation.
Ryan_SLC,
"Just don't know where we recommend untreated wood in any exterior application"
That's where the misunderstanding lies. A rain-screen is not an exterior application any more than the vent space under a roof - where even more vulnerable materials (like cardboard baffles) do fine.
Although it is outside the WRB.
DC,
I think our code is useful in looking at this. It makes the distinction between the First and Second Layer of Protection. The assumption is that the cladding should be designed as a complete answer to excluding any bulk water infiltration, and the cavity is there primarily as a capillary break, to aid drying, and deal with small incidental leaks. If the cladding were designed in a way that it required a second WRB to be installed to stop water getting into the rain-screen gap, our code would require it.
I've rarely seen pine/fir exposed to the elements rot out if it can dry, though I often see film forming finishes fail on it. I think that tends to sway opinion towards these materials.
I have 10+ year old doug fir planter boxes sitting on dirt, full of often wet soil that have lasted except for one corner of one box that sees the least sun and the most runoff. Kind of impressive, at least to me.
In a way, a rain screen is a like the perfect environment for wood. Each member is well spaced from the next, and there is a constant flow of air 24/7/365 to equilibrate to the surroundings.
Hi Ryan,
Speaking from my own home building experience here... I've used cedar furring strips for rain screens on all my builds and they have not fallen apart yet (granted I've only been building for about 20 years). In the past, I've used 3/8" thick strips but moving forward I plan to go with 1/2" for increased ventilation. I supposed you could paint the furring strips for added protection or use Shou Sugi Ban to further protect them, but I think it is unnecessary. I would recommend adding aluminum screening at the bottom and top of the exterior cladding, to prevent bug intrusion. Hope that helps. Good luck!
On a side note, I've used the Mortair material as an air gap for cedar shingles on a couple projects and it worked great! Easy to install, easy to shingle over, built in bug screen on the bottom. Would definitely use it again for cedar shingles.
Not sure what kind of "Hardie" you're installing as a lot of people call any manufactured siding "Hardie" so can't offer any advice there.
RE: Mortar- I'm unclear what exactly "engineered polymeric" is, but sounds like plastic to me. I'm in favor of avoiding plastics wherever possible, and instead use wood which is a perfect material for building houses.