What’s the Best Approach for a Rainscreen?
Can furring strips be applied horizontally as well as vertically?
Aaron Vander Meulen is building a house whose exterior walls will consist of 2x4 framing with cellulose insulationThermal insulation made from recycled newspaper or other wastepaper; often treated with borates for fire and insect protection., bracing, 2 in. of extruded polystyrene (XPSExtruded polystyrene. Highly insulating, water-resistant rigid foam insulation that is widely used above and below grade, such as on exterior walls and underneath concrete floor slabs. In North America, XPS is made with ozone-depleting HCFC-142b. XPS has higher density and R-value and lower vapor permeability than EPS rigid insulation.) foam, furring strips and, finally, Extira siding, an exterior grade wood composite.
Meulen is leaning toward horizontal rather than vertical furring strips because they’ll make it easier to install the 2-ft. by 4 ft. panels.
“Running the furring strips horizontally allows the panels to be fastened in a location the makes sense for the panels, as well as allowing some customization of panel size,” he writes in a Q&A post at GreenBuildingAdvisor. “What am I missing/overlooking?”
Horizontal furring is a problem
Installing the furring horizontally might make it easier to put up the siding, but it will to cut down on the flow of air behind the siding and will block the drainage of any water that gets past the siding.
Torsten Hansen suggests offsetting the furring away from the foam to allow back-venting of the panels. But William Geary suggests even this won’t be enough.
“The horizontal furring strips are the problem if you don't provide for (1) sufficient drainage for the rainscreenConstruction detail appropriate for all but the driest climates to prevent moisture entry and to extend the life of siding and sheathing materials; most commonly produced by installing thin strapping to hold the siding away from the sheathing by a quarter-inch to three-quarters of an inch. (this will be blocked by the horizontal strips), and (2) sufficient vertical airflow behind the siding (this also will be blocked by the horizontal furring strips,” Geary writes. “I doubt you can cut enough kerfs to provide for adequate drainage AND airflow.”
Instead, Geary suggests one of three options:
“Whatever you do, make sure you detail the rainscreen properly at the bottom of the wall to allow drainage and airflow and at the top of the wall to allow airflow, and use bug screen,” Geary says.
Horizontal furring is no problem
GBAGreenBuildingAdvisor.com senior editor Martin Holladay has a couple of suggestions — careful flashing and substituting polyisocyanurate foam for the XPS — but on the issue of horizontal vs. vertical furring, Holladay says that Meulen is unlikely to run into a problem. “I think your horizontal furring strips will work fine,” Holladay writes. “Of course you won't get vertical ventilation channels, but your siding will still dry out after a rainstorm — it just might take a few hours longer to dry. Not a big deal.”
Holladay recognizes that horizontal furring won’t be as effective as furring applied vertically, but he also points out that siding can be installed over horizontal furring without suffering moisture damage. In fact, the siding on his own house was installed that way 30 years ago, and there’s no evidence of any problems.
“I also know that for decades, siding was installed tight to the sheathing, and most such walls performed adequately,” Holladay says. “Installing an air space will be a huge improvement. The amount of bulk water that gets past siding is small. It dries out by evaporation (diffusion). This drying is accelerated by sunlight and by daily changes in temperature. These temperature changes have the effect of a pumping action that aids air exchange in the air space behind the siding.”
The choice of siding may be a factor
Although Holladay’s house shows no signs of problems, the particular type of siding that Meulen has in mind could be a game-changer, Geary says. “I don't know what type of siding Martin has but I suspect it is much more vapor permeable than Extira which is a resin-molded MDF,” he writes. “There's no way you're going to get much if any vapor diffusionMovement of water vapor through a material; water vapor can diffuse through even solid materials if the permeability is high enough. through that material.
“Plus, comparing siding installed flush to wood sheathing (presumably separated by tar paper or not) is a LOT different from the situation where the sheathing is XPS or polyiso foam. The vapor profiles of these wall systems are entirely different!”
Meulen’s proposed wall section creates a “sandwich” of two vapor impermeable layers, the foam and the siding, with not much air circulation between them.
Moreover, he adds, the use of Extira siding may be a problem in its own right. A look at the manufacturer’s website finds the material is recommended for use in signs and trim work, as well as a variety of other exterior uses, but not specifically for siding.
“I also note that they provide only a 5-year warranty and they have all kinds of warnings about painting and not letting your siding come in contact with standing water,” Geary says. “I suggest you speak with the manufacturer directly about your application. It might be fine but I'm not sure it's a good idea.”
Why not, he adds, use fiber cement siding instead? Meulen has used the material for exterior applications before, and the fiber-cement siding he’s seen isn’t as smooth as he’d like it.
Geary's information about Extira's low permeance caused Holladay to back-pedal. Holladay says that Meulen might be smart to take Geary’s points into consideration. “I think that you need to research the points raised by William and should perhaps consider a more conventional siding material,” he says.
Our expert’s opinion
GBA technical director Peter Yost added these points:
Free-draining furring strips. In addition to Coravent “honeycombed” manufactured furring strips, there is BattensPlus. Note that for both types of manufactured furring strips, you are likely to run into the issue of sufficiently fastening the wall claddingMaterials used on the roof and walls to enclose a house, providing protection against weather. to the furring strip. While simply fastening the cladding to the furring strip meets code when the wooden furring strip is attached directly to the wall framing, attaching the cladding to just the manufactured “honeycombed” furring will not suffice nor meet the building code; you will need to fasten cladding with fasteners that are long enough to go through the furring strip and fasten to the wall framing.
Furring strips versus spacer mesh. The other way to create a free-draining space (in lieu of furring strips) is a manufactured spacer mesh, such as Benjamin Obdyke’s HomeSlicker. But of course, this material is not inexpensive (probably about $0.40 a square foot). And both approaches create significantly deeper, more difficult, and more labor-intensive flashing details at penetrations.
Where is the WRB? In this discussion, there is no explicit discussion or reference to the weather-resistive barrier (WRB). I think it assumes that the dedicated WRB is the exterior face of the rigid insulation, with seams taped or Z-flashed. There is a new product from Benjamin Obdyke, HydroGap, that combines the rainscreen gap with a housewrap. Although the 1 millimeter “nubs” of HydroGap may seem minimal, research indicates that a 1 mm gap results in at least 90% effective free drainage. And that 1 mm thickness means NO significant additional depth to the wall assembly and no difficult, deeper flashing details at penetrations.
I just wrote a product review on HydroGap for Environmental Building News’s February issue. The product is being launched at NAHB’s International Builder’s Show in Orlando with product availability nationwide at about the same time.
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