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What’s the Best Approach for a Rainscreen?

Can furring strips be applied horizontally as well as vertically?

Posted on Feb 27 2012 by Scott Gibson

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:

  • Run the furring strips vertically — the best option.
  • If the furring strips must go on horizontally, find some way to create a 3/8-inch air space between the furring strips and the rigid foam.
  • Use Cor-a-Vent horizontally to facilitate air and moisture movement behind the siding.

“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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  1. FHB
1.
Mon, 02/27/2012 - 09:52

Life on the bleeding edge
by James Morgan

Helpful? 1

Boy. Rainscreen or no rainscreen, looking back at the picture on the OP those long horizontal flush joints look awfully vulnerable to me, especially with a milled top edge to the siding material. If I were the contractor on a project where the client was insisting on this look I'd plug a serious disclaimer into my warranty document and recommend regular pre-emptive maintenance inspections. And to give the assembly a fighting chance I'd also recommend protecting the walls with a darn good roof overhang with no more than a single story height exposure, and making sure the bottom of the siding was well above the splash zone, certainly more than the code-minimum 8".

I see no mention of a location of this project - maybe I missed it - that could make a difference. If the location is in a dry climate with infrequent moisture events it'd probably be OK. In an exposed location subject to wind-driven rain I'd stay away from this altogether. Signage is signage. It's not expected to last forever. I'd sure want more than a 5-year warranty on an integral part of the weather enclosure of any home I was professionally responsible for.


2.
Mon, 02/27/2012 - 10:08

Caulked seams
by James Morgan

Helpful? 0

I also note this comment in the manufacturer's documentation:

"When designing products made with Extira, always allow for the natural expansion or contraction of Extira due to changes in its moisture content and assess interaction with other components in the final product. Like other wood composite material, the moisture content of Extira will change based on the ambient relative humidity and environmental conditions to which it is exposed."

In a trim piece that movement will be only a small fraction of a millimeter, while in a 2' vertical panel width movement could easily be enough to destabilize a caulked seam in a fairly short period of time - again, depending on local climate conditions. There's a reason traditional siding materials almost invariable rely on lapped joints which are free to move.


3.
Mon, 02/27/2012 - 10:56

Siding
by Doug McEvers

Helpful? 0

I would consider a rabbeted bevel cedar siding, it is still affordable and if primed on all sides before installation will hold paint for a long time.


4.
Mon, 02/27/2012 - 11:19

Edited Mon, 02/27/2012 - 11:22.

alternative
by mike eliason

Helpful? 0

alternative option...
vertical battens w/ a layer of horizontal counter battens. for rainscreen w/ vertical siding, is our preferred approach. detail is used throughout much of EU for walls (and especially roofs)
http://baublog.pichuraconsult.de/__oneclick_uploads/2009/06/sdc11469a.jpg


5.
Tue, 02/28/2012 - 13:31

Edited Tue, 02/28/2012 - 14:51.

Mike, the failing point of
by aj builder, Upstate NY Zone 6a

Helpful? 0

Mike, the failing point of the system in your picture is batten failure due to drying out the wood till it is brittle unstable junk.

But for sure until the battens or roofing fail, that roof will be dry!

To add longevity the battens should be beefier.


6.
Wed, 02/29/2012 - 08:43

Home slicker and flashing details
by Douglas Horgan

Helpful? 0

We use "home slicker"/"cedar breather"/"whatever the same mesh material is called from some other supplier", precisely because it's thin enough that conventional flashing & window details work fine.
I'm looking forward to seeing this new Obdyke drainage wrap too, but the 1/4" deformed mesh materials work very well and are easy to detail.
I also agree with the specific concern over the siding materials in this thread, and with Martin's general point that any space behind siding will allow it to perform a lot better than none, even if the space isn't detailed for a vertical flow of air. Just changing from a sandwich of siding-wrap-sheathing with no gap, to an assembly with some gap, seems to make a huge difference.


7.
Thu, 03/01/2012 - 03:49

Horizontal vs vertical furring strips
by Deon Deetlefs

Helpful? 0

Just a question: - why not apply the furrings strips diagonally? - this will allow drainage and airflow required.


8.
Thu, 03/01/2012 - 09:11

Edited Sun, 12/09/2012 - 13:32.

Rain screen Furring Strips placed horizontally
by Greg Albracht

Helpful? 0

Hello Martin, Greg Albracht here, we have a new Rainscreen strip that works fantastic for this exact application. It WILL allow air movement both vertically and horizontally. Its our Siding Master(TM) Product that you reviewed years ago in Energy Update Magazine.
This is far superior to wood furring strips, plastic batten or those expensive mesh rain screen products that can cause problems with wavy siding if nailed too tight. Our system includes top and bottom vents (Screened). James Hardie(R) Siding has referred many large companies to us who have used the Furring Master and who have re-ordered.
Patent Pending Furring Master (TM) is by far the BEST Choice for any rain screen application, that is not our opinion that is our customers.

New Furring Master (Two) Aluminum PVC Coated Furring Strips (Hat Channel)
https://sites.google.com/site/gorillagutterguard/our-products/furring-ma...

Furring Master Website:
http://sidingmaster.com/Furringmasterfibercement.htm


9.
Sat, 03/03/2012 - 12:12

Question for Greg
by Garth Sproule 7B

Helpful? 0

Greg
Will your furring system work over a thick layer of foam insulation? Do your strips hold nails?


10.
Sun, 03/04/2012 - 06:48

Question for Greg re: Furring Master over Thick foam
by Greg Albracht

Helpful? 0

Hello Garth, yes the Furring-Master(TM) can be installed over thicker foams. We use the 20 gauge Furring master that we just came out with for this application. Yes you can apply the cladding to the Furring Strip on the 20 gauge. On our 22 gauge you must penetrate into the framing so the 22 gauge would not be the correct strip you need for this application. The 20 gauge is. You will need to use self drilling screws. What type of cladding are you applying over the rain screen? Please call me direct with any questions at 402-686-4257


11.
Mon, 02/10/2014 - 20:47

Horizontal furring strips are no problem
by Edith Gawler

Helpful? 0

if you want vertical siding for your exterior layer, just install two layers of furring strips; the first is vertical, installed directly over the interior studs. Then apply a second layer horizontally, which will receive your exterior boards. The best part of this system is that you don't need to rely on expensive metal or plastic furring strips, and you don't need to rely on pressure treated wood. The double layer of furring strips creates a very airy cavity behind the exterior wall, allowing air to move vertically and horizontally at will.
But i am also curious... has anyone heard of installing the furring strips diagonally? Pros/cons of doing this?


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