William Costello is building a new house in southwest Virginia that will be framed with 2x6s and will include up to 2 inches of exterior rigid foam insulation. He plans on installing 3/4-inch thick plywood furring strips on top of the exterior foam, and then will side the house with LP SmartSide lap siding designed specifically for houses with 24-inch on-center framing.
It all sounded straightforward enough until Costello took a close look at the installation instructions from LP Building Products.
“I discovered that LP’s installation instructions specify 1.5-inch by 3.5-inch furring strips for foam greater than 1 inch in thickness,” Costello explains in a Q&A post. “So I guess that would basically mean a 2×4 stud for every 2×6 stud in the wall. This is more material and expense than we had bargained for, and perhaps also more increase in wall thickness than we want for window installation.”
He double-checked with LP to make sure that’s what would be required to maintain his warranty, and indeed that’s the requirement. There’s a chance he could keep the thinner furring strips if he used fasteners for the siding that were long enough to bite into the 2×6 framing, a rep told him, but he’d need written approval. That would leave him with buying more expensive fasteners, so the option isn’t necessarily very appealing.
Costello sees several options: install the siding according to LP specs; install it his way and hopes he never has to make a warranty claim; use 1 inch of polyiso insulation instead of the 1 1/2-inch to 2-inch EPS and forget the vented rainscreen; or use extra long fasteners for the siding.
What’s his best option?
Next time, plan ahead
Costello’s dilemma is a good example of why better planning is important, replies GBA senior editor Martin Holladay.
“I don’t know what stage of construction you are at, or whether you have purchased your siding yet,” he writes. “But your case is a good example of the need to plan ahead: It’s important not to select a siding until you’re sure that the siding installation details will work for your planned wall assembly.”
And what about the options Costello sees? Holladay would not recommend ignoring the manufacturer’s installation instructions, in part because doing so amounts to a building code violation. And switching to polyiso insulation and skipping the rainscreen gap has its own problems.
“It’s a very bad idea to butt the back of your siding tight to a layer of rigid foam,” Holladay says. “You need the rainscreen gap for siding longevity.”
Finally, he says, don’t use the extra-long siding fasteners with the 1×4 furring unless you are able to get the OK from the manufacturer. “This approach sounds expensive because of the cost of the long screws,” he adds.
Tinkering with the original plan
John Clark offers another suggestion: a product called Home Slicker made by Benjamin Obdyke. “Slicker can be applied to the insulation and then you can attach the siding directly to the studs through the foam,” Clark says.
Costello wonders whether switching from LP SmartSide to fiber-cement siding or even vinyl is worth considering. “Another option might be Hardie board,” Costello says of siding made by James Hardie . “I think I’ve read that Hardie only requires 1/2 inch of furring strip ‘bite’ for the fastener, so our 3/4-inch furring would be more than sufficient. Not sure about 24-inch centers though. Worst case we’ll have to go to vinyl, which we really don’t like aesthetically, but I gather we’d be able to fasten it through even 2 inches of foam right to the stud, and still have air space behind of course.”
Brendan Albano writes that fiber-cement siding over 1×4 furring at 24-inch centers is common. “You’ll of course want to confirm that the manufacturer approves it and all that, but if there is a Hardie product that fits your aesthetic, that might be a good way to go without having to alter much else,” he says.
Clark points out that vinyl siding doesn’t need a rainscreen gap at all, and recalls a recommendation from the Building Science Corporation for a minimum gap of 1/4 inch for all siding other than vinyl. “Perhaps more is better,” he adds, “but it’s not necessary; 1×4 furring strips are just readily available.”
Writing from New Jersey, Jonathan Lawrence says his own wall design included 3 inches of Roxul mineral wool, 2×4 furring, and Hardie Artisan siding.
“I recall that the furring and fastener requirements were the same for Roxul and foam,” Lawrence says. “The Artisan siding is about twice the weight of regular Hardie, which required me to go with 2×4 furring because I needed 1.75 inches of fastening depth on both the furring and the stud. Had I gone with regular Hardie, I could have used 1×4’s. We also spec’d 7.5-inch HeadLoK fasteners spaced 16 inches on-center vertically and horizontally.”
LP weighs in
Matt Vrazel, who identifies himself as the North American Field Technical Manager for LP Building Products, directs Costello to a technical note on the topic, the same document GBA reader Douglas Epperly had flagged for Costello’s attention.
“This document specifies using 1×4 southern pine with a specific gravity of at least 0.55 as furring strips,” Vrazel says. “The average specific gravity across the four major species of southern pine (loblolly, longleaf, shortleaf, and slash) is about 0.55. The specific gravity is important because it is directly correlated with fastener withdrawal resistance. Therefore, using a furring strip with a lower specific gravity will have lower wind load resistance.”
While the document requires furring strips every 16 inches for 36 Series Strand SmartSide products, 76 series SmartSide lap is rated for 24-inch on-center framing, he adds.
“You also mentioned using a 1-inch thick polyiso insulation board without furring strips and thus eliminating the rainscreen,” Vrazel adds. “You can use this option in combination with a drainable housewrap between the insulation board and siding, which will create a drainage plane behind the siding. With this design, you’ll be able to fasten the 76 Series Strand SmartSide through the foam directly to the 24-inch on-center framing according to the installation instructions.
Specific gravity spec raises questions
The specific gravity of wood changes with its moisture content, replies Charlie Sullivan, so the specification Vrazel cites may needs some clarification.
“The same strip of southern yellow pine might be above 0.55 specific gravity at 14% moisture content and below 0.55 at 6% moisture content,” Sullivan says. “Is your spec for a specific moisture content? And more generally, is your expectation that everyone should be measuring and checking that spec, or is it enough to be sure it’s really southern yellow pine, and not some other generic unspecified SPF species that would have a lower specific gravity?”
Further, Steve Knapp points out, southern yellow pine may not be readily available in all parts of the country: “Would strips of CDX Douglas fir plywood work as a furring material?” he asks. “It supposedly has a specific gravity of .57.”
Lawrence says that the specific gravity of furring came up in his research on using Hardie fiber cement siding. He took his cue from a technical bulletin on fasteners from Hardie, which required spruce-pine-fir furring with a specific gravity of 0.42 or greater.
“So back to Hardie and another technical report to determine the required fastener penetration and thereby the furring thickness,” Lawrence adds. “It was almost a circular exercise, but now I know how they determine all of this. However, I discovered the easiest thing to do is call Tech Support, they already know the answer, so unless you really want to figure this out on your own, I would call LP tech support first.”
Our expert’s opinion
GBA technical director Peter Yost had this to say:
Even if you don’t have to comply with the International Residential Code, you might want to consult two tables from the 2015 IRC:
- Table R703.15.1, “Cladding Minimum Fastening Requirements for Direct Attachment Over Foam Plastic Sheathing to Support Cladding Weight.” The table is reproduced below as Image #2.
- Table R703.15.2, “Furring Minimum Fastening Requirements for Application Over Foam Plastic Sheathing to Support Cladding Weight.” The table is reproduced below as Image #3.
The IRC provides comprehensive guidance on this whole issue:
(1) Following manufacturer requirements: You must follow any and all manufacturer installation instructions and requirements.
(2) Specific gravity of framing/furring: Footnote (a) in both tables for cladding and furring fastening requirements (Table R703.15.1 and Table R703.15.2) requires “spruce-pine-fir (SPF) or any wood species with a specific gravity 0.42 or greater in accordance with American Wood Council National Design Specification,” which states the specific gravity for oven-dry moisture content.
(3) Using plywood for furring in place of spruce-pine-fir: Footnote (a) for Table R703.15.2 (see Image #2 below) states “…any wood species,” so it’s unclear if the code allows 3/4-inch plywood furring strips to be substituted for 1x spruce-pine-fir, but I can’t see a performance reason for not allowing this substitution.
(4) Covering all aspects of the approach: Table R703.15.2 gives clear guidance on every aspect of spacing, attachment, fasteners, and maximum sheathing thickness, including the required compressive strength of the rigid insulation, even though the table is titled for foam plastic sheathing; see footnote (c).
Just my cut: If there are no furring strips, I find that attaching cladding through just about any thickness of rigid insulation unnecessarily difficult; far better to attach the furring strips through the continuous insulation layer to the underlying framing, and then attach the cladding to the furring.
A free-draining and convective-drying 3/4-inch gap is needed for just about any wall cladding in an area like Herndon, Virginia, with 42 inches on average of rain a year. But with an OSB-based cladding like LP SmartSide, it’s especially important.