Wall assemblies that incorporate rigid foam insulation over exterior sheathing, followed by furring strips and siding, are becoming common. The extra layer of insulation helps reduce thermal bridging through wood framing, and the furring strips create a ventilation space behind the siding that promotes drying.
But writing in a Q&A post at Green Building Advisor, Sonny Chatum wonders whether this wall system, often called a “vented rainscreen,” is compatible with vinyl siding.
“I am referring to a general wall assembly with several inches of rigid foam over studs, followed by 1-by or ripped 3/4-in. plywood furring strips attached with HeadLOK screws into studs, say on 16 in. centers — so that there are voids, or vented, areas between strips,” Chatum says. “How does the siding installation hold up if installed over such strips? Is it good to make the strips wider than, say, 4 inches so the vented areas are less and support for the vinyl is more? CertainTeed vinyl siding installation instructions seem to be against this vented approach, but at the same time their website points to a technical bulletin from FastenMaster that certainly supports the furring strip approach — so they are confusing.”
It’s an intriguing question that could potentially affect many builders and homeowners. Is it OK or not?
Industry group seems to back idea
Several posts direct Chatum to online articles about the correct way of attaching furring strips to the wall over a layer of foam, but it is the vinyl siding itself rather than the furring strips that has Chatum concerned.
Specifically, he wonders whether the void behind the siding between the furring “presents a problem for the lasting integrity of a vinyl siding installation.”
On this question, GBA senior editor Martin Holladay offers a link to a Vinyl Siding Institute installation guide that seems to support this approach. “Installing vinyl siding over furring strips is very common,” Holladay writes, “especially over concrete blocks.” Indeed, two illustrations pulled from the installation guide show exactly that: vinyl siding attached to furring strips applied over block or concrete (see Images #2 and #3, above).
But no clear word from manufacturers
Chatum had seen the manual in the past, but his more recent research has turned up an entirely different answer, and phone calls to CertainTeed muddied the waters even more.
He was told in one phone call that applying siding over furring strips was fine, as long as the gaps between strips were filled in with 3/4-inch-thick insulation, creating a continuously flat substrate.
But, citing various sections of CertainTeed installation manual, Chatum calls attention to directions that seem to rule out the practice. The manual says vinyl siding “must be applied over rigid sheathing that provides a smooth, flat surface,” and that vinyl siding should “never be applied over open furring strips or studs.”
“Despite the mixed positions, my guess is that CertainTeed (and others) just don’t want to extend themselves beyond acceptance of anything short of a ‘perfect’ substrate,” Chatum says. “But I’m sure someone has installed vinyl siding over the furring strips, and has probably experienced good results, if the other important installation details were followed…I can’t be the first.”
If CertainTeed seems to be suggesting it’s not a good idea, Holladay finds installation instructions from another manufacturer that embrace it. He provides a link to a guide from Mastic Vinyl Siding that show furring strips with rigid foam between them. Oddly, the foam isn’t as thick as the furring strips, so the wall isn’t totally flat.
Now we’re really confused
Chatum’s research has prompted calls to three CertainTeed reps, each of whom has something slightly different to say:
- One didn’t know and refers Chatum to someone at corporate headquarters (who did not return his call).
- One who saw no problem with applying siding over furring strips.
- One who said it was OK as long as the spaces between furring strips were filled in with 3/4-in rigid foam.
Matt Gibson, the manager of contractor programs for CertainTeed’s siding group, wrote in an e-mail to GBA that “vinyl siding must be installed over a rigid sheathing that provides a smooth, flat surface, or an underlayment (such as wood, wood composition, rigid foam or fiber sheathing) that is no more than 1 inch thick. Vinyl siding cannot be applied directly to studs or lathe strips.” Gibson’s ruling seems to preclude the installation of CertainTeed vinyl siding on vertical furring strips installed over a layer of rigid foam.
“It seems odd that GBA for years has been promoting the furring strip approach to be used under ‘cladding,’” Chatum writes. “but there does not seem to be any history with vinyl siding as the cladding.”
That’s a good point, given the fact that according to the U.S. Census Bureau, vinyl siding is the principal exterior wall material on 36% of new single-family houses — more than any other type of wall cladding and four times as common as wood.
“Yes, this issue gets more surprising (scarier) all the time,” Chatum adds, “even beyond my disbelief that it really doesn’t seem to have been encountered or addressed before.”
Our expert’s opinion
GBA technical director Peter Yost identifies three separate issues:
- Does vinyl siding need furring strips to be considered a vented rain screen assembly?
- What sort of planar support does vinyl siding need in addition to an adequate fastening schedule?
- If I don’t need the furring strips for venting, do I need them for fastening?
On the first question, let’s take a look at Building Science Corporation’s Building Materials Property Table. The vapor permeability of vinyl siding is listed as “70” perms.
What? Can you think of anything less water- or vapor-permeable than a piece of PVC about an 1/8-inch thick? And what do those quotation marks mean?
Well, a square of 1/8-inch thick PVC is essentially vapor-impermeable: 0.0 perms. But in an installed assembly, there is so much air movement around and behind the loosely fastened siding that the vinyl siding assembly acts as if its vapor permeability is “70” perms. That means that even without any dedicated air space furring strips might provide, there is plenty of space for bulk water to drain between the siding and the WRB and for air to move quite freely behind the cladding.
Because vinyl siding fasteners can’t be “set” (allowing lots of movement with the material’s thermal contraction and expansion), I think of vinyl siding as being “hung” on rather than attached to the rest of the wall assembly.
NOTE: The only wall cladding without an air space behind the cladding in all of the hundreds of GBA details is vinyl siding.
And this last aspect of vinyl siding’s installation relates to the second question: how much planar support does the siding need, given that it is already not “pinned” in constant contact with the field of the wall it covers? It seems as though simply following the manufacturer’s fastening schedule would be all that is needed to adequately ensure that the cladding keeps its place on the wall.
This third question is tougher to answer—at least from my desk it is! Both will work: 5 to 6 inch nails or screws to connect the vinyl siding to the framing versus regular fasteners to the furring strips; but neither is a clear winner in my book. I bet the aggravation factor favors the furring.
But let’s keep installed vinyl siding’s moisture management and drying potential separate from considerations of proper attachment to the rest of the wall assembly. BSC has answered the first question; let the manufacturers give their answer(s) on the second.
And you all in the field can answer the third question a hell of a lot better than this desk jockey can.