Helpful? 2

Can Vinyl Siding be Applied Over Furring Strips?

The vinyl siding industry doesn’t seem to offer consistent advice, as one homeowner discovers

Posted on Apr 18 2012 by Scott Gibson

Wall assemblies that incorporate rigid foam insulation over exterior sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. , followed by furring strips and siding, are becoming common. The extra layer of insulation helps reduce thermal bridgingHeat flow that occurs across more conductive components in an otherwise well-insulated material, resulting in disproportionately significant heat loss. For example, steel studs in an insulated wall dramatically reduce the overall energy performance of the wall, because of thermal bridging through the steel. 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 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. ,” is compatible with vinylCommon term for polyvinyl chloride (PVC). In chemistry, vinyl refers to a carbon-and-hydrogen group (H2C=CH–) that attaches to another functional group, such as chlorine (vinyl chloride) or acetate (vinyl acetate). 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, GBAGreenBuildingAdvisor.com 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, an illustration pulled from the installation guide shows exactly that: vinyl siding attached to furring strips applied over block or concrete.

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 striops were filled in with 3/4-in rigid foam.

Matt Gibson, the manager of contractor programs forCertainTeed'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:

  1. 1. Does vinyl siding need furring strips to be considered a vented rain screen assembly?
  2. 2. What sort of planar support does vinyl siding need in addition to an adequate fastening schedule?
  3. 3. 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.


Tags: , ,

Image Credits:

  1. Carol Collins
1.
Wed, 04/18/2012 - 12:11

pre-notched furring strips.
by David Taormina

Helpful? 0

You can get furring strips pre notched into eps. You will probably want a wrb over the foam so you just have to be careful that your nailing ino the strips through the paper. then simply apply the viynl siding like a regular job. This is perfect for viynl but i would'nt do fiber cement this way.


2.
Wed, 04/18/2012 - 16:28

ICF
by Randy Ross

Helpful? 0

we build mostly ICF homes 30 - 40% have viynl siding
vert straping 16" oc 12 years + and have seen no problems so far
personaly i wouldnt use viynl on a dog house but customors choice


3.
Thu, 04/19/2012 - 08:01

Siding by myself with the furring already in place-steel siding?
by Sonny Chatum

Helpful? 0

Scott, thanks for following up, and there's still questions, as you note. Over the years, it seems that GBA has strongly supported the furring strip approach, but perhaps has been somewhat casual about the implications of specific cladding types applied this way. I sort-of quietly felt that GBA "owed" us readers a more in-depth look at the situation. One question is, do you have any history of success/failure with the vinyl job shown in the picture you used?

Thanks, Randy R., for the 12 yrs. of experience. So, in your experience, none of the vinyl has taken on a wavy appearance because of any vinly sinking into the spaces? What cladding over furring strips do you use the most? I live in the dog house that I think I'm going to put vinyl on. I should be able to install vinyl all by myself, which is one reason I "like" it. Furring is already on, and the Alpha B. is biting at the fur, so I'm looking for a comfort zone.

For now, I've started filling between furring strips with 3/4" foam, because I just don't trust the vinyl not to get wavy over the open areas. Look closer at the Mastic instructions that Martin cited
(http://www.mastic.com/Homeowner/Learning_Center/Install_Instructions/Ins...), page 17: "Do not apply vinyl siding directly to furring strips without sheathing, because the siding may conform around the furred areas causing an uneven appearance." That would be cute--it might "catch on."

I know it's more expensive, but does anyone want to talk experiences with steel siding? Can I do it myself (assuming competency)? The Rollex instruction manual shows an installer going right at it with installing the steel siding over furring strips.


4.
Thu, 04/19/2012 - 14:32

doghouse
by Randy Ross

Helpful? 0

never had a problem with wavy siding as long as the plane of the straping is true
the siding should remain nice and strait the 1/2 " bend on the butt should hold its shape
as for straping i rip 5/8 or 3/4 whatevers on sale presure treated plywood into 3 inch strips
i have found plain staping cups and warps and throws out the wall
(word of caution dont use iron or galvinized screews with pt lumber)
stainless steal is the best fastener or ceramic coated


5.
Fri, 04/20/2012 - 07:28

Furing under vinyl
by Walter Gayeski

Helpful? 0

What timing. I just decided to do vinyl over strapping. I was thinking that would reduce the amount of nail holes through the 2" iso over the sheathing. I was looking at this product (http://www.alside.com/index.aspx?page=54) as it has a reinforcement strip. Just a bit of insurance against sagging. Each 8' fur strip will be attached with three screws. The siding nails will pierce the exterior aluminum skin on the iso but not the interior skin. Please comment before I start cutting checks!


6.
Fri, 04/20/2012 - 16:42

Vinyl Siding can be purchased
by aj builder, Upstate NY Zone 6a

Helpful? 0

Vinyl Siding can be purchased that has a foam backer, and is superstiff. Of course it is meant to be tight to sheathing and part of the insulation package. It is by far the best looking vinyl. And it is as expensive as all other choices such as fibercement or cedar, etc.

A home with lots of exterior foam is expensive to build. So, one might as well upgrade one's siding choice to a top shelf product. To save costs if really needed go with T 1-11 which done right can look great.


7.
Tue, 04/24/2012 - 18:33

struggling with same questions
by Dirk Gently

Helpful? 0

Great timing on this discussion for me also since I am currently trying to decide what to do with vinyl/foam. I am only trying to cut down of thermal bridging for my 2x6 construction heated garage and wanted to do 1" polyiso but now know that is not enough to keep dew point outside wall... I contacted Mastic vinyl company to find out if I could nailor screw thru 1-3/8 polyiso with regular vinyl OR if I could nail their insulated siding thru 1" of foam.....I have recieved no respose.
I am so undecided at this pointthat I am considering attaching the foam to the interior of the wall. I did this on my home several years ago and have beeen happy with it. If anyone has any links to interior foam discussion it would be much appreciated.
My concerns about strapping over foam are more due to it warping and the fact that I did not figure. the foam/strapping into my overhang design (too late now and I dont want to lose any.
vinyl siding handbook say max thickness foam under vinyl is 1".
Wondering what all posters above did with housewrap (WRB). under foam. over foam or ommited completely.


8.
Wed, 04/25/2012 - 21:54

vinyl over strapping
by Jim Bannon

Helpful? 0

Firstly, good luck with trying to get any guidance from the vinyl manufacturers. I have tried and get pretty much the same response as mentioned above. For them it seems, it's caveat emptor and CYA.

We have done several vinyl siding jobs over foam with strapping with no problems (none yet anyway). The most recent was 2" of Thermax over pine board sheathing. After stripping the original wood siding, we wrapped the house with tyvek, taping all seams. We did this mostly to protect the house from the elements during the ridgid foam installation process. Over the tyvek, we attached 2" of ridgid foam using occasional 3" roofing nails and roofing tins. We would later be installing vertical strapping 16" oc, so we didn't go overboard with the roofers and tins. All foam seams were taped with aluminum tape.

On the outside corners we "strapped" with 8" wide rips of 3/4 plywood to give good nailing for the 6" vinyl corner boards. In the field, and around all windows, we just ran standard drywall strapping. All strapping was attached with 4" GRK screws. Since the sheathing was 1" pine boards, we weren't to concerned about hitting the framing. We put a screw about every 4' or so. Along the bottom of the walls between the strapping, we stapled strips of ridge vent material (Cobra vent) to keep out critters and allow drainage.

I think the vinyl was Certainteed, since this is what our siding guy generally uses.

We discussed putting a second layer of tyvek over the foam (before the strapping went up), but opted not to as we couldn't figure out a way to attach it and figured the foil facing was a perfectly suitable WRB, and in the event of a failure, there was a layer of tyvek under that.

It's been over a year and the vinyl shows no signs of warping or sagging between the strapping. The entire process was fairly labor intensive, especially detailing around windows but we wouldn't hesitate to do it again.


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