How To Combine Board and Batten Siding With Exterior Rigid Foam?
Exterior foam helps minimize thermal bridging and improve energy efficiency. The question is how to keep costs down and avoid moisture problems
Claire Remsberg, an architect in the Rocky Mountain region, is working on a house where the main goals are to limit 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 the 2x6 wood frame and to beef up wall R-values. Plans call for vertical wood siding over a layer of rigid foam insulation.
If that sounds more or less straightforward, the details are not. The contractor has limited experience working with rigid exterior insulation, Remsberg writes, and has concerns that installing siding directly over the foam may not be a great idea.
Remsberg has explored a number of wall assemblies but admits that "nothing is looking quite right yet." Her request for ideas on how to detail the wall is the subject of this week's Q&A spotlight.
Create an air space with strapping
Wood, fiber-cement or plywood siding should not be installed directly over the foam, writes senior editor Martin Holladay.
"I wouldn't hesitate to install 1x3 or 2x4 horizontal strapping, 24 inches on center, on top of the foam," Holladay recommends. "Screw the strapping through the foam to the studs."
Architect Jesse Thompson seconds Holladay's idea of applying the siding over horizontal strapping. Reverse board-and-batten, he adds, is another option. Putting on battens first, followed by the wider siding, makes a natural rain screen and gives a "crisp, sharp profile that is very distinctive."
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Michael Chandler, a builder and GBA advisor, has used scraps of 1/2-in. oriented strand board as furring to separate a rigid foam exterior and vertical siding. The strips of OSB are held in place with galvanized ring shank nails rather than screws. The siding is nailed through the OSB and foam into 1-in. planking that is attached to the framing, creating a SIP(SIP) Building panel usually made of oriented strand board (OSB) skins surrounding a core of expanded polystyrene (EPS) foam insulation. SIPs can be erected very quickly with a crane to create an energy-efficient, sturdy home. -like foam sandwich.
But, Chandler says, be sure to add an insect barrier at the bottom of the wall. To that end, his company bought a large sheet metal brake so the crew can produce custom Z-flashing.
Yes, but foam has risks, too
Attaching the siding to the building is but one issue.
According to some (but by no means all) builders, rigid foam insulation on the exterior of the building ("outsulation," as it's called) carries with it an increased risk of water damage inside exterior walls, because the foam can trap moisture and prevent walls from drying to the exterior.
Remsberg herself points up this issue: "On moisture management, my concern is that the outsulation does not prohibit the wall from breathing properly," she says. "The choice of insulation type is also a topic which I welcome comments on. As this is mostly a heating climate, I do not want to create a vapor barrier in the outer portion of the wall."
Michael Maines points out that with 5 1/2 in. of dense-pack cellulose in the walls (R-21), in a typical winter temperature scenario, condensation is possible if the foam were less than 1 in. thick.
"Your variables may be different, but you have to be careful with exterior foam in a cold climate," Maines says. "Done right, it's great. Done wrong, even the best rain-screen detail won't do anything to save you from moisture problems. If you're even considering foam I hope you don't have a poly vapor barrier on the inside of the wall assembly."
Robert Riversong adds that dense-pack cellulose is a good idea when wall assemblies include rigid foam on the exterior. Cellulose, he says, "has far better moisture absorption and release properties than almost any other insulation (except straw or end-grain wood). But, again, the caveat is: no interior vapor barrier (1 perm OK)."
Or, get rid of the rain screen
The idea behind a rain-screen is to provide an air space behind the siding so it can dry out. Maines suggests a product made by Cor-a-Vent will help encourage air flow and promote drying. But in this case, the climate where Remsberg is building is so arid that 1x3 battens nailed through foam into the studs would be just fine.
"I agree with Michael that, in your arid climate, a rain-screen is not necessary, provided the siding is back-sealed and there is proper integration between [the water-resistive barrierSometimes also called the weather-resistive barrier, this layer of any wall assembly is the material interior to the wall cladding that forms a secondary drainage plane for liquid water that makes it past the cladding. This layer can be building paper, housewrap, or even a fluid-applied material.] and flashings," Riversong adds.
"But your builder is correct that siding problems were not uncommon when applied directly over rigid foam board. So some spacing, with provisions for good nailing (1½ in. into framing for smooth nails or 1¼ in. into framing for ring shank)," he adds. "Be aware that excessive nailing through exterior foam reduces its insulating capacity, particularly at the studs that it's supposed to be thermally breaking."
Tom Schirber would go further. He recommends something called the Exterior Thermal Moisture Management System (ETMMS), which he says is an adaption of the Canadian PERSIST approach that does not include a rain-screen.
Schirber writes that for the best results, ETMMS incorporates "drainage planePath that water would take over the building envelope. Concealed drainage-plane materials, such as building paper or housewrap, are designed to shed water that penetrates the building’s cladding. Drainage planes are installed to overlap in shingle fashion (weatherlap) so that water flows downward and away from the building envelope./vapor barrier/air barrier all in one product on the building structure (outside sheathing) and on the foundation."
His choice is Perm-A-Barrier by Grace Construction Products. This barrier is followed by rigid foam strapping and then siding.
There's no need, he says, for housewrap beneath the siding or an interior vapor retarder. The technique was addressed by Joseph Lstiburek in an article called The Perfect Wall .
It works on any structure and in any climate and is so good, Schirber adds, "it is a bit silly to use the conventional methods."
Nothing, even the Perfect Wall, is ever perfect
Yes, Riversong adds, we know know that convection, not vapor diffusion, accounts for most of the movement of water vapor through the walls. Hence the gradual abandonment of interior poly vapor barriers in favor of effective air barriers.
According to Riversong, the problem with systems that slow or eliminate outward drying is that they can keep the building envelopeExterior components of a house that provide protection from colder (and warmer) outdoor temperatures and precipitation; includes the house foundation, framed exterior walls, roof or ceiling, and insulation, and air sealing materials. warm enough to encourage mold and decay, and bulk moisture that finds a way in has no way of getting out.
"Science and technology has yet to become 'smart' enough to outwit the gremlins of Mother Nature," he says. "When we (re)learn that She is still far more powerful than we, we will return to some of those 'common wisdom' approaches to working WITH the forces of nature rather than thinking we can outwit them with our cleverness (what the Greeks understood as fatal hubris)."
We asked GBA advisor Peter Yost for an expert opinion:
Building Science Corp.'s general rule of thumb is that a vented rain-screen isn't necessary in climates with less than 20 inches of rain annually. One caveat is for sites subject to wind-driven rain and snow. That ups the ante and means less than 20 inches of rain or snow annually might still warrant the vented or ventilated space.
A caveat for board-and-batten claddings
The vertical wood-to-wood contact in the direction of water flow means more moisture being held at the wood overlaps than for most other wood claddings. So, a ventilated air space will help keep the board and battens from differentially drying, shrinking, and cracking. It's sort of the 150+ year approach versus the 50+ year approach.
And if your project is in a wildfire zone, use heavy-duty screening to keep embers and burning brands from getting behind the wall cladding and into the vented space.
On securing the spacers to the framing and the cladding to the spacers:
The building code says that wood cladding fasteners must penetrate the studs, generally an inch of penetration (they weren't really thinking of our high performance exterior insulation systems apparently). BSC is engaged in research to show that connecting the furring to the studs and the cladding to the furring introduces no shear issues. It is unlikely that your building inspector will fret over this, but BSC work to date has shown NO shift across the rigid insulation with any cladding systems with furring strips creating a vented, drained space.
Getting ventilation (not venting) with horizontal furring strips:
if you have openings top and bottom with vertical siding and horizontal furring, how do you get air flow or ventilation behind the cladding? It is not clear that honeycombed materials such as Cor-a-vent or Battensplus have the holding power for cladding fasteners equivalent to wood furring strips. Either sufficiently fasten the cladding through to framing as spacing allows or work with the manufacturer of the honeycombed spacer to test and verify holding power with appropriate fasteners.
Using exterior foam insulation:
We only put insulation in cavities because the space is there and it is the least expensive location; it is not the best location. Exterior insulation "warms" the framing cavity and helps to prevent interstitial condensation. Use the "dewpoint" test to determine what thicknesses of exterior and cavity insulation in combination keep you out of trouble for your climate, your assemblies, and your interior set points (temperature and relative humidity).
And build every home with a decent hygrometerA device that measures relative humidity of air. Mechanical hygrometers that rely on a coil of thin metal are not terribly accurate; electronic hygrometers available at most electronic or hardware stores are usually accurate to about plus or minus 2 - 3%. for your clients -- they need to know both temperature and relative humidity for the best "comfort" level for your structure, as well as their own comfort.
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