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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

Posted on Sep 1 2010 by Scott Gibson

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.

Sep 1, 2010 10:30 PM ET

rigid over floor assembly
by steven bumpus

i was just contemplating today a concern if I use 2-2" layers of foil polyiso and furr- strips/siding to extend a possible 5" horizontally past sheathing/floor assembly of house. It would seem the astetics of shadowing and potential for bug/aviary nests or other creatures to build underneath base of assembly could be a problem and I haven't seen it addressed in past. Am I missing something, or is there an alternate fix to keep basement/sheathing planes close to each other? I'm planning a poured superior type basement, and don't want to add insul. board to exterior of concrete.thots?

Sep 2, 2010 1:34 PM ET

Basement/Sheathing Planes
by Kevin Dickson

The insulation board on the exterior of the concrete basement wall is advantageous. Of course, use XPS, and you then must protect it. But it helps capture the advantages of having the thermal mass inside.

It's not to difficult to install, because the backfill holds it in place, so you might not need fasteners except at the top, and then only if more than 12" is exposed. It sounds like you won't need to insulate all the way down to the footing, so the expense is minimal.

Sep 2, 2010 11:19 PM ET

I used Board & Batten siding over 2" foam insulation
by John MacDougall

My new house has 2x6 wall framing with blown cellulose in the cavities and 2" of xps insulation on the outside of the osb sheathing. The horizontal blocking is 2x3" on top of 1" of xps foam for a total of 2.5" thick. This creates a 1/2" air space behind the rough sawn local spruce siding. This seemed like the best way to create a thermal barrier and a rain screen which is probably not required here in the Yukon with an annual rainfall of 10 - 12" (semi-arid climate). I furred out the windows by using screwing the nailing fins to 1/2" plywood with 1.5" xps underneath so everything lines up. Lots of tedious work putting it all together but the results are nice.

Sep 3, 2010 7:43 AM ET

I went the XPS route 24 years
by steve bumpus

I went the XPS route 24 years ago, and it's required route maintenance( reapplications of fiberous concrete product) which never sat well with me! I was hoping this time for the concrete maint. free look, and besides, I'd need 4" XPS against wall to somewhat match the siding plane of walls with the 4" foil, which somehow just doesn't seem right. Not sure if floor assembly build with recessed shelf around perimeter of basement wall is good, since I'd be looking to get full surface support for house. I may need to put out specific question or when i go to architect.

Sep 4, 2010 10:30 PM ET

Ventilation space behind the starpping
by Rob Susz

The horizontal furring is a challenge, but easily overcome. I have never personally used cor-a-vent, but have a few suggestions:

First, use strips of dimpled drainage mat (like Delta MS) there are various thicknesses and crush resistances available. I've used dimple mat all over a building. Cut strips of the dimple may and place between the furring strip and the foam sheathing.

Second, if that assembly is too bulky, use a layer of plastic mesh poultry/livestock fence. It will not create a very large airspace, but will be better than no airspace behind the furring.

And lastly, you can use horizontal bands of shorter furring strips that are set diagonally. It is a bit more work, but if the above options are not acceptable, this will let you use solid wood furring with openings top and bottom.

If you are committed to the concept of exterior foam board you will make it work. Period. Cavity insulation doesn't even enter my head anymore.

My new addition has 4 inches of polyiso on the walls, and 7 1/2" on the roof. No cavity insulation. The rest of the house is getting a layer of 2 " polyiso around the outside over densepacked 2x4 walls. We added 1500 sf (doubled the house) and gas bill increased by $50 per month in the worst month. Just over a 20% increase for 100% more floor space.

Existing home is 1840's era and has no exterior sheathing either, so some details get very interesting.

Sep 6, 2010 9:18 AM ET

Vapor barrier and climate
by Timothy Betts

Here in Wisconsin, we are struggling with the concept of where to place the vapor barrier, either inside or outside the walls. After a few years of wrestling with this myself, I think the answer depends upon the severity of your climate. If you live in an extreme Winter climate such as the upper Mid-West or New England states, then you need to place the vapor barrier on the inside of the house because condensation will take place inside the wall cavity when the outside temps surpass the thermal capacity of your insulation (remember, we need to plan for -30* F in the winter). Since the "moisture drive" is from the inside of the house in the Winter, then we need to keep the moisture from entering the wall cavity in the first place. As a remodeling contractor, I have been faced with the problem of what to do with old farm houses framed with 2x4 walls. The best solution I have found is to stuff the walls with blown fiberglass behind netting, then lay 1/2" of poly-iso foam on the inside of the walls and tape it completely, then hang the sheet rock. The 1/2" of foam provides a thermal break on the studs and the foil tape gives me a completely sealed envelope that prevents moisure from entering the wall, and any that may get in from the outside will still have a path to exit the wall before mold develops.

Sep 6, 2010 11:25 AM ET

Stucco over exterior rigid insulation?
by davars

I am reading with interest these discussions regarding venting/drainage between rigid insulation and exterior siding. Has anyone had experience applying a cement-base stucco over rigid insulation? I am considering this application for a house on the Front Range of Colorado. I am looking for some measure of fire protection and a low-maintenance finish. It seems like the dimpled (Delta drain ?) product fastened with the dimples facing the rigid foam would prevent the cement stucco filling the small cavities between the dimples, thus preserving the ventilation and moisture drainage channels. Of course care is needed to keep the channels open at the top and bottom for full ventilation. My main question is: in the field, how practical is it to do this? With the concern for placing fasteners into the studs which are not visible behind the insulation and dimpled vent/rain screen, how likely is it that the crew will actually be able to accurately get the fasteners into the studs? Wouldn't it require careful transfer of measurements to locate each stud...involving extra time and labor? Then, how well do the fasteners hold the expanded metal lath or chicken wire to the plastic delta drain? Id appreciate any comment.

Sep 6, 2010 11:54 AM ET

Foam on exterior
by Vin Caruso

We built our 2x6 passive solar house over 20 years ago in sw Michigan with foam on ext. and 6mil poly on int., and fiberglass bats. After opening the walls from inside to out on many locations in recent years, for various reasons, we found no signs moisture problems at all. The foam has alum. coatings on both sides it to reflect heat in or out depending on the season. We have T111 sheathing nailed thru the 1/2 foam to the studs.

We are very happy with the very low energy costs and comfort. Highly recommend adding a heat recovery ventilator (HRV), like what we did, to bring in fresh air, vent polluted air and reduce interior moisture (humidity can be an issue even in 10-0 degree weather). HRV should be required on all new homes.

Sep 6, 2010 1:02 PM ET

by Joe

Would SIPS eliminate some of these concerns in an addittion or new construction?

Sep 6, 2010 7:43 PM ET

Double studding.
by James Richard Tyrer

Has anyone considered that it might be better do double stud the wall and put the foal insulation in the middle of the wall?

Sep 7, 2010 5:11 AM ET

Response to James Richard Tyrer
by Martin Holladay

James Richard Tyrer,
As long as you understand the basics, there are many ways to build a wall. But if you deviate from tried-and-true wall assemblies (for example, those suggested by Joe Lstiburek in his various climate-specific books), you had better understand vapor drive and basic building science principles.

A thick, double stud wall with foam insulation in the middle might work fine, or it might be a disaster. It depends on the climate, the exact location of the foam, the R-value of the insulation on either side of the foam, and even on the type of siding you choose.

Such a foam layer might end up being a condensing layer in the summer, in the winter, or both. So do your calculations before you experiment!

Sep 7, 2010 12:02 PM ET

For davars - Stucco over foam
by WesM

Take a look at a product called K-Lath. You could use furring strips (1X4's) to create the air space and apply the K-Lath over the furring. I believe this is one technique they use in the Vancouver B.C. area, for a rain screen stucco application, after the leaky condo debacle.

Sep 7, 2010 4:18 PM ET

Verticle siding with battens
by Clayton Mahan

I designed and built a house over ten years ago in the Pacific NW West of the Cascades. To those that don't know this translates into rainy winters and that means moisture but not real extreme cold. This location is also very close to Puget sound. I used green douglas fir (air dried) 2X6 framing with conventional high R fiberglass insulation. On the outside I used a five ply half inch plywood and not OSB. This had the Tyvex over it and then I used a 12" Hardiboard siding vertically applied.

I left about a 3/8" space between boards to keep each one straight and for a space to shoot in the batten nails. They were 1 1/2 finishing nails. Now at the time the Hardi product was not waranteed for this application unless I treated each board as you would a panel. Say a 4 X 8 foot panel. This means nailing all around the edges 8 or 12 inch spacing. Well now you know why I used plywood. I discussed it with a rep and I was able to shoot both staples and nails through the siding with just a dampening of the edge. The 1 3/4" battens covered the fasteners. I'll discuss painting in a moment.
The airspace under the battens between them and the Tyvex has proven to be worth the effort. I also used clear cedar for these battens as well as the trim. I'll back up. I was told that Hardi tested the siding in winds up to 90 miles per hour. You get this gusting on the Oregon and Washington Coast a few times during the winter. They told be they fastened the siding as per recommended spacing, horizontally applied, but didn't hit the studs as recommended, just the 1/2" OSB. The failure at 90 miles per hour was the flapping of the board pulling out the nails and breaking. Therefore this vertical application elliminates the flapping and applies way more fasteners plus battens.

On painting; first the Hardi siding was painted with a Luxon SW primer made for cement board. Then the battens were applied so that the SW A100 wood primer wood slop over the Luxon and not the other way around. Two coats of primer was used over the Hardi. Then two coats of their Super paint on top. Well just this year, ten years later I hand washed the house and painted one top coat again. After washing it looked new and I really did not have to paint. There was no moistur blowing the paint off like you find in a pine siding for example.

The inside of the walls have 6 mil poly applied before 5/8 sheetrock for a vapor barrier and as a sheetrock gasket if you will. This little poly layer does have a thermal bridging affect. If I ever had to insulate more I would sandwich foam between two layers of half inch plywood on the outside. Say 3/4" foam board. Then I could use four ply plywood.

The walls are double blocked, your welcome fire department, reducing some in wall airflow and all this wood does reduce the overall insulation value but I can say that this house does not show the thermal bridging stud patters through the siding like many on cold damp mornings do. Keep in mind too we expect earthquakes.

For this board and batten look I can say that this has proven an excellent alternative and the added wood for the framing still saved a few trees in that the siding didn't eat them up. If a stained wood look is required I would shoot the wood in with just a 1/4" batten. But I would treat the back of the wood first to slow the curl.

Sep 8, 2010 3:14 PM ET

Riversong - Against Outsulation??
by Sean Wiens

I do not understand why Mr Riversong is so against exterior insulation. He talks about bulk moisture (liquid water) being trapped inside the wall cavity because the resulting vapour cannot diffuse through the insulation panels to the outside. He states this will raise the likelihood of mould growth. But if this system is done right, this moisture can diffuse to the INSIDE of the dwelling and then be carried away by mechanical ventilation. What is the difference? How will one way prevent mould vs. the other way? As far as I know, neither system will be able to handle liquid water well and part of a healthy system relies on proper maintenance to ensure liquid water does NOT enter the wall cavity

Sep 19, 2010 9:22 PM ET

Is it really necessary?
by Travis Thompson

The REM Design modeling I did on a recently completed efficent 2700 sf home showed only $68 per year in energy savings in adding 1" of XPS foam over a R-22 wall assembly. The air leakage was assumed unchanged between the two assemblies. I do not think the added expense and effort combined with possible drying issues is worth the it with the exterior foam. I've done it before but exterior insulation is a major PIA to get the drainage, insect, drying, etc. details "right".

Oct 17, 2010 6:22 PM ET

Foam board, batten, solid concrete block wall, moisture
by PJL

I have two solid block (1950's) exterior (north facing and west facing) walls in a bungalow bedroom. These are very cold walls and often any attempt to warm the room results in condensation. The walls are plastered, semi gloss painted, in good condition and with no signs of damp.
This solution has been recommended:
25mm battens with K18 insulation boards fixed to them (K18: 25mm foam+12.5mm sheet rock)
My concern: Because there is a 25mm air gap between the insulation board and the wall, is this an invitation for moisture to get trapped in there and cause problems down the road?
Are there any precautions to take with this solution?

Oct 17, 2010 9:00 PM ET

Response to PJL
by Martin Holladay

What's your climate?

Oct 18, 2010 2:33 PM ET

by PJL

NW Ireland. Wet windy etc. 'Nuff said?

Oct 18, 2010 3:46 PM ET

Second response to PJL
by Martin Holladay

25 mm of foam is a little less than one inch. If it's EPS foam, that might be R-3.5. If it's XPS foam, that might be R-5. Such a meager amount of wall insulation wouldn't meet the energy code in any location in the U.S. -- even in Florida, where homes are barely heated.

Here in the U.S., building codes require much more wall insulation. I don't think your suggested wall has any problems having to do with trapping moisture; but it would certainly have insufficient insulation. What are the minimum requirements for wall insulation in Ireland?

Oct 18, 2010 5:23 PM ET

Respond to Martin Halliday by PJL
by PJL

Sorry, my misprint: the batten air space is 25mm and the foam + sheet rock is 62.5mm (50mm + 12.5).

Code insulation is only required here on new builds. (Not sure what that requirement is. A solid block concrete exterior wall would, obviously, be illegal these days.)

So you do not think this air space would eventually trap moisture and harbor mould? Would making it air-tight keep it "safe"?

Oct 19, 2010 4:16 AM ET

Third response to PJL
by Martin Holladay

Your block wall is vapor permeable. It is therefore able to dry to the exterior. As long as wind-driven rain doesn't enter through cracks as a liquid -- in other words, as long as it is watertight -- you should be fine.

If liquid water is running down the interior of your wall, you have to address water entry before proceeding.

You want to add as much rigid foam as you can. 50 mm is better than 25 mm, of course, but 100 mm would be much better.

Seal the gaps between the foam panels to make them air tight; also seal the bottoms and tops. If you have any electrical receptacles in the wall, try to make your electrical boxes airtight.

Oct 19, 2010 6:07 AM ET

response to Martin Holladay's third response
by PJL

Thanks. Yes the block wall is in good condition outside and in. I am renting and the landlord does not have a big investment in this house. If I can get a few degrees from this treatment I'll be happy. The main problem I have is that when trying to add heat to the room, just to take the chill out, condensation forms on these cold walls. I sleep in this room which compounds the condensation problem. I have a dehumidifier in the room and wipe the lower half of the walls with bleach every 6 weeks or so to make sure no mold can form. To stop the condensation is the main goal. My main question is really: if I fix the board with pegs and adhesive directly to the wall would it be safer than using battens and an airspace. So I think you have answered the the 25mm airspace should not be a problem so long as it's airtight and the walls are not leaking from the outside.

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