Good exterior wall design? No plywood!
Last Post 28 Nov 2017 11:53 AM by WadeWebb. 15 Replies.
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loghomebuilderUser is Offline
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18 Aug 2017 05:19 PM
I am building a log home which requires the Gable ends to be stick framed. This is unique in that it is an exterior wall but it is not structural. My idea is this. 2x4 walls, with 1 or 2" of eps foam screwed to the extrior. Then horizontal furring strips, creating a rain screen, with board and batten siding on the exterior. From the interior, closed cell foam sprayed into the bays. This gives it extra ridgidity. And again, horizontal furring strips, so that I can fasten board and batten on the interior. This would be super insulated and complete air barrier and thermal break. Also it requires no plywood. Is this a good idea or bad idea? Eps foam or xps foam for this application? Also, anything I need to know about the rain screen?
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18 Aug 2017 07:27 PM
Are you certain it isn’t structural and why have a gable when you could have full hip? Anyhow...

Using some plywood or OSB would create more rigidity and strength than foam. Furring strips should be vertical and have vents at top and bottom for rain screen. Why do you need horizontal furring strips on interior to fasten siding? Can’t you just fasten it to the studs?

And I wouldn't call this assembly super insulated and sealed either. But if you are building with logs this doesn't really matter since the rest of the building assembly will leak like a sieve anyways.
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loghomebuilderUser is Offline
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18 Aug 2017 09:26 PM
The horizontal furring strips is for the board and batten siding, need something to screw to. Board and batten will be on inside too.

Spray foam plus rigid foam board not super insulated? I think it is.
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18 Aug 2017 11:04 PM
This is a green building site. 2x4 anything isn't SUPER insulated...just insulated.
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loghomebuilderUser is Offline
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19 Aug 2017 04:48 PM
Every study I have read said that more than 3" of spray foam is unessecary due to dimminishing returns. I've even read one study that put 3" of spray foam at more like an R 54 when compared to fiberglass Batts.

Then adding 2" more of foam for a total thermal break. Idk, tons of R value, no thermal bridging, completely air sealed.... I'm just wondering if I am over looking anything with regard to skipping the plywood. I don't believe I need it for structural reasons and if it is not there, it can't mold or rot due to condensation.
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19 Aug 2017 08:54 PM
3.5” of foam in place urethane is about R20 and 2” of EPS panel is about R10. So you are looking at about R30 total and perhaps minus something for stud thermal bridging (between the interior siding surface and the interior EPS panel service) depending on the stud spacing, if you have any studs.

We have a building assembly moisture analysis calculator on our website that will tell you if, where, and how much the moisture accumulation rate will be within a building assembly given your climate conditions:

Borst Building Assembly Moisture Analysis Calculator

When you say "it is an exterior wall but isn't structural", that implies that this wall won't see any loads at all. If it is an exterior wall, it will at least see wind loads which are often not insignificant.

I am not aware of any building code that would allow not having some sheathing (plywood or OSB) to strengthen an exterior stick built wall. Have you considered using SIPs for this application?
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20 Aug 2017 07:00 PM
When he talks about installing board and batten siding, I'm assuming he is talking about using 1x12 or similar width lumber installed vertically with 1x2 strips covering the vertical joints between the boards. Since this is a log home, I would assume he is using rough cut lumber in which case the actual thickness is at or near 1"; although, he may be using something like western cedar that may only be 3/4" thick. Regardless, this provides substantial strength to the assembly especially considering that it is a non-load bearing wall in the gables.

I can rarely see the justification for using spray foam as cavity insulation in stud walls if damp sprayed cellulose is available. You will likely never get back the ~5x added expense of spray foam and the overall wall assembly R value will only be marginally greater than that of sprayed cellulose due to the wood studs passing through the cavity insulation every 16"-24". Plus, if you are installing board and batten on the inside, you can go ahead and install your horizontal interior strapping before spraying the cellulose in the cavity in which case the the cellulose will be the full depth of the vertical studs plus the horizontal strapping thickness. This will likely give you a higher overall wall R value that just spraying foam in the stud cavities, especially if you are using 2x4 horizontal strapping (similar to a Mooney Wall).


My recommendation would be to air seal the joints in the rigid foam insulation and spray in cellulose in the cavities if available in your area. If you install horizontal strapping (2x4 ?) on the outside of the foam to support your board-and-batten siding, I would make shallow vertical saw cuts in the front and back side of the horizontal strapping every 6" or so to insure that no water can stand or accumulate on this horizontal ledge created by the strapping.

You might also consider installing 4x8x1" foam board sheets vertically on your studs, then install 2x4 horizontal straps with a 24" gap between them, then install 4x8x1" sheets of foam board ripped in half (2x8x1") horizontally between the strapping. Insure the vertical joints in the inner and outer layers of foam don't line up. Caulk the inner layer foam joints just before installing the outer layer of foam to seal them together. Then I would caulk the outer layer of foam to the horizontal strapping. It might actually work better to install the first horizontal strap, caulk the top edge where the outer foam will sit, then install a 2' wide strip of foam, then caulk the top edge of the foam, then install the next horizontal strap and so on and so on. This would also leave you with a 1/2" air gap between the foam and siding. I would definitely make shallow vertical saw cuts in the outside edge of the strapping where it meets the siding to allow moisture and air movement.
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20 Aug 2017 07:20 PM
Diagonal steel strapping allows the building of structural walls with no plywood or OSB and indeed, such a design will reduce moisture concerns. But racking requirements vary with location.

Discussion of prescriptive code compliance is here: https://www.strongtie.com
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20 Aug 2017 08:53 PM
Oh really? Can you please point to where in section 602.10 it permits using metal strapping for shear bracing walls?

While I believe it could certainly be engineered, I don’t personally know of any engineers who would readily sign off on this approach...too much variability in fastening robustness over time. There is some general simplified information on wall bracing options here, but even it points you to an engineer if you are considering going down the metal strapping path:

Options for Shear Bracing Foam Sheathed Walls

I also have little doubt that some builders may use this approach if codes and inspections are not an obstacle to improve their profit margin. However, I would like to see something more definitive before endorsing this approach as a general prescriptive option.
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21 Aug 2017 12:33 AM
Arkie, your suggestions have much merit although they are somewhat labor intensive. Also especially good to emphasize the need for vertical channels for rain screen functionality.

When he said board and batten, I just envisioned Hardie Board panel fastened to studs and Hardie Board battens put over the panel seams and whatever OC stud spacing. You are likely correct that since this is a log cabin, he will likely be using real board and batten. We always use the Hardie Board fiber cement because it doesn’t ever rot and it provides a high degree of fire resistance which is important in the wildfire risk areas we build. We also frequently use the Benjamin Obdyke rainscreen products (e.g., Slicker Max).
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21 Aug 2017 09:54 AM
I second using SIP's, especially steel-skinned ones to get away from plywood or OSB.
loghomebuilderUser is Offline
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22 Aug 2017 11:32 AM
Yes I am using real board and batten. True 1" thick larch boards. A strong and rot resistant wood Inside and out. There will be no racking forces on the wall, or yes I would use plywood. Only wind forces. And I think with the 2x4 strapping and board and batten it would be strong enough. I didn't think cellulose would best because it could escape the wall cavity with all the gaps in the board and batten. Or is this not a concern with damp spray? I've used dense pack cellulose before and that can get everywhere. The wall is 38 foot wide and goes from about 9 ft at the peak down to zero at the corners.
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22 Aug 2017 02:43 PM
While an expensive and labor intensive way to construct a wall, I think it will work okay. Do be careful in applying the spray foam as recommended as many a house fire has started from the exothermic curing reaction when too much was applied at once.
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22 Aug 2017 07:04 PM
Damp sprayed cellulose has water activated adhesive (typically starch) to bind the fibers together and to whatever it comes in contact with while wet. While still damp, the surface is then skimmed smooth to wall framing. It is locked in pretty solid when fully dry with virtually no dust or loose fibers if done correctly. If the wall is air sealed at the exterior foam, I can see no reason the cellulose would escape the cavity even with large gaps in the board and batten on the inside.

Another advantage to cellulose is increased fire resistance.

http://www.greenbuildingadvisor.com/articles/dept/green-building-blog/insulating-damp-spray-cellulose
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22 Nov 2017 05:50 PM
Posted By arkie6 on 20 Aug 2017 07:00 PM

...........I can rarely see the justification for using spray foam as cavity insulation in stud walls if damp sprayed cellulose is available. .............

The log home builder I worked for back in 1983 used cellulose in his gabled ends.

The reasoning as I recall was that no where else in the construction was there a vapor barrier and cellulose didn't need one.

The moisture absorption and release of moisture of the logs was near equal, so that one was not feeding/beeeding moisture into the other at any given point in time.

And of course the non-synthetic nature of log homes just sort of scorns foam. 

They did use foam board below grade, had to be XPS back in those days, EPS wasn't what it is today..
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WadeWebbUser is Offline
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28 Nov 2017 11:53 AM
This is an excellent idea to create a good exterior wall design! Thanks for sharing!
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