# Exterior Sheathing advise for board & batten



## LowbanksArcher (Sep 3, 2016)

Looking for advise/confirmation on my plan here.
We're planning to replace our siding with pine board and batten. 12"x1" boards. Currently, house is 1/2" Sheathing nailed directly to the studs.
My plan:
- run 1"x4" strapping over top of the sheathing, 24" apart, for a nailing surface for the vertical board and batten.
- add 1" rigid insulation sheets between the strapping to fill the void and add R value to the walls.
- wrap with house wrap / tyvek.
- ready for board and batten siding.

Thoughts? Do I need the housewrap?


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## johneh (Sep 3, 2016)

If you run the strapping horizontal it will work
Why are you adding foam ? Extra insulation ?
Yes use the house wrap.


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## Lake Girl (Sep 3, 2016)

Since you are doing siding, I would also seal windows, doors and rim joists with this:  https://www.menards.com/main/doors-...-flashing-tape-butyl-base/p-1444426702337.htm  Stops wind/water infiltration at those weak points...


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## LowbanksArcher (Sep 3, 2016)

Yes, the foam is to add insulation. Framing is 2x4 with likely R12 worth of fiberglass. I figure adding an extra layer of insulation shouldn't hurt. 
Thanks for the advise Lake Girl. I'll have a look for that stuff.


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## Lake Girl (Sep 3, 2016)

Got the major thumbs up from an engineer when we used that on the sills and over the nailing flange/tyvek transition on our condo when we replaced windows as part of an exterior wall rehab.  The only windows that were done properly!


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## iron (Sep 4, 2016)

careful adding insulation on the outside of your sheathing. you could be creating a situation where the sheathing is then in a semi-heated space and it collects moisture. i'm not an expert, but do recall reading about issues with this.


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## LowbanksArcher (Sep 5, 2016)

Lake Girl said:


> Got the major thumbs up from an engineer when we used that on the sills and over the nailing flange/tyvek transition on our condo when we replaced windows as part of an exterior wall rehab.  The only windows that were done properly!


Yep. I'll definitely pick some of this up. I see Home Hardware and Home Depot have their versions of it. tyvek and blueskin. I'm replacing windows as well. Adding this stuff on top of the nailing fin on the new windows will work great.


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## LowbanksArcher (Sep 5, 2016)

iron said:


> careful adding insulation on the outside of your sheathing. you could be creating a situation where the sheathing is then in a semi-heated space and it collects moisture. i'm not an expert, but do recall reading about issues with this.


I'm curious about this as well. New building code here will now require all homes to have external rigid foam insulation added. I'm thinking that as long as there is no air gap for semi heated air, there shouldn't be any condensation. Even the 1" pine boards themselves would have an R1 value. Then the 1" foam would add another R3.75


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## DickRussell (Sep 5, 2016)

LowbanksArcher said:


> ....Then the 1" foam would add another R3.75



That's too little R value for the exterior. Here is an interesting read for how thick the foam needs to be: http://www.greenbuildingadvisor.com...lating-minimum-thickness-rigid-foam-sheathing


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## LowbanksArcher (Sep 5, 2016)

DickRussell said:


> That's too little R value for the exterior. Here is an interesting read for how thick the foam needs to be: http://www.greenbuildingadvisor.com...lating-minimum-thickness-rigid-foam-sheathing


Interesting. In my case there is already R12 of batts in the walls. Also, anyone installing vinyl siding around here will add 3/8" thick foam first.   I wonder if that could cause the same issues


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## DickRussell (Sep 6, 2016)

LowbanksArcher said:


> Interesting. In my case there is already R12 of batts in the walls. Also, anyone installing vinyl siding around here will add 3/8" thick foam first.   I wonder if that could cause the same issues



When you add exterior closed-cell foam with little ability to let water vapor diffuse through it, you prevent the ability of the wall assembly to dry to the exterior, which you need in winter, because the interior air nearly always is more humid than the outside air. Adding an exterior foam layer also results in the inside surface of the sheathing becoming warmer. The idea is to have that surface become warm enough to be above the dew point of interior air, so that moisture accumulating within the cavity won't condense on and be absorbed by the sheathing and adjacent framing. The steady-state temperature profile across the wall assembly can be calculated from the R values of the layers making up the wall and the design outside air temperature. The more cavity insulation you have, the more exterior insulation you need to keep the sheathing above the dew point.

As to the adding of 3/8" foam under vinyl siding, if it's foil-faced foam it's a bad idea. The foil facing is a vapor barrier on the wrong side of the wall assembly for a heating climate, and that thin layer of foam doesn't provide anywhere near enough insulation to keep the sheathing above the dew point. When doing this works without problem, it likely is due to there being so much air leakage through the wall assembly (older construction) that accumulated moisture can dry by convective flow before it becomes a problem.


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