Robert Ellenberg
BRONZE MEMBER
I have been researching this for several weeks and it just seems to get more and more complicated so I thought I’d throw out what I am finding for discussion. The problems and conflicts I am finding apply to colder climates (zone 6 or greater) but to some degree in zone 5 as well.
As a general statement it is accepted that rot inside of walls has been caused by moisture. A lot of it is from exterior water penetration and therefore sealing the exterior with a well detailed house wrap and providing a rain screed space can eliminate most of that water getting in. However, when you seal the exterior any water than gets inside the wall from within (moisture that can no longer go through a wall that is sealed) can condense in cold climates and cause the rot. So, in colder climates, exterior foam sheathing seems to be encouraged to increase wall insulation (reduce energy) and having a layer of continuous insulation over the studs greatly reduces thermal bridging.
However the wall must be able to breathe and dry out if moisture gets inside. The IRC used to require inside vapor retarders. It has been recognized that with foam and or house wrap on the exterior, that a class I or II vapor retarder on the interior did not allow a way for moisture that built up in the wall to dry out (LOTS OF DISCUSSIONS ABOUT THIS SUBJECT ON NUMEROUS FORUMS). A 2007 supplement reduced the requirement for the interior vapor retarder as table N1102.5.1 and then in the 2009 IRC it was incorporated in R601.3.1. These tables state that if you have a 2x4 wall sheathed in foam in zone 5 that it must be at least R5 (generally 1” thick) and R7.5 in zone 6. Thicker walls (2x6) require even thicker foam.
How do you get that foam on there and meet fastening requirements for sheathing and siding? I have studied R703 and table R703.4. The longest siding nails available are 3” (hand drives at that). I wrote James Hardi and they sent me 3 technical bulletins, 2 deal with attaching siding to furring strips and one to SIPS but nothing on fastening over foam sheathing though it was implied that the foam sheathing would be under the furring strips. They also very specifically disclaim any responsibility for how the furring strips are attached and I don’t find that in the code.
To further complicate things, table N1102.1 seems to approve 1” foam over a 2x4 R13 wall in zone 6, which is specifically prohibited now in R601.3.1.
From a code standpoint, it appears you can’t sheath the exterior wall in foam and then meet the fastening requirements for attaching the siding. The code doesn’t require the exterior foam (that I can find) but I thought the purpose was to reduce the possibility of the sheathing and wood framing from getting wet behind the exterior house wrap in cold climates which IS now required.
Unless I am missing something, here is what I see as the options (per the code):
No foam on the exterior which will almost guarantee condensation on the warm side of the sheathing and possibly start to rot the wall.
Foam sheathing over studs as spelled out above ranging from 1” to over 2”. Hardi (and I presume others) give requirements for the thickness of furring strips for anchoring siding so they will have to be at least 1.5” thick to meet IRC nail penetration and even more for Hardi specs (cannot be 1x2 furring).
Long screws to fasten the furring strips through the foam and into the studs would have to be speced by an engineer since there is no prescriptive code guidelines on how to do this and get adequate holding strength to in turn hold them and the siding.
Somebody tell me where I am wrong on this analysis.
As a general statement it is accepted that rot inside of walls has been caused by moisture. A lot of it is from exterior water penetration and therefore sealing the exterior with a well detailed house wrap and providing a rain screed space can eliminate most of that water getting in. However, when you seal the exterior any water than gets inside the wall from within (moisture that can no longer go through a wall that is sealed) can condense in cold climates and cause the rot. So, in colder climates, exterior foam sheathing seems to be encouraged to increase wall insulation (reduce energy) and having a layer of continuous insulation over the studs greatly reduces thermal bridging.
However the wall must be able to breathe and dry out if moisture gets inside. The IRC used to require inside vapor retarders. It has been recognized that with foam and or house wrap on the exterior, that a class I or II vapor retarder on the interior did not allow a way for moisture that built up in the wall to dry out (LOTS OF DISCUSSIONS ABOUT THIS SUBJECT ON NUMEROUS FORUMS). A 2007 supplement reduced the requirement for the interior vapor retarder as table N1102.5.1 and then in the 2009 IRC it was incorporated in R601.3.1. These tables state that if you have a 2x4 wall sheathed in foam in zone 5 that it must be at least R5 (generally 1” thick) and R7.5 in zone 6. Thicker walls (2x6) require even thicker foam.
How do you get that foam on there and meet fastening requirements for sheathing and siding? I have studied R703 and table R703.4. The longest siding nails available are 3” (hand drives at that). I wrote James Hardi and they sent me 3 technical bulletins, 2 deal with attaching siding to furring strips and one to SIPS but nothing on fastening over foam sheathing though it was implied that the foam sheathing would be under the furring strips. They also very specifically disclaim any responsibility for how the furring strips are attached and I don’t find that in the code.
To further complicate things, table N1102.1 seems to approve 1” foam over a 2x4 R13 wall in zone 6, which is specifically prohibited now in R601.3.1.
From a code standpoint, it appears you can’t sheath the exterior wall in foam and then meet the fastening requirements for attaching the siding. The code doesn’t require the exterior foam (that I can find) but I thought the purpose was to reduce the possibility of the sheathing and wood framing from getting wet behind the exterior house wrap in cold climates which IS now required.
Unless I am missing something, here is what I see as the options (per the code):
No foam on the exterior which will almost guarantee condensation on the warm side of the sheathing and possibly start to rot the wall.
Foam sheathing over studs as spelled out above ranging from 1” to over 2”. Hardi (and I presume others) give requirements for the thickness of furring strips for anchoring siding so they will have to be at least 1.5” thick to meet IRC nail penetration and even more for Hardi specs (cannot be 1x2 furring).
Long screws to fasten the furring strips through the foam and into the studs would have to be speced by an engineer since there is no prescriptive code guidelines on how to do this and get adequate holding strength to in turn hold them and the siding.
Somebody tell me where I am wrong on this analysis.
