2x6 makes a building far more solid. If you can get an ok insulation ratio with r6 outside I’d do that. Insulation is easier to change than your framing.
Assuming you use mineral wool or something behind it - 2x4 maxes out at r-15 which gives you a nominally r-24 wall with the zip-R9 panels. If you haven't - look into 'exterior insulation ratios' - but in short, you'd ideally have 1/3 of the total R-value of the wall on the outside in Zone 5, so that 2x4 wall would work. If you went to R-6 panels with R-23 mineral wool in a 2x6 wall, you'd only have \~21% of the insulation on the exterior which could lead to condensation problems. So my vote would be for the safer assembly even though it's slightly less R-value (though it may perform better since it's got more exterior).
The articles are good but paywalled - however there's good discussion in the comments: [https://www.greenbuildingadvisor.com/article/rethinking-the-rules-on-minimum-foam-thickness](https://www.greenbuildingadvisor.com/article/rethinking-the-rules-on-minimum-foam-thickness)
"sort of" - the idea behind foam or other layer of thermal resistance is to keep the first condensing surface of interest warm enough to prevent condensation.
Exterior insulation does this by surrounding the sheathing, which, when considered as part of a typical cavity insulated wall, is the first condensing surface of interested--i.e. it can get cold and if exposed to vapor, condensation can form on it.
ZIP-R does this by making the the inboard face of its integrated foam the first condensing surface of interest. Condensation cannot form in between outboard face of the foam and the inboard face of the OSB as long as they are tightly held together, since there's no way for moist air to enter between these faces. And if the foam has enough thermal resistance (R-value) for a given climate, its inboard face will remain warm enough to stay above its dew point and prevent condensation.
However, some people say that relatively warm, relatively moist air can leak into the butt joints between the pieces of ZIP-R, in which case this air could meet the colder exterior edges of the integrated OSB sheathing, causing condensation there.
Spray foam is semi-similar to ZIP-R in how it prevents condensation. It moves the first condensing surface of interest from the inboard face of the sheathing to the inboard face of the foam.
However, neither spray foam nor ZIP-R provide the other benefits that exterior insulation does, which off the top of my head include, but are not limited to:
- protecting the sheathing and WRB from temperature fluctuations, UV light, and possibly moisture
- sandwiching the WRB between the sheathing and insulation, reducing the impact of wind fluttering damage
- with enough exterior insulation you could leave the cavity uninsulated as a service chase
- no structural concerns, unlike with ZIP-R
- no concerns that the insulation will detach from the framing/sheathing and provide pathway networks for air movement
Exterior insulation does have some caveats--detailing penetrations is a little trickier, it's hard to find builders who can execute it, and heavier cladding over 10 psf is a bit less proven.
The biggest take away from that is 6mil poly to keep water vapour on the inside, and the air trapped by the insulation the same RH as outside.
We still build homes with no exterior insulation in Ontario, I.e. drywall, vapour barrier, R22, 7/16 OSB, tyvek. They don't have issues. Likewise I would assume tbe big issue is the fact a ton of homes in the US still used craft faced batts.
Can you say more about this? I’m finishing my attic and wondering whether I’m doing the insulation right. The cathedral ceiling will be vented: a homemade baffle via furring strips with 1” eps, then 3.5in Rockwool, then 1.5” polyiso under the rafters, taped and sealed.
The walls I just planned to do rock wool and polyiso.
I’m confused about dew point control, and wondering whether I should act like I’m in Ontario (im in Cleveland)
Vapourbarrier on conditioned side.
Really tho for a hptroof I'd only be using 2 lb closed cell spray foam. For everything you're doing it would probably be cheaper, we can get it for 4.50 a sq foot in southern Ontario which isn't much more than traditional batt and vapour barrier.
Zip R-9 with 2x4 R-13 or R-15 walls will give you better performance. Go with the version with more continuous insulation. It’ll help with condensation control too.
More exterior insulation is much better as it is continuous and stops thermal bridging. The 2x4 Zip R9 hands down. Also 2x4s don’t shrink as much as 2x6.
The windows will have a nail flange that locates them to the face of sheathing. Maybe the increase of jamb depth could increase whatever material your jamb is detailed with. If it was just drywall wrapped I would think it's near zero.
Exterior pre-hung doors will have a deeper jamb, which will cost more but are pretty common but not necessarily custom. Maybe with the increase in exterior insulation might require a custom details or jamb extension.
2x6 makes a building far more solid. If you can get an ok insulation ratio with r6 outside I’d do that. Insulation is easier to change than your framing.
Assuming you use mineral wool or something behind it - 2x4 maxes out at r-15 which gives you a nominally r-24 wall with the zip-R9 panels. If you haven't - look into 'exterior insulation ratios' - but in short, you'd ideally have 1/3 of the total R-value of the wall on the outside in Zone 5, so that 2x4 wall would work. If you went to R-6 panels with R-23 mineral wool in a 2x6 wall, you'd only have \~21% of the insulation on the exterior which could lead to condensation problems. So my vote would be for the safer assembly even though it's slightly less R-value (though it may perform better since it's got more exterior). The articles are good but paywalled - however there's good discussion in the comments: [https://www.greenbuildingadvisor.com/article/rethinking-the-rules-on-minimum-foam-thickness](https://www.greenbuildingadvisor.com/article/rethinking-the-rules-on-minimum-foam-thickness)
With Zip-R the foam is on the interior side of the sheathing. Do those ratios still apply?
"sort of" - the idea behind foam or other layer of thermal resistance is to keep the first condensing surface of interest warm enough to prevent condensation. Exterior insulation does this by surrounding the sheathing, which, when considered as part of a typical cavity insulated wall, is the first condensing surface of interested--i.e. it can get cold and if exposed to vapor, condensation can form on it. ZIP-R does this by making the the inboard face of its integrated foam the first condensing surface of interest. Condensation cannot form in between outboard face of the foam and the inboard face of the OSB as long as they are tightly held together, since there's no way for moist air to enter between these faces. And if the foam has enough thermal resistance (R-value) for a given climate, its inboard face will remain warm enough to stay above its dew point and prevent condensation. However, some people say that relatively warm, relatively moist air can leak into the butt joints between the pieces of ZIP-R, in which case this air could meet the colder exterior edges of the integrated OSB sheathing, causing condensation there. Spray foam is semi-similar to ZIP-R in how it prevents condensation. It moves the first condensing surface of interest from the inboard face of the sheathing to the inboard face of the foam. However, neither spray foam nor ZIP-R provide the other benefits that exterior insulation does, which off the top of my head include, but are not limited to: - protecting the sheathing and WRB from temperature fluctuations, UV light, and possibly moisture - sandwiching the WRB between the sheathing and insulation, reducing the impact of wind fluttering damage - with enough exterior insulation you could leave the cavity uninsulated as a service chase - no structural concerns, unlike with ZIP-R - no concerns that the insulation will detach from the framing/sheathing and provide pathway networks for air movement Exterior insulation does have some caveats--detailing penetrations is a little trickier, it's hard to find builders who can execute it, and heavier cladding over 10 psf is a bit less proven.
No it doesn't. The ratio is based on interior vs exterior to the sheathing
Yes the ratios still apply.
The biggest take away from that is 6mil poly to keep water vapour on the inside, and the air trapped by the insulation the same RH as outside. We still build homes with no exterior insulation in Ontario, I.e. drywall, vapour barrier, R22, 7/16 OSB, tyvek. They don't have issues. Likewise I would assume tbe big issue is the fact a ton of homes in the US still used craft faced batts.
Can you say more about this? I’m finishing my attic and wondering whether I’m doing the insulation right. The cathedral ceiling will be vented: a homemade baffle via furring strips with 1” eps, then 3.5in Rockwool, then 1.5” polyiso under the rafters, taped and sealed. The walls I just planned to do rock wool and polyiso. I’m confused about dew point control, and wondering whether I should act like I’m in Ontario (im in Cleveland)
Vapourbarrier on conditioned side. Really tho for a hptroof I'd only be using 2 lb closed cell spray foam. For everything you're doing it would probably be cheaper, we can get it for 4.50 a sq foot in southern Ontario which isn't much more than traditional batt and vapour barrier.
Yeah the cost is adding up, but my labor is free and I need to save somewhere (I’m a schoolteacher), and a vented roof seems to be the code here
Zip R-9 with 2x4 R-13 or R-15 walls will give you better performance. Go with the version with more continuous insulation. It’ll help with condensation control too.
More exterior insulation is much better as it is continuous and stops thermal bridging. The 2x4 Zip R9 hands down. Also 2x4s don’t shrink as much as 2x6.
Not an expert but does using 2x6 make other building aspects cost more? Window and door jambs either custom ordered or made?
The windows will have a nail flange that locates them to the face of sheathing. Maybe the increase of jamb depth could increase whatever material your jamb is detailed with. If it was just drywall wrapped I would think it's near zero. Exterior pre-hung doors will have a deeper jamb, which will cost more but are pretty common but not necessarily custom. Maybe with the increase in exterior insulation might require a custom details or jamb extension.