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Old 12-11-2009, 11:28 AM   #16
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Using rigid foam insulation to acheive R-19 in true 2x4 exterior walls?


I'm still confused. Your post seems to contradict itself.

"In cold climates, air barriers and vapor retarders are installed on the interior of the building assemblies. And building assemblies are designed to dry to the exterior..."

"a polyethylene vapor retarder (between the framing and the drywall) is not recommended because it limits a wall's ability to dry to the inside"

Living in Minnesota, with somewhere around 8000 heating degree days, a vapor barrier is necessary on the warm (inside) of the wall. Thus, it would dry to the exterior. I'm not sure what you are getting at with your post. Are you saying that foam (interior) would not be wise in my climate? Or just that IF foam is used (on the interior), an additional poly VB is not necessary?

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Old 12-11-2009, 01:14 PM   #17
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Using rigid foam insulation to acheive R-19 in true 2x4 exterior walls?


I am confused too - not by the science of insulation but by those cut-and-paste posts that Gary seems to provide as typically they are taken out of their context and a link is commonly lost. I respect Gary's input, it's always helpful, but sometimes a bit extra is needed. Let's see if I can help:

"In cold climates, air barriers and vapor retarders are installed on the interior of the building assemblies. And building assemblies are designed to dry to the exterior..."

A 'building assembly' is basically the combination of exterior cladding, building paper, walls, studs, drywall or any combination of such elements. In cold climates, you install vapour barriers or vapour retarders on the warm side of the building assembly i.e indoors. Now, in cold climates usually the heating system causes a positive air pressure on the inside of your home which causes the warm moist air outwards. So we say the assembly 'dries to the outside'. That's for cold climates....

"a polyethylene vapor retarder (between the framing and the drywall) is not recommended because it limits a wall's ability to dry to the inside"

Now in some climates, "vapour barriers" are replaced by "vapour retarders" and can sometiimes be placed elsewhere from what we do in cold climates.

Finally, if you spray-foam the inside of your walls, there is no need for a vapour barrier because spray-foam (closed) is a vapour barrier in itself. If you use any other type of insulation, then there would be a need for one...

Hope this helps...
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Old 12-11-2009, 01:29 PM   #18
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Using rigid foam insulation to acheive R-19 in true 2x4 exterior walls?


Hello,

I am assuming from your post that your building inspector is referring to compliance with the latest International Energy Conservation Code which would be 2006 or 2009 depending on what your state has adopted. Without knowing where in Minnesota you are, I can say the Minnesota straddles climate zone 6 and 7, 6 being the southern and 7 being the northern.

Your building inspector is likely requiring compliance with the prescriptive chart in chapter four which says that minimum wall insulation for zone 6 has to be EITHER R19 cavity OR R13cavity +R5 continous. The first being a typical R19 Batt or blow and the second referring to an R13 cavity batt or blow + an R5 insulated sheathing on the outside. Without knowing how extensive your remodel was, I assume that you cant do th R5 sheathing on the outside. So, you need to make your R19 on the cavity alone. You might have to spray an inch of foam (or rigid) and then do the remaining area with an R13 batt (some compression though). Depending on the foam type, you may end up with an R19 cavity. If you strip the exterior sheathing (or boards) as part of your gut, then just go with the R13 cavity and put up the R5 insulated sheathing on the outside of the studs.

Hope that helps or if nothing else provides a bit of education as to what the inspector expects.
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Old 12-14-2009, 06:44 PM   #19
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Using rigid foam insulation to acheive R-19 in true 2x4 exterior walls?


I appreciate all the replies. I love to learn as much as possible. But, I guess I should be more blunt about what I would specifically like to know.

What is the cheapest way to achieve R-19 in a TRUE 2x4 exterior wall, without removing or covering exterior lap siding?


Currently there is no insulation of any kind. And, all stud walls will be completely exposed from the interior.
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Old 12-15-2009, 07:43 AM   #20
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Using rigid foam insulation to acheive R-19 in true 2x4 exterior walls?


IMO, foil-backed polyisocyanurate sheets, at R7 per inch, is the only way you'll get R19 in a 4" deep cavity - and still have some space to spare. Three sheets stacked on top of the other...

Two 2" XPS sheets will reach R20 but at exactly 4" thick - but then you might have trouble putting the gyproc on.

I don't know what materials cost in your town, so can't say. But if those are the only criteria, then you don't have much choice in getting R19 in 4" deep. That's why they're now doing to 2x6 walls, where getting R19 is a lot cheaper to reach...
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Old 12-15-2009, 10:04 AM   #21
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Using rigid foam insulation to acheive R-19 in true 2x4 exterior walls?


If you could give up 3/4 inch you could fur out the wall and have icynene foam sprayed in (they have a foam that's R4 per inch). I don't know how much that option would cost in comparison to the rigid sheets. But it would sure be a lot faster.

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