I've heard many of you talk about this subject before, but I can't seem to find exactly what I'm looking for in previous posts. What is the general rule for the percentage of exterior foam board (what % of total R-value) that needs to be outside of the sheathing to not worry about vapor retarders on the inside? I swear I've heard 50%, but I can't seem to find it in previous posts. For example, if I have R20 in cellulose between the studs, then do I need to have R10 on the exterior? To complicate it more, that R-20 in the walls is center cavity, not whole wall. Is the calculation of the % required using whole wall or center cavity values? For example, if the whole wall R (2X6 w/cellulose) is R14, then will R7 of foam board be good enough? I'm going to give WUFI a try and see if I can figure it out for my specific situation, but was wondering what the rule of thumb was. My situation - Seacoast of NH, Zone 5 (colder side). Will have 2X6 walls with dense pack cellulose. Exterior foam is a given - was planning on 2" of XPS with latex paint on the drywall. Not much permeability in XPS at that thickness anyway, so I'm starting to figure why not go with 2" of foil-faced polyiso and get an extra R3 out of the whole thing. If I do this I want to make sure I've got enough R on the outside to stop condensation on the warm side of the sheathing. I'd think the R13 would be more than enough and the foil-face wouldn't be an issue since the wall can dry to the interior. Thanks for your thoughts....

I know this question has been answered in the forum before. It was probably Dana1 that answered it and the link came with a chart that had specific R values necessary to move the dew point into the rigid insulation (if that explanatiion makes sense). I know that where I am in northern RI 2" of XPS was going to be a little more than what I need to aviod a condensation issue. Air sealing is a must.
FWIW
Mat

To meet IRC min-spec in zone 5A you'd need at least R7.5 on the exterior of 2x6 construction. In zone 6 the IRC specifies R11.25 min.

With 2" of XPS (R10) you're fine, but with iso you'd be better than fine.

The whole-wall R is irrelevant in a dew-point calc- it's the center-cavity R that determines the temperature at the sheathing mid-way between studs where the condensation risk is- it's always going to be warmer near the studs.

See P77 (.pdf pagination) of this document:

http://www.iccsafe.org/cs/codes/Documents/2007-08cycle/2007Supplement/10-2007%20IRC-RB%20Supplement%20Final-7-13-07.pdf

For a bit of discussion about where it comes from see also:

http://www.greenbuildingadvisor.com/blogs/dept/musings/are-dew-point-calculations-really-necessary

http://www.greenbuildingadvisor.com/blogs/dept/musings/calculating-minimum-thickness-rigid-foam-sheathing

Monitor the interior humidity during the winter and don't let it get over 40%RH/ @ 70F for extended periods in mid-winter- very tight houses will run much higher mid-winter humidity than what you may be used to due to a lower natural ventilation rate. At 50% interior RH the sheathing would be below the dew point MUCH of the time in January. Running an ERV or HRV under dehumidistat control to 30-35% works fine in winter, but with the higher outdoor dew points of late-spring/summer you'll need to just duty-cycle it. Higher interior RH (up to ~60%) is fine in summer, since the sheathing & insulation would all be well above the dew point during that time.

Highest risk is January, but as long as the sheathing is above the dew point of the interior air at least half the time, you're golden, with extremely low risk of accumulating rot-levels of moisture accumulation in the sheathing. The mean binned hourly outdoor temp in Portsmouth is +25F, (see: http://weatherspark.com/#!dashboard;a=USA/NH/Portsmouth ) and the dew point of 70F/ 30%RH air at sea level is ~37F. With R10 on the exterior and R20 in the stud bays the average temp at the sheathing is ~1/3 of the difference between 25F & 70F above the outdoor temp:

25F + ((70F-25F) * R10/(R10+R20)) = about 37F.

Even if you're accumulating some during the coldest month, the rest of the winter it's drying (if slowly), and the rest of the year it's drying rapidly. You'd still be OK at R7.5 on the exterior, but I'd build with R10 or more for the extra margin. At R13 you'd be OK even with 40% RH interior air (dew point of 44.5F), but holding the line at 35% (dew point= 41F) would be better.

Humidity between 30%RH-50% is what's recommended for human health & comfort by medical professionals. Going below 30% for extended periods enables airborne virus transmission, going above 50% allows dust mites to breed. But keeping interior humidity to the lower side is always healthier for a wood-sheathed building, and actively adding humidity in winter above 35% can lead to higher mold spore counts the rest of the year.

Yes - exactly what I was looking for. Thanks.

So if I have enough foam on the outside (and leave the inside as the drying side), then I really don't need to worry about if the foam will allow drying to the outside? I could use EPS, XPS, or even foil faced polyiso since the drying will be to the inside. I want to keep my options open for what type of foam to use as I plan on using recycled foam board from a place like Insulation Depot and want to be prepared to take advantage of a good deal.

I'll have a minimum of 2" of foam regardless, and will spec the house for that thickness.

When the foam is cheap reclaimed goods, for the same wall thickness as 2x6 + 2", going with 2x4" + 4" buys you HUGE dew-point margin and a higher whole-wall R-value:

With 2x4 16" o.c. cellulose + 4" of exterior iso you're at a whole wall of ~R34

With 2x6 24" o.c. cellulose + 2" of exterior iso at ~R26

That's a 30% difference in R value (25% improvement on U-value) at the same wall thickness.

Framing costs are about the same in either package (same number of board feet of lumber, with somewhat fewer board cuts with the 2x6, but it's "in the noise" on cost.) Detailing around windows is the same for "innie" installations, and not that much different for "outie". The installation labor costs of the foam are about the same if one-layer a bit more if you go with a pair of staggered seam 2" layers, and the fastener cost adds a bit too, but when the foam is discounted that deeply it's still a reasonable cost/benefit to go with the thicker foam. And you'd have about 2/3 of the center-cavity R value outside the sheathing for effectively zero condensing hours at the sheathing per year.

The permeance of roofing iso depends entirely on the facers, and some would be about the same as 2" of XPS, but don't count on it. Building the stackup to dry toward the interior is fine at the thicknesses you're talking, even if it does end up with a modest about of capacity to dry toward the exterior.