I am about to build on a 85" rainfall, 6000 HDD climate with no cooling system (I can withstand a little heat in summer) http://en.climate-data.org/location/147654/ I'm thinking of (from outside in) * 1" wood siding * 1" of air gap with 1x3" furring * asphalt paper as WRB and air barrier * 3" 2pcf EPS as R10 sheathing, * A 10" double frame wall filled with a total * of R-24 polyester insulation * asphalt paper as air barrier and vapor retarder * 3-4" interior wood finish The permeability of the 3" 2pcf density is about 0,6 perms, semi-impermeable. So, I am betting on drying mostly to the interior, although In my heating climate vapor diffusion will mostly travel inside-out... Any ideas?

Just any FYI, but 2pcf EPS foam (Type IX) comes in at around R4.4/inch at 75F. And as the temperature goes down, the R value goes up. At 25 degrees F, that foam has an R value of approximately 5.0/inch. So, if your target exterior insulation value is R10 when it is cold, you could get by with 2" of 2pcf EPS foam and save ~1/3 of the cost.

Arkie- That 25F number in the EPS performance rating tables is when the temperature halfway through the foam is +25F.  In his climate & stackup it'll run about R4.3/inch, maybe R4.4/inch, never R4.5 or R5.

Not even the 99th percentile outdoor temperature is as low as 25F in Malalcahuello, Chile, let alone the mid-foam temp at the 99% temp according to the climate tables listed on his page link:

http://en.climate-data.org/location/147654/








That wall stackup will be fine in that climate- no need for vapor retarders are required at any layer, but air tightness is important on both the foam layer and the interior layer, to avoid infiltration thermal bypasses.

I didn't even look at the climate data link he posted. I just saw that he said he had 6000 HDD and assumed it was much colder than that. Where I live in Arkansas, we have 3200 annual HDD and the 99% winter design temp is 15F-20F.

Here is the link were I got the EPS thermal properties: http://benchmarkfoam.com/eps-properties/

Based on his winter minimum temperature of approximately 34F, I would expect the exterior foam to be somewhere between 40F and 75F, especially given that he has 10" of insulation between the foam and the conditioned space. That would result in the 2# EPS foam R value being somewhere between R4.35 and R4.76 per inch per the linked Benchmark Foam chart.

Maybe I could do with a 2" EPS, but I if 1" inch more is possible, I prefer to try it (just to get more thermal insulation)... I would even try 4 inches of EPS as a sheathing.... what do you think about that?

At a certain point the cost for insulation is negated due to less and less thermal gain. For example (extreme) if you were already at R100, the cost to go to R200 would be insane yet the thermal benefit would be minimal. If your roof is a weak zone, more R in the wall is over kill. If you have a lot of windows that negate the wall, more R in the wall is pointless. Etc... so the cost-vs-R equation is one only you can really determine makes sense or not. I would spend the time and energy on better air sealing so you have less air leaking and get an air exchange unit (ERV) to promote healthy air in the home.

I agree with you, Surfsup. Ihave a lot of windows (about 40% of my total wall area), although I plan to insulate them with thermal shutters on night. In Chile we don't see mechanical ventilation systems on houses (just commercial or industrial), but maybe it's time to try to find one.
Thanks