Why can't one put less exterior rigid (EPS), and use a VB on the inside wall? This will dry to the interior, and the exterior since EPS is fairly permeable... If one thinks about it, homes have been built for decades with this concept (without the external) and there will be condensation in the wall every winter that dries. So why couldn't one just put however much EPS they want on the exterior and be done with it?

The reason I ask is EPS is sooooo much less expensive than XPS but I would be below the recommended R value necessary for Zone 5 (R7.5) if I used 1.5" of EPS (approx. R5.0)... Wouldn't that still be considerably better than standard construction (and easier and way less expensive for me if I go to only 1" EPS)

Yes, as long as you keep the exterior EPS UNFACED, and Type-I or Type-II (1.0 or 1.5lbs per cubic foot density) the assembly can still dry toward the exterior. At 1.5" Type-I EPS has a vapor permeance of about 3 perms, Type-II EPS would have vapor permeance of about 2 perms. The vapor permeance of the CDX or OSB is about 1 perm, so a higher permeance foam won't be too much of a hindrance for drying toward the exterior.

But it will hinder the drying somewhat more than the classic #15 felt, which runs about 5 perms. That means you have to be extra-certain that the poly vapor barrier is truly air-tight- sealed to the framing with acoustic sealant caulk, sealing any seams with either caulk or tape only in locations that are directly supported by the framing (where it can't flex and unseal over time), and sealing all of the electrical & plumbing connections to the vapor barrier.

An even more forgiving approach would be to use a "smart" vapor retarder like Certainteed MemBrain or Intello Plus, etc. in place of poly, which would run about 1 perm or a bit less in winter when the interior relative humidity is less than 35%, but becomes vapor open when the sheathing warms and releases it's winter moisture burden into the cavity rasing it's RH to above 35%. That way it dries more quickly than it takes on moisture.

Cellulose (vs fiberglass) also helps with moisture management. EPS (of any thickness) + cellulose + MemBrain is usually hard to beat. Different tapes, gaskets, caulks and foams aren't equal, so pick (and apply) carefully. Caulks and spray foams may not continue to seal when a small gap opens up by 10x.

Well i was planning to do 1-1/2 inch XPS with applegate insulation with intelloplus membrane. Maybe now i will go 1-1/2 in EPS unfaced type two, Applegate cellulose with intelloplus membrane instead. I think this will save me $4k.

I will still caulk the sheathing/stud seams and do my best to tape/caulk all intelloplus seams and electrical/plumbing openings.

Thanks dana and jonr for your replies. Now, i feel pretty good.i lost the right to use my hvac-calc temporary license, what would the total r-value of 2x6 with this setup be? R-21-ish?

I believe i read somewhere approaching r25 you hit the inflection point of cost vs benefit so anything in the low 20s i'd be happy with.

What is the breathability of XPS at 1-1/2 in thick?

I see varying values on the internet but it looks like 1-1/2 in XPS is about 0.9 perms, similar to EPS.

I will have to weigh compressive strength (even furring strips after they are screwed in) as well.

It's not a good idea to push dew point to foam or any product unless it has anti-fungi resistance additive. Check the MSDS. Even with it not a good idea. Fungi does not care about perm rating in a stack up that actually helps transport it. Absorption %/volume is a better indicator. It is best to design for complete diffusion to the exterior that prohibits absorption with air tight interior. Use materials with pores that are too small for absorption and large enough for diffusion. Shun the use of CL 1, 11 vapor retarders except the foundation. Do not put water leads or drains in an exterior wall. Vent stacks cause moisture condensation too.

Posted By Surfsup on 16 Apr 2014 08:35 PM
I see varying values on the internet but it looks like 1-1/2 in XPS is about 0.9 perms, similar to EPS.

They aren't really very similar- you have to read the specs carefully.

XPS is tighter than EPS, often rated 0.8-1.5 perms (max) @ 1", which would be about 0.7- 1 perm @ 1.5".  Specs for permeance on XPS is often a maximum, not a typical ,and with no specified minimum. eg:

1 lbs density XPS rated 1.1 perms max @ 1"  At 1.5" that will be no more than 0.73 perms, and would likely be in the 0.6-0.7 perm range.

EPS permeance is sometimes specified as a "typical" number, (which means it could be a bit tighter or a bit more open) usually  specified with a max. The numbers vary with the density or "type"- but 1" Type-I goods are usaully about 5 perms (max) @ 1", Type-II about 3 perms @ 1", Type-IX about 2 perms.  You can count on Type-I EPS to be more than twice (and maybe three times) as vapor permeable as any XPS product at any given R-value, and even Type-IX will be more permeable than any XPS product at a given R-value.

Unless you insist on really over torquing the hell out of the timber screws (and risk splitting the furring)  you needn't be concerned with the compressive strength specs.  Pancake head screws 24" o.c. and 1x4 furring is a pretty rugged & reliable approach, with minimal splitting or compression issues. Going with 1x3s is always a mistake.

Dana, sounds good. What about TLPs comments above regarding anti-fungus additives? Is this a concern? I thought fungus couldn't live on/in foam since foam has no food for it to grow...

Posted By jonr on 15 Apr 2014 05:06 PM
Caulks and spray foams may not continue to seal when a small gap opens up by 10x.

What do you mean? Spray foam will shrink?

Hopefully not - the primary issue is that it won't expand by much when what it is sprayed onto moves or shrinks with drying or temperature changes. Even silicon caulk has a limit that is exceeded when a 1/32" gap increases to 1/8".

that's why tape is a better solution. And EPS foam does shrink - I've opened up homes with EPS foam.

XPS also shrinks, I've never seen EPS shrink.

EPS shrinks too- count on it. (Same polymer, same reasons.)

Not sure why shrinkage of rigid foams was brought up, but according to numerous sources, EPS effectively stops shrinking after 42 days at normal temperatures.

Do you guys predrill the 1x4s? I read on a HD review for the 4.5" headlok screws, the guy was doing 1x4 furring and he said the headlocks even split the studs sometimes (personally I think he was off center on some and probably hit it close to the edge). Also should I caulk between the furring strip and tyvek before screwing it in?

we use the regular GRK wood screws with tourque head - they predrill the hole, go in smoothly, hold great. Not cheap, but they are worth it.

So if I put a strip of Tyvek over the top piece of furring strip over the nailing flange, what about the side furring strips? Should I also place another strip of Tyvek over the side furring strips? If I nail the window to the side furring strip, and only use tape, there will be wood exposed on the sides. How are you guys water-sealing a furred out window?