Wife and I just put in an order for a modular home that has a relatively novel wall stack-up. Trying to use on-line R value calculators (e.g. ekotrope) has not been easy due to the unorthodox interior placement of the thermal break. Anyone here want to give it a shot? From outside to inside runs as follows: Tyvek, 1/2" CDX, 2X6 studs 16 in. OC, 1.25 inch EPS foam strips on interior face of studs, OSB strips layered over EPS, 1/2" Drywall. Stud bays to be dense packed with cellulose. Any help figuring out the walls R-value would be much appreciated, thanks.

What, no siding, just Tyvek?

At a 25% framing fraction, with fiber cement siding the stackup without the 1.25" EPS edge strips and 1.25" more of R3.7/inch cellulose would be between R15-R16, and that's including interior & exterior air films.

If you add in the 1.25" slice of 25% EPS + 75% cellulose it at adds another ~R4.75 to that, so call it R20-ish whole wall.

Then adding in the 0.5" thick slice of of 25% OSB + 75% cellulose adds another R1.2, bringing it up to ~R21 whole-wall, or ~U0.048.

Location matters: In US climate zone 6 and higher this stackup needs a Class-II interior side vapor retarder, such as"vapor barrier" latex primer, or 2 mil nylon (Certainteed MemBrain) to keep the CDX from taking on too much wintertime moisture. In zone 5 it can get away with using standard latex paint if the siding is rainscreened, with a ventilated air space between the siding & CDX, (or just vinyl siding, which is inherently back-ventilated. (A rainscreen gap adds another R0.5-R1 to the stackup too.) In zones 4 or lower it won't need anything more vapor-tight than standard interior latex paint to protect from wintertime moisture drives.

Thanks for the quick reply Dana. Yeah, I forgot to mention siding and climate zone. Siding will be vinyl and we're in zone 5, albeit on the colder stretches of zone 5.

I wonder if anyone sells pre-made thermal break 2x4s that have 2x6 dimensions. A full covering of rigid foam is arguably a wasteful way to solve the thermal bridging issue. And full exterior foam often creates breath-ability issues that strips don't.

Posted By mothermenke on 23 May 2016 05:57 PM
Thanks for the quick reply Dana. Yeah, I forgot to mention siding and climate zone. Siding will be vinyl and we're in zone 5, albeit on the colder stretches of zone 5.

With vinyl siding knock about R0.4 off those "whole wall" numbers- it's really in the noise.

With cellulose cavity fill the cellulose "shares" the moisture burden with the sheathing, protecting it. With vinyl siding you don't really need an interior vapor retarder even at the cold edge of zone 5.  But if you're the type who wouldn't sleep at night it doesn't hurt to add 2-mil nylon under the wallboard (except for the ~13-15 cents per square foot installed cost.)

Thanks again for the help Dana, sometimes I feel this forum could be improved if they just added a "DDorsett" bat signal button at the top right.

Pre-made thermal-break studs. Interesting idea. The closest thing I can think of is I-joists used as studs. People generally then use structural sheathing inside and out. A thermally broken stud would a better drop-in replacement for a conventional stud if it didn't need the interior sheating. That might work if the outside wood part was conventional 2x4 dimensions.

The heat flow through a 3/8" wide I-joist web is pretty similar to the heat flow through a 1.5" wide strip of EPS, for the same thickness (distance the heat is flowing) in either case. The difference is that with the I-joist, there is also heat flow through the fluffy insulation filling in the space where the EPS would have been, so I think the net result is that you'd need almost double the break thickness to get the same effect with the I-joist. In the case of the example above, the thermally broken stud is about 7.5 inches thick. We could get the same effect with an I-joist like thing with a 2x4 outer, then 2.5 inches of OSB web, and then have a 2x2 (1.5 x 1.5 actual) interior strip. Of course, 2x4 + 2.5" of foam, and then a 2x2 interior stud would be even better. I'm guessing that either could be reasonably cheap if someone went into production with it. With ordinary foam, the OSB web would be stronger. You could certainly use higher density foam to make that approach stronger. One advantage of the foam approach is that it would be more compatible with high-grade batt installation.

High density type IV and type IX have a compression rating of 25 psi. However, one can't get just a few sheets of the stuff. Would foam with a 15 psi value work? That is if one is careful in attaching the furring strips over the foam.

Thank you for such useful answers.

Posted By agagent3 on 02 Jun 2016 03:20 PM
High density type IV and type IX have a compression rating of 25 psi. However, one can't get just a few sheets of the stuff. Would foam with a 15 psi value work? That is if one is careful in attaching the furring strips over the foam.

15psi goods are just fine, even under slabs (but not under footings.) Foil faced polyiso is the right stuff for interior side stud-edge strip thermal breaks due to a high R/inch at conditioned space temperatures, low global warming potential blowing agent. Securing it in place with a few cap nails and a thin bead of foam board construction adhesive works.

So the cellulose shares the moisture burden with the sheeting. I assume one would not be able to get away with that using fiberglass? How about if one goes north to a colder zone? How does that jive with IRC?

In zone 5 you'd be fine as long as the siding is back ventilated (rainscreen, or vinyl or aluminum siding) with either fiberglass or cellulose and no interior vapor retarder other than standard latex paint.  In zone 6 or higher you would need a Class-I or Class-II vapor retarder on the interior side of the insulation layer.  You could probably get away without the interior vapor retarder in zone 6 with cellulose (even though it's technically a code violation), but with fiberglass or rock wool there is no question- it needs an interior side vapor retarder, and the wallboard needs to be air-tight.

Using 2 mil nylon as the vapor retarder gives you a bit more resilience than using "vapor barrier latex" primer, and quite a bit more resilient than 4 or 6 mil polyethylene, since it becomes vapor open when the air in the cavity is damp enough to promote mold growth, offering measurably higher drying rates.

The generic rule on wall asseblies for zones 4 & higher is that a Class I or Class II vapor retarder is required.  The IRC's take on wall assemblies with only Class-III vapor retarders (like standard latex paint)  is spelled out in R702.7.1.  With vinyl siding and plywood or OSB sheathing it meets the "Vented cladding over wood structural panels" criterion in zones 5 & lower.  In Zone 6 it would work with vinyl siding if a more vapor-open sheathing than OSB or plywood is uses,  such as exterior grade gypsum board or fiberboard.

Note, kraft facers on batts are also Class-II vapor retarders, and would meet spec. But they are literally impossible to air seal, and make inspecting the insulation installation quality nearly impossible.  Broadsheet goods are more reliable, and allow the insulation job to be inspected.  The long standing standard has been 6 mil polyethylene, a Class-I vapor retarder, but the extremely low vapor retardency reduces overall resilience by blocking drying toward the interior.  The variable permeance of 2-mil nylon makes it cheap insurance. The material cost of 2-mil nylon is about 2x that of 6 mil polyethylene, but the labor is the same.