Has anyone been able to get a price for the 1 1/2" R Sheathing?  I was wonder how it prices to seperately purchasing 1" Polyiso, 7/16 OSB, and Housewrap?  My local retail store (Dunn Lumber)  that Huber list as a retailer are clueless and they have to call their distributor (Boise Cascade) for a quote and Boise is clueless about this product as well.  Looks like a great product if the price is right.

Jay

According to Huber:


"Product available in limited markets. Please call 800-933-9220 to see where you can purchase this product."

It's sorta like the world's thinnest stress-skin panel. It's fine if all you're looking for is ~R6.5 ish over fiber-insulated framed structure with no exterior foam .  That would bring a 2x6 framed cellulose insulated wall (~R14 @ 20% framing fraction) up to ~R20. 

In most of Dunn Lumber's operating are (western WA, US climate zone 4) there's an economic rationale for going to R25 whole wall performance. See:

See: http://www.buildingscience.com/docu...mate-zones

Savings  over separate housewrap/iso/sheathing are likely to be mostly in the labor costs. I'd expect to see a higher material cost.

Call me old fashion but there is something about ISO and housewrap that I prefer. I feel, although I am likely mistaken, that the perm rating on that combo is better.

Dana will likely set me straight.

With separate foam & housewrap the permeance is determined by the facers on the iso, but even most fiber facers are sub-1 perm. Foil facers are extremely low perm, which would require all drying to be toward the interior. Ideally you'd want to be able to use only a class-III vapor retarder (such as any old latex paint) on the interior to maximize drying capacity.

To be able to use wood sheathing with foil-faced iso on the exterior and a relatively high-perm interior would require more than 1" iso on 2x6 construction for climate zones 5 & up. At 1" over 2x6 framing you could use something like MemBrain on the interior up to zone 6 for 2x6 construction, (maybe even zone 7), which would dry more quickly during the spring warmup than vapor retardent paint.

More exterior R and a higher-perm interior would still be desirable. To meet spec-min on IRC 2009 for zone 6 with a wood sheathed 2x6 framing using only standard latex on the interior would take 2" of exterior iso. In zones 7-8 it would take 2.75"-3". In zone 4 (Dunn Lumber's operating area) 1" is more than enough.

I realize that vapor permeability is separate from air tightness but is there any benefit to not taping the seams on the ISO and using a higher perm rating housewrap to the exterior for the primary drainage plane and air barrier?

i.e. do you get diffusion through the seams and therefore have a better ability to dry to outside.

Not taping the seams of the iso doesn't change it's vapor permeance one bit.

With vapor diffusion it's all about surface area and differences in the WATER VAPOR PRESSURE of the air mixtures on opposite sides, and even high-permeance seams have very little surface area. If you did something like making a grid of small perforations on both facers on a 1/2" grid you could make foil-faced iso about as permeable as housewrap but short of that it's going to be fairly low perm.

With air barriers it's all about AIR PRESSURE allowing bulk movement of gases. Unsealed seams leak air.

Housewraps can (and should) be detailed as air-barriers as well, but they're more prone to damage resulting in air leaks than structural sheathing or rigid foam sheathing.

Dana,

I understand that surface area is the measurement which is why underslab insulation needs to be right against the concrete and not separated via sand layer, gravel, etc.

I just thought that some air movement between the layers would increase the permeability or drying ability.

Air movement in the micro-space between the layers would be a thermal bypass, especially if it's deep enough to provide significant drying capacity.

It's better to go air-tight and modestly vapor permeable on the exterior foam when the foam isn't thick enough to prevent moisture accumulation in the sheathing from a dew-point calc point of view with only a class-III vapor retarder on the interior.

But using impermeable foil facers can still be made to work and work well using variable perm goods on the interior such as MemBrain. It's class-II vapor retarder when it needs to be, but will dry as quickly as a class-III vapor retarder.

Air-tightness on the interior finish wall and air-retardency of the cavity fill are more important than the interior vapor retardency. A square inch of air leak to the interior of a low-density fiber filled studwall is worth a whole wall's worth of vapor diffusion through standard latex paint in terms of the volume of water transported. Using high-density fiberglass or cellulose cuts the volume of air flow from convective forces by ~90% (even more, if dense-packed). Cellulose (but not fiberglass) will also absorb and retain a good deal of the moisture that gets in from vapor diffusion or minor air leakage without losing thermal performance, protecting the susceptible wood. Making the sheathing an air-barrier also limits the amount of wind-driven infiltration of interior air into the stud cavites.

Moisture gets in by any number of paths, but it get's out mostly via vapor diffusion. If you make either side of the assembly highly vapor retardent, good design dictates giving it as much drying capacity in the other direction as possible. eg: If poly is used on the interior (as required by Canadian code) a vented rainscreen cavity between the siding and the sheathing is HUGE (and also required by Canadian code), but it means any exterior foam or facer needs to be at least 0.5 perms, and 1.5 perms would be even better. Poly interior + foil-facer exterior (or more than 2" of XPS) == moisture trap, a disaster waiting to happen- a small 1 time leak in the flashing under wind-driven rain situation might take years or even decades to dry, whereas with 0.5-2 perms in both directions it could be gone in weeks.

Dana,
Hate to bump this but I always try to get info from existing posts rather than create new ones. I am not clear on something in last post. if there were air channels inbetween foam as discussed above then it renders that foam useless (or partially useless)
If this is true then the vinyl siding which has foam eps integrated with is not worth the premium they get for it.
I bring this up because I am trying to build an energy efficient garage. So far it has:
ICF frost wall
5" high density eps under radiant concrete slab (I got a great deal on this off craigs list)
2x6 framing which I will use wet blown Cellulouse
My original plan was to put 3/4-1" xps under drywall INSIDE the building to reduce thermal bridging some......Now I am considering 1" polyiso outside (not enough for sheathing condensation I know zone 5) SO I was going to use vinyl with integrated insulation to get the dew point outside the wall.
Now I don't know if the siding with insul will work...
Any studies out there on performance difference between INSIDE vs. OUTSIDE rigid foam?? I cannot find any info on that and looked for many, many hours.

I guess I should add that I really do not want to add strapping over the exterior foam...and since vinyl siding guidelines call for it to be installed over not more than 1" foam then that is my limit..
If someone has installed vinyl over 1-3/8 foam with no problems (perhaps screws would be better?) then I would love to hear of it (or a link to the discussion)

Where do you get 1 3/8" foam?

I've installed vinyl a couple of times over 1 1/2" XPS foam, which I like because its easy and fast to buck the windows/doors with 2x4 on the flat. I used 2 1/2" roofing nails for the siding, it doesn't quite get you the penetration your supposed to have but it worked with no problems.

Basically we built the house, installed the housewrap and bucked each window and door with 2x4 nailed flat for window and door flange nailing. On the top of each buck we installed a drip cap or z flashing which went underneath the housewrap, then we cap nailed the 1 1/2" XPS foam on studs to give us reference for siding, taped the seams, installed siding after the windows and doors were in.

With window and doors the head/top membrane flashing tucked under the drip cap/z flashing that flashed the buck, and side membrane tape pieces covered the window flange exposed portion of buck and lapped onto foam. The bottom piece, which went on first, covered the interior sill and lapped down over the buck onto the foam. That way absolutely none of the wood buck, which was white wood, was exposed to the back of the vinyl.

I didn't bother sealing the bucks to the wall as I detail the wall sheathing as my air barrier. I use Certainteed vinyl solely for their studfinder hem which saves more time than you'd think and accurately gets you in the studs.

The foam in foam backed siding does little for insulation so do not buy it with that in mind.

It was intended to prevent the oil canning that cheap vinyl siding gets and to maintain its appearance. Once the idea of being "green" took off, the cross over marketing was the primary thrust.

It is not a bad upgrade as long as you are not paying through the nose for it.

Thank You so much for the replies. I could not find anyone else who has applied vinyl over anything greater than 1" thick. The big box orange store has 1-3/8" polyiso. I would prefer to use XPS as it has drying potential (although that is up for debate).
I am assuming the z flashing over the buck portion is at least an inch or so???? if you are tucking the membrane under it I am sure it is not the typical 1/4" return on store bought cap flashings.

I will review the FHB article by Rick Arnold...I think he had pics of his flashing detail.... then again he only used 3/4" foam

The only 1.5" XPS I have found is the type used for interior basement walls...it has a dado on the edge to recieve 2.5" furring to both secure the foam and give a place to screw drywall on once up. I considered using this system to attach the vinyl siding but...that would leave all these 2.5" strips of sheathing with only 3/4" foam and in danger or cold spots on the sheathing and the dreaded interior dew point.

WoW I agree with you on the siding...I talked a customer out of it a few years ago and substituted it for dense pack cellulouse blown in cavities instead. I am by no means a vinyl siding expert (I am too slow and fussy at it) but do it on occasion. I usually follow manufacturers directions (max over 1" foam) and thought I could use the foam back stuff to get me an R-1 along with 1" polyiso for a total of R-7.5 which is what I need in zone 5 with 2x6 construction.

I may start a new thread to see if anyone else applies vinyl over greather than 1" foam...that would solve all my problems if I can convince myself that it will be ok.

Many thanks again

We bent the z flashing for the buck at 4" up the wall, 1 5/8" over the 2x4 edge and a 1" leg down to tuck the membrane under.