What is wrong with the following wall assembly? Zone five 2800 ft elevation. 2x6, dense pack cellulose, osb, sto gold liquid membrane, cladding TBD. Plenty of caulk, heel truss, blown in cellulose r-50. The builders in my area are not familiar with the details of exterior foam. Thoughts? Suggestions?

Cladding is important in considering the stackup- could even be critical here!

I couldn't find a spec on the vapor permeance of the Sto Gold Membrane. Sto Gold Coat runs about 6 perms, which might be OK. Sto Gold Fill is about 18 perms, so the final total permeance would be about 5-6 perms. (See: http://www.awci.org/buyersguide/files/sto_guard-1105.pdf ) With cellulose cavity fill and no interior vapor retarder beyond standard latex paint the goop-on WRB want to test >5 perms, and it would also work better with a rainscreened/back ventilated siding for the OSB to dry into. (Ripping 3/8" ply wood to ~2" and through screwing it to the studs would provide a sufficient rainscreen gap for the siding.) Using a housewrap and air-sealing the sheathing with caulk/spray-foam/construction-adhesive (or even Sto Gold Fill) would give the OSB a better shot at drying into the rainscreen gap.

2x6 w/dense pack with no exterior foam and an R50 attic would just meet code-min for zone-5:

http://publicecodes.cyberregs.com/icod/irc/2012/icod_irc_2012_11_sec002.htm

You're looking at about an R14 whole-wall R after thermal bridging- it doesn't take a lot of foam to do a whole lot better.

At 2"/R7.8 Type-I EPS (low-density EPS often used in commercial construction) would still allow the assembly to dry toward the exterior, so long as the Gold Membrane is sufficiently permeable, and brings your whole-wall R above R20, thermally breaking the band joists & foundation sill as well as the framing. With R7.8 on the exterior you have sufficient dew point control that you don't need an interior vapor retarder (other than standard latex paint.)

If the wall thickness at 2" + 2x6 is too unwieldy for your builders, cutting back to 2x4 on the framing and putting up 2" of foil-faced iso (R12) also gets you to R20-land for whole-wall R at the same wall thickness as standard 2x6. With nearly half the R value on the exterior it doesn't matter what the permeance of the goo is (you're blocking exterior drying with foil facers on the foam anyway.)

Find a builder who is interested in learning something new. There are plenty out there who learned how to handle R13 + 5 code min with an inch of foam without missing a beat. It isn't a huge leap from there to take it to 2" and beyond. The key details to get right are the window & door flashing and the rainscreen. Depending on whether you mount the windows more flush with the siding vs more flush with the sheathing determines which side of the foam the housewrap goes on. (I'm not sure if any of the Sto goods can be applied directly to foam.)

With a foam-over use 1x3 or 1x4 furring through screwed to the studs 24" o.c. with pancake head timber screws penetrating at least 1.5" of sheathing + stud, not the ripped plywood. If superior wall flatness is of concern, use 2x furring. Rainscreens rock- they increase the lifespan of the exterior paint and siding (by a LOT), and allow both the siding and the assembly to dry. Even though it's only required by code in wet coastal B.C., they provide a real benefit that you can take to the bank in other climates as well. (The cavity between brick or stucco and wood sheathed structural walls is also a rainscreen gap, necessary due to the high moisture drives of masonry claddings.)

I read on GBA that dense packed cellulose is NOT an air barrier and should not be treated/viewed as such. Is that true?

It isn't, but the STO should be

The Sto goods are being marketed as a means of air sealing and weatherproofing the structural sheathing, but like anything the quality of the air seal depends a lot on the diligence and training of the installer.

Dense packed fiber insulation can be fairly air retardent, to the point that air leakage could be disregarded from an energy use point of view, but not from moisture transport point of view. That said, air sealing of the gypsum side is more critical when there is no foam on the exterior, and in a US climate zone-5 location when the R value of the exterior foam equals or exceeds that of the cavity fill even air-transported moisture concerns evaporate (so to speak. :-) )

No cavity fill is an air-barrier not even closed cell spray polyurethane. But the big holes matter a lot more than the small holes, and with dense packed cellulose you should be looking at all of the other stuff- caulking the studs to the sheathing, the stud plates to each other (when doulbled) and to the subfloors, etc. for dealing with gross infiltration issues. It's not that different when spray foam is used as cavity fill- the foam may seal the studs to the sheathing, but it doesn't touch the rest of the common air leakage. Air barriers are not rocket science, but the details add up. Spraying Sto Gold Fill on the fiberglass taped seams on the exterior is a pretty good approach, but won't necessarily address thermal bypasses between the framing and sheathing. Details count.

The sheathing in of the 2x4 + R12 wall is at near zero risk from interior moisture drives unless the house is kept unusually dry in winter, whereas the 2x6+ R8 wall would be at some risk from air transported moisture, but not from vapor diffusion through the paint & gypsum.

The 2x6/no-foam code-min wall has the highest moisture risk, but the risk can be substantially mitigated by back-ventilated/rainscreened siding, and would meet code without an interior vapor retarder. But without a vented rainscreen gap it would not meet code without a class-II (Certainteed MemBrain or vapor barrier latex paint) or class-I vapor retarder (poly sheeting or foil) on the interior side.

In general, if the assembly can be designed to tolerate winter moisture drives without vapor retarders it will be more resilient. The exterior foam approach is tried & true (and enshrined in code) so long as it has sufficient R value relative to the cavity fill.

Posted By Dana1 on 04 Dec 2012 05:29 PM

Dense packed fiber insulation can be fairly air retardent

No cavity fill is an air-barrier not even closed cell spray polyurethane.

They stated that the closed spray foam is the most air tight, with dense packed cellulose coming in second place and fiberglass batts coming in third. Those were the only 3 insulation materials tested in this test.

Wouldn't an ICF cavity (the 6" of concrete) be considered an air barrier?

According to Building Science:

" All wall assemblies experience a loss in thermal performance due to air movement through the assembly. This is true for all of the assemblies tested regardless of the type of insulation material used (e.g. cellulose, fiberglass, ocSPF, ccSPF, XPS). The energy impact of airflow depends on the flow path, the interaction between the air and the solid materials in the assembly, and the installed R-value of the assembly.”

The higher the wall R-Value, the more important it is to air seal it because it seems counter intuitive but the higher R-Value wall will experience greater loss than the lower R-Value wall.

What's wrong with it? It's not an ICF wall.

At risk of oversimplifying response to OP, I agree with Brad.

Our 3400 SF ICF house in north Florida (with 50 windows) heats and cools with highly satisfactory humidity control for under $400 per year, suppresses almost all outside noise, and relieves me of all termite, rot, and rodent concerns without the nuisance and cost of pest control chemicals, inspections and bonds.

What's not to like?

Not to mention how much stronger the ICF wall is for Florida hurricanes or Minnesota tornadoes.

I heated our 3200sf house with a small wood stove and backup propane furnace. Last year I used 25 gallons of propane all winter, and our range is propane too.

engineer; some ICFs are at risk for termites..........ICF Termite Damage

Posted By Lbear on 04 Dec 2012 06:22 PM

Posted By Dana1 on 04 Dec 2012 05:29 PM

Dense packed fiber insulation can be fairly air retardent

No cavity fill is an air-barrier not even closed cell spray polyurethane.

They stated that the closed spray foam is the most air tight, with dense packed cellulose coming in second place and fiberglass batts coming in third. Those were the only 3 insulation materials tested in this test.

Wouldn't an ICF cavity (the 6" of concrete) be considered an air barrier?

Show me the article.

The wall assembly air tightness test article on GBA that I saw compared mid-density fiberglass batts with facers in two different stapling orientations, wet-sprayed (not dense packed ) cellulose- typically ~2lbs dry density, open cell foam, and closed cell foam.

Yes, the closed cell foam wall was one of the tightest,  but the closed cell wall still experienced a net 23% increase in heat loss for the whole structure when subjected to a 10 pascal pressure difference across the wall, compared to a 24% increase with wet-sprayed cellulose.   The open cell foam only saw a 16% increase- so it won on whole-structure thermal performance, independent of leakage cfm, but it won on the cfm numbers too, if you study the graphs carefully.

The cfm @ 10pa of the  2lb cellulose wall was higher than the foam walls, but less than the fiberglass walls. But it's also true that going from 2lbs density to 3lbs density (something of a minimalist dense pack density) roughly triples the air retardency of the cellulose, and going to 3.5lbs (a commonly used dense-pack density in cold climates) it's passing only 1/5 the amount of air as 2lb cellulose. See the air retardency vs. density curve in this bit o marketing fluff for Spider.

Thanks for your responses. I am wondering if there is a place or an article that can detail in writing the best steps/practices to install two inch polysio foam on the exterior of OSB. I will be using casement windows, 2x6 walls ,dense pack cellulose, zone 5. Looking to make it a no brainer for my builder. Thanks again.

If he doesn't like to read, start by making him watch the movie:

http://www.greenbuildingadvisor.com...side-house

But there are many details spelled out in the numerous links on this blog page:

http://www.greenbuildingadvisor.com...-sheathing

BTW: With an insulating foam sheathing over stick-built approach, if you build the foundation with insulated concrete forms (ICF) it's convenient to align the foam sheathing and exterior EPS of the ICF for good drainage and an excellent thermal break at the foundation sill. Sometimes this means cantilevering the foundation sill off the foundation a small amount, but if it's an inch or less that usually doesn't require an engineering sign-off.

A typical ICF offering has 2.5" of exterior EPS (R10 exterior + R10 interior) but there are still some 2" (R8 + R8) versions out there, and an R16 ICF foundation is still pretty good. But if you can only get the 2.5" ICF and you're going 2" poly iso sheathing there should be no structural issues with hanging the sill plate 1/2" off the concrete.

With an exterior foam + ICF approach using "innie" windows, with the housewrap between the foam and the structural sheathing, be sure to Z-flash properly where the  bottom of the housewrap reaches the ICF or it'll drain any penetrating water toward the foundation sill rather than toward the exterior.  (one of many details to think about during the design & construction.)  While metal flashing at that point would be a thermal bridge, using copper flashing there is still a good idea in termite country.  Otherwise EPDM or similar heavy-duty membrane materials will be better insulating at that juncture.

Posted By Dana1 on 05 Dec 2012 11:07 AM

They stated that the closed spray foam is the most air tight, with dense packed cellulose coming in second place and fiberglass batts coming in third. Those were the only 3 insulation materials tested in this test.

Wouldn't an ICF cavity (the 6" of concrete) be considered an air barrier?

Yes, the closed cell foam wall was one of the tightest,  but the closed cell wall still experienced a net 23% increase in heat loss for the whole structure when subjected to a 10 pascal pressure difference across the wall, compared to a 24% increase with wet-sprayed cellulose.   The open cell foam only saw a 16% increase- so it won on whole-structure thermal performance, independent of leakage cfm, but it won on the cfm numbers too, if you study the graphs carefully.

The cfm @ 10pa of the  2lb cellulose wall was higher than the foam walls, but less than the fiberglass walls. But it's also true that going from 2lbs density to 3lbs density (something of a minimalist dense pack density) roughly triples the air retardency of the cellulose, and going to 3.5lbs (a commonly used dense-pack density in cold climates) it's passing only 1/5 the amount of air as 2lb cellulose. See the air retardency vs. density curve in this bit o marketing fluff for Spider.

I was referencing the same article. So open cell foam is more air tight, with closed cell right behind it, then cellulose and finally fiberglass? Is that what you are saying?

What about ICF (6" of concrete)? Is that not considered an air barrier?

I'ts not ME that's saying it, that's what the BSC guys' test results were on the assemblies under test, as reported in the text & graphics of that blog posting.

They didn't test an ICF wall assembly, nor did they test SIPs.

SIPs & ICFs do make pretty good air-barriers, but both will leak at the seams, just as the stick built assemblies tested did. There are fewer seams to seal in an ICF or SIP structure, but they as important for performance as in any stick built assembly.

Note in they article they state the test conditions, which were intended to simulate the defects in some real world standard practices, and not at all detailed the way high performance builders would normally do it:

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"The walls were built with a few deliberate air leaks. Each tested wall had three electrical boxes (two duplex outlet boxes and one single-gang switch box) with Romex wiring between the boxes.

Shims were used to introduce a 1/32 inch gap between the OSB sheathing and the bottom plates and top plates of the walls, as well as between the gypsum wallboard and the bottom plates and top plates of the walls. While this decision may sound odd, the deliberate gaps were included so that the walls would leak about as much air as “typical” unsealed walls. “The 1/32 inch gap is based on work done by Don Onysko of Forintek, and represents the typical gap resulting from expansion and contraction of wood framing due to seasonal wood moisture content changes,” said Schumacher. “The 1/32 inch gap sounds like a lot, but it gave us leakage rates similar to what we are seeing in the field.” According to the BSC report, “All of the assemblies … were more airtight than most real wall assemblies (e.g. 0.05 to 0.20 cfm50/ft2).” The quoted range of 0.05 to 0.20 cfm50/ft2 refers to leakage rates in opaque walls, but not walls that include a band joist, door, or window, Schumacher explained." "


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Mind you, the leakage performance was at a pressure consistent with a steady 9 mph wind, which isn't a common average wind speed, but not an insane number.

In any one of those assemblies a large fraction of the leakage was at the sill plates, just as it would be with ICF & SIP. It proves that you can't just build a piece of crap an spray cellulose or foam on it and expect it to be air tigth (not exactly news, eh? :-) ). An interesting comparison would be to compare assemblies using the same insulations built using some of the air-sealing standards that are common amongst builders who care, such as caulking between/under/over sill plates, and between sheathing & framing, etc. vs. the assemblies built with shimmed-out intentional leak points. The full report will probably show up on the Building Science website at some point in the next 12 months.

I was a bit surprised that the energy loss performance at 10pascals wind-pressure in the crappy construction with the wet-sprayed cellulose was almost identical to that of the closed cell foam- I would have thought the extra tightness of the foam would have helped, but the heat-exchanger aspects of the air flow through the fiber fill apparently made up for quite a bit of that!

I suspect the reason the oc foam tested tighter than the cc foam is that the high-expansion ratio of the oc foam filled in the shimmed cracks a bit better. I also suspect that 3lb+ dense packed cellulose would have matched or beaten the oc foam leakage, since fiber blown under pressure would find and fill those cracks whereas the damp-sprayed technique only splats it against the sheathing until it sticks, and the expanding foam only fills the space adjacent, isn't pulled toward cracks. With nothing driving the fiber toward the exfiltration points of the assembly the way dense-packing does the damp spray isn't as tight. But even dense-pack would still leak some air without caulked seams, which is why performance home builders will usually air-seal the structural wood prior to insulating.

As a less invasive retrofit it's hard to beat dense-packing cellulose for tightening up studwalls, but some of the new-school fiberglass blowing wools will in fact do just that. The smaller fiber size is able to pack tighter into the exfiltration cracks.

In re CC foam's less than expected showing I would offer that since CC doesn't expand as much it is easier for installers to fail / overlook filling small gaps

It sounds like BSC did not test the effects of changing gap width resulting from expansion and contraction of wood framing due to seasonal wood moisture content changes (or wind). I'd like to see how well spray foam handles this (does it stretch or crack).

And since they didn't test any air barrier films (like MemBrain, taped never stapled), test two air barriers (recommended) or specify if the XPS was taped or caulked, I wouldn't make any conclusions about spray foam being the best.

My guess is that taped Membrain (or house wrap) + sprayed cellulose + taped rigid foam would significantly outperform everything they tested.