We are building a chalet in Maine and our builder, who is an Eco type builder originally proposed a 2x6 wall with fiberglass in the cavity and 2" spray foam on the inside. R-value = ~28. We wanted to add some rigid board on the exterior, but cost is an issue, so no go. He is now proposing a double stud 2x4 system with cellulose throughout with an R-value of ~35. I know R values aren't the most important parameter to gauge efficiency, but what I read at buildingscience seems to indicate there is more risk at the sill plate for moisture issues if not done properly. Although our builder has good experience with regular 2x6 construction and various foams/EPS systems, this would be the 1st house he has done as a double stud. Apparently one or two of his carpenters have done them in the past, but we don't want to be the guinea pig. Then there is the lost floor space on the inside due to the thicker wall and greater recessed windows. Are there any other reasons why we should/should not go with the double study option? He said he may be willing to absorb the added cost since it promotes his business as more "green". Not sure how this is more green when you're using 2x the wood for the walls. Anyways....

I'm not sure what you mean by "...2" spray foam on the inside..." . Sprayed between lateral 2x3s mounted to the 2x6? How is the finish wall mounted?

There is no stackup with 2x6 fiberglass + 2" of foam (any foam, inside or out) that adds up to a true R28 as a whole-wall R (after the thermal bridging of the framing is factored in.) The 2x6 w/fiber cavity fill is ~ R13, at typical framing fractions, can be PUSHED to ~ R14.5-R15 with an optimum value engineering (OVE) framing design. Adding 2" of even R7/inch foam on the inside doesn't thermally break the band joists or sub-floor- it'll still be shy of R28 as whole-wall. But it's not clear how the finish wall is mounted with 2" of spray foam inside the studs.

Going with 2x6 and 2" of rigid iso on the exterior with some air sealing detailing on the structural sheathing isn't very tough to build,thermally breaks 100% of the wood, and ends up in the same R26-ish range, usually for less money.

http://www.greenbuildingadvisor.com/blogs/dept/musings/how-install-rigid-foam-sheathing

Double the rigid iso to 4" and it'll be in the R40 range.

It's even easier to build 2x4 with 2" of exterior iso (still an ~R20 wall) which is the same wall thickness as a standard 2x6 wall, which can reduce the complexity of window & door details.

Double studwalls work and aren't hard to do, if your contractors think they're up for it. It's more framing lumber, and can be more difficult to dense-pack, but it's pretty straightforward. The weak points (thermally speaking) requiring some thought is how to thermally break and moisture protect the top/bottom plates, foundation sills and band-joists. Rainscreen-vented siding buys a lot of drying capacity, and foam-insulating the interior of the band joist/sill does too. As long as it's well protected from bulk water wetting at the siding & flashing it has a huge capacity for buffering wintertime vapor-diffusion loads from the interior, and will keep the framing & sheathing dry using only standard latex finish paints as the interior vapor retarder (at least for most of US climate zone 6). Using fire rated expanded-cork insulation (something of a specialty item) as fire-blocking can allow you to get a better thermal break at the top/bottom plates and have a fairly good continuity with the attic insulation, etc.

A key detail to get right is defining the primary air barrier on all six sides of the cube, and how it is detailed at window/door openings, corners, at the top-plates/ceiling, and at the subfloor/bandjoist/foundation sill.

Double studwalls uses more framing timber, true, but 2lb spray polyurethane has a very HEAVY greenhouse footprint, since it's blown with HFC245fa (at more than 1000x the GHG potential of CO2), whereas cellulose has an ultra-low GHG footprint- lowest of any commonly used insulations. The exception on spray foam is the Icycnene MD-R-200, which uses water as a blowing agent, but it's only R5.2/inch compared to R6-7/inch HFC-blown goods. The "-R-" is important- Icynene also sells a product MD-C-200, which is uses HFC245fa as the blowing agent. But the " -R- " goods should air-seal just as well as other 2lb foams, and while vapor-retardent, is still a class-III vapor retarder at 3" with good drying potential, whereas other 2lb foams are well into class-II territory at 2".

Iso (polyisocyanurate) and EPS are blown with pentane, which has only ~7x the GHG potential of CO2, and are far more benign than the more common 2lb foams or extruded polystyrene aka "XPS" ( blown with HFC134a, which is even more damaging than HFC245fa ). Using HFC blown goods sparingly and substituting EPS or iso where appropriate is generally good policy, and can even be a cost-savings. At anything more than 2" most 2lb foams add more to the world's GHG burden over the building's lifecycle than the energy-use savings the additional R-value offsets. Closed cell spray foam is GREAT for air sealing and moisture protection as a non-wicking semi-permeable condensing surface even at 1" nominal thickness though.

Dana,

Wow, thanks, although I understand very little of the detail you highlighted. From what I recall seeing in one of builder's homes under construction was they used 6" fiberglass batts in the wall then directly sprayed 2" foam over that on the inside. Drywall was used as the finish wall. The one thing I like about the spray foam is it seals off pretty much all potential for air penetration, right? Dense pack still seems to have the potential of letting air in near outlets, etc. I am still leery of doing the double stud since it does not sound like our builder has much experience with it. It sounds like dense pack with 2x6 and 2" rigid booard exterior may be the best compromise. Pretty sure it will cost more though. So just to be sure, are you saying dense pack with 2" rigid board would be better then dense pack with 2" spray foam on the interior? thanks.

Yes, I AM saying that 2" of rigid polyiso board on the exterior outperforms 2" of spray foam on the interior, if you do it right, and it's usually less money.

Spray foam is a great sealant, but even with spray foam you get more bang/buck putting the foam on the exterior, since it fully seals and thermally breaks all of the framing wood down to the bottom of the sheathing, whereas on the interior it is interrupted & thermally bridged at the ceiling & floor even if you spray-foam the interior side of the band joists, etc.

I also don't know how you would get a flat interior wall with the irregularities of 2" of spray foam, and putting 3.5" drywall screws 12-16" o.c. through the foam defeats a lot of it's thermal breaking characteristics, since steel is about 1000x as conductive as 2lb foam.

To use 2" of spray foam on the exterior you can use pancake-head timber screws (FastenMaster HeadLok or similar) 24" o.c. (to minimize thermal bridging through the screws) to hold furring to the structural sheathing & studs, using chunks of XPS as spacers between the furring & sheathing where you put the screws. The furring then becomes a depth-guide for the foam guys, and the siding can be installed on the furring.  To meet mechanical loading the screw needs to penetrate 1.5" into something structural.

Using 1x furring is OK, but 2x goods makes for a flatter wall. The gap between the foam & siding becomes the "rainscreen" gap (screen & vent it to the exterior both top & bottom), which both the siding and wall assembly can dry into. Closed cell foam would also qualify as the weather-resistant barrier, no housewrap required.

But in some ways rigid foam is easier- DO check out the link to that blog, and click on the links on that page for more details: 

http://www.greenbuildingadvisor.com/blogs/dept/musings/how-install-rigid-foam-sheathing

Air sealing the sheathing to the framing, and the framing to the subfloor, subfloor to band joist, band joist to foundation sill etc, is easily done with acoustic-sealant caulks or 1-part low expansion foam during construction, without the need for large volumes of spray foam. Up to about 4" of exterior foam thickness it's pretty straightforward and easy, and you can stick with 24" o.c. spacing on the timber screws at that thickness for most siding types.

Taking it up to even R40 whole-wall using  5" of rigid iso on the exterior and 2x4 framing is becoming pretty common, and is a method often used in "deep energy retrofits" on existing houses.  The deeper the foam, the longer the screws and the more awkward it becomes, but people even put 6" out there.  Using reclaimed roofing iso can cut the material cost of the foam by ~2/3, sometimes more. (If you can't find local sources, Insulation Depot in Framingham MA  will drop ship if you can take a whole truckload.  You may have to wait if they don't have it in stock, but if you have a place to store it it's worth stockpiling before it becomes a bottleneck to the project.

Exterior foam and insulated concrete form (ICF) foundations with the sheathing foam alighned with the exterior EPS of the ICF makes for a continuous thermal break, keeping the foundation sill & band joist warmer & drier than most approaches to insulating them from the interior.

I should have clarified the spray foaming on the inside was performed between the studs, not covering them. So, no thermal break on the studs. I will propose to our builder dense pack with 2" rigid on the ext. I forget, but 2" R-val = 10? So theoretically, our wall with cellulose should be ~R29?

With 2" of cc foam between 2x6 studs and the other 3.5" fiberglass or celluose the whole wall R at 16"o.c spacing it would still be under R15 whole-wall.  With 24 o.c. stud spacing it would be close to R17.

On the exterior foam use iso, and you'll get R12-13 @ 2" . With XPS (the HFC134a blown stuff) you'd be at R10 @ 2".  It's safer/better to go with 2" of iso, because:

In climate zone 6 (most of ME), per IRC 2012 & IRC 2009, with only R10 on the outside of  2x6 you would still need to use an interior vapor retarder.

But with anything over R11.25 you can use standard latex paint as the interior vapor retarder and call it a day! Even though the foil facers used on most iso (but not roofing iso) are a strong vapor barrier blocking drying toward the exterior, it's sufficient R to keep the sheathing above the dew point of the conditioned space enough of the winter that the sheathing won't get wet & rot or mold.  With paint-only on the inside you have a lot of drying capacity (much more than with 2" of XPS on the exterior and a kraft-facer or vapor-barrier paint on the interior- or worse POLY), and seasonal moisture or minor bulk water incursions dry relatively quickly toward the interior during the spring.  With 2" of XPS and a vapor barrier paint or kraft facers on the interior it has less than half the drying capacity.

See: http://www.buildingscience.com/docu...uirements/

Also, with an air gap of 3/4" or more  between an exterior foil facer and the siding it adds another ~ R2 to the average thermal performance due to it's low-emissivity, and high infra-red reflectivity (the radiant-barrier effect.)  

But even omitting the foil facer you'd be looking at the mid to high-20s for whole-wall R with a 2x6 fiber-filled studwall and 2" of exterior iso, nearly 2x what you'd get with 2" of foam + 3" of fiber in the stud bays.

Double stud walls are an excellent way to build. (we're doing one now). Don't use a single plate - build two walls with individual plates and gasket the outside one (under the plate) - and under the PT sill - with building gaskets from Conservation Technology. To address two concerns: "more risk at the sill plate for moisture issues"and "Dense pack still seems to have the potential of letting air in near outlets": the building has to be air tight. Moisture travels on air - stop the air infiltration and you'll stop 90% of vapor transmission. You'll need to air seal the foundation/wall joint/rim joist area - best done with spray foam. And the exterior wall needs to be an air barrier - the easiest way is to use Huber ZIP sheathing, or you could use OSB or Advantech and tape the joints with SIGA tape - available at Maine Green Building Supply in Portland. Obviously these aren't the only air sealing techniques that you need to pay attention to, so read up on air sealing and combine that with your builder's knowledge.

Let me try this again....

We will be doing 24 oc, but you're saying only R17? Code is R19. Our builder says he will be using 2.25" cc spray foam with R13 fiberglass, totaling R28. How the heck can you get an R13 out of 3.25" glass ( I am assuming 5.5" max insulation for 2x6)? Your R17 sounds more like the actual r-val and not theoretical, but either way, how is out builder getting to R-28? What is the r-value of 2.25" cc foam? We proposed substituting the 2" rigid board exterior for the interior foam and as expected, he says due to added labor, the rigid board would be more expensive.

R-17 is the actual R value since you'll lose 20-25% of the R value of the wall due to the studs, header and plates. The code is based on the stated R value of the insulation, hence the builder's R-28. The problem is that the physics of insulating (and heating) your house requires actual (as opposed to theoretical) R value, so you while you think you are buying an R28 wall, you are actually buying R17. Exterior insulation, which covers the whole wall, not only gives you the whole rated R value of the insulation, but warms the stud cavity so that the insulation inside the studs is more effective. Yes it is more expensive up front, but the other choice is more expensive as long as you live in the house. The builder's recommendation of "flash & batt" - spray foam + batts is however better than 100% batts, but since you have the opportunity to significantly improve your house, it would be a wise move.

Interesting. So our older our that had R19 batts throughout, probably only really had an R-value of 14? Terrible. So with the builder's spray foam/batts we're not doing much better at R17. But the air infiltration I assume would be much better controlled/prevented. Well, this makes me want to reconsider the double stud wall perhaps, since he's claiming an R val of 35, which, again, understand will be less than that. But the way he was describing it, he said they would build the walls so there would be an airspace between them, effectively creating a thermal break.

If it were you, would you opt for the double stud wall or exterior foam board?

I built a house last year (1500 SF ranch with full conditioned basement) which cost the owners under $250 to heat from mid Dec to mid June. That house had 2x6/cellulose walls with 2" of XPS foam, so that method does work. This year I'm onto the "next generation", using a double stud wall to improve R value without increasing cost substantially. With a double stud wall, the space between the studs could be 10" (R35) or 12" (R42) without affecting anything costwise but the cost of the cellulose. Air sealing is probably more difficult with double stud; with exterior foam, a decent, taped foam job will help with air sealing. The best wall would be a stud wall with two layers of 2" polyisocyanurate (R-26) as it gives you R-40 or more while eliminating thermal bridging.

So with 10" space between double stud wall, that chews up interior floorspace right? That's my only holdup. I don't want to sacrifice space on the inside.

Confession of a thirty plus year insulation contractor; there is no way to "dense pack " cellulose insulation so that over the lifetime of that wall system {double stud} I could ever guarantee no settling. I've been reading these post over the years with some ammusement and also some concern that people are being sold a bill of goods on the ability of an insulation contractor to pull this off.

Well it will cost us an additional $3K if we want to go with the double stud wall. Our builder wanted $2500 for labor and $3500 for XPS to do the ext foam board, so we can't afford that.

Smart

My guess is that if you mix enough glue into cellulose, it will stay in place perfectly.

a double stud wall ... without affecting anything costwise but the cost of the cellulose

Isn't there a cost for additional framing?

smartwall: I'm not a building scientist, but there is widely distributed and verified information that properly installed dense packed cellulose does not settle. Apparently you're not one of the few insulation contractors trained in that business.
Yes a 10" wall takes up more space than a 5-1/2" wall - about 4% in the houses I've designed. You have the choice of spending $$ on foam, spending "space" on cellulose insulation or spending $$ on heat. I design houses where i show the interior dimensions before we price and build the house; to be changing the system after the fact is difficult, yes. Perhaps you could add length to the gable end to make up for it. I have no solution for gaining insulation value with no cost.

jonr: once you've decided to build a double stud wall, you can vary the depth of the wall. the cost of additional framing at that point is nil.

Well the builder would space the walls 2.25" and supposdley come up with an R35. But we'd lose 3.75" around the perimeter of the walls. On corner rooms, that can add up to several lost sqft. I think we're just going to stick with the spray foam/batts.

Posted By jzinckgra on 30 Aug 2012 08:51 AM
Interesting. So our older our that had R19 batts throughout, probably only really had an R-value of 14? Terrible. So with the builder's spray foam/batts we're not doing much better at R17. But the air infiltration I assume would be much better controlled/prevented. Well, this makes me want to reconsider the double stud wall perhaps, since he's claiming an R val of 35, which, again, understand will be less than that. But the way he was describing it, he said they would build the walls so there would be an airspace between them, effectively creating a thermal break.

If it were you, would you opt for the double stud wall or exterior foam board?

Not terrible, just the facts:  Something like 25% of the wall area is framing timber (studs, plates, headers, blocking etc) at an R-value of ~R5.5.  When you run the math on average heat loss through the wall at the wood and batts to calculate the whole-wall performance, it's about the same as if the wall were ~R13 of a uniform R-value, R14 if you used 24" o.c. studs and careful minimization of framing timber.

R17 on the foam/batts combo is a best-case number with a 20% framing fraction rather than 25%.  With a 25% framing fraction it's still shy of R15.

Air infiltration at dense-pack cellulose densities drops to irrelevant levels from a thermal performance point of view, but still affects moisture-migration (but only slightly).  Foaming (or dense-packing) the stud bays can only air-seal the stud bays, and doesn't address air leakage between the framing's doubled-up top plates or under bottom plates, etc.

The notion that dense packing "doesn't work" in the longer term may be true at 3lb density in a cold climate when there is no exterior foam, but there are now known climate & density parameters by which the settling can be predicted or even prevented:

The settling of cellulose is a function of mechanical creepage & strain from seasonal moisture cycling into & out of the cellulose.  That moisture cycling is a function of both local climate conditions and the wall stack up.  In not-so-cold temperate places with modest dew point swings like mid-coastal CA, even at a relatively low 3lb density it will go forever without settling, but in Cariboo ME or Bergen Norway it would settle measurably in a decade.   This has been carefully studied and measured in Europe, and it's possible to specify a density at which the creepage if fully elastic (= no settling) for almost any climate zone.  Much of the academic credit goes to a Dane named Torben Valdbjørn Rasmussen who has studied the problem extensively.  An English-language version outlining of some of his experimental work & analysis can be found here, and has been the basis for cellulose installation standards in some places (if not in N. America.)  There's more of his work out there if you care to dig further.

When you put 2" of foam on the outside of the sheathing, the temperature of the cold side of the cellulose is that much warmer & drier, with far less moisture cycling over a year.  For a typical ME location 2" of foam on the exterior of a 2x framing is a near-equivalent to a cellulose-only building  in Carmel CA, from a humidity cycling point of view.  With foam sheathing anything over 3lbs density should work for a VERY long time, but shoot for 3.5lbs or 4lbs if you're uncomfortable with 3lbs.

It's not clear to me exactly how that translates to very thick double-walls & Larsen trusses with rainscreened siding, but it should still be pretty favorable. With latex paint limiting the diffusion rate into a much larger mass of cellulose than conventional framing there is simply less water per unit volume or weight in the cellulose, and with the enhanced drying toward the exterior of rainscreened siding the peak moisture levels are inherently limited.