Ok, my plan is being drawn up right now at 10" double studwall.  Still figuring out insulation - I spoke with a fiberglass blowing contractor, he stated they did his dad's house as  a single studwall, blowin in fiber, with the 1/2" dow foil-faced foamboard on the inside to act as air barrier/vapor retarder.  All seams taped.  He claimed a VERY low infiltration - 0.05 I think it was.  I'm averse to putting in conventional poly, as I've heard that it is proven to deteriorate very quickly - even behind walls, in ceilings, etc.  I'm not sure how much I believe that, but it's a concern.   In addition, I could install the 4x8 sheets of foil-foam by myself - whereas poly I would have to have a sub (probably the insulation sub) install at a cost to me.  I think the cost would probably be a wash.  It would gain me an additional R-3, good air barrier without having to rely on my drywall sub to caulk/seal as in the Airtight Drywall Approach.  

The board is on sale at the local big-box for 8.65/sheet - I figure about $600 including tape for the whole house.  If I did this, I wouldn't do spray foam - it would be really redundant.  So, it's a cost savings there - but does it make sense?

Thoughts/opinions/ways I might do it better or cheaper?  Not better AND cheaper, I know that doesn't exist.  Or does it? :-)

Looks good, just make sure there is a way to top the fiberglass off from the attic for when it settles. Easiest way is to just leave the gap open so as the wall fiberglass settles, the attic fiberglass falls in to take its place. You might consider using fiberglass batts just in the area under the windows so you will never have to remove the windowsill or worry about it settling there. My father did the foil faced inside insulation, seams taped on his 2*6 stud wall home and it works well. I know you know this, just remember that air sealing the walls doesn't do much good unless you get the rest of the envelope air sealed as well, including tricky spots like around the floor joists.

Thanks for the input - just like everything else, I'm going to overkill the tricky spots - I plan on caulking and gasketing all concrete/wood, wood/wood joint areas - rimboard, joists, subfloor, bottom plate, etc. Also will definitely be sprayfoaming the joist cavities. Will dense fill fiberglass settle? Installer (obviously) claims it won't.

Whether & how much blown fiber insulations settle depends a lot on the true density and the installer's abliity to acheive it with any consistency. In a double-wall installation this can be difficult to assess, since it can blow/fill-in laterally quite a distance without studs to block it. It may be worth installing blown-insulation screening every few feet connecting the two studwalls to provide barrier to dense-pack against.) The PassiveHouse uses engineered beam "studs" to form reasonably sized cavites that can support dense-packing pressures. Larsen Trusses don't have that, but if you stapled the purpose-designed screening in place you can still dense-pack it for a more consistent result.

I'm less familiar with dense-packing performance of blown glass & blown rock wool than dense-packed cellulose, or what densities are required to eliminate settling. In a 10' tall cavity it takes about 3lbs/ft^3 or higher to completely eliminate gravitational settling of cellulose (but saturating wetting events could still cause some shrinking.) Many dense-pack cellulose installers go for 3.5lbs/ft^3 average just to ensure that even with variations they're getting 3+ everywhere. Glass & mineral fibers offer no hygric buffering and less thermal-mass than cellulose, but the same lack of hygric buffering makes them less susceptible to shrinkage/damage from wetting events (but in flood or hurricane saturation you'd want to remove/replace quickly anyway, just as you would cellulose).

On the better & cheaper front, since you have a huge thermal-break with the framing due to the insulation between the walls, going with foil faced rigid insulation with FSK taped seams for an air (and vapor) barrier is more expensive and labor intensive than using flexible non-insulating sheet goods. NON-perforated aluminized WOVEN polyester/nylon (or woven polyethlene-not but not sheet polyethylene) radiant barrier has extremely low vapor permeability and excellent air-barrier characteristics. If you use the 4' rolls stapled to the studs with 3" of overlap sealed with mastic it'll be as-good or better an air-barrier than a whole bunch of cut up 4x8' sheets of foil-faced iso with caulked interfaces and FSK taped joints for a whole lot less money & labor. The total length of seam will be less, and the joint sealing method more secure. You can roll it and staple it to corners without creating any special seam-sealing issues, and it can be cut/punched/trimmed very precisely around plumbing ventilation & electrical penetrations with knives/scissors, whatever... It can be folded and stapled/mastic-sealed to the sub-flooring at the bottom to lap under the finish flooring for a no-caulk perfect air-stop solution there too. You don't have to worry about providing a 1/2" gypsum or equivalent thermal barrier to meet code using RB either, the way you have to with iso. It's a bit more ruggedized against accidental penetration during construction than the thin foil on iso-board is too. (Brushing/troweling on a stripe of mastic is less work than careful beads of caulk or taping.)

Shop around, it's ~$150/1000ft^2 in 250x4' rolls. so you'll be looking at material costs half that of the thinnest iso board. If you really need/want the extra R3, make your wall 0.75-1" thicker, eh? Just be sure to specify "woven" and "vapor barrier" or "low permeability", you'll find the right stuff.

For Wisconsin, I'd suggest NOT using a foil-faced or polyethylene vapor barrier. Just drywall with vapor retarder primer will give you a 1-perm vapor retarder, low enough so that it would take a year of sub-zero dewpoint outside air to bring the wood to saturation (even ignoring the hygric capacity of the cellulose if that is used in the cavity). The 1-perm level lets the cavity dry to the inside on those occasions when that has to happen, due to outside conditions. A vapor BARRIER prevents the drying to the inside.

You should have an air barrier to minimize air infiltration and movement of inside air into the cavity during winter conditions, but poly isn't what you should use except in the coldest climates. The ADA technique works, but as you note, care must be taken to use sealant when the drywall is installed.

Dana1, can you provide a link to info on that sheet material you suggest using? What is the perm rating of it?

The other type of sheet good that might be used is Certainteed's MemBrain. It has the 1 perm rating at low humidity, but that increases dramatically when the humidity in the cavity goes up, to permit inward drying.

The "breathable" perforated versions have permeability in the 10-50 perms range, the "vapor barrier" non-perforated versions are on the order of 1-5 hundredths of a perm (0.01-0.05 perms)- very comparable to 6 or 10 mil polyethylene but more durable in a wall. (Don't use it as ground-contact vapor retarder though- the aluminizing will deteriorate pretty rapidly in a chemically active wetting situation.) It's basically 2 layers of aluminum foil heat-laminated to a thin synthetic-fiber fabric- how permeable is aluminum foil? (Not very!)

The perforated versions probably perform well-enough as an air-barrier when applied directly under gypsum board. (The intersection of gaps in the gypsum and the perforations in the radiant barrier can't add up to even 1/10 of a square inch per hundred square feet of wall area.) If you want to maintain some inward-drying capability one could use the perforated version without much impact on overall infiltration. The perforated stuff is too permeable to meet spec for cold/very-cold climates unless used in combination with vapor retardent paints on the interior.

If used with 1/2" thick furring strips between the RB and the gypsum it adds another effective-R5 or so to the wall (not that it's necessarily worth the trouble in a superinsulated wall stackup.)

Some specs taken at random from the web (there are literally dozens of comparable competitors):

http://www.raflect.com/raflect/information/C105/specifications

Non-Perforated Ra-flect™ VB Radiant Barrier - Specifications:

* Standard Size: 48" wide by 250 feet (1,000 Sq. Ft.)
* ENERGY STAR® qualified
* Double Sided Radiant Barrier
* Contains 99% pure Aluminum
* Emissivity: 0.02 (ASTM C1371-98)
* Reflectivity: 98% (ASTM C1371-98)
* Class A / Class 1 Fire Rating
* Flame Spread: 20 (ASTM E84)
* Smoke Development: 30 (ASTM E84)
* Clean and Non-Toxic - GREEN
* Resists Mildew
* Emittance value: .02 (Blocks 98% of Radiant Heat)
>* Water Vapor Permeability (Vapor Barrier): 0.005 g/m²/24hr<<<<<<<<<<<<<<<<<<<<<<<<<
* Thickness: 5.3 mil ± 5%
* Elongation (Machine Direction): 45%
* Elongation (Cross Direction): 39.1%
* Tensile Strength (Machine Direction): 257 lb/in²
* Tensile Strength (Cross Direction): 200 lb/in²
* Mullen Burst Strength: 272 psi
* Temperature Range: -140°F to 212°F




....or....

http://www.energyefficientsolutions.com/rbspecs.asp

ARMA FOIL , ARMA FOIL-VB

ARMA FOIL™ is a high strength radiant barrier. It does not tear easily and is well suited for holding staples in all positions. It is available as either Perforated (ARMA FOIL™) or Non-Perforated (ARMA FOIL-VB™) and is double sided for maximum savings in both summer and winter.

* Standard Size: 48" wide by 125 or 250 feet
* ENERGY STAR® qualified
* Double Sided Radiant Barrier
* Contains 99% pure Aluminum
* Emissivity: 0.03 (ASTM C1371-98)
* Reflectivity: 97% (ASTM C1371-98)
* Class A / Class 1 Fire Rating
* Flame Spread: 10 (ASTM E84)
* Smoke Development: 10 (ASTM E84)
* Clean and Non-Toxic
* Resists Mildew
* Emittance value of .03 (Blocks 97% of Radiant Heat)
* Water Vapor Permeability (ARMA FOIL™): 70 g/m²/24hr
> * Water Vapor Permeability (ARMA FOIL-VB™): 0.02 perms<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
* Thickness: 5 mil
* Tensile Strength (Machine Direction):80.4 lb/in
* Tensile Strength (Cross Direction):65.4 lb/in
* Mullen Burst Strength: 160 psi

Depends on your expected timeframe for the house. Remember, fiberglass is glass, which is actually a liquid, so with enough time(like 2000 years), it will turn into a pool on the bottom of the bay. ;-)

With a good install with absoulutely no gaps and proper density througout, you probably wont have any setteling for a long time. I personally don't have that much faith, but if you have gotten ahold of an excelent insulator, more power to you. But even on a proven and properly installed dense pack fiberglass insulation system, in a few decades you just might need to top the stud bays off. Not having made a few simple preperations for this now will turn this into a nightmare you or your children might have to face. Bottom line is it shouldn't settle if done right, but who wants to gamble on it?

I'll tell you an easier way (maybe, depends on insulation) to incorperate the foil faced foam. Put the foam in between the studs, and tape the joints there, and you dont have any wires, pipes, nails, ect going though the foam, as they will all be in the inner set of studs, and your sheathing will be on the outer set of studs. For my next house, I will be using a double stud wall, fiberglass batts in the inside and outside, and R-max foam board between.

Dana1 - something like this?

http://www.neutralexistence.com/shop/raflect-radiant-barrier-vapor-barrier/

Dick - I'm not going to get involved in the vapor barrier vs vapor retarder issue - code here is vapor barrier (ok, now it's a vapor retarder < 1.0 perm, but you know what I mean). My thought is that Listuribek recommends having one side of your wall assembly as more permeable than the other. With OSB sheathing, that's fairly low perm - I don't believe I want to install a higher perm system on my interior wall, considering I'm in northern Wisconsin - my climate is very near that of Duluth/Int. Falls/Canada. I plan on having good quality ventilation/heat recovery - I think I'd rather take my chances at not pounding moisture into that cavity where it doesn't want to dry to the exterior, than rely on a 'huge hygric buffer'.

aardvarcus - how do you plan on installing the foam board in between the studs? I like the idea of having it behind all the penetrations - but I can see that being a massive pain to install, tape, and caulk!

Yeah, that's the type of RB/vapor retarder I'm talking about but...


...you're putting the low-permeabilty side of the wall on the far less-favorable side!

In cold/very-cold climates like yours you're better off making the EXTERIOR of the wall assmebly more permeable, since your drying-to-interior season is short, and the liklihood of even building, let alone maintaining over time a perfect interior air barrier is somewhere between slim & none.   Even if you do, allowing a hygric load to build in the wall from interior 8 months of the year means you have to keep the interior humidity QUITE low (nosebleed low?) all summer to remove that load.  (This is why it's code in Canada to put polyethylene on the interior side, as often as THAT gets screwed up in practice.)  Alternatively I s'pose you could keep the interior RH under 15% all winter to slow the build up, but that's the antithesis of human-comfort (25-30%+ feels a WHOLE lot better!)

In a weather driven wetting event OSB lets the water into the wall just fine- it's permeability goes way up when wet, then falls below 1 once it dries- it's something of a humidity-diode in the wrong direction.  In heating dominated climates higher permeability sheathing is preferable, (Huber ZipSystem, GP Stedi-R, Dow Sturdy-R etc ), as are low permeability air-tight interior walls.  The colder you are the more serioiusly you need to take this.

I agree that there ultimately will be events that will result in some water entering the cavity, however infrequent they may be. To minimize entry from rain events, a minimal rain screen separation of siding from housewrap is prudent for new construction. Given the inevitability of water entry at some point, preventing drying to the interior totally is not such a good idea, except in the coldest of climates.

My comment about needing a very, very long period of cold/dry outside weather (far longer than even Wisconsin winters), is based on a calculation anyone can do for a 1 perm vapor retarder. In absolute terms, the rate of water vapor entry is really quite low. The little amount of water that enters by just diffusion through the VR over a even few months of bitter weather is easily absorbed safely by the wood framing and sheathing, even ignoring the rate of diffusion through the sheathing to the outside. Movement of water into the cavity by convective flow of leaking air typically is much greater and is one reason why the air movement barrier is so important to get right. Making the VR even less permeable than "very low" doesn't really buy you much protection, but it can cost you some needed drying at other times.

Will a vapor BARRIER on the inside work OK in northern Wisconsin? Probably yes, if the exterior is detailed well and the sheathing has at least some permeability. Maybe even definitely so if the air barrier is somewhat leaky so that some small amount of air leaks through the cavity at times.

well ok - now I'm REALLY confused. The standard building practice here, and I think probably in every cold climate in north america, is OSB wall sheathing, insulated studwall, vapor retarder (99.99% of the time it's 4-6mil poly), then drywall. You are both telling me that this is not the way to build??

richntiff,
In your above description of a wall, the vapor retarder is the major question. I have lost track of where you plan to build, but location in the key to that issue. Check out Building Sciences reference manuals for your geographic area. But basically, poly vapor barrier should NOT be used in any but the coldest climates. If your area has any significant amount of summer that requires AC, then you should not use poly.

Wes,

I'm in northern Wisconsin - the lower edge basically of climate zone 7 - similar to Duluth, MN. I'd estimate we have 10-15 days per summer where AC is 'needed' - now for us, that's temps above 90. I'm going back to my Certainteed MemBrain plan - then I'd have a built in safety net, the MemBrain will allow the assembly to dry inwards when needed.

I've been unsuccessful so far in finding a local dealer for MemBrain so I can get a budget price for it. Do you have any approximate cost figures for it? I'm leaning toward MemBrain myself, over the use of just VR primer paint on the drywall for two reasons. First, according to humidity against the membrain, its permeance will increase even more should the need arise. Second, having just VR primer on the drywall still requires ADA, which in turn means that the drywall installer must be supervised completely to make sure sealant is used around the perimeter and at electrical polypans, for air barrier integrity. If a transparent membrain is used as the VR, visual inspection of the installation is possible before the drywall goes up. Of course, the drywall installer still has to take care not to damage the MemBrain material.

One final thought on the 'need to dry inwards' at times: when the house construction is finished, the wood may still have a lot of moisture in it that has to dry out. The 'need' to dry inward may be there at the outset.

MemBrain would certainly be a better choice than a high-permeability interior.  But high-permeability sheathing and/or a cavity-type drain-plane promote drying toward exterior is still a very good idea.

Other issues of note- in northern WI you're in the "Moderate" zone for weather driven wetting, which means a gap (but not necessarily a full vented cavity or rainscreen) is recommended between the exterior siding and the drain-plane material (housewrap, felt, whatever).  See figure 2 in this document:  http://www.eere.energy.gov/buildings/building_america/pdfs/db/33288.pdf

But nearby you in Canada they don't mess around- a 10mm rainscreen layer is now considered best-practice (or code?) everywhere.   Something to consider...

In single & 1.5 story (dormered) structures, weather driven wetting through siding can be mitigated a LOT by going to 2' overhangs on the roofs (which also mitigates foundation bulk-moisture issues as well.) The only real down side to the overhangs (beyond cost) is greater suceptibility to roof damage from hurricane-force winds. (Do you see 100mph+ winds in your neighborhood very often?)  At 2 stories long overhangs still protect the upper portion from wetting, but the lower 1/4, not so much.

BTW: In a super-insulated house with heat-recovery ventilation your new-improved "needed" AC days will now likely be zero (VERY likely if you build in a lot of thermal mass inside of, or as-part-of the insulation boundary.), as long as you don't over-glaze the S and W sides.

Dick,

I found an in insulation supplier fairly near me that will sell me MemBrain for $140 per 9.5x100' roll. Not too bad of a price I don't think.

Dana1 - thanks for that, I am building 2' overhangs, and am tentatively planning on a rainscreen type treatment behind the siding. Still working on those details as well - either 1/2" furring or a rainscreen housewrap product.

Also checking on insulated structural sheathing that has higher perm - like Stedi-R. Does this stuff meet code for racking without requiring corner let-in bracing? If so, I'm sold....

The Stedi-R installation instructions has fastener & fastener spacing specs called out for use as structural sheathing (at which point I think it meets code in most places without special framing requirements.  Call 'em or email 'em to find out if there's any question about a particular structural spec you need to meet.) 

IIRC ZipSystem also meets structural specs similar to OSB. (Look that one up yerself! )

Sturdy-R is NOT a structural material in the same way as the wood-fiber based high-perm sheathings are.

What fiberglass insulation proponents fail to mention is that its performance goes DOWN as temperature goes DOWN.
Why? R factor testing is done at 70 F.
As T goes down, so does the R factor. (Fiberglass : R3.5 to R2 @ 0F)
In contrast, expanded polystyrene (EPS) performance goes UP as temperature goes DOWN. (R3.5 to R4.75 @ 0 F)
For a wintry climate, your best insulation bang for the buck is EPS.

See insulations compared chart on:
http://www.thermasave.us/

True (sorta), but let's not exaggerate, eh?  The more rational scoop:

Fiberglass loses R value with increasing delta-T due to induced convection currents within the material.  The degree of heat increase heat transfer due to this factor varies dramatically with orientation (and density, and thickness...more on that later).  The worst configuration is low density & thin layers, cold side up/warm side down.  (Say, a 3.5" R13 batt in the floor of an attic.)  The delta-T per inch within the batt is quite high, and it's resistance to air flow low, so when warmed from below the air in the batt nearest the bottom heats up, gets lighter, and flows freely upward as cooler air from above sinks and displaces it. 

This is the ONLY configuration where R3.5/inch goes to R2/inch at 0F on the cold side.  The Oak Ridge National Labs data show less loss in R value than that using low-density loose-fill FG (not batts) in an attic floor configuration, but at R25 & R30 nominal thicknesses.  (Further tests show that topping it with even 3" of blown cellose is enough to restore R-value by better blocking the convection currents.)

If you make fiberglass 25 inches deep (even if low density blown fiberglass), the problem is much reduced- there's more fiberglass impeding air flow (5x the thickness), and less localized delta-T to induce the air flow in the first place.  There is still some loss in R-value, but not nearly that of thin batts of low density. Going to high-density batts also retards air flow better, and suffers less R-loss with delta-T.

In a warm-side UP configuration with an air-barrier on the warm side there's almost ZERO induced convection (the warm air is in the top of the batt, and the natural buoancy keeps it there.  The decrease in R-value at higher delta-Ts in that configuration is negligible. (The batting under staple-up radiant works pretty well at blocking conducted heat losses. Being translucent to radiated heat it still underperforms other insulations of equivalent  R-value by a signifcant amount sometime though, but that's unrelated to R-value, which measures it's conducted heat transfer.)

In walls, thickness & density also counts.  In a 3.5" (2x4"construction) wall cavity, the warm side causes upward motion of the air, the cool side downward motion, but lateral motion is limited to only the turbulence induced by the fibers from the up/down flow.  This micro-turbulence creates some mixing & lateral motion, but it's not huge.  The thicker the wall, the less real mixing goes on, and the current has to traverse the entire height of the wall cavity, not the thickness of the batt.  R-value is still lost with higher delta-T (far more than in the under-radiant floor configuration) but not nearly as much as in the attic floor configuration.

But here we're talking about DENSE PACKED fiberglass (with much higher resistance to air flow), in WALL (not a ceiling), a SUBSTANTIALLY THICK wall to boot.  I don't think one needs to be much concerned about it dropping from R50 to R48 when it's 0F out.  Sure, the steady state heat loss will be greater under those conditions than if dense-packed cellulose or foam, were used, but not even 10% (or 40% more as implied by your R3.5/R2 example.)  In a 2x4 studwall with cheap R11 batts, yeah, it'll be more than 10% lower, but not here, not even close.

That said, I'm not a big fan of fiberglass, even dense-packed (particularly in dual stud-wall construction) For roughly equal money cellulose offers a whole lot more- lower air infiltration (depends on actual density), HUGE hygric buffering capacity to protect framing structures from moisture problems, and a significant performance boost due to it's much higher thermal mass.  At about 1/6th that of concrete per unit volume, a 12" thick wall of dense-packed (3lbs/ft^3) cellulose has the same thermal-mass a 2" concrete slab of equal area- it's not nothing, and it will be measurable in the heating/cooling bills.

EPS is great & all, but it's the antithesis of a hygric buffer, and has near-zero thermal mass to speak of.  It's great for getting high R-value in roofs (where loading is an issue), and great for blocking thermal bridging in studwalls, but in a dual-studwall configuration? Don't think there's enough value-added here...  

When EPS SIPs come in true R50+ clear-wall values they might be a worth a second look for superinsulation, but I haven't seen 'em yet.  There are enough manufacturing issues around fabricating even R25 SIPs to make me think R50 SIPs will be a long time coming...