Could someone please help me out with explaining how this wall system will function in terms of vapor/ moisture path?

The walls are timber framed with full a full 4"insulation cavity provided on the exterior of the timber frame that will be filled with dense pack cellulose.
The insulation cavity will be sheathed on it's exterior with 2 layers of 1" xps foam board, joints staggered and sealed. Housewrap on exterior of xps.

Interior paneling will be a mix of wood paneling, plaster, and possibly brick in the kitchen.

Exterior siding is undecided.

Thanks

where are you?

Location and local climate are critical for determining how much sheathing R you need to protect the sheathing from dwelling too many seasonal hours below the interior dew point. The IRC has prescriptive levels that assume 3.5" (2x4 framing) cavities, as well as 5.5" (2x6 framing) cavities based on US climate zones, but you can probably interpolate for a full 4".

http://publicecodes.cyberregs.com/icod/irc/2012/icod_irc_2012_7_sec002_par025.htm

If it's not too late, any chance I can convince you to move to polyiso instead of XPS? The blowing agents used for XPS have more than 100x the global warming potential of those used for polyiso at identical-R. The performance per inch is comparable (polyiso is even ahead of XPS at identical thicknesses until it drops below 20C/-4F or so), and foil-faced polyiso is easier to air-seal than XPS. At 2" XPS is more vapor open, which allows more drying capacity for the sheathing toward the interior, but if you have sufficient R for dew point control it won't need to, just keep the interior sides reasonably vapor-permeable (no polyethylene sheeting, no vinyl or foil wallpapers, etc.)

You'll definitely need an AIR barrier between the wood paneling or brick and the dense-pack under any circumstances, (Thin gypsum works, if properly detailed as an air barrier.)

you may want to install your air barrier from the exterior so it is not interrupted by the beams and posts. will the stud cavity sit on the floor, the foundation or will it hang from the timber frame?

My location is northern New York. West central Adirondacks to be more specific.
What I've outlined in my post is what my engineer has specified.

Xps was specified and is what I used for below my slab for both it's psi rating and it's performance in wet conditions, but why not eps or, as you responded, polyiso if it has the same vapor block characteristics?  I would have thought xps would be unnecessary for the wall, but I'm just a builder and have no training in engineering.

The engineer has shown 1/2 drywall on my plan and not having asked the question I'm fairly certain that it is included as an air barrier though I am not planning on using any drywall.

According to the building science corporation 2" of xps is considered to be vapor semi-impermiable, so my assumption is that once I get an effective vapor barrier between my dense pack and paneling I can panel the interior and side the exterior as I please.

In other words, since the wall cavity is effectively sealed off from any vapor transmission from either side, and is therefore not a wall which would need to breathe as in strawbale, paneling and siding don't have the critical function as in an example like strawbale. Or am I off on this? The technical details on this issue that I've read tend to make my head hurt.

Thanks

Posted By Bob I on 18 Jun 2013 12:42 PM
you may want to install your air barrier from the exterior so it is not interrupted by the beams and posts. will the stud cavity sit on the floor, the foundation or will it hang from the timber frame?

The insulation cavity is provided by 2x4's that run horizontally on the exterior of the timbers.

Do not use poly on the interior walls!! you can never be certain that moisture will not get into the wall, so the wall should be built to allow it to dry to the inside or outside or preferably both. the ideal would be to use an interior air barrier between the cellulose and the paneling. However, do not confuse the air barrier with the vapor barrier. Water vapor can go through a permeable air barrier. The other confusing issue is that if you have wiring and outlets in the wall, that means you have holes in the barrier.

you should also have an exterior air barrier which could be the foam. It should be taped and sealed at the top and bottom of the walls so air does not get behind it there.

XPS is somewhat permeable so is more forgiving than polysio which is moisture impermeable.

which means you cannot get a good (long term) seal with an air barrier (or a vapor barrier should you use one). And that makes the exterior air barrier more important. Since you cannot get a high enough R value with the 2x4s to use an exterior impermeable foam like polyiso, either go with 2x6's or 8's, or stick with the XPS. This is an ideal place for a Larsen Truss where you could get 10 or 12" of dense pack cellulose in the cavity and forget about the foam all together.

An exact zip code is necessary to determine whether you are in zone 5 or zone 6, but R10 is sufficient for dew point control at 3.5" cellulose depth in either condition. If it were a full-dimension 4", it's a bit marginal for zone 6, but still OK. But at altitude in the Adirondacks it's closer to a zone-7 climate, at which point R10 isn't good enough for 4", but would still meet the IRC prescriptive bare minimum for 3.5".

You need both the interior & exterior to be AIR-tight for thermal performance, but air tightness on the interior side is far more important for protecting the sheathing in your climate. You absolutely DON'T want a vapor barrier between the cellulose an the interior- it needs to be air-tight, but at least moderately permeable to water vapor. At 2" XPS is about 0.5-0.6 perms- a class-II vapor retarder, and not very breathable. If you then put something vapor-impermeable on the interior side the assembly becomes far less resilient, since any moisture that gets in would then take months of optimal weather to leave, if drying through 2" of XPS was the only path. Air-tight gypsum on the interior side of the 2x4 cavites with a coat of standard latex primer is about 5-6 perms, literally an order of magnitude higher drying capacity than 2" of XPS, and that's where you want to be- air tight, vapor semi-permeable. All wall assemblies need to have the capacity to dry at reasonable rates, since no assembly is perfect. There's good, better, and best, but there's no such thing as perfect in the construction world. If you needed perfection for the assembly to survive the next 100 years, you're already dead. But it sounds like you'll be OK on this one.

A key distinction to be clear about is the difference between an air barrier and a vapor barrier. The "vapor" in building science terms is always referring to water vapor. Many materials are air-barriers at the bulk-macroscopic level that still allow water vapor molecules to move fairly readily. And even when not detailed to be air tight, vapor barriers still block the diffusion path for water vapor, even if air-pressure differences would result in air moving through the assembly. A slashed up sheet of poly is still a substantial vapor barrier, since vapor diffusion is about the surface area x vapor pressure (humidity level) difference, but it's a terrible air-barrier, since air can pass through the slashes when subjected to air pressure differences.

Like Bob, I prefer to spec the sheathing as the primary air barrier for thermal performance, since it is more protected from incidental breaches over time. But reasonably good air tightness on the interior side is important for moisture control at the sheathing unless you bump up the sheathing-R in your climate to about a 50/50 foam-R/cellulose-R. 3.5" of 3.5lb dense packed cellulose is about R12.5, so it wouldn't take much. (Changing to 2" of iso instead of XPS would get you there.)

(edited to add:)

Air-transported moisture can be orders of magnitude greater than transport by diffusion only, which is why air tightness on the interior is important.  In winter there will be many periods where the sheathing is below the dew point of the interior air, and will adsorb much of the moisture in the cavity. That lowers the vapor pressure in the cavity relative to that of conditioned space, and water vapor will diffuse through the paint on your gypsum into the cavity when that happens, but it's at a slow & controlled rate.  But if there are air leaks and convective paths from the interior it'll be a LOT more moisture.  A square inch of air leak is worth a whole wall's worth of vapor diffusion through 5-perm paint.  When the sheathing warms up with warmer weather, it releases it's adsorbed moisture into the cavity, raising the vapor pressure inside the cavity relative to the conditioned space air, and as long as it's semi-permeable it'll all get out well before summer when prime mold-growing temperatures begin in earnest.   The cellulose will help, since it can buffer quite a bit of moisture in it's hollow fibers without affecting performance, helping to even-out the moisture flows in & out of the wall cavities.

I agree, you want an exterior AND an interior air barrier of some type. EPS is much less expensive per R and more breathable.

EPS is a bit harder to air seal unless it's manufactured with facer, in which case it's in the same boat with polyiso, only thicker. EPS shrinkage over time & thermal cycling can sometimes be pretty substantial too, just like XPS- layering is important for thermally breaking the eventual gaps.

So what would be a good wall design for Iowa (50648) using 2x6's? Current thought is plywood exterior followed by 2" of spray foam, then fiberglass batts with NO vapor barrier. I get really confused over this vapor barrier vs. no vapor barrier. Does anyone have a "cheat sheet" if you will, that can help in the decision making process?

Some general principles are that spray foam doesn't address thermal bridging and in most of the US, a vapor barrier on the exterior side is non ideal. And IMO, cellulose (dense pack or moist sprayed) is better than fiberglass.

Posted By agagent3 on 19 Jun 2013 10:29 AM
So what would be a good wall design for Iowa (50648) using 2x6's? Current thought is plywood exterior followed by 2" of spray foam, then fiberglass batts with NO vapor barrier. I get really confused over this vapor barrier vs. no vapor barrier. Does anyone have a "cheat sheet" if you will, that can help in the decision making process?

The IRC publishes the cheat-sheets on how much foam-R you need to use standard paint as the interior vapor retarder.  They have them for both walls & roofs.  Zip 50648 is on the cool edge of US climate zone 5.

If you're putting the 2" of closed cell foam on the inside of the wall cavity it's essentially the same as 2x4 construction, since there is 3.5" of fiber insulation. But with the severe thermal bridging of the studs that 2" of foam is pretty much wasted, since it adds only ~R1 in the "whole-wall-R" performance.  A 1" flash-foam on the interior would be sufficient for protecting the sheathing and air sealing, and you could compress R21 high-density fiberglass batts into the 4.5" cavity, which would yield about R18 (center cavity) for the fiber, R6 (center cavity) for the foam which is a ratio that still cuts it for dew point control in zone 5.

Allowing R1 for siding + sheathing + gypsum, at a 25% framing fraction (typical for 16" o.c. framing) the differences in whole-wall performance are:

R21 fiberglass, no foam:   R14.5
R23 rock wool, no foam:  R15.2

1" ccSPF + compressed R21 fg:  R15.3

2" ccSPF + R13 fg:  R15.5
2" ccSPF + R15HD fg:  R16

But if you added 1.5" of foam  (less foam than a 2" interior job) to the OUTSIDE of the sheathing you'd have about R10 on the exterior, with plenty of dew-point margin, even with R22-R23 rock-wool (preferred over low-density or even high-density fiberglass) and a whole-wall-R of about R24-R25. 

If you did it with 2" of rigid polyiso on the exterior you'd be at R27-R28 whole-wall, or about double the performance for a similar amount of money.

Bottom line- save the high R/inch foam for the exterior- as cavity fill it's a waste of money.

On the exterior it's easier to use rigid foam than closed cell spray.  It can be tacked in place to hold it while you install 1x furring through-screwed to the studs with pancake head timber screws 24" o.c. on which to hang the siding.  The plywood should be detailed as the primary air barrier, with mastic sealed seams &/or glued to the framing at the edges with construction adhesive, and caulk it to the framing inside each stud bay prior to adding the batts.  Similarly, a bead of caulk under the bottom plate as it goes up, and between doubled-up top plates makes it air-tight. 

Whether the housewrap or felt goes between the exterior foam & sheathing, or on the exterior side of the foam depends on how you intend to install the windows. If the exterior glass is to be approximately even with the siding, the housewrap goes on the extirior side of the foam, to lap continuously with the window flashing. If they're to be even with the plywood, the housewrap goes between the foam & plywood.

When adding more than an inch of exterior foam it's better to do it in two layers, with seams overlapped by a foot or so, with taped seams on both layers for air tightness.  Foil faced polyisocyanurate (eg Thermax) is preferable to XPS (pink, blue, green) for ease of air sealing, but also for environmental footprint reasons. The blowing agents used for XPS  (and those used for closed cell spray foam) are powerful greenhouse gases, whereas polyiso and EPS are blown with pentane, with less than 1% of the global warming potential  of XPS or closed cell spray foam. 

Posted By Dana1 on 18 Jun 2013 01:46 PM
An exact zip code is necessary to determine whether you are in zone 5 or zone 6, but R10 is sufficient for dew point control at 3.5" cellulose depth in either condition. If it were a full-dimension 4", it's a bit marginal for zone 6, but still OK. But at altitude in the Adirondacks it's closer to a zone-7 climate, at which point R10 isn't good enough for 4", but would still meet the IRC prescriptive bare minimum for 3.5".

You need both the interior & exterior to be AIR-tight for thermal performance, but air tightness on the interior side is far more important for protecting the sheathing in your climate. You absolutely DON'T want a vapor barrier between the cellulose an the interior- it needs to be air-tight, but at least moderately permeable to water vapor. At 2" XPS is about 0.5-0.6 perms- a class-II vapor retarder, and not very breathable. If you then put something vapor-impermeable on the interior side the assembly becomes far less resilient, since any moisture that gets in would then take months of optimal weather to leave, if drying through 2" of XPS was the only path. Air-tight gypsum on the interior side of the 2x4 cavites with a coat of standard latex primer is about 5-6 perms, literally an order of magnitude higher drying capacity than 2" of XPS, and that's where you want to be- air tight, vapor semi-permeable. All wall assemblies need to have the capacity to dry at reasonable rates, since no assembly is perfect. There's good, better, and best, but there's no such thing as perfect in the construction world. If you needed perfection for the assembly to survive the next 100 years, you're already dead. But it sounds like you'll be OK on this one.

A key distinction to be clear about is the difference between an air barrier and a vapor barrier. The "vapor" in building science terms is always referring to water vapor. Many materials are air-barriers at the bulk-macroscopic level that still allow water vapor molecules to move fairly readily. And even when not detailed to be air tight, vapor barriers still block the diffusion path for water vapor, even if air-pressure differences would result in air moving through the assembly. A slashed up sheet of poly is still a substantial vapor barrier, since vapor diffusion is about the surface area x vapor pressure (humidity level) difference, but it's a terrible air-barrier, since air can pass through the slashes when subjected to air pressure differences.

Like Bob, I prefer to spec the sheathing as the primary air barrier for thermal performance, since it is more protected from incidental breaches over time. But reasonably good air tightness on the interior side is important for moisture control at the sheathing unless you bump up the sheathing-R in your climate to about a 50/50 foam-R/cellulose-R. 3.5" of 3.5lb dense packed cellulose is about R12.5, so it wouldn't take much. (Changing to 2" of iso instead of XPS would get you there.)

(edited to add:)

Air-transported moisture can be orders of magnitude greater than transport by diffusion only, which is why air tightness on the interior is important.  In winter there will be many periods where the sheathing is below the dew point of the interior air, and will adsorb much of the moisture in the cavity. That lowers the vapor pressure in the cavity relative to that of conditioned space, and water vapor will diffuse through the paint on your gypsum into the cavity when that happens, but it's at a slow & controlled rate.  But if there are air leaks and convective paths from the interior it'll be a LOT more moisture.  A square inch of air leak is worth a whole wall's worth of vapor diffusion through 5-perm paint.  When the sheathing warms up with warmer weather, it releases it's adsorbed moisture into the cavity, raising the vapor pressure inside the cavity relative to the conditioned space air, and as long as it's semi-permeable it'll all get out well before summer when prime mold-growing temperatures begin in earnest.   The cellulose will help, since it can buffer quite a bit of moisture in it's hollow fibers without affecting performance, helping to even-out the moisture flows in & out of the wall cavities.

A couple follow ups... what is an ideal interior air barrier besides drywall? My plan for paneling so far is brick in the kitchen, and a mix of plaster and wood paneling throughout the rest of house.

My zip is 13420. Would 2) 1" layers of iso perform better in terms of r value and air barrier then the specified 2) 1" layers of xps? The cavity provided by the 2x's for the dense pack is a full 4".

And am I going the wrong route here to begin with - I'd originally planned 4" of closed cell foam to be sprayed in the cavity with the foamboard sheathing and my engineer convinced me to consider dense pack cellulose.

And here's the kicker. My plan so far, given my back holds out, is to veneer the exterior with stone. How does this affect everything?

Zip code 13420 is on the warm edge of climate zone 6.  At your binned hourly January temp of ~22-23F (see:  http://weatherspark.com/#!dashboard;a=USA/NY/Old_Forge )you'll definitely be getting more R-value out of double-layered 1" iso than double-layered 1" XPS, and it'll be easier to air seal reliably (using purpose-made 2" FSK tape.)  Either would work from a dew point control point of view with the full-dimension 4", but more is always better.

When thermally bridged by the framing closed cell foam is a huge waste of money. Worse yet, unless you shopped around very carefully the closed cell foam would be blown with HFC245fa, with 1000 x CO2 type global warming potential (GWP) and the lifecycle environmental hit would be larger than the energy use it's offsetting. If you're using closed cell foam in any part of the structure, in your area you may be able to find an installer for Aloha Energy's 1.8lb foam, which is blown with water at a VERY low GWP. (A.E. is based in Saratoga Springs NY, but they're a very small player in the closed-cell foam biz, and don't have 100s of installers out there like the big boyz.)

Even if you got your framing fraction down to 15% (possible, with 24" spacing with no doubled-up elements) with  4" of closed cell foam you're looking at about R15 average for that layer (not counting the sheathing or gypsum) after the thermal bridging of the framing is factored in. With dense packed cellulose you'd be at about R11 average for that layer.  How much are you willing to pay for that R4 difference? 

You can beat that performance upgrade by adding another inch of thickness to the rigid iso on the exterior, usually at lower cost for the additional exterior foam + 4" dense-pack than the ~$4/square-foot you'd be paying for closed cell foam, and you wouldn't be sandwiching the sheathing into a "dries practically never" situation with low-perm foam on both sides.

OSB makes for a durable and vapor retardent air-barrier, when the seams & penetrations are detailed as an air barrier, but it's not any cheaper than gypsum + primer.  If you want to use a plastic film type product, Certainteed MemBrain is the right stuff, since it's low-permeance when it needs to be during winter when the air is drier, but becomes fairly vapor-open when the humidity in the  wall cavity rises and needs to dry.   It's pretty rugged stuff as far as plastic films go, but it's not nearly as robust as wallboard or OSB.

Stone veneer can hold quite a bit of dew/rain moisture, which gets released in intense bursts once the sun shines on it.  With the exterior foam that won't be an issue though, since even XPS is fairly vapor retardent, and foil faced iso is a true vapor barrier.  Without the foam it would be an issue, since much of that moisture would end up in the structural wood with only #15 felt (or worse, housewrap) on the exterior.

The masonry ties become a problem for stone veneer on 2" foam though- a lot of masonry contractors will all but refuse the project with foam more than 1" thick.  I'm not sure where the foam thickness limits are before you have to go to full-custom ties, but it's a cost adder even at 2". If you're building for 100+ year sustainabilty, $tainle$$ ma$onry tie$ would be in order. How committed are you to the stone?

Back in Iowa (50648). I follow the IRC chart for walls but got a bit confused on the ceiling section. So for an attic truss when planning a living space in it, how does it get insulated? The walls as per IRC chart? And the ceiling/attic? The attic over the living space it to be ventilated.

ideal air barrier? 475 High Performance Building Supply.com and SIGA dealers both sell permeable fabrics and the tapes to use with them.

Posted By agagent3 on 19 Jun 2013 05:36 PM
Back in Iowa (50648). I follow the IRC chart for walls but got a bit confused on the ceiling section. So for an attic truss when planning a living space in it, how does it get insulated? The walls as per IRC chart? And the ceiling/attic? The attic over the living space it to be ventilated.

That's what you get for stepping on somebody else's thread with your own questions, but I'll answer anyway. :-)

With a trussed attic with living space within it, you could leave a 2" soffit-to-ridge vent gap between the roof deck and insulation, and insulate without foam, with an interior side vapor retarder, but that's can be cumbersome (but not impossible) to properly  air-seal. The IRC prescriptive levels in Chapter 8 are for when you want to insulate with some portion above the roof deck, or in contact with the underside of the roof deck using combinations of air-impermeable insulation (foam, typically) and fiber, with a Class-3 vapor retarder (like standard latex paint) on the interior side. 

The IRC prescriptive-R levels for foam under the roof deck are currently the same as for above the roof deck, but there is strong WUFI simulation evidence that you can use much less on the interior side if the foam is sufficiently vapor retardent, such as 2lb polyurethane foam.

Posted By Bob I on 19 Jun 2013 05:56 PM
ideal air barrier? 475 High Performance Building Supply.com and SIGA dealers both sell permeable fabrics and the tapes to use with them.

Simply vapor permeable isn't ideal- it has to be around or under 5 US perms in winter to really cut it at his R-ratio and climate.  (I could run a WUFI sim on his stackup to verify where my gut is telling me it needs to be, but I won't. :-) ) I haven't seen really good comparative specs on the SIGA product to estimate where that lives. (What means 5 meters of air diffusion equivalent thickness in US-perms terms, anyway?  I haven't read EN 1931 to try to figure it out, have you?)  With drying to the exterior impeded by the foam but insufficient foam to stay above the dew point of the interior air all the time, some level of vapor retardency on the interior is mandatory, even with the buffering capacity of 4" of 3.5lb cellulose.  This is a more severe climate than most of populated Europe. If he had R15+ on the exterior I wouldn't sweat it though.