Explain the correct usage of rigid insulation on the exterior of the building.

Adding exterior rigid foam board is one of several ways to add R-value and reduce thermal bridging, and it has become a favorite among builders who want a quick and easy way to meet those goals. It is also touted by building scientists as a way to decrease the condensation potential on the interior surface of the plywood or OSB sheathing by moving it toward the warmer-in-winter side of the thermal gradient.

As you suggest, there are potential problems. In a cold climate, the vapor drive and vapor-laden air movement is primarily from the inside to the outside of the thermal envelope. Given that no wall system can either be built or remain perfect for the life of the house, it is wise to allow a wall to "breathe" or allow water vapor permeance to the exterior.

Only foil-faced foam board (polyisocyanurate) is a vapor barrier. But the more commonly-used XPS (extruded polystyrene, or Styrofoam) has a vapor permeance of 1.1 per inch. This places it just a tad short of the conventional definition of a vapor retarder (perm = 1). But in a cold climate, 2" of exterior foam is typically required to prevent the sheathing from reaching the dew point, and 2" of XPS has a perm rating that places it squarely within the vapor retarder (class II) category.

It has long been considered good practice, in a cold-climate home, to have an exterior "skin" that is at least 5 times as vapor permeable as the interior "skin". Summertime vapor drive, at least when the sun is shining on a wall, is from outside to in. But a layer of exterior foam will significantly reduce the temperature-driven moisture movement toward the interior.

Long-term studies (2 years) have demonstrated that, in the event of minor exterior rain penetration, a wall system with exterior foam does not ever dry out and, because it is warmer in winter, remains above the mold danger zone (temperature and humidity), whereas a wall without exterior foam has opportunities for cyclical drying.

Other methods of increasing R-value and reducing thermal bridging include, double-frame systems, cross-hatched walls, and interior foam board. The first two can be insulated with dense-pack cellulose, the most green of all conventional insulations. There is much debate over whether any petrochemical foam can legitimately be considered a green building material.

So then.....where is this building practice used most effectively? What climate zones?

I have never used the system but have heard it recommended by a number of building scientist. John Straube, I believe, recommended it for virtually every climate zone. Then I was wondering whether it was a good idea to tape or caulk the joints. Or it would seem that you would need an additional vapor barrier like Tyvek on the outside of the foam. Then rainscreen spacing. Seems easier and safer to just stagger your wall studs to eliminate thermal bridging.

It is primarily a cold-climate strategy. Straube and Lstiburek would not recommend this strategy for all climate zones (perhaps ICC zone 4A, certainly 5 and above and possibly 3-B). If the exterior foam board is used as an air barrier or weather barrier, then joints should be sealed.

Housewrap (weather resistant barrier - WRB) like Tyvek is NOT a vapor barrier - quite the contrary, it is one of the most vapor permeable building materials available. It is relatively resistant to liquid water and air. Be careful about applying a polymeric WRB outside of exterior foam board and under a rainscreen, as this leaves it vulnerable to air pressure differentials.

Thanks Martin. Both systems, PERSIST and the REMOTE seem like good systems, especially in cold climates....but sounds like it would be fine elsewhere as well. Seems important to stress the need for good mechanical ventilation to provide fresh air and help to deal with indoor humidity. Also seems that this method would be good for the PassiveHaus standard as well, especially in retrofit situations.

Not sure I understand the risk related to using additional insulation in the stud cavities. I heard Straube say that he would use cellulose in the cavities in almost any climate for added R value.

Yes, in standard framing, I think I understand the condensation risk, especially in cold climates. Warm air infiltrating through insulation, contacting the cold exterior sheathing and condensing.

But, my question was within the PERSIST wall system, why is it risky to place more insulation in the stud cavities? The exterior sheathing is already kept warm, reducing the chance of any condensation, I think. Are you saying that additional insulation in the cavities isolates the exterior sheathing, increasing the chance of condensation? Maybe I am missing something.

Martin,

Conrad was asking about adding exterior foam board to increase the wall R-value and create a thermal break. He was not addressing the REMOTE or PERSIST systems, which are very different beasts than the conventional exterior foam supplement.

Lstiburek was responding to your question about the PERSIST system. In his "Designs that Work" series, he does not include exterior foam in climate zones other than the ones I listed above. Your article also credited Chris Makepeace with the same assesment, but I wouldn't give much credence to an "expert" who doesn't seem to understand how a blower door test works. If your quote is correct, he seems to think that a tight house cannot be depressurized to 50 Pa, whereas a tight house is the easiest to pressure down - a loose house cannot be depressurized.

The one point on this with which I would agree is " it's important to get the details right", and it's important that the details stay right for the life of the house. It makes much more sense to build an envelope which is forgiving enough to handle the inevitable leakage or occasional condensation that will occur at some point (or many) in the life of a building. It is for that reason that Straube recommends cellulose, as it is extremely resilient and forgiving of occasional moisture, and helps buffer indoor relative humidity (as do most natural materials).

Unfortunately, the current trend in so-called "green" building is exactly the opposite. It is a logical extension of a blind faith in technology to solve the problems that earlier technology created, and is based on an assumption of perfection and infallability. Nature doesn't work that way - it creates sustainability through an innate ability to respond to known and unknown threats through flexibility and accomodation. If we were wise, we'd build our homes with the same resilience.

Robert,

What then is the "conventional exterior foam application".....I see lots of pictures of builders applying exterior foam in places like Texas, etc. How is it different from PERSIST/REMOTE? Do they put an impermeable membrane between the foam and the sheathing?

Also, I agree with you...tight houses are very easy to depressurize to 50 pascals. Not sure where Chris Makepeace was coming from on that one.

Can anyone comment on this assembly? Cold climate (Zone 6) low budget house. It's currently framed 24" OVE and sheathed and I would like to continue assembly as follows from in to out - 5/8" drywall (airtight), dense-pack cellulose (2x6), 1/2" OSB, Tyvek, 5/8" foil-faced polyiso (foil-out), Delta dry (3/8" thick ventilation mat (vent's both sides) gapped wood siding. The rigid doesn't get me below dew-point at sheathing for all temps, but a little helps right? window flashing is back at tyvek. tyvek is taped rigid is not. the thought is that the air gap from delta dry helps make foil polyiso more effective (radiant w/ airspace) and allows siding to breathe. Interior will not have specific vapor barrier at this point. This seems to be vapor barrier on wrong side of assembly but I'm not planning on putting one in if i go w/o foam, relying on airtight drywall and dry climate to cover me. other option is 1/2" XPS but i had the thought that foil-faced polyiso would maximize air gap i am already creating? Thanks for any input.

I'd say you're asking for trouble. In IRC climate zone 6 (I assume you're out west in B-dry zone?), 2" of exterior foam is typically necessary to keep the sheathing above the dew point most of the heating season.

You're using relatively impermeable and relatively vulnerable OSB for sheathing and then covering it in an impermeable foil-faced foam board with a relatively impermeable rainscreen membrane (perm=0.385). Even with the air-tight drywall system and good indoor humidity control, you're almost certainly going to get some condensation on the sheathing, accumulating during the heating season and with insufficient drying potential during the summer due to air gap and radiant barrier.

The radiant airspace between the foam board and the vented rainscreen adds nothing to the thermal envelope, since that's vented space. If you want to use the outsulation technique, I would recommend 2" of EPS if that's available, or XPS if it's not (EPS allows more drying potential to the outside). In any case, do not use interior vapor barrier.

You can also increase the R-value of the walls and reduce thermal bridging by using a cross-hatched framing system, with 2x2s perpendicular to the studs. Those can be placed 16" oc and 1/2" drywall used (I've commonly installed 1/2" drywall on 24" oc framing with dense-pack and no bulging or screw-popping).

Conrad,

The REMOTE/PERSIST systems are just that: building systems, requiring very careful integration and detailing of the weather barriers, air barriers, and thermal barriers. They typically use bituthane self-adhering membranes over the sheathing as an air barrier, integrated with ceiling air barrier. And they use very thick exterior foam layers, often as the only thermal barrier.

In "conventional" construction, builders often simply add exterior foam board for additional R-value, for thermal break, and for weather barrier and/or air barrier. But simply adding another layer of material, without integrating it into a whole-house system, including indoor humidity control, vapor diffusion control, and drying pathways is the kind of piece-meal approach that can create more problems than it solves.

Thanks RR,
I do like the idea of simplifying and internal strapping may prove easier with my timing. I am in Idaho @ 6000' and framing now. I really like the use of cellulose as insulation, it;s fairly benign compared to foams. I'd like to hear your thoughts on the roof. I have a trussed cathedral ceiling, 20" parallel chord heels that turn horizontal (think collar tie), ridge to collar tie depth is 5'. (2/3 of single gable roof is parallel chord depth only) Would you recommend 4" urethane and remainder of cavity blown full with cellulose and unventilated (R-72 to R-120+) - or 2" baffled ventilation gap, 18" of cellulose throughout (R-54) and a small ventilated attic? I could strap or put rigid at ceiling also to boost R. I 've heard that majority of heat loss is out roof and also that my elevation and cold climate (county borders Zone 7) is best served by R-72 min at roof. Thanks again. By the way, other parts are R-20 radiant SOG and DHW via electric boiler with 2-coil tank supplemented by boiler and evac tube. Woodstove, good solar gain. 30% unlimited tax credit on solar right now, wow. Your help is appreciated.

Heat moves equally in all directions. Only warm fluids (liquid or gas) move upward. So there is no more heat loss through the roof than through any other part of the thermal envelope unless you have a leaky house with consequent air temperature stratification. There can be more summer heat gain through the roof, however, because of the more direct angle of incidence.

For that reason, and to avoid ice dams and allow evaporative drying and drainage of possible roof leaks, I recommend vented roof systems. Given the depth of roof trusses, it would be reasonable to install continuous, sturdy vent baffles, continuous soffit and ridge vents (the only venting system that works, which requires wind-baffled ridge vents such as Lomanco OR4 or Air Vent Shingle Vent II), and dense-pack cellulose. You can also sheath the trusses with 3/8" CDX, cover that with housewrap, install 2x2 sleepers on top of each truss and then sheathing and roofing to create a weathertight, self-draining vent cavity and allow a full 20" of insulation.

18" of cellulose should give you an R-68 roof (R-3.8/in), but that would be excessive without bringing the walls up to at least R-40 which would require a double-wall framing system, either staggered studs or Skekatchewan or modified Larsen Truss (which is my system: http://www.builditsolar.com/Projects/SolarHomes/LarsenTruss/LarsenTruss.htm)

You can use the flash & fill system, but I prefer cellulose to any foam product and I keep my exterior skin at least 5x as vapor permeable as the interior, which means avoiding OSB and plywood (fiberboard or sawn lumber is good).

Here's a related question from a new reader. I've got a client who needs to re-side and would like to upgrade the insulation. 6B climate zone, 7961 HDD, 1166 CDD, 12 ' precip/yr. Typical mid 80's construction, 1/2" GWB, VB (currently being verified, but probable) 2x6 R-19 fiberglass, cdx, paper (felt?) and siding. The client was assuming EFIS which suits the house style.
Question: Would a cement based finish such as TESS (www.tessfinishes.com) with a perm rating of 72 over 2-3" of XPS (perm 1.1/inch) (they also specifiy Tyvec stucco wrap) over the existing sheathing safely allow moisture to migrate outward?

thanks, Jim Baerg

Can anyone advise on proper insulation of exterior walls and roofs in Gulf Coast wet and always humid climate. Moisture tends to drive inward. Do ERV's work well in this type of climate?

Thanks,

HJC

Robert,

Thanks. It seems that you are saying the only 'safe' application of exterior foam on the framing means to follow the REMOTE/PERSIST methods....that is, carefully installing a weather barrier on top of the sheathing, before applying the necessary outsulation.

Applying foam insulation on the sheathing, without the weather barrier, may certainly work in certain climates....but it can be risky due to the needs of allowing the inside humidity to escape and allowing moisture to dry to the exterior by diffusion.....but it does seem that you would need something over the foam because that in now the drain screen behind the exterior cladding, especially on the vertical joints where it is difficult to seal with a z-bar type flashing...unless the existing tapes work well for sealing joints over a long period of time....just the life of the siding!

Jim,

2"-3" of XPS (perm 0.55-0.37)is an effective vapor retarder (vapor semi-impermeable) and will not allow the wall assembly to dry to the outside effectively. It's kind of silly to place a 72 perm coating (is it really that high?) over a relatively impermeable backing. Use EPS instead.

P.S. I assume you meant 12" of annual precipitation, if you're in a "B" or dry climate zone.

Conrad,

No, I'm not an advocate of the REMOTE/PERSIST systems, though they do include a more systematic approach to integrating the three necessary barriers: thermal, air, and moisture. What those systems use over the sheathing is not simply a "weather barrier", which typically means housewrap, but an air/vapor barrier. The foam board is used as the weather barrier as well as thermal barrier. And I'm not an advocate of relying on any kind of tape to last the life of the house. In fact, in water pressure tests on housewrap, the common failure points are the taped diagonal slits on the above-window flaps.

All envelopes that rely on foam or any form of plastic tend to expect that all systems will be installed perfectly and remain perfect over the life of the house. This assumption is necessary, since most of these buildings have little capacity for diffusive drying and little or no moisture storage (buffering) capacity.

What I do advocate are building envelopes that are tolerant of occasional moisture, allowing safe storage (buffering) and diffusive drying in both directions. This requires natural building materials, which are preferable for many reasons in green construction, such as wood, cellulose insulation, straw bale, cob, earthen plasters and natural paints. Such materials are not only highly moisture tolerant (breathable) but offer additional indoor air quality improvements such as negative ions.

Thanks Martin and Robert for your input.

Seems that Lstiburek recommends a housewrap under the foam, on top of structural sheathing (if it is required)....to act as a secondary drainage plain... if and when water goes past the foam, thru the joints I assume. Not sure how you would provide the secondary drainage plane, if you are able to attach the foam directly to framing without the use of plywood or OSB sheathing. Maybe you then put the housewrap on the outside of the foam.

Of course, I always recommend a drainage plane space behind the cladding....by attaching vertical furring strips for siding attachment. Perhaps then, that furring would be attached over the taped vertical seams on the foam and you could use a simple Z-bar flashing on the horizontal joints. That would be pretty tight, I think.

Anyway, all these systems need to be considered carefully within each climatic region. with great attention to details around all penetrations.........

If using the housewrap as the air barrier material for the wall system, then it must be protected from positive and negative pressure differentials and installed in such a way as to avoid deflection. For that reason, I believe, Lstiburek recommends placing it between sheathing and exterior foam board.

If there is no structural sheathing, the housewrap can be applied under the foam board if dense-pack insulation is installed in the cavities to keep the air barrier from flexing. Otherwise, it's best to apply the housewrap/air barrier outside the foam board, but only if it's sandwiched by the cladding (i.e. no rainscreen) unless the foam is used as the air barrier material.

I would not "always recommend a drainage plane space" under cladding. That's required only in high rain exposure areas. In most climate zones, back-priming the siding is sufficient for long-term durability, assuming the thermal envelope can breathe in both directions. A rainscreen complicates door and window installation and falshing integration and can leave the housewrap vulnerable to pressure deflection.

While self-adhering flashing tape and rainscreens are useful technologies, they've both become overused as panaceas for moisture problems. The KISS principle still applies.

I am finding these postings very interesting...if not a little over my head.

I am currently framing a new house for myself. OVE principles with 7/16 osb followed by tyvek , 1" stryofoam board, and then cedar siding. Am i hearing you right that condensation may be a problem?

I live in southern oregon. A semi dry climate with hot dry summers and wet winters. Should I skip the foam? I love the idea of the thermal break but spending my life savings on a future mold-pit scares the h*ll out of me! Thanks for any insite.

What's the rest of the thermal envelope? Drywall, poly vapor barrier, cavity insulation, wall thickness, rainscreen? What kind of HVAC system? Whole house ventilation? Full basement, crawl space, slab on grade? Ventilated roof/attic?

I need the whole picture to offer any useful advice.

Robert,
its drywall, fiberglass batts, 2x6, 7/16"osb, 1" styrofoam, tyvek, cedar siding. Mitsubishi minisplit ductless heat/ac. ventilation is bathfans (1 on 1st floor and 1 on 2nd )on 24 hr timers, and slot vent windows in bedrooms and one in the living room.

crawlspace. vented roof (combo of stick framed 12" i joists and trusses).

Currently in framing stage with osb installed. What do you think of the system as a whole? any changes youd recommend? Thanks, jason

You didn't list interior vapor barrier - that's good, since you're reducing the outward drying potential with the perm 1.1 foam board.

Assuming the crawlspace is dry, I'll go along with Martin's assessment - with the caveat that careful attention must be paid to exterior detailing and flashing integration (which becomes more problematic with a rainscreen) to prevent water leakage. Tests have demonstrated that water intrusion into foam-sheathed walls often remains long enough, in the warmer wall environment, to cause mold problems.

If you must use fiberglass, I would at least suggest using unfaced batts to avoid any interior vapor retarder. You would be much better off, however, with dense-pack cellulose in the stud cavities. Not only is it a completely non-toxic, low embodied energy, fire resistant and largely recycled product, but it also manages potential water intrusion far better than glass fibers by pulling it away from wood framing and sheathing and allowing slow release. Use cellulose treated only with borates (not ammonium sulfate), and your walls will also be mold-resistant.

Cedar siding should also not be placed directly over polymeric housewraps, since the surfactants in the wood can make the polyolefins more permeable to liquid water. However traditional 15# felt underlayment works fine and often works better than plastic housewraps.

I'm doing a remodel on a 1928 bungalow. At present, most walls remain the original 2 by 4 with 3/4 fir t&g or shiplap inside and out. I'm trying to retain this construction to avoid going down the major seismic retrofit route with the building department. It's odd construction with non-uniform stud spacing.

The plan at present (from the inside) is 1/2 inch or 5/8 inch drywall detailed as ADA (with vapor barrier paint only if the bldg dept makes me), 3/4 fir shiplap interior cladding, densepack cellulose in the wall cavity, 3/4 fir t&g fir exterior cladding, 15 lb felt, 1 inch XPS, 1/2 PT ply furring/air space, fiber cement lap siding. ( Most manufacturers seem to suggest not back priming fiber cement) For what it's worth, the fir cladding is nothing like air tight.

I'm trying to make the felt serve as an innermost WRB with details to route water out to the exterior of the foam sheathing. I've constructed a model of the extant wall to work out the details of the design. The foam (1 inch Foamular t&g) will be taped (vertical) and Z-flashed (horizontal) and treated as a second WRB with window//door details flashed at that layer. I'm essentially trying to adapt the Lstiburek Wall Section 1 design from his Guide to Insulating Sheathing.

The house is located in Portland, OR. It is about 550 sq ft with a basement (it is relatively dry and I'm using large overhangs and somewhat improved drainage at the top of the vintage concrete) and will incorporate a Panasonic ERV, an LG Art Cool mini-split heat pump with direct vent, natural gas wall heat for backup. It will have soffit/ridge ventilated comp roof with about 18 inches of cellulose. I'm pretty sure I understand the issue with the ratio of inter cavity insulation to exterior foam and that it means paying careful attention to the interior air barrier and humidity levels. My thinking is that the storage capacity of the cellulose and felt might also assist in prevent the exterior fir cladding or inner foam surface from becoming the first condensing surface.

I realize that retaining the present dual clad construction presents additional challenges but the option gets close to completely rebuilding the structure. Even now, some of the wall sections will have to be rebuilt as shear panels with plywood sheathing on the exterior to meet seismic requirements.

A related question has to do with the detail at the top of the rainscreen. I would have a trim detail at the top of the wall to conceal the upper rainscreen vent but I also have the soffit vents just above that. I note that some Canadian references state that venting the wall into the roof is a bad idea so I'm wondering how to isolate the upper rainscreen vent from the soffit vents (or if the mixing of outside air with the rainscreen 'exhaust' would mitigate any issue with that air finding its way into the soffit vent. (I recently noticed an APA publication illustrating a rainscreen not vented at the top - another possibility . . . )

This may or may not sound completely crazy but no one I know is familiar with any of the particular issues involved so I'm hoping some of you can provide some feedback. My apologies for the lengthy initial post, but it seems like more detail is better on this forum.

You might want to post this in a new thread instead of at the end of someone else's lengthy one (or ask the moderator to move it).

You seem to understand the issues involved and have come up with a functional wall system that should work well in your climate while keeping the integrity of the bungalow. An inch of exterior foam should be sufficient for your "warm sheathing" approach, combined with the ADA, a vapor-open interior and hygroscopic cellulose insulation in the cavity.

It appears that the fiber-cement siding manufacturers' recommendations against back-priming are to allow the drying of process moisture from manufacture as much as to allow drying from environmental moisture. I would backprime any cellulosic siding that is not applied to a rainscreen, but leaving it unprimed with a drainage space is fine. I would recommend flashing with felt gaskets or aluminum step flashing at butt joints.

In your marine climate, it may be better to not top vent the rainscreen (which makes it a drainscreen), both to prevent extra moisture from entering your soffit vents and to prevent ground moisture from venting up behind the unsealed back of the siding.

I've heard a lot of recommendations for OSB, Tyvek, and insulated sheathing on exterior walls. My question is... Where is the drainage plane? I hope you are not relying on the Tyvek as your drainage plane, because drainage aint going to occur at that layer. Sandwiching Tyvek between OSB and rigid insulation will not allow for water to drain. Perhaps the insulated sheathing is the drainage plane... but then what is the point of the Tyvek?
Shouldn't you be using a Stucco Wrap or Drain Wrap?

very interesting reading.. we are framing 2 houses in vancouver bc wet coastal climate, and trying to decide on insulation methods as well.. have had some discussion on using a Roxul mineral fibre semi rigid board instead of SM due to the Roxul's permeability qualities.. Roxul is interested in getting some residential market for what is traditionally a commercial product.. may be a little more difficult to use as the density is quite a bit less and we would need to use 2 inch to get a R value of 8 or 8. We like the fact that it may be better at drying and perhaps it is a "greener " product but are still puzzling through it and looking for other thoughts. We are also thinking air tight drywall , have an Arxx basement, radiant heat with an eneready hrv and still finalizing a solar thermal system for domestic hot water and radiant heat assist.

Exterior remodel project for a 1960's 2-story colonial in central New England. Scope includes replacing windows and doors, siding and exterior trim, and applying a layer of exterior wall insulation and roof replacement. Wall construction is plaster/drywall, 2 x 4 frame 16" OC with fiberglass batt R11, 3/4" plank sheathing. We want to add 1" Foil-faced iso-board R7 over the sheathing, tape all seams, and install pre-finished cement board siding. All door and window openings will be flashed with bitithane.
Do we install house wrap atop the sheathing or atop the new insulation board? Is vertical strapping over the insulation board recommended, or not? Would R5 'Dow' board be preferred for the exterior insulation?

your water-resistant barrier (WRB).

Martin,
I have noticed that you have not adopted Joe's new Vocabulary.
Check out this Vocabulary Insight
http://www.buildingscience.com/documents/insights/bsi-024-vocabulary/?to...

Joe's proposed vocabulary shift has not yet been widely adopted.

I know .... I have barely seen it being promoted.
I heard about it through his Newsletter
I really like It .
I think it helps me to visualize

Richard,

In central New England I would not recommend foil-faced exterior insulation board, even if you used 2". That creates a wrong-side vapor barrier during the heating season and an excellent condensation plane at the inner foil face. If you used unfaced fiberglass between the studs and no interior vapor barrier, then the wall assembly could breathe to the inside for eventual drying during summer months, but in a cold climate a wall should be able to breathe to some degree outward.

Extruded polystyrene insulation board (styrofoam) at least allows some vapor permeance, whereas foil-faced insulation has zero permeance, but you should use a minimum of 2" of exterior foam to reduce the likelihood of condensation.

True that Robert Riversong's opinion concerning vapor tightness is a minority opinion in North America.
Not so true in Europe.

Almost every modern European assembly that I have observed (by internet) seems to be vapor permeable.
Here is just one example:
http://www.natural-building.co.uk/PDF/Pavatex/090216_Technical_Manual_PA...
I have looked at MANY and can provide many more links
The Europeans seem to have more respect for water than "we" do.

If it were just Riversong... I might dismiss him as one of those Vermont Hippies ;-)
I am not sure that Robert is "correct" ...but what if he is....

Intersting how Europeans seem to have no problem using a wall system that provides a warm side (vapour permeable) rigid board for racking resistance and air control.
I really like the apparent simplicity and continuity of such systems.
I feel like I'm butting my head against a brick wall trying to find similar approaches to building here in NA.
John, I would love to see more examples please.

Lucas,
here is a video showing modular construction ..airtight and vapor open...
mineral wool outsulation
http://www.youtube.com/watch?v=wBQHBr7zn0M&feature=related

here is webpage... airtight ...vapor open wall
notice...no peel & stick....ventilated cladding...metal sills
http://live.pege.org/2006-building-salzburg/passive-house-wall.htm

Here is an Outsulation Retroftit
http://www.passivhaustagung.de/Passive_House_E/modernisation.htm
appears(not sure) to be vapor open graphite enhanced EPS
Notice the Mineral fiber above windows

When I was searching for Passivhaus info...I did not see any examples that used foil or vapor barriers.

I think I have gathered through these posts what is "ideal". Just want to double check. Framing in Central VT- 2x6 blown in cellulose. House ontop of ICF foundation. I am still not sure if it is better to place the 2" xps (all seams taped) inside or outside of the 1/2" plywood. Then vertical strapping (drainage plane) and then spruce clapboards.

Adam,

There is no "ideal" building system. Each approach has benefits and liabilities. The important thing is to understand the hygro-thermal dynamics so as to build on the strengths and not exacerbate the weaknesses.

If you're in Central VT and free this weekend, there's still room in my Hygro-Thermal Engineering class at Yestermorrow in Warren.http://www.yestermorrow.org/courses/detail/hygro-thermal-engineering-managing-moisture-in-the-home?StartDate=2010-02-15&SortColumn=StartDate&SortDir=ASC

With the wall system you've described, you'd want the plywood sheathing on the frame for shear resistance and to keep in on the warm side of the outsulation. If you're using the XPS also as a weather barrier (with no housewrap), then you might want to use two layers of 1" board with offset and taped seams. If you're using housewrap, that's somewhat less important, but the housewrap will be somewhat vulnerable to pressure differentials since it won't be tightly sandwiched between solid surfaces. And bulging polymeric housewrap is susceptible to condensation on the backside.

Drainscreen or not, make sure you backprime your clapboards and seal the endgrain before installing. And, if you haven't already poured your foundation and it's a full basement with conditioned space, you might consider the ThermoMass system with the foam sandwiched in the middle of the concrete where it's not vulnerable to UV, insect or physical damage and doesn't require additional coating. With ICFs, you lose most of the thermal mass benefit of the concrete. With the inverse - concrete/foam/concrete - you get almost as much dynamic mass benefit as with exterior-only insulation.

Robert, I think there was a space missing before your link try this
http://www.yestermorrow.org/courses/detail/hygro-thermal-engineering-man...

Thanks John. I saw that after I posted and there is STILL no edit function. I guess the webmeisters here expect all us "greenies" to be perfect ;-)