You know by now that we’re all about finding the best bang for our buck when it comes to energy efficient construction. That’s how this whole thing started. It’s been a while since we’ve posted on advancements in our building philosophy, so why not talk about our next super-insulated wall assembly that we think could just possibly be the most affordable wall assembly that achieves maximum R-value and minimum thermal bridging with the least impact on your budget.
This is a bold statement, so I expect some comments and rebuttals. Here is my rationale.
THE Wall Assembly
- Double stud wall, 2×4′s on 24″ centers.
- Dense-packed cellulose in the wall cavity.
- OSB exterior sheathing.
- R-10 continuous exterior insulation, taped at all seams.
Here’s why this wall assembly is chalk full of awesomeness:
- Double stud walls on 24″ centers is the easiest and cheapest built thick wall cavity. Both material costs and labor costs are extremely low compared to alternatives (TJI’s for instance).
- Cellulose is economical while also being the best green insulation product known to man (see future post on cellulose superiority). It’s cheap. It barely has a carbon footprint. It fills all voids. It retards air flow like a champ. It has high thermal mass compared to alternatives like foam and fiberglass. Did I mention it’s affordable? This is important.
- OSB is cheap and makes it easier to attach cladding and laterally brace your structure.
- Continuous exterior insulation is the icing on the cake. It’s not uber cheap or low in the carbon footprint category, but used sparingly, it can be quite effective. It eliminates any thermal bridging worries that might have you up at night. When taped, it also can virtually eliminate air penetration and double as a water resistive barrier allowing one product to serve three important roles.
The next Postgreen Home, the 2 point 5 beta, will incorporate a 9″ thick double stud wall filled with dense packed cellulose (R-30) and sheathed in OSB and capped off with 1.55″ (R-10) of exterior Poly-iso rigid insulation. This will result in a wall with an R value of 40 and absolutely no thermal bridging. This will be our best wall assembly to date and most likely the wall we will stick with going forward. [Please forgive discrepancies from this description and the image above]
Inferior Alternatives?
One of my hopes in posting this hypothesis is that one of you guys out there will prove us wrong and give us a better wall assembly that packs more bang for the buck. Here are some of the other options I will shoot down in advance:
- Excessive exterior insulation – As we stated above, rigid insulation is not cheap and has a larger carbon footprint than many other insulations. Therefore it should be used as sparingly as possible. R-10 is the minimum amount needed in most US climates to mitigate dew point issues that could cause moisture in your walls. Lastly, going over 2″ in thickness in exterior insulation will drive up the cost of your fasteners and labor required to attach your exterior cladding to your framing underneath.
- Spray foam – While spray foam could be argued to be one of the more effective insulation options out there, it’s by far the most expensive and one of the most damaging to the environment with it’s ozone killing component and high carbon footprint.
- Any type of batt insulation – Seriously?
- ICF’s - Expensive, built of harmful foam, hard to get a high r-value and filled with concrete.
- Big ass SIPs – These have the same problems as excessive amounts of foam as that is what the core is comprised of. Also, have you ever tried to lift a 10″ – 12″ roof SIP into place as a wall? They’re really heavy and unwieldy, adding to your install costs.
- Blown-in Fiberglass – Substituting fiberglass for cellulose would decrease your costs slightly while achieving the same R-value. The big downsides are that it has a much higher carbon footprint, virtually no thermal mass and does not resist airflow, crippling it’s advertised R-value in the face of any air movement through your walls.
Let the comments roll.
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three thoughts…
1. we have a lot of clients that have seen OSB delaminate and are deathly scared of using it, especially in a seismic zone. makes a great air barrier, though.
2. wouldn’t modified larsen trusses be cheaper (construction + install)? the few projects i know that used them banged them out quickly, and then installed them in a relatively short time. OVE-double stud walls would seem to take longer (though this is purely speculative) and use more material ($$)
3. regarding rigid – is it EPS that isn’t glued down so it can be recycled?
ICFs just suck. i’d rather use CLT.
Mike,
what does CLT stand for? I am not sure what you are talking about.
Chad,
I have done a lot of research and pricing on affordable super insulated construction and have come up with exactly the same wall assembly being the best value option. This is how I am planning to build my own home.
I would make two changes – and I’ve commented about them here before.
I’d use 2×8 studs instead of a double 2×4 wall. The foam layer is sufficient to mitigate thermal bridging, and the labor for framing a wall twice will more than pay for the material.
I would keep the vapor barrier to the inside, forget about taping and sealing the foam, and add a layer of building paper to provide the first layer of flashing to the cladding system. Sorry – I still think its risky to move the vapor barrier to the exterior. And the detailing of openings without the building paper layer is tricky and error prone – which if it leads to moisture in the wall will defeat your efforts to keep the wall cavity dry.
This achieves 99% of your goals, and is in a format that is familiar to any contractor. No new processes, no straying into unfamiliar ground, less barrier to adoption. Your builder has history with you on this, which is good. But for creating a wall system that home builders can widely adopt, mine is better.
Mike – OSB is pretty common here and I haven’t heard of the delaminating issue you are speaking of.
We’ve looked at the Larsen Truss before, but honestly I don’t see how it could be easier than the double stud we are using. There really isn’t less lumber since you need all of the lateral bracing and it uses a lot of techniques and details that standard framing crews are not familiar with. The beauty of a 2×4 double stud wall is that it’s simply adding another standard wall that anybody knows how to frame…
Lastly, I’m pretty sure we wouldn’t be allowed to do the balloon framing shown in the Larsen Truss system here in our attached rowhomes in Philly. There are a lot of fire code regulations that would probably prevent this.
The rigid used is a poly-iso foam that can achieve R-10.1 in 1.55″ thickness. It would probably be mechanically fastened, not glued.
What would be the material cost of this wall (US/ft2)?
I think you’d have to quote locally for the best answer. There are very few components to price. Cellulose, 2×4′s, OSB sheathing and 1.55″ thick DOW Thermax or similar Poly-Iso board.
Chad,
I am based in Barcelona, Spain and as you can see from our website I am also working on a LEED Platinum project. It was more to have a rough idea. We will be working with local suppliers and have similar, but different solutions. It was just to get an idea. By the way, I love what your guys are doing.
Emmanuel
Greg – I think we could go back and forth on these issues forever, so we may have to agree to disagree. In your Lagom House you are basically showing the same wall detail with a 2×8 or staggered 2×4 wall on 24″ centers and 2″ of rigid on the outside. The rigid is your vapor barrier to the outside on this design.
If you have your vapor barrier on the inside, they you might as well move the rigid to the inside of the assembly. You’re going to have some difficulty making this continuous at your decks where your floor trusses are coming all the way out to your sheathing.
I’m going to argue that two 2×4 walls will perform better and cost less to build than a single 2×8 wall. The 2×8 walls will be very heavy, expensive and non-standard to the framing crew. Adding a few extra standard 2×4 walls will probably not impact your framing labor numbers much if at all because they are normal walls. You have complete thermal separation at not only every stud, but every top and bottom plate as well.
Flashing windows and doors should be no more difficult than with normal house wraps if you are using quality window flashing tapes and products. Most crews hate house wraps anyways and love to eliminate them if possible. If they hate a process, what are the chances of them doing it 100% accurately?
Neither of us are going to definitively answer the debate over where the vapor barrier should be located. Many others have been arguing over this for years. I’m mainly arguing for the improved performance and reduced cost and complexity of our method detailed above over others.
Emmanual – I’m sure we will do this calc at some point in the near future, and when we do I’ll try to remember to post it here in the comments for you. Thanks for the interest!
If my understanding of your wall system is correct there is no vapor barrier on the warm side. Assuming that’s true – by using the poly-iso board on the exterior you limit your wall drying capacity only to the interior.
Now…after several layers of paint the vapor permeance of a wall can decrease substantially so you end up with low vapor permeance on both sides (actually almost zero permeance on the exterior).
I do not think R10 foam is enough to completely avoid condensation in the wall cavity so when you’ll get that it will dry very slow. Cellulose will help a lot by keeping the moisture away from the wood but assuming a worst case scenario where the poly iso tape breakes and compromises your air barrier I think some moisture related problems can appear.
I’m in the planning stages of my own house (we’ll start construction next spring) and like you I reached the same conclusion – double wall with cellulose insulation is hard to beat when it comes to cost/R value (for a high R value wall).
what I’ll do differently – I’ll try to let the walls dry to the exterior as well as to the interior and since it is a double wall already I think the thermal bridge problem is already solved without any extra foam on the exterior.
I’ll build a thicker wall (12″) and I’ll use vaproshield air barrier on the exterior and a “smart” vapor barrier on the interior (changes vapor permeance according the the humidity in the wall – check Certainteed). In this way I can get all the advantages of a double wall system but I still feel that I’m protected if something goes wrong. Regarding the budget – as you said – that polyiso is not cheap.
I already talked too much
good luck
Chad,
Our numbers back you up as well, exterior air-barrier double stud walls have been the lowest cost site-built thermally broken high-R wall.
2×8′s are expensive and heavy to work with, and the thermal bridging is a significant downgrade in insulation value, even with exterior rigid foam. Chad’s wall (9″ double stud + R-10) has a total R-35.9, a 2×8 wall with R-10 rigid is only R-29.1 (assuming 20% framing factor, can be better with careful framing).
We’re trying hard to not use the exterior rigid foam with walls like this, it adds a lot of complexity at window flashing details and adds a fair amount of cost. But, the rim joist becomes a significant thermal bridge at that point and you have to resort to tricks like insetting it 2″ and adding rigid, but then you run into bearing issues with a 2×4 bearing wall.
The ease of description and familiarity to framing crews is a significant part of its low cost in my opinion (build a 2×4 house, add an extra wall on the inside…).
In theory interior air barriers are better in a cold climate, but you have to have a high-perm exterior sheathing to make it work (GP stedi-R, Cellotex, diagonal board sheathing), and the air sealing is much more difficult with an interior air barrier unless you add an additional 2×3 wall to the inside, which adds yet more cost.
At this point, we are trying to eliminate vapor barriers in our projects. Vapor retarders, yes, but vapor barriers have been vastly oversold and look unnecessary except in severe climates. I would much rather have assemblies that are vapor open in all directions.
Emmanual, a local builder priced out standard height 12″ double 2×4 wall w/ cellulose vs. 2×6 w/ fiberglass, it was an upgrade of extra $12.50 per linear foot.
jeremiah
CLT = cross laminated timber – structure/finish/air barrier/sexy
http://bruteforcecollaborative.wordpress.com/2010/02/11/kreuzlagenholz-cross-laminated-timber/
chad,
ran a quick WUFI model on the assembly and it’s showing really significant moisutre buildup that doesn’t dry out at the cellulose/OSB plane. have you guys looked at that?
I bet my downpayment that’s because of the external foam layer which is not thick enough to stop the condensation (a la Building Science method or REMOTE or PERSIST) but is vapor impermeable enough to really slow down drying.
mike
if you find the time – it would be interesting to see how the wall performs in WUFI without the foam layer, just with a vapor permeable air barrier.
thank you
the external foam is thick enough to stop the condensation, all moisture buildup appears to be from the interior. it looks like it wants to be diffuse open to exterior, but hits the polyiso and just builds up between the last bit of cellulose and OSB.
i’ll run a few permutations tonight after dinner.
I love it. WUFI calcs! Great comment thread going here. We did actually have a WUFI calc run and didn’t think it looked too bad. I’d be interested to see your video or snapshots of the points that are troubling you. What time of the year were they, how long did they last and did it dry out eventually?
Another reason we like using a bit of exterior rigid is because it’s free real estate here in the city. It can overhang outside the property line. Every inch of width in a 15′ wide rowhome is precious…
chad, i will try and email films. i’ve never tried to export, so we’ll see if this works.
ran 3 variations:
1 as design, w/ vapor barrier @ interior (best performance)
1 without vapor barrier, lots of moisture content @ OSB/cellulose
1 without vapor barrier or polyiso, which would probably melt away.
moisture content in last 2 seems to be cumulative, never really drying out.
without the polyiso, the water content of the OSB/cellulose is off the charts.
ok…I ran some WUFI simulation myself and I got the same results as mike. your system builds up moisture if you do not control the interior humidity.
however, if you keep the interior humidity below 50 % your wall appears to be ok in my simulations. an HRV driven by a humidistat should keep everything ok.
I was wondering if anyone was taking interior humidity control into consideration. We use ERV’s instead of HRV’s to help control humidity levels further indoors. We also use HVAC equipment that has dehumidifying settings only. In addition, we strive for Passive House levels of air tightness which helps to keep moist air from infiltrating the building in the first place.
The WUFI analysis is a great to see, but the proof of the moisture tolerance and success of dense-packed cellulose wall assemblies has to be first and foremost the built collection of houses that have now survived several decades without their sheathing turning to oatmeal (in the words of Martin Holladay…).
We have piles of structures all through northern New England and Canada with no exterior foam, cold sheathing all through the winter, and no epidemic of rotting sheathing, as long as there is good air control at the air barrier. In your much milder climate, you just aren’t going to have interior moisture problems in wintertime with these kinds of assemblies if you testing them for air leakage mid-construction with a blower door.
Winter moisture diffusion is a very weak process, I think we need to keep our focus on stopping leaks from the outside from damaging our buildings, it’s a much more powerful process, and one we still get wrong over and over again by screwing up flashing details. The reverse moisture drive in summer in air conditioned buildings with interior vapor barriers is a much scarier issue (http://www.greenbuildingadvisor.com/blogs/dept/musings/when-sunshine-drives-moisture-walls).
Jesse raises the question of which of the two stud walls is loadbearing. If you wish to avoid the thermal bridge of the floor joists as Chad describes, then it would make sense that it is the interior wall. But code would still call for fire stop blocking at the floor level. Doesn’t this add a thermal bridge?
We use the interior studs as load bearing to eliminate this thermal bridging and the subfloor ties the two stud walls together while acting as a fire stop.
Excellent value engineering as usual from Chad. The differences in cost and performance compared to the current Building Science Corporation recommendations are hardly worth debating*
1. Will the siding also cover the foundation insulation without a discontinuity (or step)?
There’s an “air barrier” thread where experienced practitioners recommend the Zip system that 100K used on the Skinny House, with housewrap. They say it’s easier to do a reliable job of air sealing: http://www.greenbuildingadvisor.com/blogs/dept/musings/airtight-wall-and-roof-sheathing?page=1
than by taping the foam.
Did you guys have installation or cost problems with ZIP wall?
*The BSC wall section can be studied in the May issue of JLC or http://greenbuildingindenver.blogspot.com/2010/05/walls-for-new-construction-state-of-art.html
I’d love to see the discussion continue here. I’m very interested to know if the WUFI analysis is really showing a problem because I also agree this appears to be a very well designed wall. It seems to hit all the right levels of economy, performance, and constructibility. The possibility of moisture build up seems to be the only challenge. One additional question, would you use foil faced or unfaced polyiso for the exterior insulation? That would affect the vapor migration significantly.
That said, I’d be hesitant to not use a product like Tyvek drainwrap under the exterior insulation. EIFS systems like STO, the REMOTE and PERSIST systems, and most rain-screen systems use some sort of additional drainage plane because taped foam isn’t substantial enough and needs a backup system. Zipwall could work though and be a higher performing option.
There is a lot more analysis, including extensive WUFI runs, here at Building Science’s High-R wall case study: http://www.buildingscience.com/documents/reports/rr-0903-building-america-special-research-project-high-r-walls
BSC’s focus is often on changing the practices of production home building, so some of their caveats are aimed at builders who are not carefully shell testing their houses to ensure that there are no significant leaks that can drive moisture into their shells (if you are building houses that blower door less than 1.0 ACH50 you are probably in a different category).
Wow Jesse, this is an awesomely thorough document I have not come across yet. They seem to be advocating the assemblies with spray foam in them the most, which has just been too costly for us to incorporate to date.
They don’t have our exact wall assembly in the report, but do speak highly of the hybrid approach of exterior insulation and cellulose in a deep cavity. They note that there is risk in some of these assemblies, but nothing to cause you to eliminate them, especially if you are focusing on air tightness. Jesse is alluding to this in his last comment.
We’re trying to make both the exterior rigid and the interior drywall as air tight as possible. By doing this and keeping the interior humidity in check via ventilation and mechanical means, I have no concerns about the durability of our wall assembly. I may see if I can figure out a good way to post our WUFI calculations done by a consultant recently as well for those interested.
BSC has been shifting gears on spray foam rapidly now that the Global Warming Potential of US blowing agents in closed cell is becoming so apparent. Daniel Bergey from BSC had an in depth presentation about it at the NESEA conference this year. BSC does talk a lot about how foolproof closed cell is, they seem to have seen far fewer failures than they’ve seen from other insulation methods.
Even so, this doesn’t make me want to specify closed cell at all, the problems with toxicity, cost and irreversibility are enough good reasons not to use it, let alone the problem that the US blowing agents fry the atmosphere.
I agree that a hybrid wall system may be the better choice in your case due to the narrow lot where you gain some real estate from foam. however, if we are to talk about “the most affordable and effective wall assembly” on a regular lot and if we are to belive WUFI, I think there are systems that perform as or better for less money.
a wider wall, between 12″-16″ filled with cellulose with an air barrier on the exterior and a vapor barrier on the interior it is probably better from the $/R value point of view that the hybrid system.
also, according to WUFI such a wall will not build up moisture even without humidity control which is not the case with the foam wall so the mecanicals can be probably simplified too. in a heating dominated climate the bulk of the drying happens to the exterior and by using the foam you cancel that.
another plus in eliminating the foam is that one doesn’t have to rely on the foam tape to mantain the integrity of the air barrier.
you can tell by now that I’m not a big fan of the foam even if I think there are places where we do not have viable replacements.
great find, jesse. that seems to jive with a lot of things i picked up during passivhaus training. although the conclusions of the article seem to be that double stud and larsen truss walls are at a significant disadvantage when it comes to moisture potential.
the 9.5″ double stud wall w/ cellulose has 3x the hours of potential condensation as a 2x6OVE w/ 4″ EPS strapped to exterior.
Mike,
I agree, there is more theoretical moisture risk to an exterior air barrier wall in a cold climate than an exterior insulated wall. The body of built projects seems to be saying that it turns out to not be too high a risk as long as you use cellulose for your cavity insulation (which can buffer moisture and later dry out).
But, it’s clearly a cheaper US solution based on conventional stick framing practice that ignores the proper wood wall construction for a cold climate: interior air barrier, vapor open exterior sheathing. This construction system is typically more expensive than exterior air barrier construction (ZIP wall), but it makes better technical sense.
Since we can’t get thick structural fiberboard here to use as vapor open exterior sheathing like the Euro’s do, old fashioned diagonal pine sheathing would work well.
Better wall, outside to inside:
Siding
Furring
Asphalt Felt WRB
Diagonal T&G 1x pine boards
double stud or larsen truss cavity
Plywood / OSB air barrier (formaldehyde free)
2×3 chase / wiring cavity (possibly dense-packed for additional R)
GWB, painted.
btw, I’m in phase two next week…
That’s a nice wall assembly. Probably uber efficient, but not affordable or easy to convince a crew to install properly. I love Passive House, but if we can’t find methods to build it affordably in the US, it will never grow enough to make the impact it is in Europe.
ha, for some reason, i was already under the impression you were certified.yeah, that interior chase just bothers me from a purist standpoint. dan whitmore used homasote for a fiberboard-like material.
the euros also have some really nifty wall assemblies with cellulose-filled prefab panels and cross laminated timber, a few inches of mineral wool w/ uber slick rear-ventilated rainscreen.
chad, we just had this discussion at work – i think it’ll be two things w/ passive house in the states. affordability is definitely key, and the europeans kind of mastered this on larger multifamily projects (the first PH is a 4-plex). not looking like a thermos is the other. fugly superinsulated boxes won’t win the hearts and minds, no matter how much money they save.
Nothing like a wall type Friday night.
For extreme economy in wall construction, check out Robert Riversong’s Larsen Truss system: http://www.builditsolar.com/Projects/SolarHomes/LarsenTruss/LarsenTruss.htm No sheet sheathing at all to the exterior, just shiplap siding with diagonal strap bracing. 2×3 balloon framed exterior framing, interior bearing wall, air tight drywall (doesn’t try for less than 2.0 ACH50, however). When you draw it out in plan it’s almost bizarrely minimalist.
It’s heavily influenced by rural stick building practices (green lumber!), and could run into all kinds of problems in a high scrutiny or urban environment, but it’s extremely resource frugal and quite clever. It’s the antithesis of the complex multi-layer german wood wall, but breathable to the outside just the same.
dan’s project is semi-urban and uses a modified larsen truss, and got close to a 0.6ACH50, and was done in a way so he can still access air barrier and fix problematic areas (e.g. a wooden beam w/ a crack in it…)
dan’s wall (from in to out):
GWB (i think)
OSB (air barrier)
modified larsen truss/cellulose
homasote
rear ventilated rainscreen
http://existingresources.files.wordpress.com/2010/04/seattle-ph-larsen-trusses-in-the-rain.jpg
Many of the experts cite how hard it is to build an effective air barrier on the inside of the wall. Sounds like Don has been pretty successful, however.
Green,
which experts? almost all of the passivhaus projects i’ve been looking at the last year have interior air barrier and routinely blow sub 0.6ACH50. if your going after the ‘airtight drywall’ approach then i can see why that would be so difficult.
you can check out some of these projects here, just click on the objektsuche tab above, and then one of the states. after selecting a project, wall construction data would be under the ‘Energetische Kenndaten’ tab:
http://igpassivhaus.cuisine.at/surface_new/start.htm
Some well-regarded guys in this article: http://www.greenbuildingadvisor.com/blogs/dept/musings/airtight-wall-and-roof-sheathing?page=1
Thanks again for the info. Mind you, Dan’s wall will cost somewhat more than Chad’s wall. The tradeoff is that Dan’s wall dries to the outside, an approach I can definitely understand in Seattle.
What a great post and great set of comments.
In the past my biggest concern with cellulose was settling over time, which you do not address in your post. Have any advances been made to control this in you wall system?
As you know, great care must be taken to keep the dew point out of the wall insulated with cellulose. I have been in long term debates with many people in the business about the advisability of vapor barriers either inside or outside. With an outside vapor barrier, I too have concerns about a winter time migration of interior moisture in your wall.
I think that this would certainly at minimum dictate a strong warning in the home owners manual against the use of humidifiers in these homes. I have seen a ten year old untaped SIP house virtually destroyed by the extensive use of a wintertime humidifier.
green,
cost more in labor? materials? embodied energy? LCA?
as green architects/builders, i feel that all these things have to be weighed. but heck, i’d rather build without insulation at all and utilize process energy, themal storage and solar air collectors:
http://bruteforcecollaborative.wordpress.com/2010/02/12/das-kybernetische-prinzip-the-cybernetic-principle/
moderns,
dense pack cellulose should not be settling, unless it is soaking wet, in which case you’ll got other problems. dan stapled mesh to his trusses to prevent cross flow when blowing the cellulose. also makes calculating how much should be in each bay a little easier, i would think.
poor installation of dense pack is another problem altogether.
This is somewhat very useful post to build up a Effective Super-Insulated Wall. When we make a house increasingly energy efficient with conventional materials, we reach a point at which the house can be called “super-insulated”, which is going to be very much essential now a days. Much of what we think of as super-insulating techniques are simply commonsense building methods carried out with an unusual degree of care and attention to detail. We suggest that you take a long look at both super-insulating and passive solar building systems, and use whatever techniques from both approaches that seem appropriate to your situation.
Looks like a great scheme to me! I work for McGraw-Hill and the enthusiasm we’ve had from builders and homeowners regarding exterior insulation tells me that you are not alone, either.
Chad, sort of off the flow of the topic, but regarding extending the floor deck to the outside stud to serve as a fire stop, in the past fire blocking at floor levels had to be a minimum of a 2x framing member. You could not use the floor deck. I don’t know if that has changed under some local codes I tend to think not.
The reason I asked the question is because I suspect that you can not avoid this particular thermal bridge at the floor level.
You may be correct Greg, although we have not gotten tagged on this by our building plan examiners yet. I’ll have to look into.
Given the comments, a double stud with ZIP System on the outside and a thinner poly-iso board in the center of the wall is growing on me as possible alternative. Air tightness on the outside, middle and inside. Dries to the outside and inside. Dense-pack cellulose plus cellulose or rock-wool batts.
If its any help I just cracked open the IRC. Section R602.8 is fireblocking, thickness is discussed in R602.8.1 Materials. Phila code may differ.
I’ve seen attention to this rise as fireblocking foam has come into common use at all penetrations.
Chad,
The perm of ZIP feels too low to count on drying through it, I assume it’s an impermeable layer in our calcs.
In your climate I don’t think that’s necessarily a bad thing, you will get a lot of solar driven moisture trying to get inside your building during the summer (rainstorm, then hot sun baking your siding to 100+ degrees) that is much stronger force than any wintertime moisture drive could dream of.
Right. Let’s not forget we always use a vented rainscreen on the outside which is integral to this discussion. The wall will be cooler and dry out much better than a typical US house without rainscreen. This seems to be key to most all European Passivhaus wall assemblies as well.
is good to see that you’re giving so much tought to the air tightness but I think ZIP system over plywood is not the best choice. I couldn’t find permeability numbers on their website but DuPont wants us to belive that ZIP permeability is less than 1 perm. (look for tyvek vs ZIP document on DuPont website)
now you have two low permeability layers in your wall assembly and an exterior air barrier based on some tape with only 15 years warranty. We’ll see what WUFI says on this one
.
apparently you really want to use some type of foam somewhere in your wall. why not use 1.5′-2′ of open cell sprayed foam on a plywood sheating? much more permeable than ZIP, better air barrier than ZIP, cheaper than ZIP+foam…..win, win, win
rear ventilated rainscreens are key to more than just passivhaus – outside of masonry i can’t think of any euro buildings that don’t use them.
plus, they tend to look really awesome… at least the way they detail them. aahhhh, memories.
Adrian,
The open cell has all the negatives of a vapor open material without any of the positive benefits you gain from a natural fiber material like cellulose.
It can retain water, but can’t release it (sponge-like internal structure), but still lets vapor pass through it to the exterior sheathing where it can cause moisture damage.
It’s also expensive and is made of extremely hazardous chemicals that have to be correctly combined on-site in an uncontrolled environment to safely become inert and become safe to live next to. See this discussion for more on this issue, and the ugly consequences of the (thankfully) rare times when things go wrong during installation: http://www.greenbuildingadvisor.com/blogs/dept/green-building-blog/does-spray-foam-insulation-out-gas-poisonous-fumes
Board stock foam at least manages to avoid the on-site chemistry management problem inherent in spray foam application.
Sorry if this is an ill-informed inquiry.
Would talk about wet-spayed cellulose vs blown in dense-packed.
I would think it would be more thorough in covering the wall cavity.
But maybe, the wet-pack isn’t “dense-pack” when it dries.
apart from the density wet sprayed cellulose at 12″ depth needs a lot of time to dry (I’d say weeks but don’t take my word for it) and it is not a good ideea to cover it before it is dry. that can delay the schedule significantly.