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Most Affordable & Effective Super-Insulated Wall Assembly?

by Chad Ludeman on July 16, 2010 · 81 comments

in envelope,Two Point Five Beta

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.

Double Stud cellulose wall with exterior rigid insulation

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:

  1. 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.
  2. 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.
  3. Any type of batt insulation – Seriously?
  4. ICF’s - Expensive, built of harmful foam, hard to get a high r-value and filled with concrete.
  5. 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.
  6. 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.

If you enjoyed reading this post I can promise you'll love our new writing over at Postgreen Homes. Yeah, we know that's the same thing your favorite band said and their new album is nowhere near as good as their early stuff, but seriously, we are actually still getting better.

There also isn't much conversation to be had here . . . at least not with us. So come on over to the Postgreen Homes Blog and tell us what you think of our new(ish) digs and crazy ideas. We will be sure to tell you what we think of your opinion.

{ 2 trackbacks }

Blown In Cellulose – The Ultimate “Green” Insulation? — 100K House Blog
August 16, 2010 at 8:21 pm
Innovative Passive House in Lafayette | Jetson Green
December 21, 2010 at 1:18 am

{ 79 comments… read them below or add one }

1 Peter Lee August 11, 2010 at 11:49 am

What I was trying to get at is, if one method was better in the long run. Yes, if you’re a builder, time is of the essence.

In my case, I plan on gutting a house and I’ll be on my own schedule. If the wet-spray had the advantage, then I’d be willing to put up with the drying period.

So is there an advantage with the wet-spray, do you still achieve a “dense-pack”?
I would think that you can see and confirm that every square inch is covered.
With blown in dense-pack, I could imagine pockets where the worker didn’t really do the job correctly (there’s a lot to cover and it gets monotonous) and the dense-pack really isn’t very dense in spots.

2 Cletus August 11, 2010 at 6:33 pm

Followed the blog for a while … great work.

Now I’m looking to build and among many questions, wonder about the cellulose insulation. You specify dense-pack. In new construction does this mean wet/spray applied?

Does anyone have any experience with wet cellulose application and galvanized metal studs? I’ve heard some warning about corrosion with wiring and plumbing – might also be fiberglass disinformation.

Also considering denim.

I’m in Southern California, so insulation not as high stakes as when I was in Minnesota – but I’d like to get as close as possible to a passive house – cool nights and solar mass days.

3 Robert Riversong August 13, 2010 at 4:04 pm

Good try, but enclosing the entire house with 2″ of petrochemicals is hardly “used sparingly”. Used around the foundation and under the slab (where other materials don’t perform adequately) IS sparing use of a high-footprint and expensive insulation material. Eliminating a full foundation and building on a shallow, frost-protected grade beam further limits use of high-impact materials.

“Double stud walls on 24? centers is [NOT] the easiest and cheapest built thick wall cavity.” There are ways to design a double-stud wall appropriately (such as using the inner wall to bear the floor decks and the outer wall to bear the roof, to avoid the thermal bridging and air-barrier problems with exterior rim joists), but a Riversong Truss wall (modified Larsen Truss) is easier, less expensive and more resource-efficient as well as solving all the thermal bridging and load path issues common to double-wall systems.

I get R-45 from a cellulose-filled 12″ wall without resorting to petrochemicals and non-vapor-permeable exterior skins (most drying in a cold climate is to the outside), and can often do this with no exterior sheathing (metal T-bracing where codes permit, which is most of the country) such that I end up using no more wood than a sheathed 2×6 house and get much better thermal performance. The deep window boxes are an additional benefit.

4 Chad Ludeman August 16, 2010 at 3:52 pm


I’m so glad (and honored) that you stopped by to comment. I don’t disagree with your comment and wall assembly and have been giving it a lot of thought lately.

I’m not totally sold on the need for all of the cross bracing between the two stud walls. We are currently looking into basically the same detailed wall, except we are bringing the subfloor all the way out to the exterior of the outside stud wall which ties the two walls together with minimal thermal bridging at each floor deck.

In addition to this we are considering sheathing the entire inside of the outer stud wall with OSB to act as our primary air barrier that would tie into the subfloor as well as OSB under the roof deck TJI’s. This creates a Passive House level air barrier that is very easy to detail and tape all the way around the envelope from inside the house.

What makes our goals so difficult is that we are trying to create a Passive House level wall (Super insulated, Super tight and zero thermal bridging) on a budget that can be built quickly in a production setting.

Most of the Passive House walls we are seeing in our research from overseas use this middle layer of OSB as their primary air barrier. The difference is most of them are using a 12-16″ thick wall made of TJI’s with sheathing on the outside and OSB on the inside. They then build another 2x wall inside that that houses all electrical runs, is load bearing for the floor decks and gives a nice wall to drywall that does not need to be airtight. We believe TJI walls are too difficult and expensive in the US and your modified Larsen Truss walls or the one I have described above in this comment would be superior in the states.

Thoughts? Anyone?

5 August 16, 2010 at 4:27 pm

My thoughts are that Joe L. has probably seen and studied the inside OSB method but isn’t pursuing it because of cost.

“What makes our goals so difficult is that we are trying to create a Passive House level wall … on a budget that can be built quickly in a production setting.” Amen. These wall details are starting to consume too much of our innovation horsepower.

Remember the slab insulation debate? At about R40, these walls start to approach a tipping point where the smarter dollars might well be spent on solar PV. It’s even harder to guess at where the infiltration tightness (ACH@50pa) specification crosses that line. And these tipping points are economic moving targets that are highly climate dependent, producing different answers for different locations and costs.

It’s absolutely fascinating to follow this evolutionary process. Just don’t forget that a residential envelope can be too tight and too well insulated due to economics.

Progress is being made: we can eliminate a few technologies in our search:
1. Fiberglass insulation (any type)
2. Sprayed foam insulation
3. SIPs

Mr. Riversong makes strong arguments against exterior foam, but like Joe L., I still think it’s on the table for many climates.

6 Adrian August 16, 2010 at 4:39 pm

(check also the images from the comments section)

in my opinion is hard to go simpler or cheaper than this (maybe with mr Riversong method) and if we’re talking about airtight – they ve managed to get to 0.36 ACH @ 50 Pa.

7 Nick Furfaro January 7, 2011 at 9:17 pm

Very interesting discussion…just the type of stuff I spend my days thinking about.

I’m in favor of the limited use of exterior foam as well. In addition to it’s contribution to insulating and eliminating thermal bridging, exterior foam can raise the temperature of the interior surface of the sheathing, shifting the dewpoint outside of the cavity wall and allowing much more flexibility in the choice of interior vapor retarder.

8 Robert Riversong January 7, 2011 at 10:08 pm

Actually, there isn’t much flexibility in vapor retarder in a cold climate – the code requires no more than 1 perm. But except in a swimming pool enclosure, vapor diffusion is not a significant issue – it’s air movement.

With controlled indoor humidity and a tight envelope that limits convective moisture transfer, the temperature of the sheathing is not an issue. And, with a highly hygroscopic and moisture-buffering insulation like cellulose, minor incidental condensation can be stored, redistributed and released without consequence – particularly if the structural envelope can dry to the outside.

But, with exterior foam board, there is little to no drying potential to the outside, so moisture accumulation becomes a much more serious problem. And, because the exterior foam board maintains the sheathing at a higher temperature, any amount of moisture accumulation can result in mold or decay. Unintended consequences.

A breatheable wall is a durable wall.

9 Nick Furfaro January 8, 2011 at 2:41 pm


Some valid points you bring up.

Regarding my comment about vapor retarder flexibility; I’m in British Columbia, and our code allows for some flexibility in both permeability and location of the vapor retarder…Sorry for the misunderstanding!
I couldn’t agree more that air movement contributes far more to vapor transport than diffusion does. Specifying an interior air barrier in a cold climate is typically my first line of defense, when it can be installed effectively. In some projects (timber frame structure with 2×4 infill walls and exterior foam insulation) the air barrier is likely to become compromised due to wood movement. In this type of scenario, I would switch to a different strategy; Move the air barrier to the outside of the sheathing where it’s far simpler to install in a continuous manner. Of course there could still be convective looping of air from one interior location to another, but if the sheathing temp. is above the dew point that shouldn’t be too much of a problem, especially if the cavity insulation has some hygroscopic buffering capacity as cellulose does.
Also, all else being equal, raising the temp. of the sheathing will lower its relative humidity. Would this not lessen the chance of mold growth? I have to admit I’m not 100% sure on this one, but it seems right…

My thoughts on this are to try to build some forgiveness into the wall assembly. We all know that buildings change with time.(different owners or tenants, different uses, additions, renovations, etc…) I’m a strong advocate for the Airtight drywall approach, but I certainly can’t guarantee that the air barrier will still be intact and fully functional in 20 years…never mind 100 years! If we’re not building with longevity in mind, we’re missing one of the key components of a sustainable building system. I would rather trade a bit of material efficiency for a little redundancy in my walls if it means they will stay functional over time and with changing conditions.

10 Robert Riversong January 8, 2011 at 3:47 pm

“raising the temp. of the sheathing will lower its relative humidity”


Sheathing doesn’t have relative humidity, only air. The relative humidity (RH) of the air adjacent to the sheathing determines the equilibrium moisture content (EMC) of the sheathing (or other hygroscopic material). For instance, 80% RH results in an EMC of about 16.5% which is enough to initiate and sustain mold growth. And the local RH at any point within a thermal envelope depends on the vapor pressure on each side and the relative vapor resistance (inverse of perm) of each element.

The EMC of the sheathing also depends on the permeance, hygroscopicity and capillarity of adjacent materials on both sides as well as the combined drying potential due to thermal and convective forces.

The problem with the “warm sheathing” approach is that it relies on the assumed perfection of all layers for the life of the building. By both restricting drying to the exterior with relatively impermeable layers (ext. foam) and reducing inward solar-driven heat flux with a combination of a convective rainscreen gap and low-conductive layers (ext. foam), the drying potential of the sheathing is reduced to the point at which any potential failure (leak) will result in the long-term accumulation which leads to decay organism growth (rot).

The only two studies I’m aware of that included a deliberately-introduced leak event – one in a foam-sheathed wall and the other in a bituthene/spray-foam sandwiched roof deck – demonstrated a high probability of catastrophic decay.

So, Joe Lstiburek’s “perfect wall” and other foam-based envelopes are theoretically wonderful but practically problematic. A “fail-safe” envelope is one that, if it ever gets wet, can safely store moisture and dry in both directions before the moisture becomes a problem.

Remember, even saturated sheathing is not a problem as long as it’s too cold to grow mold or decay fungi and can dry out before it gets warm enough to become an incubator.

11 Nick Furfaro January 9, 2011 at 3:40 pm


Thanks for the clarification about RH, EMC, etc…This certainly helps me to see the big picture more clearly.

Of course I must agree that relying on the “assumed perfection of all layers for the life of the building” is a mistake to be avoided. I would go one step further and suggest that it’s probably unwise to assume the perfection of even one layer for the life of the building, including the air barrier. I suppose I’m just wary of trusting that the continuity of the airtight drywall will withstand new owners, remodeling, additions, etc…Although I can see that your approach (Vapor-open, with dense-pack cellulose) would appear to be quite forgiving in the case of air leakage condensation…I’m curious as to what your climate is like with regards to Design Days, RH, etc…

I’m very interested to follow developments in the area of affordable & effective super-insulated walls. The comment “we are bringing the subfloor all the way out to the exterior of the outside stud wall which ties the two walls together with minimal thermal bridging at each floor deck. ” by Chad L. made me think of the current work being done in Edmonton, Alberta on both the Riverdale and the Mill Creek net zero houses (see link)

By using separated wall plates tied together with 3/8″ osb plates, they’ve traded what seems to be minor thermal bridging for the benefit of maintaining the traditional workflow for the framing crew, in that the double walls are raised in the standard fashion. I wonder about the labor costs of this method vs the truss-style wall?
(The exterior sheathing could be omitted if other bracing were included, but provides a surface for stucco cladding…)

12 Josh Rockwell February 3, 2011 at 7:07 pm

Have you guys seen/heard of a company called EcoSteel. They use pre-insulated steel wall panels with up to a R-48 value. is their website. Living in Northern Utah I know all to well the value of thermal efficiency (This week it reach -20 for a low).


13 Tony March 23, 2011 at 1:01 pm

I am having trouble with your graphic view of your wall. Could you re-do using standard drafting technics. I’ve never seen 2X4s depicted like an A/C current.

Not trying to hurt you but help you help me and others.


14 scott wells April 8, 2011 at 1:17 pm

I need someone to clarify the moisture and condensation issues presented in differing posts regarding vapor barriers. In particular, what would be the moisture issues in an exterior wall if the wall was built with moisture barriers on both sides? I am sure there are issues that could be addressed but it seems more important to prevent air and moisture flow from BOTH sides of living space. How much does a wall need to really “breathe” if the wall system itself is completely air and moisture impermeable from both sides?

Let me be more specific as to what I have in mind and then I will accept any advice from those with more authority, which I will assume is anyone with experience in the area of super insulated wall construction.

Planning a small (1000 sf) single level home on a sealed crawlspace in cold weather (northern Wisconsin). Let’s say the foundation walls are ICF, and a strong moisture barrier to seal the crawlspace floor. Throughout the house, including the crawlspace, an independent air exchange system between interior and exterior. Otherwise airtight as much as possible.

Why not, for example any type of simple wall system, say 2×6 or even 2×8 @24″ OC, filled with loose fill insulation, with vapor barriers (plastic or insulated sheathing panels) on both the interior and exterior walls, effectively eliminating airflow through the wall? The exterior foam panels could meet flush and seal with the foam from the ICF foundation walls, and the interior of every room sealed on the inside wall with a vapor barrier of plastic or foamboard. Again, air flow within the home and to the outside controlled by a ducted air exchange system.

What type of moisture could build up in a wall system this super tight? If any, could a simpler wall system like this be somehow modified to eliminate or reduce this moisture? I am not trying to be flip, but never mind the cost or environmental impact of all that rigid foam insulation for sake of this line of questioning, just address the impact of making the exterior walls moisture impermeable and/or insulated on both sides. This would be a much simpler wall system than double wall systems; easier to build despite higher initial costs. It just seems to me that allowing the wall to “breathe”, either to the inside or to the outside, means easier heat transfer to the cold and invites moisture into the wall cavity. Sealing both inside and outside should reduce moisture transfer to the interior of the wall, but I am open to the suggestion that I am completely off base here and would like to understand why. Thanks in advance for your consideration of my limited knowledge here when responding.

15 Barney Coverdale May 21, 2012 at 9:41 am

Chad – useful stuff, thanks. Just one thing: could you expand in some detail on the following from your original post:

Any type of batt insulation – seriously?

Thanks. Barney

16 Sam July 30, 2012 at 2:50 pm

Has the Riversong Truss wall been built in Southern California? I curious about Earthquake loads and being able to install stucco without the plywood sheeting.

17 Maurice Huet December 14, 2012 at 3:31 pm

You seem to focus on the insulation in the walls but forget that the worst heat losses or gains will be through your windows which might be R5. The heat losses will find their way to the worst insulated surface and penetrate there. R30 or R40 will not make much difference when the heat penetrates the R5 windows. Your money is best spent on the best balance of insulation envelope.

18 Chad Ludeman December 14, 2012 at 3:46 pm

I’m not sure what this means Maurice? Are you suggesting a home with no windows?

19 Barney December 15, 2012 at 6:20 am

Sure, in cool to cold climates, high-spec windows will always lose more heat than high-spec walls. But windows also gain heat from the sun. Triple glazed, unshaded, south facing windows at latitudes up to about 55 degrees generally have an annual net gain. Walls do not.

20 Robert Nemoyer March 23, 2013 at 10:39 pm

I see a lot of concern about the weakness of windows in your superinsulated homes and suggestions about superwindows. But doesn’t make more sense just to use window panels to raise the insulation level of the windows. If your north windows are kept small and you add thermal shutters wouldn’t this work as well as high r windows? And wouldn’t it be much cheaper. I am building for myself and plan on both exterior thermal shutters that can be closed and pictures on with 2 inches of foam behind them.
The pictures will slide over to cover the windows when it is cold and especially when the room is not in use.

21 tom sutherland August 9, 2013 at 6:02 pm

have old 2×4 constructed house in anchorage ak…fiberglass battes in between studds foundation concrete block goes down about 6 feet to footing not insulated split level house some are superinsulating using foam panels to 4-6 inches which is put on outside of studes….any suggestion of best way to superinsulate old home?

22 Russell August 26, 2013 at 12:41 pm

Fantastic job
Great to see someone pointing out the inconsistency of using foam, to save the environment. I’ll add in it’s also a fire hazard, mostly from the huge amounts of smoke it creates, but also some foams flamability when it reaches it’s flash point, which is well within what is found in home fires. These are hazards not just to occupants, but to firemen and the community at large.
I’ve seen glass wool / thermafiber type high density “board” insulation used in place of exterior foam, and I’ll be trying it out on an building I’m refitting as a guest house. Same R value, and, it fireproofs the structural wall too! That’s thermafibers big market, firestopping and fire proofing. If you’re interested, you’ll find the blow by blow posted on roughdesigns in 2014.

23 Dave Brice November 4, 2013 at 11:56 am

Uh, where’s your rain screen at the exterior? Appears omitted. In high wet climate regions of the country, a rain screen is needed.

24 Frank February 21, 2014 at 12:38 am

I think Maurice makes an interesting point about windows.

Depending on the amount of glass in a wall (and it’s r-value), there must be a point of diminishing return in the wall cavities r-value (or perhaps not); where increasingly higher r-values within the wall cavity loose their impact on the overall r-value of the wall because of limitations in the glass.

How do we assess the optimal r-value of a wall, given the area and r-value of the windows that exist in the wall?

In the case of a wall that is all glass: Optimal r would be that of the glass (r=4?).
In the case of a wall with no glass: Optimal r would presumably be unlimited.
In the case of a wall that is 50% glass: Anyone?

25 Glen Wither December 16, 2015 at 12:14 am

I am going to build a one bedroom retirement home for my wife and me next year, near Ottawa Ontario, which gets as cold as Helsinki, and it will be built to a passive house standard. I am also doing most of the lighter work myself, including the framing. I am not as worried about time or adding a little extra for long-term insurance. That is an advantage some do not enjoy. I was a young practicing architect with the Saskatchewan Housing Corporation and CMHC when Harold Orr and John Larsen, and others were experimenting in real projects. GBA has a great article on the Larsen truss history which also quotes the hard nosed John Larsen, a good read for some folks here. Having also been an investigator on the wall failures on BC cooperatives, I worry about keeping walls dry, and I am also suspicious of any plastics nailed to the outside of wood framing. While I completely understand why Robert Riversong’s modifications to Larsen’s design have considerable merit, in my situation I lean toward using 6 mil poly over a taped sheathing on the outboard side of a 2×4 insulated wall, and then hanging a Larsen assembly off that bearing wall. That way, the air and vapour barriers are at less risk of damage from occupant use or other damage, and the assembly is affordable and simple to built in order to get a roof on quickly. I have done enough building and investigations to know how thin barriers can be easily degraded by workmanship or use in service. I like good and simple, and learned years ago how engineering for that last 20 percent of perfection in both business and architecture can be costly and a bad return on investment. Yes, I understand value engineering and why passive house uses Design software to configure an envelope. It seems to me that a lot of smart people are chasing that last 20 percent and it’s getting complicated and for many folks..confusing, or worse, beyond the reach of many. If we can’t design a good house without the PHPP software at our desk, one wonders how far we can take this concept. Thoughts anyone?

26 tom barthelemy January 7, 2016 at 5:49 pm

I’m in the final stages of designing a high-performance home for a client. We have tall walls – 12′-0 and 10′-0 and I’ve been sorting out the alternatives. I’ve also come to the conclusion that double-stud is the way to go, but without the foam layer on the exterior. My reasoning is that we can get quite “normal” methodologies on the interior and the exterior while coming up with an R40 wall that dries to the exterior – which foam on the exterior can only make more difficult.

Money is definitely a factor on this project and we have to have the BS Detector set at 9+ when people maintain that this or that comes at no additional cost. I’ve done a couple houses lately with the exterior sheetfoam method. They do work as advertised, but mygawd did we go over that shell a lot of times. Enough OSB to achieve shear values, two layers of foam [Northern Minnesota] a system of 3/4 rainscreen, blocking around all the windows & doors, then [finally] the siding.

My scheme for the walls is 5/8 drywall, a tight tight TIGHT 8 mil air barrier, 2-2×4 framing, likely denspak cellulose, 1/2″ cdx, sealed drainplane housewrap, and the siding. There are adjustments, here and there – I’ll use 2×6 studs for the bearing wall on the 12′-0 areas, for instance.

One tweak that I’ve never seen addressed came to me when I did my own doublestud house. I’m definitely a 24″ o.c. guy whenever possible and the code/engineering guys said 2×4 would be OK, but I was not happy about how “boomy” the walls were so I added horizontal cross blocking at the midpoint [I had 8'-0 walls with the inner and outer studs alternating on layout] reasoning that the walls would stiffen. I was correct on that point but was stunned at just how much improvement I got. An engineer later told me that I had come up with something pretty close to a 2×8 wall on 12″ centers. Definitely recommend that approach.

Well, i could go on but…..gotta go shovel some snow.

27 tom barthelemy January 7, 2016 at 6:05 pm

Ok OK…it’s not dark yet and I see a couple issues on this string that I gotta coment on.

First the windows. Keep in mind that your windows bring in the no-cost heat of the sun. They are an energy plus, not an energy minus. It is quite true that a thermal treatment will radically improve their performance when the sun is down, but that doesn’t mean that you shouldn’t bring it on with the glass.

Second the perennial air/vapor barrier question. A wall needs to dry to one side or the other as certain amount of water is going to get into the cavity no matter how hard you try to keep it out – and I speak as a guy who has been fanatical about sealing the interior for the past 40 years. However the side that needs the barrier changes as you go from north to south. A Florida home should have the barrier on the exterior as the AC will result in a cooler indoor environment than exterior. In Minnesota the barrier is on the inside and the system will dry to the outside.

Then, just to keep things from getting too predictable, if you go for the exterior XPS approach and put on enough of the stuff [3" where I live] then the dewpoint will occur in the foam layer which means you don’t need a barrier at all.

28 Sarah Cobb February 4, 2016 at 2:39 pm

OSB is not a reliable air barrier.
Why not use something like the sonoclimat eco4 for bracing? It’s super green, no nasty foam.

29 Matt McDowell December 13, 2016 at 10:14 pm

I was leaning toward closed cell foam to insulate a 54′x60′ net zero shop I’m building in northern Michigan, but your wall design looks like it has serious possibilities. My building will be steel frame with wood 2 x 4 inner and outer purlins on the flat for walls, and top chord bearing open web trusses for roof purlins which I’m hoping to pitch 12/12 for pv panels. I want the interior walls to be exterior plywood and the taped exterior panels to be 1″ poly-iso and the resulting 10″ bay filled with cellulose. After reading some of the comments about moisture trapped in the wall cavity, I wonder if I should skip the poly-iso and just go with plywood. The outer layer on the shop will be metal. My concern with moisture has to do with the structural metal frame embedded in what is essentially a cellulose filled box. The shop will have a small wash bay in one corner and a lot of lumber so the moisture potential is there. The windows will be few, mostly on the north wall and there will be some sort of ERV. I wonder if anyone has experience with something similar, and if the interior plywood will mitigate the potential moisture issues caused by the exterior foam board.

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