Location of WRB in Wall Assembly with Rigid Insulation
In a wall assembly with exterior rigid insulation should the WRB be placed between the exterior sheathing and the insulation or outside of the insulation? This topic has been discussed on nearly every green building website and in every forum that I have encountered. There seems to never be a definitive answer.
I am confident that the WRB should be placed on the sheathing to the inside of the rigid insulation, but it would be great to get some confirmation from an accurate building science perspective. Also, it would be helpful to understand why there is so much confusion on this topic.
The general consensus seems to be that is should be placed as I stated, but then there are products such as Dow Styrofoam SIS that place the integrated WRB on the outside – this is just an example because I understand that this product is the sheathing/insulation/WRB all in one, but why is it alright to place the WRB outside of the insulation in this application?
My question is pertaining to climate zone 4, but I assume that the rational is consistent across the country.
Any building science/technical input that may help me and others answer this often asked question would be great.
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While I teach building science and hygro-thermal engineering principles, it requires only common sense to make this determination.
The function of the weather-resistant barrier (WRB) is to keep the weather (rain and wind) away from the structural frame and the thermal barrier of a house. It is often referred to as a "secondary drainage plane" behind the cladding, because it will capture and drain any pressure-driven water before it gets beyond this outer perimeter.
To function as intended, the WRB has to be thoroughly integrated with all exterior flashings, and placing it directly behind the cladding makes this far simpler and more effective.
For a variety of hygro-thermal reasons, I am not an advocate of exterior foam board in cold climates, particularly any that act as a vapor barrier, such as foil-faced polyiso or XPS greater than 1". But, for those who choose to use this quick and easy technique for interrupting thermal bridges, the choice should be clear.
The ideal cold-climate thermal envelope, from my perspective, from the inside to outside, would be: interior finish/air barrier, thermal barrier integrated into structure with thermal breaks, weather barrier, cladding.
Perhaps the reason that you have not yet obtained a definitive answer to this question is that there is no definitive answer -- and no need for one. The WRB can go in either location; or you can install housewrap in both locations if you want, with one of the two chosen to be the primary WRB (integrated with flashings) and the other installed as insurance.
You can even use the foam itself as a WRB.
Each of these choices has advantages and disadvantages, and each has its advocates. But I can't think of any significant building science reasons to stop the discussion and declare that there is only one way to do it.
I agree with Robert that the WRB should be integrated with flashing.
I don't think the answer is so obvious.
Here is a different perspective explaining why a "crinkly" wrap should be considered BETWEEN OSB and Rigid Insulation
Though I don't like the idea of sandwiching a drainage plane between structural sheathing and exterior foam, there is a reason why Building Science.com recommends to "mind the gap" as you will read in the article John Brooks points out.
That extra bit of space that products like Tyvek Drainwrap provides reduces hydrostatic pressures allows for any moisture that gets to the structural sheathing to dry out. If using OSB as your sheathing, this "drying" potential becomes important.
I like the idea of using Drainwrap between the OSB and foam, but putting the primary drainage plane (secondary if you consider the cladding to be the primary drainage plane) to the exterior of the foam board, and the Drainwrap becomes secondary. Of course ALL flashing should integrate with this primary drainage plane. The details are tricky.
This debate ignores some fundamental principles. When we built with durable materials, these "quandaries" didn't exist.
When we use non-durable materials, like OSB, and then encapsulate it with relatively non-permeable secondary sheathings, such as XPS, then we need to over-complicate our building system in order to compensate for the moisture-intolerance of a highly manufactured material and the vapor-resistance of what should ideally be a breathable layer.
What ever happened to KISS?
Yup... I am firmly in the camp of do it once right verses ... make up a difficult system that then needs a difficult solution. KISS is the answer here. OSB with sheet foam tight to it is asking for trouble in wet areas for sure. Have seen it done and have seen it fail.
I appreciate all of the responses and the discussion. I understand that there is no definitive answer, but, in my opinion, this is part of the reason the many architectural designers (like myself), homeowners, and contractors are hesitant to accept many green building practices - I actually touched on this in a blog entry (http://blog.cleantechies.com/2010/01/12/green-building-changes-green-mainstream/) some time ago if anyone is interested.
I know that there are several green building/building science experts that have posted to this thread. To get right to the point... if you were hired as a consultant to make recommendations for a new home in Nashville, TN what would the "perfect" wall assembly involve (using outsulation) if money and material availability weren't an issue (assuming wood framing and Hardie lap siding). This may help me, and others, to better understand some of the principles that you are discussing.
This thread has gone a little off topic (from where WRB, to why outsulation). Since it's heading there . . . .
Damned if you do, damned if you don't?
In another study from Building Science Corporation (John Straube and Jonathan Smegal), they evaluated various wall systems with hygro-thermal modeling software:
The three highest rated walls were:
1) Advanced framing with exterior insulation
2) Closed cell spray foam (filling entire cavity)
3) EIFS wall
Modeling and calculations don't tell the whole story. But the conclusion basically points the way toward exterior insulations: warm the sheathing, raise the dewpoint, eliminate interstitial condensation, reduce summer vapor drive.
The system with the greatest number of hours of potential condensation was the Truss Wall (I know their model wall system is a different version than yours, Robert).
Their analysis looked at wintertime condensation, summer vapor drive, and wetting events.
That last item seems to be the sticking point for those opponents of exterior insulation: limited exterior drying potential.
But it really can't be as simple as:
Use exterior insulation, wall deteriorates from exterior wetting events and limited drying potential.
Don't use exterior insulation, wall deteriorates from internal condensation on cold sheathing.
I don't think there is anything simple about a "modern" house. Back before insulation and WRBs and triple-pane windows and high-efficiency appliances, yes - houses were simple. Now, they are extremely complicated (why else would we need building scientists and schools and codes?).
I'd like to think that both methods can be done well, or poorly. No?
I would be grateful for any real-world examples of the problems INHERENT to either system (i.e. pictures or case studies, not anecdotal evidence). I believe that a lot of what gets passed off as a system problem is just poor execution / lack of craftsmanship.
"What would the "perfect" wall assembly involve (using outsulation) if money and material availability weren't an issue (assuming wood framing and Hardie lap siding)"
First, you're already assuming the answer by the specifications you've already listed.
Second, you've got to significantly refine your question to get a meaningful answer.
What does "perfect" mean?
The most "green" (whatever that means)?
The best hygrothermal performance in that climate zone?
The most durable (which may be different from above)?
The most energy-efficient?
The most cost-effective (I know, you said "if money weren't an issue" but it always is)?
The most buildable?
The most sellable?
The most profitable (isn't that really the bottom line for most builders?)?
I think there is a problem even in framing the question that way. Should we aim for "perfect"? Joe Lstiburek thinks he's designed the "perfect wall", but I see it as little more than taking a trend to its ultimate (and ridiculous) conclusion. And I see the entire spray foam, outsulation, hermetically-sealed house as an abomination to nature and life. "Perfect" only within a dysfunctional and unsustainable paradigm which is driving us to the brink.
We won't get the right answers if we continue to ask the wrong questions, or even if we ask the right questions with the wrong mindset.
I do appreciate your tackling these issues, and your blog is a great example of how muddled this discussion inevitably becomes when it occurs within old paradigm thinking.
Your blog presented a plethora of terms which may be considered synomynous when, in fact, they are polar opposites, or shadows of each other.
"Green" and "sustainable" used interchangeably. What passes for green is almost never even close to sustainable, let alone regenerative of earth, place, community and spirit.
"Knowledge" and "credentials". Credentials are mostly marketing tools. Knowledge, experience and perspective are what counts and should be "credential" enough.
"Property appreciation" and "value". The real value of a house is that it provides shelter. We cannot build sustainably as long as homes are seen as investments (a gamble which no longer seems to pay off).
"The psychology of buying". That's a term straight out of Madison Avenue and initiated by Edward Bernays, nephew of Sigmund Freud, who used his uncle's psychological theories to manipulate people into buying what they did not need (his book was appropriately titled Propaganda).
"Client-driven movement", "to go mainstream", "programs", "training", "credentials", "codes", "products"... These are all elements of a dying paradigm. There are dozens of books and articles coming out about the death of capitalism and the birth of an entirely new communitarian paradigm based on the biological imperative of evolution (Darwin, by the way, understood late in his life that evolution was fueled far more by cooperation than competition, but his followers ignored that because it didn't fit their social agenda).
You come closest to making the leap when you conclude that we need to transform the way we think about our homes. In fact, we need to transform the way we think. And that transformation is already occurring. But those who continue in the old paradigm will truly be the ones "left behind" when the SHIFT hits the fan.
I understand your point... thanks for being direct. I didn't intend to ask the "wrong question" or have the "wrong mindset". My initial question did involve recommendations on the placement of the WRB in an outsulation assembly. However, even within this post there are competing veiwpoints.
The framework of my question came from a common scenario that I often see as a designer - a client wants a recommendation of an "energy efficient" assembly. Wood framing is the most common in my area, Hardie is often demanded, and, from my research, outsulation seems like a viable option in this area. I understand that there is no "perfect" assembly - my point was to ask individual experts what their "perfect" assembly would be. I did not intend to assume the answer. I was trying to illicit opinions regarding cavity insulation (cellulose, foam, etc.), location of WRB, primary and secondary WRBs, furring channels for Hardie, OSB and rigid, only rigid (with bracing, etc.), products (such as ZIP, Dow Styrofoam SIS, etc.)........
To refine the question... I guess I am looking for the best hygrothermal performance, durability, and energy efficiency.
Jed - I believe there seems to never be a definitive answer from a building science perspective because each way will more or less perform the same from a vapor diffusion perspective. So my thought process would then take me to next thing to think about - what is the main purpose of the WRB? To keep bulk water out of the wall assembly. So why not put it on the outside? Who is going to install the WRB? The framer? The sider? You? Unless you are doing it yourself - take the path that is less confusing and less can go wrong. Installation details are so important - seems easier to visulize everything if it is in the same plane. If your window flange/flashing in on the exterior of the rigid foam and your WRB is on the inside, you have just created several problem areas for potential leaks. Why make things any more confusing than they need to be?
If you are dead set on using foam - why even install the OSB in the first place. DOW SIS will satisfy all your lateral stability reqirements. If you are uneasy about the structural qualities of this new product (as I am) then install let in bracing to supplement or just use plywood on the corners and in that case you can put a WRB in both places if it will allow you to sleep better at night.
With so many new products and the similarities between them all there isn't always a simple good, better, best solution - it always comes back to your own experience and judgment - after all, you are the one that will have to warrant this home.
Jed... if you read this whole thread... you have your answer everwhere and no where. Ultimately you have to decide on what you are putting in your wall, then make sure every aspect of the design works with every other. Rot... comes from many uses and misuses of materials and homes. Basically from my experience over many decades, the tighter you seal up a home the more you have to watch out for damage where there will be leaks in your seals and the owners have to understand where they are inputting moisture. We have a builder here that has built with layers of external foam for decades. Call him up, he loves to teach and is very affordable. He is the most knowledgeable guy I know... for doing exterior foam and running into and solving the issues that arise.
Bruce Brownell builds super tight... but... makes sure to handle moisture intrusion.
http://www.aaepassivesolar.com/faq.html#What is passive solar?
IMO... just don't call present day foam a green product.
ADKJAC, your friend apparently builds R-26 envelopes (4" Thermax @ R-6.5/inch according to manufacturer) and claims R-38.
Having read through his website, I also question the rest of what he claims for his houses.
Here's a wall: 2x6 framing, plywood sheathing, wrinkled housewrap, two layers of 2-inch-thick polyisocyanurate with staggered seams, another layer of housewrap (the actual WRB) integrated with the flashing for your outie windows, vertical 1x4 strapping 16 inches o.c. screwed through to the studs, Hardie lap siding.
WOW .. that's easy for you to say
It sounds similar to the wall in the June issue of JLC (plus 3 more layers).
Is this the 100 year wall?
Because it will take you 100 years to pay for it?
Since the strapping on your wall is 16" O.C. then are you saying that the 2x6 studs are 16" O.C.?
Yes. I like to see siding supported every 16 inches. But 24 inches will work.
Listen, Jed invited people to submit examples of "perfect walls." If you want to play, it's your turn.
Obviously, I'm not paying for it. Keystrokes on my computer are cheap.
You may not be able to enjoy "keystroking" after you wrap your "perfect" house in foil, which attenuates cell phone and wireless LAN signals. But the good news is the aliens won't be able to read your thoughts.
If cost were no object....
I would suggest Image 2 from this Blog
In addition to the illustration......
I would include Metal Sills and Shingle Lapped Building Paper near all window and door openings.
I would put ALL the insulation on the outside and frame the wall with 2x4 at 16 oc so that the strapping was also at 16 oc ( I share your feelings about support for the siding)
Since there is no cavity insulation...16 OC is not a penalty ..Ignore OVE
Extra wood will only improve the R-value (AND the Thermal Mass INSIDE the thermal boundary).
What the heck...let's throw the project scrap wood into the cavity too(less to landfill).
Now affordable is another matter......
It's possible to order polyiso with a variety of facings. If you don't want foil-faced polyiso, buy it without the foil.
Martin, I don't want polyiso, even with a smiley face on it.
The perfect wall? No detail exists in isolation. Here in central N. Carolina you can't talk about a perfect wall without talking about a perfect roof overhang and a perfect ground detail, for those not infrequent moments when the gutter overflows. Get that right and the WRB is just backup. Get it wrong and the WRB is just wishful thinking.
Robert... you live near to many homes designed by Brownell. There is one just North of me that was just put up. The owner also has PV mounted in his yard. The builder is usually not Brownell, he is mostly the design end. I have toured homes with him. They are very evenly conditioned.... meaning comfortable. He may be getting R from wall cavity air spaces etc... if interested you should meet him, amazing person full of engineering knowledge to the point of easy overload. I don't think anyone on this site could stump him, not even close. And as to experience, decades of designing, building and checking results on dozens of his own designs. And don't think he hasn't researched what others do and say... he has. Last note..... I have not seen him in awhile. I may have to call soon myself to check up on Bruce and
One other tidbit I remember... owners heat these homes with tiny woodstoves... easily. Any larger and they overheat. Bruce originally I think was aiming for no heat system needed... and found that a wee bit tricky... yaa know... codes... long overcast periods... etc.
Just a bit more about Brownell... As most anyone who has read up on lowering one's BTU needs knows.... sealing up air leaks... is a huge part of reducing energy demand when compared to a typical 1970 built home. Brownell does work hard to seal his homes. Very tight numbers posted on his site, and I have seen how much effort the contractors put in fitting sheet foam, buying foam that is more exactly sized even at a higher cost, taping seems, staggering seems... on and on. Then I believe he works hard to wrap insulation and connect it all around all 7 sides of the box we call a home. He has been insulating this way for years longer than the Passive House German invasion to the US. Just some fodder for all to play with.
What is your name... Is it Bill?
The Brownwell link was interesting .. but I did not see any construction photos or details.
Do you have any links to construction photos or details?
Since this thread started off talking about walls.....
How about wall details? wall to window,wall to ceiling, wall to floor
Apparently ADJACK's name is Bill Robinson (he slipped up once and posted using his real name), and he's easily impressed with nonsense.
Brownell's site, called aaepassivesolar.com depicts a house system that's very active solar, with fan-forced 150-ton subslab storage system. He describes this system running on a "small fan" and yet claims 3 ACH of internal air movement.
He claims his R-38 walls are 7 to 9 times as insulated as "typical new homes", which would put those homes at R-4.2 to R-5.4.
And he claims 45%-50% indoor RH with no apparent fresh air exchange, just recirculation and filtering of interior air. In the northeast climate, 50% RH would result in window condensation.
I wonder what other miracles this designer incorporates into these homes?
John... I think I will contact the Brownells next week and see if they desire to contribute here themselves. Quoting his site... and questioning the facts... just simply call them Robert, John.
As to me... Robert.... How many times do I have to let you know how the two names came to be. I had signed up twice over the years with two different computers... caused me issues... didn't think it would make you lose sleep or need to chase my posts around noting my name. What gives? We both like building.. I enjoy info discovery per websites and discussions and have enjoyed letting others know that I post from the Adirondacks... hence the name.. a fun name yaa know.. fun... like "fun" Robert... we can meet anytime... I am just human my friend, not worth calling out for the rest of our lives. Some of my posts may be of interest to some... and I am sure yours are to some. Can't you go with that Robert? I can. No biggy really. Life is short... The sand box is small... Lets all just get along and leave the sand kicking to others?
Brownell... Contact time... Much smarter to me than chopping him to shreds here with out his chiming in.
I like Brownell homes... that I have seen. but... I am not a fan of foam if cellulose or Spyder fit the bill. Just like you actually Robert. We have way more in common than not.
adk (adirondacks area) jac.. is part of my actual name... and yes.. Robert can post for anyone my other nickname... and yes... anyone can hunt anyone down on the internet these days... we all should thank nature for one thing... death... one hell of a way to be done. Until then... smiles everyone... really...life is short enough?
Wall details... OK.. As has been pointed out loud and clear... professional services need to be paid for right? A paid member here has access to wall details. I suggest John you contact Brownells via their website info and see about what details you may obtain and what professional fees may be involved. I just know of them and am not able to speak for them.
I do have another builder/designer that builds very similarly and know that he is available for profesional services to do with timberframing and insulating with sheet foam also. Contact me for his info.
Enjoy your day men... and may all your wall assemblies stay dry.
I think you took my question the wrong way.
I would like to call everyone by their first name
And I am interested in your comments and the builder that you suggested.
Read my post again ...please.
I wish everyone would at least sign off with their first name .. if for some reason it is not the same as your screen name.
John Brooks (Dallas,Texas) Hot/Mixed Humid
John... I love yaa for that post... I have a zillion nicknames... Bill, aj.. and too many more... adkjac... will... hey you... the list is endless.... glad to be a friend... see you in New Orleans sometime.
Robert... First and foremost... you are da man... bar none... crazy bigtime all out green. Best thing going for a site ... to do with green.
So.,... no editing... oh well... Robert green jedi master:
I am a firm believer that many people can be part of this site for discussion. There is no need to email me and let me know what you think of my ability to stay on topic.. form a sentence... or whatever that isn't on topic. That and you harping about people's names is third grade stuff don't you agree too about this??? ... you are sharing a wealth of info when you post info. You are doing nothing for yourself or others when you go off topic and after the poster. Hit the ignore button with me if you are able to find it on your puter. If not... walk away for a day... tend to the garden... play with the pup?
Now.. as to Brownell's claims.. I am cutting and pasting you own post from another sister site... where you state that 50% humidity is not an issue for window condensation... am also doing the same for what you said about Brownell above in this thread.... I looked up charts online too.... and low e windows seem to be a good choice for stopping condensation here in Northern NY... Brownell's area.
"And he claims 45%-50% indoor RH with no apparent fresh air exchange, just recirculation and filtering of interior air. In the northeast climate, 50% RH would result in window condensation"
Riversong from Fine Homebuilding "by Riversong in reply to Anonymous [original] on Wed, 12/24/2008 - 03:45
"Normal comfortable humidity levels in the high 30's will show moisture on the glass when there's no air movement. No?
No. A double-glazed lowE argon window should not condense at 0Â° outside until the indoor humidity is about 60%. The inside surface temperature of the glazing should be 55Â°, which would require 70Â° air at 60% RH for condensation to occur, let alone frost.
You MIGHT have a humidity problem (you have to measure it to be sure), but 70Â° indoor air at 25% RH will condense on a window that's cold enough to freeze.
The real problem you have is either
1. thick curtains or shades or shutters that effectively seal the windows from the indoor air, or more likely
2. significant air leakage around the window that's allowing the inside surface to drop below freezing.
There's no way a new efficient window should ever freeze on the inside.
Check weatherstripping and closing hardware, and make sure the rough openings were properly air-sealed."
Design * * Build * * Renovate * * Consult
Solar & Super-Insulated Healthy Homes
Edited 12/23/2008 9:53 pm ET by Riversong
Edited 12/23/2008 9:54 pm ET by Riversong
Edited 12/23/2008 10:05 pm ET by Riversong
Design * * Build * * Renovate * * Consult
Solar & Super-Insulated Healthy Homes
OK... reading this.... I wonder if you are having issues with Brownell due to competitive issues.... (you both seem to teach, advise, design, build homes...)
We are all human... I know this post will not make your day... I truly apologize. Here's the deal... I met Bruce a few times over the years and he is one hell of a man... And he has or had been giving his whole life to building homes that use less E... since we went thru the gas rationing days back in the 70s I believe.
I would think you would at least be respectful of Bruce especially since you have never met him or contacted him. Your post above contradicted your own thoughts on moisture Robert... just so you could attack Adirondack Alternative... IMHO. The not so informative side of you IMHO.
You who works these topics with lots of facts... should be ready to deal with your own mistakes... and should be much more a gentleman to those that are not you. Including those that post anonymous or as Santa Claus... If someone posts as "fill in the blank" then answer back to "fill in the blank" or ignore and go take one of your weekly baths to calm down.
Remember... I like you... and yes I know.. you may just not like me... I can live with that till we can settle this over a game of choice... ping pong... or sking or vbvall are my choices... or just share a few brews from you local ... Otter Creek Brewery
back to facts... living.. and fun till we die...and even then.. there just might be ski hills in heaven and jacuzzis in hell! life after and beyond green... as it should be someday.
ADKJAC UPSTATE NY, who thinks he's GOD:
I won't dignify that post with a response.
God... me... I was kidding about you actually... the issue is your behavior Robert. Rise above yourself if you can... post nice to all people.
And admit your mistakes. As in posted above.
You shoulld apologize to the Brownells too.
What were you saying about staying on topic?
This is not a forum to discuss personalities. I sent you a private email to take that off-line, but you would rather abuse this forum.
Robert.... on topic.... Brownells... 50% RH works according to you once and one time it does not. Please clarify.
And the forum will stay on topic the less you worry about someone's name and the less you post like you did above to do with doubting the facts posted at Brownell's site with no regard for your own thoughts previously posted. When you disagree with someone be a gentleman. And then we all will stay on topic and the forum will not be abused.
Follow your own advise Robert.
So... RH confusion... please on topic clarify.
On topic... called Brownells... Bruce is still at it... but is out of town this week.
Mentioned this site... windows... etc... got some laughs. They stand behind their homes and the owners are happy campers.
If you like foam sheet goods for insulation, these people have done more than anyone I know and are successful at it for decades. Decades! I think they know how to deal with moisture and indoor air by now.
Robert... any chance you will acknowledge that Bruce may be worthy? You really should meet him. You could talk for years.
Since you insist on "poking" this rather than let it fade into the oblivion it deserves, I'l respond (just to put it to bed, not because it deserves a response).
There is no inconsistency between my statements on this forum and the ones you quoted out of context on another that I long ago left because of irresponsible and abusive participants and moderators (a situation I was glad to discover was not - until recently - present on this site).
I made no judgement on Bruce Brownell or his work, only on the website that you linked to, which was more hype than substance and contained some apparent BS (not Building Science). So I will neither retract those comments nor apologize for them.
Similarly, I don't know either you or your work, only what you choose to publish on this forum. And it is your words which I respond to - also commonly more hyperbole than substance, more ellipsoid than square, far more meandering than straight to the point, and occasionally inappropriate.
Since you seem to have made this your home away from home, you might consider some self control and limiting your responses to subjects that you actually have expertise in.
Robert... you are a zen green master for sure. I will let this go since you somehow feel your posts were different. Agree to disagree with yaa.
Bruce is a wonderful person.
Try to meet him someday.
In your wall description -
- which/where is the air barrier? Is using the Airtight Drywall Approach necessary here?
In a wall assembly built like the one I described, the air barrier could be a the plywood sheathing -- if you want to do it this way, tape the seams of the plywood with high quality tape -- or it could be at the drywall, using the Airtight Drywall Approach.
Could the "hygric redistribution" layer (wrinkle wrap) be the air barrier as well?
There is nothing "simple" about detailing outsulation.
I can just about get on the outsulation train until I attempt to draw out all the details and or study the details by others.
I think that all the details that I have seen so far are awkward at best.
I am attracted to the lure of "the perfect wall" (the Darkside) but I keep returning to simple is better.
and.... Outie windows ;-)...