I am looking at different wall sections for my new house. (Southern Ontario Canada)
What do people think of this idea that was posted on a polyurethane insulation web page?
“The best insulated brick house you can buy is one where the brick is laid a half inch away from the wood studs with no sheathing,” xxxx said. “You then spray polyurethane two inches thick onto the back side of the brick. The reason that I specify two inches rather than the inch and a quarter is that I want a little more grip on the brick and studs. Polyurethane sprayed in that manner ties the brick to the studs better than any brick ties you can nail to the studs.
“So, the wall literally becomes one piece,” xxxx concluded. “The brick — for the first time, because of the polyurethane — actually becomes its own total weather service. Now that’s a big benefit!”
Was wandering what would happen to any water that got thru the brick?
Thanks
Replies
System would fail miserably.
The perm rate of PUF on masonry blocks would be in excess of 30 ng/Pa.s.m2.
Rot out the studs in no time.
Gabe
Gabe,
I certainly don't claim to have expertize in the area, but do want to understand. If polyurethane sprayed foam is closed cell and impervious to water vapor, how would the water get from the bricks (which would stay wet much longer this way) to the studs? Or is the water vapor source coming from the inside of the wall/building and being trapped by the sprayed in polyurethane?
Stan
Smijal
Several factors come into play with this application.
Adhesion between the masonry and PUR is compromised when the water wicks through the masonry brick.
Masonry or clay bricks need the air space to dry out properly. In a winter climate, you don't want your bricks to be wet in minus 30 weather.
In order to get a true air barrier with PUR foam you need about 3 inches of thickness or you apply paint over it.
New wood already contains moisture and sealing it on all sides may cause it to detoriate prematuraly.
These are a few of the initial reasons for my comments.
We spray foam just about every day here at site and while I'm a believer in the benefits of foam, I also recognise the limitation of any product.
Gabe
Mr. Martel's comments are apt and to the point. The success of rainscreen walls (walls with a draining airspace behind the exterior wall finish) is well proven in southern Ontario- both through scientific studies (NRC), and in many, many built examples.
Reading the description of this strange wall assembly, I also wonder about the lateral resistance method it proposes. Perhaps diagonal bracing is used on the studs, but it reads almost as if they intend to transfer lateral loads from the studs through the insulation to the brick wythe. Not something I would feel comfortable proposing.
In response to one of the preceding posts, I think a distinction has to be made between two types of clients who wish to educate themselves- at least two types of which I am aware.
I am obliged and willing to explain/clarify all issues that a client raises to ensure that they have a clear understanding of the project they are paying for. If a client has a concern (whether that concern comes from accurate information or not) I would expect them to raise it, and they, in turn, should expect me to address it to their satisfaction.
It is unfair, however, to expect me to provide condensed building code, building science, construction costs, assembly detailing and practical jobsite concerns lectures, all to explain to a a client why their proposal will not work and has to be changed, all in order to have the client return with a revised plan that introduces a whole new set of problems that require changes and subsequent condensed lectures, all in order to have them return with another.....
There is a fine line between the client who wishes to be knowledgeable about his/her project and to participate in its shaping, and the client who wishes to design and construct buildings without exerting the considerable effort and suffering the considerable expense of obtaining an architectural/ construction/ design/ project management education- either by institution, by apprenticeship, or through a life's worth of experience.
I am sure that we have all had our experiences with both.
There's a funny irony in your posting today....I spent the afternoon holding back my temper trying to educate an architect with a 50's attitude and education. He wanted me to install the foam before my roof was sealed. He thought that the foam would be safe in the rain.
I worked on one of the two first buildings in Ottawa to embrace modern building envelope science and practices. The NRC was having difficulty educating architects who confused air barriers with vapour barriers.
Architects are notorious for detailing perfection and then expecting it to be achieved successfully in the field.
My aim has always been to take the achievable and then try to perfect it.
Architects would be well advised to copy this ideal.
Gabe
Gabe wrote:
"My aim has always been to take the achievable and then try to perfect it. Architects would be well advised to copy this ideal."
I totally agree with this point-of-view.T. Jeffery Clarke
Quidvis Recte Factum Quamvis Humile Praeclarum
Somehow Jeff, that ideal has gained credibility with your approval.
Thanks
Gabe
Frugalarch,
While I understand your distinction between these two theoretical types of individuals I do not agree that there are only two broad classes to categorize them into. Agreeably, the person who wants an individualized education and to revise and review on a continual basis is a very unwanted customer. However, being on the consumers end, I have heard so many different versions of how to/how not to, and also read these in building trade magazines over the years, that perhaps it might be of benefit to think of many consumers as having been duped and wanting more information to make better decisions. Usually the contractor is long gone after the the problems arise and the consumer is left with the problem and the cost of rectifying it. Perhaps I am being a little too sensitive and reading into your comments too much, but it seems you would like your clients to ask a question, hear the answer, and nod in simple agreement. I have heard experts opinions in the industry be proven wrong later enough times to be that trusting.
Smijal,
I apologize, for I have obviously not expressed myself clearly.
I did not mean to imply that there are only two types of clients. I meant to note that, in my experience of clients who wish to educate themselves about architecture and construction through the design process and construction of their own building(s), there appear to be two distinct types, that I find the approach of one useful, and that I find the approach of the other counter-productive.
I did not mean to leave the impression that I want clients to blindly accept design professionals' responses to and explanations of their questions and concerns. As written earlier, I feel obliged to answer clients' questions to their satisfaction. If a client is uncomfortable with an issue once it has been explained to the best of my understanding and ability, this discomfort must be addressed before the project can continue. The client must either gain reassurance elsewhere, seek different counsel, or choose to trust my and my colleagues' experience and judgement over their own. Clearly, it would be improper for us to continue to develop a design that, in our professional opinion, contains one or more significant flaws.
This discussion on the answering of clients' questions is, however, somewhat removed from the intended point of my previous post.
Designing buildings is complicated and involved affair. Decisions are typically made for a number of interrelated reasons, and seldom to address a single concern.
As noted earlier, designers juggle code concerns, site planning concerns, ergonomics, costs, construction sequencing, the spatial experience throughout the building, the practical concerns of getting things built, program, etc, all at the same time. Each decision made reverberates to some extent in each of these spheres. Failure of a design decision to adequately address any one concern may lead to a failure of the design as a whole.
As a result, in order to design well one must have a working knowledge of most of the factors that will exert influence over the design, and some skill in the juggling and balancing of these influences in order to produce a desirable end result.
A client without this working knowledge and skill who wishes to maintain control over the design process by providing their own design solution and asking for its direct translation into a set of working drawings places the designer so asked in an impossible position.
Given the client's inexperience, the solution is likely to contain one or more oversights that make it impossible/impractical to develop into a complete solution. Not knowing the motivations behind the proposed solution, the designer can only point out its failings.
This 'pointing out' typically requires an explanation of the proposal's oversights, and subsequent justification of this explanation. These are the 'condensed lectures' I was refering to. Largely, they are simply explanations as to why the client cannot or should not do what they are proposing to do. Armed with this 'new' knowlege, such clients typically leave, adjust their proposals to suit, and return with revisions in hand.
Because design decisions must address several concerns at once, the changes made to address the first set of raised concerns often fail to address yet another set of concerns, concerns that may even have been well satisfied in the previous proposal.
These new oversights are explained, and subsequent justifications of the explanations are offered. Armed with this 'new' knowledge, these clients leave and......
Through this frustrating process the client usually ends up with a mutated design containing little of the original proposal's promise. What it contains, instead, is a lengthy story for each visitor about the chain of events that 'forced' its awkward, poorly resolved and/or poorly performing spaces to end up the way they are.
This is not useful for the client. This is not useful for us. It takes a lot of time, and produces poor results. It is poor business practice. It results in poor architecture.
A large financial and emotional commitment is required to realise a new building. As with all commitments, the larger it is, the harder it becomes to relinquish control over it to another.
Allowing a skilled professional to develop a proposal that addresses the clients' needs for a building should not, however, be seen as a relinquishing of control over the project.
Greater influence over the design can be exerted by the client in determining the project's intentions, and assessing the suitability of design proposals in meeting those intentions.
The impossibility of such a request aside, I maintain that it is not reasonable for a client to expect me to provide a condensed education in architecture, building science and construction practices simply because he/she is unwilling to trust me with the translation of his/her ideas into a built form, or because he/she fancies himself/herself a designer, but is unprepared to exert the considerable effort required to become a competent one.
I honestly believe that such clients should either gain the expertise required to design competently themselves, or seek out a design professional that they are prepared to trust.
Again, I am always willing to explain to a client the reasons why certain design decisions were made, and others were not. This is a productive process that increases the shared understanding of the project.
We encourage as much client input as we can in developing our designs. The correct design solution for each client lies, to a large extent, imbedded within them and the program around which the building is to be built. Good designers use their knowledge, skills and experience to extract this solution from the client and develop it to its full potential. To open up possibilities that were never expected.
It is most gratifying to hear a client say "that is exactly what I had in my head. I know it doesn't look like my drawings and clippings, and it doesn't match my description, but it is exactly what I had in my head."
There is a significant difference between controlling the appearance of the final building, and attempting to control the design process that will produce it.
I recognize that there are poor and incompetent architects, designers, technologists and contractors currently at work. If you do not trust the abilities of the person designing your building, I would recommend that you end the relationship as amicably as possible, and find another that you do trust.
If, as your last sentence seems to suggest, you have seen so much architecture and building science debunked as to be unwilling to trust any design professional, I think the path that you must take is quite clear.
But remember...
"Doctors get to bury their mistakes. Architects can only advise their clients to plant ivy."
Your clairification proves I read you wrong. Thanks for the explaination.
My last remark about seeing many "experts" opinions later debunked was only to serve the agrument again readily accepting everything proposed without taking it (and the proposee) with a grain of salt. This is not to say there isn't trust, just not blind trust. All the years of living have taught me to be a little more cautious, and carefully pick those I work with (when I have a choice) caefully. Despite there being only a few circuses, there tend to be many "clowns" out there.
It sounds as if you are an architecht, my comments were aimed at builders/contractors. From your explainations, it sounds as if being one of your clients would be a rewarding experience.
Stan
>If polyurethane sprayed foam is closed cell and impervious to water vapor
Plus this isn't literally true. I won't absorb water, but it can absorb water _vapor_. Foamers have told me of cases where a humidity difference inside v outside, for example a cold, dry climate, can draw the inside moisture through the foam. And a manager of a refrigerated fleet told me the same thing--they weighed trucks over time to see how much moisture was absorbed by the foam, or pulled through the foam and trapped between it and an impervious material and it was amazing how much water weight they gained. Made a big difference in mileage, he said.
Interesting about Polyurethane foams lack of resistance to water vapor. That clears up a lot of questions. Thanks.
Sounds idiotic.
I can't see how foam would have a long (or maybe not even short) term structural benefit in holding the weight of the brick.
Furthermore, the airspace normally left between the brick and the sheathing is to evacuate water driven past the bricks. If the brick cannot dry out, the clay can hold too much moisture. This will weaken the brick and mortar with our S.O. freeze/thaw cycles.
Good example of why insulation subcontractors should not be hired to design homes, or why roofing contractors should not be retained to do the electrical wiring, or why my materials delivery man is not cooking my dinner....
Piffin,
You may be missing out on a great meal or then again........
Gabe
This is also one that I should put in "peeves:" Homeowners and clients that get goofy information off the web and then try to sell it to the builder. So then I have to convince them (politely of course) that they're full of #&*%!
I guess medical doctors get the same type of thing from patients who get all "knowledgeable" about some new cure-all (and expensive) medication that they insist on having.
Notchman,
Nobody can be an expert in everything. Where exactly do expect homeowners to get information? And why are you so offended when they present the ideas to you? We all know there are hacks in every profession. Shouldn't the consumer try to learn as much as possible about all of the alternatives before building? Shouldn't they also consult with a knowledgable professional (or two) such as yourself to help weed out the good information from bad? The internet is a great tool for gathering information. Good information and bad information. Can you really blame someone for trying to come up with alternatives to the generally poor design/building practices that constitute the majority of what is/has been used in the United States?
Scott, Ideas being presented do not offend me. INSISTENCE by some customers on the use of a material or a method that I can recognize from experience, training or common sense to be inferior, needlessly time consuming, overpriced or just an invitation to future problems simply because one saw it on a website does annoy me. I confess it's not an everyday problem, but it does come up.
The people I've encountered who pull this seem to take information found on TV, magazines, newspapers and the internet as gospel and, to change their thinking or to educate them a little, sometimes takes a lot of time and patience, which I have a lot of, but I do have my limits.
There is a lot of good information on the internet, don't get me wrong, but when a homeowner who has little or no building experience hires me for my experience and my reputation and my integrity, then begins to dictate to me or prod me from a position of being ill-informed, I am, and have every right to be, offended.
Notchman,
Well said. I stand corrected. I guess I was just a little on edge from watching the addition going up across the street and noting all of the things that are going to cause some future homeowner to pull their hair out (not to mention what the current homeowner is going through). I apologize for my tone.
Happy Friday!
Thanks! I, too, have those days and make those observations and end up getting verbally a little edgy.
Before there's too much misinformation about PUR foam I should point out that closed cell PUR foam does not act like a sponge and when properly applied does become an extremelly efficient vapour barrier.
The original posting was about installing foam on the wrong side of the wall system.
Masonry walls leak moisture like a sieve and must be allowed to do so and to dry out as well.
Mess with this fact and the wall will self destruct.
Gabe
newby1:
Most building codes require a vapor barrier between masonry products, brick, stone,stucco,etc. and wood. Also in my area, East Tennessee, you must have weep-holes in brick or stone walls. Both are needed to protect the wood from moisture. I think you would have big problem because the urathane based insulation would act as a spong.
Charlie
My understanding is that it can be more like a trap than a sponge. A friend has a hunk of it in a jar of water for 20 years and it's absorbed nothing. However, supposedly vapor can pass through it under certain conditions and create problems at the junction of the foam and the adjoining material.
I'm just trying to grasp the issue and implications of vapor drive and hope I'm not overstating or misstating. It's a complicated issue that is the source of disagreement amongst people I know who've been working with it for two decades, so I'm not gonna catch all the subtlties in my relatively few years.
Hi Polyurethane Foam Insulation Folks,
The specific question was one I have never heard of or contemplated [spayed foam behind brick]
I am very interested in sprayed in foam insulation for my old house and have done as much reading, research and conversation as I've been able to. I am leaning toward this type of insulation solution for my [stick built] home for a number of reasons, the biggest con being cost.
The point that has been batted around by Gabe, who sounds like a foam installer, and a few others is that foam transmits vapor. My research shows that it is impermeable, providing a wind barrier, and a vapor barrier all in one shot. [forget about Tyvek on the outside and Tu-Tuf or 6 mil poly on the inside]. The reading that I have done on this is not lay stuff, I bought the 'newsletter' for $70 and the subscription is $2,500 per year! This is not to say that high cost makes it real, it is to say that with costs that high, only a professional rag could survive.
click on Energy Design Update http://www.aspenpublishers.com/environment.asp to see the subscription for yourself....
So I officially dispute your statement that foam is porous. I think the actual term is 'closed cell'. As to the brains or stupidity of bonding it to brick, I'd have to ask my mason.
What are the foam guys opinions on pros and cons of polyurethane vs. lcynene?
Lastly, here is a chart from more research from another source: http://www.buildingperformancesolutions.com/Comparison.htm
/P
GENERIC INSULATION COMPARISON CHART
DESIRABLE FEATURE
Fiberglass Batts
Wet Sprayed Cellulose
Urethane Foam
Icynene Foam
Perfect fit every time
X
Air barrier not required
X
X
Won't permit convection
X
X
X
Impermeable to moisture
X
X
That's not what I said!
Kindly re-read my comments and apply a little comprehension.
Gabe
All materials are permeable to greater or lesser degrees. Sheet steel is permeable, but at a rate of vapour transfer so low that it is termed impermeable. Because of this, building scientists get upset over references to 'vapour barriers', and insist they be more properly refered to as 'vapour retarders.'
Polyurethane foam has a very low permeability, low enough to be a very effective vapour retarder if applied correctly, in a suitable thickness. 6 mil polyethylene sheet has a lower permeance than a few inches of foam in laboratory testing. Allowing for the vagaries of polyethylene installation, the differences between the two as retarders is insignificant to the overall assembly performance. Both can be detailed and constructed to function well. There are advantages and disadvantages to each.
I believe that the fleet trucks Cloud Hidden was referring to are insulated with expanded polystyrene- the white beads pressed into boards that line coolers, form wig stand heads, etc. This material is inexpensive and insulates well, but it has a high and widely varying permeability, somewhere on a par with concrete. As such, it requires a separate vapour retarder. Without a retarder, vapour migrates through the board until it hits its dewpoint, condenses, and eventually waterlogs the insulation.
It should be noted that Mr. Martel (who, I would wager, does not install foam for a living- not directly, anyways) did not argue that polyurethane foam had a high permeability. He actually argued the opposite. More importantly, though, his initial observations of the assembly remain valid.
-If one wraps studs in a vapour retarding foam on three sides and paints the drywall on the fourth side, all moisture present in those studs from the start of construction, mudding, etc, cannot escape. This is not desirable.
- Wind driven rain can be expected to penetrate brick veneer. Measures should be taken to ensure the rapid drying of this brick- typically in the form of a draining air cavity behind. Sealing the back face of brick with foam acts to the contrary- it ensures the brick will have difficulty drying. This is not desirable.
The functioning of an entire wall assembly is not a mason's responsibility, but a mason will know good detailing practices. It will be difficult to find a mason prepared to argue that brick should be left wet for extended periods of time, particularly in cold climate conditions.
I appreciate the information on insulation provided by a previous post, but would caution against the focussing of research solely on an insulation component alone- walls should be evaluated as assemblies, examining the interface between components, and their overall performance.
A lot of product literature can be found suggesting, by check marks in boxes, that competitors' products offer no single advantage over the product featured. Often times an examination of the categories selected to receive checkmarks, and those categories conspicuously absent, is more enlightening than the checkmarks themselves.
As a final note, polyurethane foam is fairly dimensionally stable, independent of temperature. How the brick in the proposed assembly is to expand and contract over a long period of time while remaining attached to the foam is unclear.
>I believe that the fleet trucks Cloud Hidden was referring to are insulated with expanded polystyrene
Specifically not true. Polyurethane foam. I know the difference and the ex-fleet manager knows the difference.
And Pablo, "porous" is your word and too strong. But even closed cell foam has microscopic openings. They're too small for drops of water to pass, but not for water vapor under the right conditions (big humidity difference on one side v the other and a small enough thickness of foam). The houses I design and have built require PUF. We have a clue how it behaves under what conditions. The rule of thumb is that 3 inches will be sufficient to avoid vapor drive in most cases, but in cold dry climates, either 4 inches or a breathable exterior coating so that vapor doesn't get trapped and pool.
As for PUF v Icynene, depends on the application. One of my friends is one of the largest foamers in the Southeast. He sprays both and will recommend one or the other based on the density requirements and several other factors. In a frame house, he (and I) would spec icynene typically. For roof systems, poly. For our Monolithic Domes, poly.
Cloud Hidden,
I apologize for having put words in your mouth. The phenomenon you've described runs counter to all of the information I have read or heard about polyurethane, and to my experience with it. It would be much appreciated if you provide further details.
No apologies necessary. I think we're all just trying for an accurate understanding of the issues. It's a complex subject, and one where the experts occassionally disagree, and I ain't one of them! Just an interested observer and user and specifier of the product...
http://www.resintechnology.com/TB-RT2031.html and then find the "vapor drive" reference.
http://www.monolithicdome.com/articles/coatings/ The guy who wrote this is a friend, and if questions come up, I can probably get him to answer. He's been foaming since the 70's.
Or google --urethane foam "vapor drive"--
There are a bunch of references to it, though no single complete explanation. My conclusion is that you have to think about sources of moisture and then decide whether you want a permeable coating or a true barrier next to your PUF.
Cloud Hidden,
I don't know what's worse. The amount of information or misinformation on the internet. The internet has made experts out of every wingnut with a little time on his or her hands.
Perm ratings for one can be easily misinterpreted. As can just about every other test ratings given any product on the market.
Wall systems are tested to specifics as are each of it's components.
The big question remains......will the product stand up to or perform as intended in the real world or in your specific application?
A few months ago I redesigned the insulation of a large building.
The insulation specified was foil backed semi-rigid insulation.
Our building also had over 3000 welded or mechanical connections for the precast and curtain wall assemblies.
To try and seal the insulation and try to have continuety in the VB would have required 16,000 ft of tape and incrediable attention to detail.
This insulation past all of the technical criteria for the project but it would have failed miserably in the field.
Switching to PUR gave us a better seal and insulation value than could ever have been possible with foil and tape over irregular surfaces therefore a much better building envelope.
You can look at product data until you're blue in the face BUT unless you truly understand the value of what you're reading and have the experience to know it's limitations you will NEVER be able to consistently build a good assembly that will perform as intended.
I have a friend that's a cardiologist and that would make me a......
Gabe
As with my other posts in this thread, I'll offer the disclaimer that I am not a PUR expert and don't do more with it personally than use it in domes and grind it to shape (a particularly nasty way to spend a day) and coat it to protect it. So I pass on the info from the four foamers I know, guys using it daily since the 70's and 80's, be/c they have more credibility with me than I do with me. Their consensus seems to be:
1. Polyurethane foam is subject to vapor drive under certain conditions.
2. You either need a barrier to make sure that moisture isn't introduced into the system or you need a coating permeable enough to let any moisture escape, because you especially don't want to trap water within the system.
I think you and I agree on the wonders of polyurethane. And I agree with you that the entire assembly and application must be thought out. A cold storage facility will have different behaviors than a residence than a flat roof assembly on a school. I'm only really familiar with one application, the thin-shelled concrete dome, and on that we use a vapor barrier on the outside and humidity controls on the inside, and it seems to work perfectly well.
I'm keeping kept out of the whole "use it with brick" discussion be/c I don't know squat about that assembly. My only info relates to that bit someone mentioned about it not being able to absorb water. If not used properly, I believe it can and will. At least, that's what I've been told by people who've seen it happen.
Have you seen failures in PUR systems? What caused them? What happened? What was the fix?
Edited 6/9/2002 5:27:54 PM ET by Cloud Hidden
Since the topic of polyurethane foam and vapour diffusion has been raised, I started asking around, talking to detailers, reps, gc's, reading through the technical literature and reports that are available. Thought I should offer up what I had found for the consideration of all.
It appears that in applying puf to a surface, the line of interface between the puf and the substrate provides the greatest contribution to the puf's vapour retarding capabilities. Makes sense if you think about it- the foam cells attach to the wall surface forming what is essentially a thin continuous film of very low permeance- much greater resistance to transmission than that of several inches of the open cell structure.
While the open cell structure does provide resistance to vapour transfer, the single, largest and most important drop in the assembly appears to occur at the interface of substrate and foam.
If applied properly, most puf's should provide enough resistance to vapour transfer through this line of interface to act as a Code recognized vapour retarder.
The most important question is then which side of the foam this line of interface occurs on.
All of my experience with puf has been as Mr. Martel described, with the foam applied to the exterior side of the substrate, locating the line of interface (in southern Ontario) to the warm side of the foam.
This, I suspect, is why I had never heard of the failure of puf that you described. In fact the BASF products that we typically specify have gone out of their way to demonstrate through independent testing authorities that their puf, if applied this way, is an excellent vapour retarder and insulating package, all in one. They have even done core sample testing of their installations from the seventies applied in this mamer to prove that they experience virtually no moisture gain. I would suspect that they have undertaken this aggressive third party testing approach in response to the stories of moisture gain in puf that have been circling around. The product performs exceptionally well, and in cases like that described by Mr. Martel, is almost the ideal product for the job.
If the foam is applied to the interior of the substrate, however, the line of greatest vapour transmission resistance is located to the cold side of the insulation, and the system fails. Vapour migrates through to the dewpoint and condenses. Basic building science principles (I'm not sure what the references to solar pressure and vapour drive in one of the links that you posted were talking about). Just the simple science of vapour moving from areas of high pressure to areas of low pressure, and, if reaching its dewpoint temperature along the way, condensing in the insulation.
There is a long list of agricultural buildings here in southern Ontario that have done exactly that. Spray foam on the inside of metal barn siding, fill the building with hogs over the winter and the water gained in the walls is remarkable. Like water filled sponges just as you described.
All of which is to say that the original assembly that started this thread is an extremely poor one. Its line of interface is at the brick, on the cold side of the insulation. Effectively sealing the back side of the brick to boot. The foam will clog with water over time. It also provides another line of interface at the stud faces. Trapping moisture in the studs while the foam around slowly fills with water. Not a good idea at all in a cold climate location.
Thanks for the discussion and resulting enlightenment.
>Just the simple science of vapour moving from areas of high pressure to areas of low pressure, and, if reaching its dewpoint temperature along the way, condensing in the insulation.
I think this is all anyone meant, and you worded it better than I did. I've fretted over not being as clear as I would have liked on this, and you did a better job than I. I totally agree with you and with Gabe. (Gawd, I hope you each think you agree with the other.) I love what poly does, but closed cell or not, vapor drive is something that can happen under the conditions you mention if the system isn't designed and constructed properly. Thanks, FRUGALARCH.
What's the harm, Frenchy? Well, if it turns cold, the vapor that's condensed in the outer layers of foam can freeze, expand, and split the foam. That can lead to leaks that compromise the building in the way FRUGALARCH detailed. At least that's my understanding. Caveat emptor. As to why a piece of poly in a glass jar of water wouldn't eventually sink, there's none of the pressure imbalance FRUGALARCH discussed, so the conditions to move the vapor and then have it condense inside the foam wouldn't exist.
As to what might happen in your case or the brick case, I won't even venture a thought. That's beyond my comfort zone. I'm really just trying to understand myself how it behaves and under what conditions, so for interpretations and recommendations I'll happily defer to the many people with expertise.
> Sealing the back face of brick with foam acts to the contrary- it
> ensures the brick will have difficulty drying. This is not desirable.
Isn't the main function of the air gap in a veneered frame wall to keep liquid water away from the other components? In the worst case, a brick wall that is sealed on one side will take twice as long to dry as one that is open to the air on both sides. Are we really designing so close to the edge that a factor of two in drying time is going to yield an unacceptable wall?
> As a final note, polyurethane foam is fairly dimensionally stable,
> independent of temperature. How the brick in the proposed assembly
> is to expand and contract over a long period of time while
> remaining attached to the foam is unclear.
How stable is fairly stable? According to the Brick Industry Organization (www.brickinfo.org/pdfs/24g.pdf), the coefficient of thermal expansion for clay masonry is 3.6 microinches per inch per degree F. For a temperature range of -40F to 110F, the dimensional change in an 8 foot brick wall would be less than 1/16". I couldn't find a number for polyurethane foamed in place.
In a rainscreen wall, the majority of water hits the veneer wythe and is shed down the exterior face. Empirical evidence shows that water will penetrate through a brick wythe. The first strategy in dealing with water that has penetrated is simply to get it out of the assembly as soon as possible. Hence an airspace with moisture barrier. Water runs down the inside of the brick face, down the moisture barrier, across a flashing and out through weepers at the bottom course. Water penetrating a brick wythe that is sealed on the back side cannot escape, but remains trapped at the inside face of the brick until it is dispersed by capillary action through the 'closed' assembly to a site of evaporation.
The second strategy is to get dampened brick dry. The air cavity permits air circulation, encouraging evaporation from the interior surface. A wettened surface that has a vapour/ moisture retarder placed over it does not take twice as long to dry as one that is exposed to air movement. As noted before, it has to rely upon capillary movement through the width of the brick to the face where evaporation is taking place. If you wish to observe the difference in drying time between the two, soak two long pieces of masonry (like a sill or precast curb) in water, remove them both to dry, but take one and wrap half of its length up in cling-wrap, covering all sides and the end. Time how long it takes until the surface under the cling-wrap is visibly dry.
The third strategy is to reduce moisture penetration into the brick assembly in the first place. If the brick wythe is sealed, the pressure difference across the brick wythe is based on the interior room pressure and the exterior wind pressure, or, in the case of corners, parapets, etc., on the pressure difference between windward and leeward sides. Rain that coats the surface of the brick is forced by this pressure difference into the wall- sucked through joints and cracks, and drawn by accelerated wicking action.
With an air cavity, air entering the cavity equalizes the pressure across the brick, eliminating pressurized penetration of water into the brick itself. The pressure differences now occur at the face of the moisture barrier on the other side of the airspace, where less free water is present by virtue of the brick veneer. The end result, less water gets into the brick, and water that does get into the brick does not enter the brick as deeply, and will thus dry more readily. This is, of course, of more concern on taller, larger structures where more significant and prolonged wind pressure patterns are likely to occur.
The combined effect of these three principles is remarkable. The NRC has published numerous technical bulletins comparing rainscreen assemblies with monolothic assemblies, and evaluating their performances.
If you would prefer personal experience of these phenomena, take a rainscreen brick wall, caulk up all the weepers and watch what happens.
The concern expressed over differences in dimensional stability between the brick and the foam is less a matter of the actual distance of expansion, and more a matter of the force exerted by this differential movement, the ability of each component to absorb this force (obviously, the brick is of no concern here), and, most importantly, the ability of the plane of adhesion to resist this force.
The temperature range you have given may appear wide at first glance, but it is not unreasonable (here in Canada, anyways) when you consider that an unshaded south facing brick wall will rise in temperature to considerably above the surrounding summer air temperature.
On the other hand, one sixteenth of an inch seems terribly small (not that it really matters, but we should bear in mind that walls expand along their length as well as their height). It is not the distance that matters, however, but the force exerted in expanding (which, with brick, is considerable enough to tear apart assemblies if not accounted for properly) and whether it is greater than the foam's ability to maintain its own integrity, and (again more importantly) its ability to adhere to the brick surface. If this force of expansion exceeds the strength of bond between the foam and the brick, the assembly will, of course, choose the path of least resistance and delaminate. Over years of having the brick dampened, I would expect this bond to continue to deteriorate until delamination was certain to occur.
Is this delamination a large concern? It wouldn't be, except, of course, for the fact that the proposed assembly is relying solely upon this adhesion to tie the bricks back to the rest of the assembly.
If it begins to delaminate, of course, it will also create conditions that are ideal for the wicking and collection of water in the assembly at the point where it is least desired.
Well, those are my thoughts on the matter, anyways. Clarifications, corrections, comments always welcome.
Thanks, Cloud Hidden, for the information you provided.
If foam will absorb moisture, why won't it sooner or later sink? is some moisture absorbsion so bad? I wood gets wet all of the time, in my house for example in the basement there are exposed joists plates etc. they've absorbed moisture for 100 plus years and seem sound. Places where moisture would be "trapped" like in the fiberglass insulation between walls where there was a vapor barrier on the outside and plastic sheeting on the inside are fine also.
I'm not trying to argue, just understand. I'm currently building a home with SIPS and based on what you are saying they will fail if I put masonary on the outside of them. Did I missunderstand you ?
Mr. Martel,
Thanks for adding a closing note of practical clarity to the discussion.
Okham's razor in a nutshell.