Aurutõkkest siiski veel, sest sellest kogu see teema ju algaski:
https://www.greenbuildingadvisor.com/ar ... -air-leaks
I guess I need to make a presentation slide that says in big bold letters: Capillary materials do not exhibit condensation at the dewpoint. Take a can of beer from the fridge on a muggy day. Condensation. Liquid water. Take a block of 2x4 from the same fridge and set it next to the can of beer. Same temperature, same surrounding vapor pressure. No condensation. No liquid water. Something happens to the 2x4 but it isn’t condensation. Sorption happens. Sorption also happens at vapor pressures above and below the dewpoint. Get picky? Below freezing materials are far less sorptive, but they exhibit frosting, not condensation. Fully saturated materials may exhibit condensation (liquid water) but they’re no longer capillary materials. So I’ll repeat: Capillary materials do not exhibit condensation at the dewpoint.
I read your post here in Westford, Mass., where I just heard Bill Rose give a presentation on a variety of building science topics. He held up a mirror and asked the audience, "Can you get condensation on a mirror?" and he answered his own question: "Of course you can get condensation on a mirror."
Then he held up a sponge. "Can you get condensation on a sponge?" And he answered his own question, "Well, yes -- you can get condensation on a sponge -- if you use the sponge to wipe the mirror."
Then he told a story. "On a hot humid day, I take a beer out of the refrigerator. I put the cold beer on the table. Do I get condensation on the beer can? Of course. Now I go back to the refrigerator and I take a chunk of 2x4 that I keep in the fridge and I take that out and put it on the table. Do I get condensation on the 2x4?"
Finally, he said, "So I got a call from someone who said, "I've got a problem, Bill. I'm getting condensation in the inside of my wall cavities." Bill answered, "You've got condensation in your walls? Why are you building your wall out of beer cans?"
He advised the audience that the word "condensation" is often misused to talk about sorption.
"One of the problems in the building industry is that we have a spreading "cult-like" mentality that worships at the "church of polyethylene". This cult views the answer to all moisture problems as the installation of a polyethylene vapor barrier condom on the inside of buildings. This cult is responsible for many more building failures than building successes. It's time that the cult deprogramming started." -Joe Lstiburek, Ph.D., P.Eng.,
Ironically for Canadians, the first technical reference I can find of inward vapor drives trapped by poly causing rotting walls in cold climates, is from the Division of Build Research, dating to the early 1960's. At the University of Waterloo we were measuring and photographing test wall rotting starting in the early 1990s (I attach the photo of a brick veneer wall in Waterloo Canada: brick, Tyvek, fiberglass poly). At about this time the Swedes were also publishing results of rotting walls and mold due to warm weather condensation in their cold climate.
A major change in the last 20 years is the proliferation of air conditioning in cold climates. This is almost a prerequsite for moisture problems to occur due to poly. And AC is common in Minneapolis, Buffalo, Toronto, Ottawa, etc.
https://www.greenbuildingadvisor.com/ar ... into-walls
Because of inward solar vapor drive, vapor diffusion from the outside inward is often more worrisome than vapor diffusion from the inside outward — so you need a good vapor barrier strategy
Builders have worried about wintertime vapor diffusion ever since 1938, when Tyler Stewart Rogers published an influential article on condensation in the Architectural Record. Rogers’ article, “Preventing Condensation in Insulated Structures,” included this advice: “A vapor barrier undoubtedly should be employed on the warm side of any insulation as the first step in minimizing condensation.”
Rogers’ recommendation, which was eventually incorporated into most model building codes, was established dogma for over 40 years. Eventually, though, building scientists discovered that interior vapor barriers were causing more problems than they were solving.
Interior vapor barriers are rarely necessary, since wintertime vapor diffusion rarely leads to problems in walls or ceilings. A different phenomenon — summertime vapor diffusion — turns out to be a far more serious matter.
D
uring the 1990s, summertime vapor diffusion began to wreak havoc with hundreds of North American homes. This epidemic in rotting walls was brought on by two changes in building practice: The first was the widespread adoption of air conditioning, while the second was one unleashed by Rogers himself: the use of interior polyethylene vapor barriers.
Rogers conceived of interior vapor barriers as a defense against the diffusion of water vapor from the interior of a home into cold wall cavities. Rogers failed to foresee that these vapor barriers would eventually be cooled by air conditioning — thereby turning into condensing surfaces that began dripping water into walls during the summer.
One early
victim of this type of diffusion was Cincinnati builder Zaring Homes. In the
mid-1990s, Zaring Homes was a thriving mid-size builder that completed over 1,500 new homes a year. But the company’s expansion plans came to a screeching halt
in 1999 when dozens of its new homes developed mold and extensive rot.
The first signs of the disaster surfaced in July 1999, when homeowners at Zaring’s Parkside development in Mason, Ohio, first began complaining of wet carpets. These moisture problems emerged only ten weeks after the first residents moved in to the new neighborhood. When inspection holes were cut into the drywall, workers discovered 1/4 inch of standing water in the bottom of the stud cavities. “We were able to wring water out of the fiberglass insulation,” said Stephen Vamosi, a consulting architect at Intertech Design in Cincinnati.
Consultants concluded that water vapor was being driven inward from the damp brick veneer through permeable fiberboard wall sheathing (Celotex). During the summer months, when the homes at Parkside were all air conditioned, moisture was condensing on the back of the polyethylene sheeting installed behind the drywall.
“Zaring Homes went out of business because they had a $20 to $50 million liability,” said building scientist Joseph Lstiburek. “Hundreds of homes were potentially involved. To fix the problems would probably cost $60,000 to $70,000 per home. It was a spectacular failure, and they are out of business.” (For more on Listiburek’s view of inward solar vapor drive, see Solar-Driven Moisture in Brick Veneer.)
I
nward solar vapor drive problems require four elements
The phenomenon that destroyed Zaring’s walls came to be known as inward solar vapor drive. The classic disaster requires four elements:
A “reservoir” cladding — that is, siding that can hold significant amounts of water;
Permeable wall sheathing like Celotex or Homosote (that is, fiberboard);
A polyethylene vapor barrier on the interior of the wall; and
An air-conditioned interior.
Problems with inward solar vapor drive show up first on elevations that get the most sun exposure; north walls are usually immune to the problem.
Whenever a wall separates environments at different temperatures and moisture conditions, the direction of the vapor drive is from the hot, moist side toward the cool, dry side. After a soaking rainstorm, the sun eventually comes out to bake the damp siding. When it comes to driving vapor, the sun is a powerful motor.
The heat of the sun easily drives the moisture in damp siding through housewrap and permeable wall sheathing. The first cold surface that the vapor encounters is usually the polyethylene behind the drywall. That’s where the moisture condenses; it runs down the poly and pools at the bottom of the wall cavity. It doesn’t take long before mold begins to grow and the walls begin to rot.
Once the phenomenon of inward solar vapor drive was well understood, it was identified as one of the main mechanisms causing a cluster of wall-rot problems in EIFS-clad homes in North Carolina. Inward solar vapor drive is also blamed for many of the “leaky condo” problems in stucco-clad multifamily buildings in Vancouver, British Columbia.
Data from a 2003-2004 wall-drying study by building scientists John Straube, Eric Burnett, and Randy Van Straaten confirmed the phenomenon of inward solar vapor drive.
“Inward vapor drive resdistributes moisture quite dramatically,” said Straube. “Some people have said, ‘Summer condensation on the interior does not occur.’ But summer condensation does happen, even in Ottawa.”
For decades, builders have worried about vapor diffusion into walls from the indoors during the winter. But if a home has air conditioning, vapor diffusion into walls from the outdoors is a much bigger problem.
According to Straube, “Solar-driven vapor is much more important” than winter diffusion from the interior. He continued, “The moisture is coming from the other side of the assembly.”
After only 10 weeks of occupancy, some of the Zaring homes were so wet that most of the brick veneer, sheathing, insulation, and drywall had to be removed and demolished.
Never include interior polyethylene or vinyl wallpaper in an air-conditioned home. If your building inspector insists on a vapor retarder that comes in a roll, choose a smart retarder like MemBrain.
John Straube: “The whole reason we’re talking about vapor barriers is not because vapor diffusion control is so important, but because people believe it is so important. The question comes up, have we seen diffusion-related building failures? And the answer is, very few — maybe in rooms with a swimming pool. Assuming that the vapor came from the inside, you would have to have a very high load before you would see a problem. I think that solar-driven vapor is much more important. The moisture is coming from the other side of the assembly.”
Everyone wants good performance. No problem there. Prescriptive requirements are intended as shortcuts to good performance, and they facilitate commerce. They should be allowed to remain in effect only if: 1) their subject is critically important, 2) they are necessary, 3) they are sufficient, and 4) if the link between the prescription and the performance outcome is continually policed. In my opinion, all four are open to question. That said, we might imagine a future in which the building code sections that address the vapor barrier would all go blank. I bet most readers would be able to design excellent buildings that perform well and are quite durable, without using the word “vapor barrier” at any point in the process.