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.
Something is rotten in Denmark
During 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.
Zaring Homes goes bankrupt
As with many scientific discoveries, it took a series of disasters to fully illuminate the phenomenon of summertime vapor diffusion.
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.)
Inward 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.
Reservoir claddings include brick veneer, stucco, manufactured stone, fiber-cement siding, and (to a lesser extent) wood siding. Although wall failures with permeable sidings like Celotex are particularly spectacular, inward solar vapor drive is also a factor in the failure of walls sheathed with less permeable types of sheathing, especially OSB.
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.
Moisture and temperature probes confirm the phenomenon
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.”
Worry about diffusion from the outside in, not the inside out
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.”
Avoiding problems caused by inward solar vapor drive
If the components of a wall assembly are poorly chosen, as they clearly were at the Parkside development built by Zaring Homes, there may be no faster mechanism for destroying a house than inward solar vapor drive. 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.
But once you understand inward solar vapor drive, it’s relatively easy to choose building details to avoid problems. Here are a variety of ways to reduce risks; of course, you’ll probably only need to adopt one or at most two of the following measures to avoid problems.
- 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.
- Avoid high-permeance sheathings like fiberboard behind reservoir claddings. Instead, specify foam sheathing — especially behind brick veneer, stucco, or manufactured stone.
- Homes with asphalt felt experience fewer problems with inward solar vapor drive than homes with plastic housewrap.
- Consider the use of a water-resistant barrier (WRB) that is impermeable to water vapor. The best-known vapor-impermeable WRB is Delta-Dry. Delta-Dry is made of stiff high-density polyethylene formed into a 5/16-inch-thick egg-carton configuration. The three-dimensional WRB creates two air spaces: one between the siding and the WRB, and the other between the WRB and the sheathing. Unlike high-permeance housewraps, Delta-Dry depends on air movement (ventilation) to dry the gap between the Delta-Dry and the sheathing.
- Walls with a rainscreen gap between the siding and the sheathing experience much less inward moisture transfer than walls without a gap.
- Ventilated rainscreen gaps are more effective at limiting inward moisture transfer than unventilated rainscreen gaps.
- More vapor drive problems occur in homes with dark-colored siding than light-colored siding.
- When specifying stucco, choose a traditional stucco formulation without modern polymeric admixtures, since stuccos with these admixtures dry much more slowly than traditional stucco formulations.
- Choose a siding (like vinyl siding) that is not a moisture reservoir.
I’d like to thank architect Steve Bostwick, one of the consultants who investigated the Zaring Homes disaster, for graciously sharing his photos. I’d also like to thank William Rose, whose historical research has highlighted Tyler Stewart Rogers’ role in establishing the idea that the warm-in-winter side of wall insulation should be protected by a vapor barrier. Rose is a building researcher at the University of Illinois in Urbana-Champaign.
Last week’s blog: “Using Ceiling Fans to Keep Cool Without AC.”