Diagnosing Moisture Problems in Wall Assemblies

Building Science Digests

BSD-108: Investigating and Diagnosing Moisture Problems

By Joseph Lstiburek
Created: 2006/10/27

Introduction

Water is supposed to be easy to understand.  It only comes in four forms or states [1]. And the rules governing water movement are straightforward:

  1. Water runs downhill due to gravity.
  2. Air carrying water vapor goes from areas of higher air pressure to areas of lower air pressure.
  3. Water in the vapor form diffuses from warm to cold driven by the thermal gradient.
  4. Water in the vapor form diffuses from more to less driven by the concentration gradient.

    Ok, not always straightforward:

  5. Water in a porous material diffuses on pore surfaces from more to less along the concentration gradient in the form of adsorbed water. When there is a lot of it and it fills the pores it is sometimes referred to as capillary water. In this way it can move upwards against the force of gravity – or sideways long distances. Just remember that porous materials “suck” and you’ll be ok.

Water always changes its behavior, because its form is never constant. Evaporation, condensation, capillary suction, gravitational flow, vapor diffusion and mass flow of moist air are all happening at the same time inside building cavities and inside materials.

Water plays clever tricks on us by changing forms and methods of movement along its flow path.  It’s devious and treacherous and laughs at us simple-minded humans whenever it can get away with it. We have to fight it by knowing its tricks. We’ll begin with some simple stuff – diagnosing and finding rain leaks. Then we’ll get into some clever tricks water can play on us.

Rain, Rain Go Away…

Rain falls out of the sky, mostly straight down. Sometimes the wind smacks it against the walls. Diagnosing rainwater leaks is not that complicated. If things get wet after a rain, it’s probably the rain. Start at the wet spot and work backwards and upwards. This gravity thing is pretty predictable. Now, there is a catch – water likes to stick to things and it can run horizontally quite a long ways because nothing is ever completely flat. And that pesky wind can blow it uphill a ways – sometimes over things that are an inch or two high – like sills, flashings and ledges.

The best time to diagnose rainwater leaks is when it is raining. Duh. And unless you are superman and can see into walls, you should be prepared to make lots of holes to see inside of assemblies. Now, it doesn’t always rain when you are asked to look. So you can make your own rain when you need to. A garden hose works real well. Consultants get embarrassed when they charge lots of money while using a garden hose so they use a spray rack instead to make it seem more “technical” and “scientific.” But a leak is a leak whether the water comes from a hose or a calibrated spray rack spraying calibrated water at a calibrated air pressure.  In fairness to consultants, standardized tests can come in real handy once you know the flow path and you want to know if the leaking window you found meets industry standards. Just remember to be “gentle” when you use a hose – you don’t want the momentum of the water spray forcing water into the building. The secret is misting the surface and letting gravity do the dirty deed. So fire hoses are out.

Sometimes a building only leaks rainwater when it is windy. And there isn’t always wind around when you need it – like when you are on-site looking for the leak. So make your own wind.  Instead of blowing against the building from the outside, suck air out of the building from the inside to simulate wind driven rain. Turn off the supply air, and crank up the exhaust. You should probably only do this with adult supervision, but you get the picture.

Most of the time rainwater leaks are straightforward to diagnose, but sometimes finding moisture due to rainfall is not such an obvious process.

Clever Water Tricks I – The Old Rain on the Brick Sun Trick

Up to now, the story has been pretty simple. Let’s complicate it by splashing rainwater on a brick veneer. Brick is a sponge. Brick wicks water into itself because it is porous. The mortar in the joints between bricks is also a sponge.  Mortar wicks water into itself because it is porous. There are also cracks between the brick and the mortar. These cracks are also “pores” and they also wick water. Remember, porous materials suck. Think of a brick veneer as a moisture reservoir that is filled during a rainstorm. So now we have this wet sponge on the outside of your building. The sun comes out. The sun beats down on the wet brick on the southwest side and makes the water in the brick hot. How hot? Probably 120 degrees F? Let’s go to the psychometric chart. Find where 120 degrees F crosses the saturation curve (100 percent RH). Hey, we’re off the chart.  We have to go to the steam tables. Wow. Any guesses what direction the water in the brick wants to go? Did I mention the building this brick veneer is enclosing is air-conditioned?

The brick is wetter than the rest of the wall and wetter than the inside. The brick is hotter than the rest of the wall and hotter than the inside. The water in the brick is driven inwards out of the brick into the airspace where it turns into a vapor. Some of the water also goes to the outside, because the brick is also wetter and hotter than the outside. But let’s go back to the airspace behind the brick. It’s not likely that the airspace is free from mortar droppings and vented at the bottom and at the top so that ventilation air can flush the water vapor driven into the airspace out of the brick to the outside. Even if the cavity is clean it’s rarely vented.  It may be drained, but it is seldom vented – at least not by design – although it should always be [2]. It may be vented accidentally – and this accidental venting saves a lot of buildings.  It seems that we are more often lucky rather than smart.

So inwards goes the water vapor, traveling along the temperature gradient and along the concentration gradient.  How far it travels depends on what’s in its way.  If it runs into something impermeable like foam sheathing or a rubber membrane on the backside of the cavity behind the brick veneer, the vapor won’t travel far.  If it hits foam sheathing or a rubber membrane the vapor condenses, turns back into a liquid and runs down the back side of the airspace, hopefully to a flashing where it is directed out of the wall to the outside.

But what happens if it runs into a building paper or a housewrap that breathes? The heat driven vapor blows through it like a hot knife through butter. What is the building paper or housewrap installed over? Probably a gypsum sheathing – highly permeable to vapor. So vapor diffuses right through it. Next comes fiberglass cavity insulation, which can’t stop the vapor – it’s permeable too. The vapor goes all the way in until it hits the plastic vapor barrier. Not a good idea to put a plastic vapor barrier on the inside of a brick veneer wall that sees rain and sun.  The vapor condenses on the plastic vapor barrier and runs down the wall to sit in the bottom plate track (Photograph 1). Now we have a full range of problems to choose from: corrosion, mold, odors or staining. You can get the same effect by installing a vinyl wall covering rather than the plastic vapor barrier (Figure 1). Just ask the hotel industry about this practice (Photograph 2).

photo_01: interior poly

Photograph 1: Interior Polyethylene Vapor Barrier – Condensation from inwardly driven moisture

figure_01: inward moisture drying

Figure 1: Inward moisture drive due to solar radiation

photo_02: vinyl wallcovering

Photograph 2: Vinyl Wallcovering – Mold due to inwardly driven moisture trapped by the vapor impermeability of the vinyl wallcovering
In this example the water started out as a liquid governed by gravity (the rain on the brick veneer) and was pulled into the porous brick veneer by capillarity. It was driven from the brick veneer and converted into a vapor in the airspace behind the brick by the energy added by the incident solar radiation [3]. Once in the cavity it traveled along the concentration gradient and thermal gradient through the wall assembly materials by the process of vapor diffusion until it condensed back into a liquid at the interior of the exterior wall assembly. Once in the liquid form again, it ran down the wall to the bottom plate. Two rules can prevent this common problem sequence:

Rule Number One: Never install a vapor barrier on the inside of a wall assembly, which has a moisture reservoir cladding and a vapor permeable combination of sheathing and building paper.

Rule Number Two: Always vent claddings, especially reservoir claddings. Remember, that in order to vent the cladding you need an air gap behind the material along with an air inlet, an air outlet and a clear path connecting the two.

This is important to your homes health!

When Sunshine Drives Moisture Into Walls

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When Sunshine Drives Moisture 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

Posted on Jul 2 by Martin Holladay, GBA Advisor

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Forgetting the air space behind the bricks sure didn’t help. After only 10 weeks of occupancy, some new homes built by Zaring of Cincinnati were so wet that most of the brick veneer, sheathing, insulation, and drywall had to be removed and demolished. A portion of the defective walls were sheathed with Celotex fiberboard, which is so vapor-permeable that moisture held in the brick veneer was easily driven into the wall cavity when sun shone on the bricks.

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;
  • 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 multi-family 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 Staaten 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 Homosote or Celotex. 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.”