Moisture—Fighting a Subtle Intruder
June 2009Water, in large quantities, can be devastating. From Noah’s Ark in biblical times, to the Johnstown (Pa.) Flood in 1889, to the 2004 tsunami in the Indian Ocean, water can be as much of a troublesome element as fire.
But in smaller quantities, water can be good. You drink a couple of glasses of it every day to stay healthy. You wash your car with a bucket full of it, or a load of clothes with a bit more.
In even smaller quantities—when it is condensed and diffused, and commonly called moisture, it can cause as much heartache as a flood or other act of nature, especially in the construction industry.
Moisture can be a villain that everyone faces at some point or other.
In a country such as the United States, with many different climates and weather-zones, fighting moisture intrusion and subsequent damage calls for different products and tactics depending on where in the country the war is being waged.
Part of this article surveys many of AWCI’s contractors to establish prevalent use of tactics and products in various areas, but first, let’s review some basics.
Three Water Flavors
Moisture, which is to say water, comes in three different flavors: solid (ice), liquid (water) and gas (vapor).
Most contractors have a decent handle on the solid variety, and—especially in the South—have little or no trouble keeping it out of buildings. Many also have a fairly good handle on the liquid form. If anything is causing headaches nowadays, it is vapor.
Maria Spinu, Ph.D., of DuPont Building Innovation, has made a brilliant career of studying and combating vapors especially, and has this to share about it—and the dew point temperature—in an announcement for one of her 2005 lectures:
"Water exists on earth in three physical states that can undergo reversible phase transformations. Dew point temperature is the onset of the vapor-to-liquid transformation known as condensation.
"Moisture problems in buildings are the result of liquid water accumulation within the building enclosure. The sources of liquid water within a building enclosure include liquid water intrusion or condensation of excess water vapor transported by air currents or through diffusion.
"Dew point temperature is the temperature at which the concentration of water vapor reaches its saturation and will condense on cold surfaces to form droplets of water. We often see condensation on windows or other cool building surfaces. This so-called surface condensation is not much of a problem. However, when condensation occurs within a building assembly (interstitial condensation) it can lead to moisture problems, which range from building durability and performance, to indoor environmental quality (IEQ).”
The purpose of a building’s envelope is to keep the internal environment separate—and protected—from external conditions. This includes keeping the cold (or the heat) out and the heat (or the cold) in. Another purpose of the envelope is to keep moisture out.
According to the McGraw-Hill Construction’s Continuing Education Center’s course on air barriers (January 2006), "Moisture, when it does enter the building, moves through the envelope as liquid water or as water vapors. The difference between the two physical states of water is the size of the molecular aggregates: liquid water exists as large molecular aggregates (up to 100 molecules at room temperature), while water vapors exist as free molecules. Consequently, the transport mechanisms are different for liquid water and water vapors.”
Liquid Water. The main source of liquid water for above-grade walls is rain, which can find its way behind the exterior cladding and so be driven into the building enclosure by four main forces:
• Gravity, which can draw water down through openings and cracks, and into the construction assembly.
• Capillary forces, which act like a sponge sucking water through small cracks and pores. Smaller cracks result in greater capillary forces.
• Rain droplets can pass through openings in the exterior cladding, driven by the momentum of the falling rain.
• The pressure differential can push or suck water through openings and cracks, into the construction assembly.
Water Vapor. When moisture enters the building as water vapor, it penetrates the envelope either by air currents or by vapor diffusion.
For vapor diffusion to occur there has to be both a driving force and a pathway. In this case, the driving force is the difference in water vapor concentration (or difference in vapor pressure) across an assembly: Water vapors flow from an area of higher concentration (higher vapor pressure) to an area of lower concentration (lower vapor pressure).
However, looking at the practical side of things, experts estimate that the amount of moisture vapor shifted by air currents can be 100 to 200 times higher than the amount shifted by diffusion, and can account for more than 98 percent of all water vapor movement through the building envelope.
The air current rule of thumb: Vapor flows from warm (high pressure) to cold (low pressure).
The Vapor Barrier
Keeping the moisture out (and away from insulation, where it can do major damage) is the purpose of the vapor barrier. The main design decision is where, exactly, to place it, especially since vapor, reaching a vapor barrier and with nowhere else to go, will eventually accumulate, reach a dew point and turn into liquid water.
Heating or Cooling Climate. In a heating climate—where a building is heated more days of the year than cooled—the vapor (by the warm-to-cold principle) will prevalently travel toward the outside. In the cooling climate the opposite it true: Vapor will predominantly travel from outside the envelope toward the inside of the building.
Since barrier membranes are usually placed adjacent to wall insulation, the issue of where, exactly, you place it is determined by the prevalent vapor direction. If you happen to place it incorrectly—i.e., at the far side of insulation—as the vapor travels, condensation is likely to occur inside the insulation and degrade it considerably over time. Fiberglass can lose as much as 70 percent of its insulating properties when wet.
It is therefore crucial to place the barrier at the near side of insulation—as the vapor travels—so that vapor hits the barrier before entering insulation.
In a heating climate, that means placing the barrier between the inside of the building and the insulation; in a cooling climate, between the outside of the building and the insulation.
Who Determines Placement?
When it comes to determining not only the risk for moisture intrusion, but the products—and their precise placement—to guard against it, the architect calls the shots.
As Bill McPherson of Central Ceilings in Massachusetts succinctly put it: "We don’t devise, or recommend, solutions. We implement them.”
This sentiment is echoed throughout the country, where it is always up to the designer or the architect to detect and solve potential moisture issues.
But there is one interesting caveat: According to Pat Arrington of Commercial Enterprises in New Mexico, the contractor license in his state lays the ultimate responsibility for any building problems at the contractor’s feet, whether he followed incorrect design advice or not. So, New Mexico contractors, beware.
Gregg Conrad, president of CSW, Inc. in North Carolina, adds to that that if he notices something wrong as far as fighting moisture goes, he would "raise a flag. Even though the designer specifies the system, and we’re only responsible for applying it properly, if there’s an inconsistency in the design we have to make them aware of it.”
An Engineering View
Jim Stump is a Portland, Maine–based engineer with Criterium Engineers, a company of consulting engineers with more than 70 offices in North America. His view on moisture problems/solutions is well worth sharing: "Of course, moisture intrusion is always through the building envelope. How that occurs here in Portland, Maine, however, is different from how it occurs in North Carolina, and certainly different from, say, Phoenix, Ariz.
"The vapor barrier needs to be on the warm side of the insulation. That is the basic criteria. So, in the South, where the warm side of the insulation is usually the outside, and you are attempting to cool the inside, the vapor barrier would be toward the outside of the building.
"In northern climates like here, it’s the opposite. The warm side of the building in the wintertime is the inside; the cool side is on the outside, so the vapor barrier should be toward the inside.
"The difficulty with design comes in climates that are in-between, states like New Mexico or the mid-Atlantic states like Virginia or North Carolina, where you get both.
"The vapor moves from hot to cold, and when it reaches the dew point it will condense, and if that happens to be in the insulation, then you have a problem.
"A critical issue when evaluating a building for moisture solutions is to view the building as an organic whole, and take all aspects into consideration.
"The climate is just one factor. You also have to evaluate airflow, design, specific location—a building on top of a hill will behave quite differently from one down in a valley—the type of heating deployed, the type of cooling used. How much sun does it get? Is it a solar building? All of these things relate to the organic whole that you need to consider.”
Fiberglass Insulation. As mentioned earlier, fiberglass degrades greatly when wet. Why precisely is that?
"Fiberglass insulation,” explains Stump, "relies on air pockets, and when it gets wet it loses those air pockets and, therefore, loses its insulation value.”
As much as 70 or 80 percent?
"It’s certainly possible. And I, unfortunately, see that phenomenon quite often.”
Wood Framing. Stump has this to offer: "Some designers suggest that when it comes to wood-framed walls, you should put a vapor barrier on both the inside and the outside to try to seal the wall.
"My experience is that, while this may look good on paper, in practice—because no barrier is ever 100 percent effective—you’re going to wind up trapping moisture inside that wall.
"You have to consider that the wood frame has quite a bit of moisture in it already, even if it is a kiln-dried piece of lumber. Over time, it will lose some of that moisture, and this has to go somewhere. If sealed in, it will eventually reach dew point and condense inside the wall.”
Taking the Country’s Moisture Fighting Pulse
What products are used where? Keeping in mind that the contractor as a rule does not choose, nor recommend, the moisture fighting weapons he deploys, the question becomes instead: Which products does he normally install (as specified by the architect)?
The brand names you would expect to hear are the names that popped out of the mouths of contractors from all over the country; it is their preferences in product type that vary.
Gabriel Castillo of Pillar Construction in Virginia likes liquid-applied membranes, which become part of the substrate. "When the liquid applied membrane dries, it hardens to a rubber-like, waterproof membrane, so you know that it will cover and seal well. There are no pores, no holes, no way for moisture to penetrate. … You just roll it on. It’s as if you were to apply a very thick paint.”
Stephen Angell, president of Cape Cod Plastering in Rhode Island, uses "a self-healing, peel-and-stick product, for exterior cladding.”
Robert Aird of Robert A. Aird, Inc. in Maryland does of lot of exterior insulation and finish systems and sees the gamut when it comes to brand names, but he cautions about maintaining the integrity of the entire system: "Some [barriers] can be used with other products, but normally they are only tested and approved to work with their own EIF system.
He goes on to say, "In the last 10 years or so, though most actively over the last three or four years, we tape all sheeting joints, we spot the screw heads, we seal all penetrations and connections to other materials, and then apply a liquid-applied barrier over the whole face of the building to create an air- and water-barrier—or a WRB, a weather resistant barrier.”
But in Florida, Eric Boulanger of Boulanger Drywall Corporation does not often see liquid applied membranes being applied.
Gerald Roach of Forks Lath & Plaster in North Dakota mostly sees the big brand names, but adds that "it’s also getting more common to do a sprayed-on or trowelled-on moisture barrier over the sheeting, especially on bigger jobs like the Wal-Marts and motels.”
Glenn Sieber of Easley & Rivers, Inc. in Pennsylvania says, "What we now see more and more of is studs, sheeting, then a spray-on or a trowel-on or a place-and-press membrane for waterproofing—then a rigid insulation.”
Richard Riley of Simpson Commercial Contracting, Inc. in Alabama: "On the exterior wall substrate we like to use a roller applied barrier, because they’re seamless. On the exterior wall—if we’re worried about moisture—we normally use an elastomeric finish.” Riley adds that all the major brands work.
The moral of this story is that if you can smell it, you missed a turn way back there, and you’re now facing damage control, literally … which usually means several pounds of cure.
The ounce of prevention is to understand how moisture travels, and how to channel its movement.
Coeur d’Alene, Idaho–based Ulf Wolf writes for the construction industry as Words & Images.