A hundred years ago, moisture was not the issue it is today. At that time, buildings had stone-and-brick-bearing walls wide enough to carry 3.5 pounds of moisture per square foot before the surface moisture reached dew point, resulting in mold and bacterial growth.
That is actually seven times the moisture-holding capacity of today’s brick and insulated metal stud walls, which can only carry 0.5 pounds per square foot before trouble ensues.
And along with them, today’s moisture issues.
The Why
These days, in the spirit of employee health and productivity, we hear more and more about Interior Environmental Quality (IEQ) and about how Microbiological Volatile Organic Compounds (MVOCs) threaten the health of building occupants.
This, more often than not, comes down to moisture.
In the mad rush to build more and more cost effectively, it is easy to overlook the fact that ultimately the success or failure of a project—if it is meant to be occupied by humans—rests on its IEQ, which encompasses indoor air quality (IAQ). And healthy, comfortable employees are invariably happier and more productive than those who are unhealthy and uncomfortable.
Unfortunately, designers and builders tend to focus on the initial cost of a project rather than on planning and building toward the value of increased user productivity and health.
Moisture is a major contributor to mold growth, unhealthy buildings and poor indoor air quality, and if high levels of moisture persist on or inside a wall or roof assembly, these will eventually lead to growth of microorganisms such as mold and bacteria, not to mention potential insect infestation. The metabolism of mold and bacteria will in turn create the MVOCs that are responsible for the musty smells in damp buildings and for the spores and cellular components that adversely affect health. MVOCs can also generate toxins that can cause health problems.
In fact, internal moisture degradation is a leading cause of premature failure of building envelopes, as persistent moisture can lead to rot, corrosion and other forms of deterioration, and can also dissolve water-soluble structures, such as gypsum drywall and mortar in masonry construction.
The How
Architect Levi Patterson of the DLR Group in Oregon confirmed that today, designers are placing a lot of emphasis on moisture management: “The best practice we follow here in Portland, and that I think is followed pretty much in all temperate climate, is the rain screen principle.”
The Rain Screen. The rain screen approach, which incorporates cladding, air cavity, drainage plane and airtight support wall, offers multiple moisture-shedding pathways.
The principle of the rain screen is to separate the plane in a wall that sheds rainwater from the plane where air infiltration is stopped. In terms of construction, this means that you’ll have an outer plane that sheds rainwater but lets air circulate freely, and an inner plane that is relatively airtight.
In terms of pressures, there is little or no pressure differential across the outer plane, and so there is no driving force to move water through it. There is, however, a significant pressure differential across the inner plane, but since there is no water present on the inner plane, none will penetrate it despite the presence of a driving pressure difference.
“There’s been some debate,” continues Patterson, “over whether you should seal your building completely, or let them breathe. We seal our buildings. That, however, has as much to do with our energy performance goals as with moisture management.
“The point is, though, that once you have it sealed up, you had better have it sealed up by proper deployment of air and vapor barriers.”
Air Barriers versus Vapor Barriers. There is a big difference between air transported moisture and vapor diffusion.
Proponents of vapor barrier systems tend to confuse these two transport mechanisms. Do they control the vapor diffusion, air transport, or both? It is not always clear. What is clear is that air transport is far more significant than vapor diffusion, accounting for approximately 90 percent of the moisture potential to penetrate the envelope.
The key here is that although air barriers—controlling the air transport of moisture—are a good idea everywhere, vapor barriers—controlling diffusion—are not.
Vapor diffusion is governed by the second law of thermodynamics. Moisture will flow by diffusion according to concentration gradient—from more to less—as well as by temperature gradient—from warm to cold. This means that vapor tends to diffuse from the inside out in the North and from the outside in the South. In the middle of the country, part of the year it goes from inside out and part of the year it goes from outside in.
It is easy to say, “Let’s put a vapor barrier on the inside up North and on the outside down South.” It is much harder to define “north” and “south.”
Walter Scarborough, a Dallas based architect consultant, is all too familiar with this issue. “Placement of vapor barriers is crucial. The location of the air barrier is not as critical.
“The vapor barrier only catches diffusion, which only comprises about 10 percent of the moisture that potentially enters the building. From research I’ve done over the years, I have concluded that you don’t need vapor barriers at all unless the project is in the North, and by that I mean Zone 6 and up (Northern Continental United States and Alaska). From Nebraska down, you just don’t need vapor barriers.
“It is also important to realize that you can never really make a mistake with an air barrier, but you sure can make one with a vapor barrier. If it’s in the wrong location, or if it’s used where it’s not needed, the vapor barrier can cause many more problems than the air barrier can even begin to match, because the water will condense on the wrong side of the vapor barrier, and wreak havoc.
“The rule of thumb says from hot to cold, and people have used that rule of thumb for years, but the problem is that when you get into the Southern states and the Southern climate, you don’t need a vapor barrier at all. What you need is the air barrier.”
Science versus Rule of Thumb. If the climate is such that a vapor barrier is called for (Climate Zone 6 and up—Zones 7 and 8 are exclusive to Alaska), you no longer have to live by the hit and miss of rule of thumb. In fact, today you had better perform a computer simulation to determine the correct location of such a barrier.
Enter WUFI (Wärme und Feuchte Instationär – Transient Heat and Moisture).
Jointly developed by Fraunhofer Institute for Building Physics in Holzkirchen, Germany, and the Oak Ridge National Laboratory (ORNL) in Oak Ridge, Tenn., the WUFI-ORNL/IBP hygrothermal model is a personal computer program for the hygrothermal (heat and moisture) analysis of building envelope constructions, specifically tailored to the needs of architects and building envelope designers.
Levi Patterson is familiar with it: “The DLR Group has signed on to the 2030 challenge—it’s a commitment to design carbon-neutral buildings by 2030—and as part of that, we are now learning WUFI.
“We will use it to simulate and measure moisture performance in the various regions where we have offices, since each region needs to establish what design—including placement of any vapor barriers—is best for them.
“How much will WUFI help? Well, it’s an improvement over rule of thumb, that’s for sure, but the accuracy of any program output is of course based on the data you put into it. Still, we recognize WUFI as a good tool to help us design better assemblies.”
Jim Stump, engineer at Criterium Engineers in Maine, does not use WUFI day-to-day, but, he says, “we will sometimes use information generated by others who use the program. WUFI is interesting, but it looks at systems in a broad sense, by zones, which are quite broad. You then have to take that data and apply it to the particular site and building you’re working on.”
It does appears, however, that WUFI is well on its way to becoming the standard in construction hygrothermal analyses with more architectural firms, as well as standards bodies like the National Multi-Housing Council, now using WUFI.
EIFS. As a side note, WUFI is the tool that the ORNL has used over the last few years in its “Exterior Wall Cladding Performance Study” to evaluate EIFS moisture-prevention efficacy across all eight climate zones. All simulations have now been completed and the final report, which from previews proves that EIFS has put any and all moisture issues behind it for good, is due out by the end of the summer.
ORNL has already completed its study of EIFS Zone 3 performance, and you can access the good news on EIMA’s Web site at www.eima.com.
The Who
It is clear that moisture management has become a critical issue in current building design and construction, but who calls the shots in this regard?
Short answer: The architect.
We will let the architects give us the long answer.
Patterson says, “We are an integrated design firm and we have mechanical, electrical, plumbing and structural engineers on our staff, and as part of the design team. But it falls on the architect to drive this, in close consultation with engineers.”
Scarborough concurs: “It’s up to the engineers to run the tests and simulations—and they will obviously influence the decision, but since any barrier deployed is an architectural component that has to be spec’ed out by the architect, he or she will have the final word.
“That said, it really ought to be a consensus decision between architect and engineer. Teamwork is crucial in this area.”
Contractor Robert Aird of Robert A. Aird, Inc. in Maryland echoes that sentiment: “When the architect is the captain bringing engineers on board, all moisture-management shots are called by that architect. But oftentimes today, the architect and engineers are hired separately by the owner, and then we can only hope that they are talking to each other.”
Stump defends the engineering community: “There is an architect of record and there is an engineer of record. The architect of record for a project would be the one dealing with water management, and would have the final responsibility for those details.
“It’s a legal issue as much as anything else. The architect of record has legal responsibility for those drawings and for the design. We, as engineers, will develop details on occasion that we stamp, which then become our responsibility. But in regard to new buildings and developing the water management details for those buildings, we really just provide input to our client.”
Final Word
Today, moisture management has become a key component in all wall assembly designs, and simulation and analysis technology is keeping pace with this importance. Not that every architect or engineer in the land is using WUFI now to determine optimum barrier placements, but the tools are there and the need for them is increasingly being recognized.
As a result, drawings and specs should grow increasingly accurate, resulting in buildings that not only will not sustain water damage over the long term, but that will also provide a great indoor environment where anyone would enjoy working.
Coeur d’Alene, Idaho–based Ulf Wolf writes for the construction industry as Words & Images.
