DINÁMICA MIGRATORIA

In a perfect world, one simply plans, designs, constructs and operates the building so that nothing ever gets wet.

No rain leaks in. No humid air leaks in. No plumbing ever leaks or breaks, no groundwater or irrigation spray ever seeps in and nothing indoors is ever cold enough to produce any condensation.

The budget is always big enough to accommodate perfection, and the entire building and its mechanical systems are constructed and operated exactly the way the owner and the designers intended.

For those of us who do not live in quite such a perfect world, a prudent risk reduction strategy has two basic principles:

1. Reduce the water and humidity loads on the building to their minimums, through budgeting and design deci-sions.

2. When parts of the building get wet indoors in spite of everybody’s best efforts, make sure the moisture dries out quickly.

The owner—not the law—makes the key decisions

Most importantly, building occupants need to understand that noth-ing in the law (at least in the USA, at present) prevents owners and designers from constructing a building which grows mold.

This could change in the future. But to date, civil authorities have not yet decided that mold or bacteria present a health or safety risk important enough to change the requirements of building codes.

Literally millions of code-compliant buildings are growing mold and bacteria, every day.¹⁰ And because “millions of buildings” suggests these buildings follow design practices which are widespread and therefore “standard” (and therefore acceptable), the building codes and standards of care which govern professional practice are not currently preventing architects and engineers from designing millions more buildings which have a high risk of growing mold.

So the decisions described below are hard ones for some owners.

At present, both the owner and the designers must decide whether or not to reduce mold risk, on their own, without being compelled to do so by civil authorities. To some owners and designers, the suggestions will be familiar, and may already be standard practice. For others, the suggestions will require some changes from the way things have been done in the past.

Suggestions for owners and Architects

The owner and the architectural designer establish the baseline risk of bugs, mold and rot, because the decisions they make have the great-est and most enduring effects on the amount of water and humidity which get into the building. These suggestions minimize those loads, and therefore they minimize the baseline risk.

Fig. 5.7 Roof overhangs reduce rain contact, and therefore reduce risk As the photo indicates, it does not take a very wide overhang to reduce the rain contact for the life of the building.

The photo also shows how much more rain soaks the wall when there is no overhang.

The volume of rain flowing down the wall surface is the principal risk factor for leaks, and therefore for bugs, mold and rot. Roof overhangs are an economical way to reduce that risk by more than 50%.

Roof overhangs, gutters and wall projections avoid 50% to 75% of the rain load on the exterior walls

Since the owner probably knows what the building should look like, he will influence what the architectural designer chooses to present as design alternatives. If the owners insist on roof overhangs, gutters and short projections above windows and doors, the risk of moisture accumulation indoors is greatly reduced.¹¹

Look at the differences in rain accumulation shown in figure 5.7.

Roof overhangs of about two feet [58cm] can prevent about half of the annual rain load from contacting the building, and therefore from dripping down the wall until it finds an open joint. As one leading building scientist has neatly summarized “If it doesn’t get wet—it can’t leak.”¹²

Reducing the total amount of rain which contacts the exterior walls reduces both the volume and the frequency of any rain leak which will accidentally get into the building through joints around wall penetrations, or through joints where different siding materials meet each other. That’s why roof overhangs, gutters and projections above windows make the building more tolerant of minor imperfections in joint design and installation. They keep rain water off the wall.

Conversely, without these details there will be a larger volume of water flowing down over all the joints, for the life of the building. With more water, every joint design and every small error by the installing contractor make the building more fragile with respect to water intru-sion, humid air infiltration and indoor moisture accumulation.

For an example of long-term durability enhanced by roof over-hangs, consider the Roman temple shown in figure 5.8.

It was built during the first century AD, in Southern France. About 2,000 years later, the sculptural detail under the overhang is still sharp and clear. Two thousand years of durability is a fairly impressive return on investment for the incremental cost of that slightly wider roof.

Install sill pan flashing under windows and doors

One building scientist with over 35 years of experience designing, constructing and investigating buildings has famously said that: “There Fig. 5.8

Evidence that roof overhangs pay long-term dividends

The short roof overhang sheltered the sculptural detail on this Roman Temple for 2,000 years. That slightly wider roof was a bargain-basement investment, considering how well it achieved long-term resistance to rain water leaks and to water-related building problems.

Fig. 5.9 Sill pan flashing under windows After roof overhangs, the best way to reduce the risk of moisture leaking into the building.

are only two kinds of windows in North America: those which leak...

and those which will leak, later.”¹³

That observation may or may not be overdrawn. But support for that opinion is provided by an engineering collaborative which inspects approximately 25,000 new buildings each year. Within that sample, 35% had water intrusion problems caused either by leakage through the windows themselves, or by leakage around those windows as a result of installation shortcomings.¹⁴

These facts suggest that—after the roof overhangs and gutters which reduce the water loads—the most important architectural contribution to reducing the remaining risk of moisture problems is to install sill pan flashing under the windows and doors.

Figure 5.9 shows two examples of sill pans. They rest on the block-ing which supports the window. They catch any rain water which gets through—or around—the window, before that water can get into the rest of the wall. The outer edge of the pan drains any water leakage out of the wall—either outside the cladding, or into a waterproofed drainage cavity between the cladding and the sheathing.

Figure 5.10 is provided courtesy of the building scientist who believes all windows will eventually leak. It shows how sill pan flash-ing can be built of self-adhered flashflash-ing membranes, and how that

type of sill flashing can be properly integrated into the layers of the exterior wall. Integration of flashing with the other layers is the next link in the chain of owners’ and architects’ decisions which keep moisture from collecting in the wall.

Clearly establish responsibility for integrating the flashing around windows, doors and balconies

Another building scientist has suggested that ”Water gets into a build-ing through the cracks between the architect and the contractor.”¹⁵ Around all exterior wall penetrations for windows, doors and balco-nies, there are joints. And along every one of those tens of thousands of feet is a crack—through which rainwater can and will leak into the building, unless there is effective flashing behind the cracks.

Effective flashing is a sheet of metal or other durable material behind the cracks which stops the water from getting further into the building, and which redirects that water back outside the building, and off the wall, entirely.

The real problem with flashing and with integrating windows and doors into wall layers is that “everybody is responsible” for making the flashing work so that it excludes water. As all readers will recognize from their own experience, when “everybody is equally responsible”, nobody is really in charge—so success is unlikely.

Fig. 5.10

Visual explanation of flashing details

& window integration reduces risks When the craftsmen on the job site have diagrams like these, it reduces the probability of leaky corners around and beside windows—the chief cause of moisture intrusion into buildings and therefore the second most common risk factor for bugs, mold and rot after the absence or presence of roof overhangs.

Architects know that all openings and all joints between different materials have cracks, and that all of these must be flashed. The cracks themselves are easy to flash effectively. Single long runs of flashing are very simple to design, and rather easy to install correctly.

But the problems come when the architect and the contractors reach the corners. Excluding water in all of those thousands of corners on a building is a highly complex problem.

Which layer goes over, and which layer goes under? How are the flashing pieces joined and sealed water-tight when they bend, fold and meet at the corners? How does the water get back out of the deep corners under windows when the window leaks? How does water stay out from under sliding doors which open onto balconies? How does water avoid getting into the wall through a mitered outside corner where two pieces of flashing meet, but are not welded? And which trades are responsible for installing the exterior wall framing, the windows, the water barrier and the exterior cladding? Which of these craftspeople is on-site, and when? And who is responsible for making sure that the installation sequence of all these different layers and different components, installed and purchased by different subcon-tractors, goes together in a way that really excludes water—especially in all those corners? Who makes the drawings which describe that sequence and define which trade is responsible for which layer, at what time? Who inspects the result, and how is water-tightness tested and documented, and when, and by whom?

Diagrams like those shown in figure 5.10 don’t happen automati-cally. Owners who are not experienced with construction realities usu-ally assume that these issues will be all be dealt with by the architect, or the contractor, or somebody, somehow. But millions of buildings with water intrusion around and through windows testify to the fact that flashing in the corners is often left to chance.

Architects are usually not eager to provide the details of how each layer integrates with all the others in the corners. Usually, the architectural drawings show wall sections alone, rather than the more informative isometrics of how each layer meets and integrates with

all others in the corners. Such drawings are numerous, complex and costly to produce. So often, the architect just provides wall sections, and leaves the corner integration undefined, assuming that will be the responsibility of the contractor, who is traditionally in charge of the “means and methods” of assembly.

The contractor looks at the wall sections provided by the architect, and then leaves the “details of construction sequencing and trade coordination” of those corners to his site superintendent. The site superintendent tells each crew foreman to “make sure he coordinates with the other trades.”

So in too many cases, successful water exclusion is under the control and guidance of the craftspeople who show up on the job site on each day when the pieces have to go together. Without drawings from the architect or from the general contractor, and without physical mock-up wall sections showing what the designers intended in the corners, the craftspeople—whose native languages are often different than the language on the drawings—are now in charge of designing and installing tens of thousands of water-tight corner joints.

Fixing flashing leaks and repairing the resulting damage in a reli-able way generally requires removing exterior cladding and perhaps windows, with a cost of millions of dollars and months of disruption.

So when such buildings leak water (provided that the leaks are visu-ally apparent), the favorite remedy of the contractor and the architect is the caulking tube, because it is relatively inexpensive and the results are visible, even if seldom effective over time. The owner is left with a fragile building.

Reducing the risk of these problems is difficult, but is most easily accomplished by the owner. The owner can specify in his program requirements which organization—either the architect or the con-tractor (but not both):

”...shall be responsible for the design and 3-D draw-ings which clearly show all layers and their installation sequence for all flashing details at all corners in addition

to all straight joints in the exterior wall. These flashing details and their defined construction sequences shall be effective in excluding water from the building, as measured by the absence of any liquid water or any elevated moisture content of any materials inside the exterior wall. Elevated moisture content shall be defined as any moisture which is sufficient to generate microbial growth, or which reduces the life of the material or assembly in question such that it must be replaced to ensure all of its functions for the useful life of the building, a time period which is defined in the contact documents.”

If the architect or contractor is wise, he will ask for additional money in his budget to accomplish this complex and time-consuming task. He will also ask for extra money to construct a mockup wall sec-tion on the job site itself, showing the physical reality of these details for the benefit and ready reference of the craftspeople.

If the owner wishes to reduce the risk of the most common cause of bugs, mold and rot in buildings, he will provide those additional funds.

Exterior cladding which drains rain and dries quickly

The selection and design of the exterior cladding is another risk-laden decision which is determined by the owner’s look-and-feel decisions, and his budget.

The cladding is the surface which first receives the rain. So first, it should shed most of that rain. Then, when some moisture gets behind it, the back side of the cladding should be an open air space, so that the water will run down the back side of the cladding instead of contacting the sheathing, which is usually more moisture-sensitive than the cladding.

Then, the sheathing should be covered by a continuous and completely sealed water barrier, so that when water gets across that air gap in some places, the water flows down the face of the barrier rather than soaking and penetrating the sheathing.

The bottom of that air gap between the cladding and the water barrier needs drain holes and flashing. And the top of that air gap needs air vents so a slow current of air can dry out any water that gets past the exterior face of the cladding. One example of brick veneer cladding, air gap and waterproof sheathing is shown in figure 5.11.

That air gap and water barrier is called a drainage plane, and the entire assembly with drains and vents is sometimes called a “rain-screen wall.” It costs more money than simply pressing the cladding up against the sheathing. It will also be more complicated to design.

But it is far more reliable in excluding water than cladding which does not have that air gap. When all layers are in direct contact, water will creep though cracks and along fasteners all the way to and through the sheathing, and then into the building.

Fig. 5.11 Air gap followed by a water barrier keeps water out of the interior wall The air gap behind the exterior cladding keeps water from contacting the sheathing, reducing the risk of water problems. Behind brick, as shown here, it’s also important to place a vapor barrier membrane over any wood-based or gypsum sheathing to protect it from the high vapor loads from the sun-heated, rain-saturated bricks.

The air gap and drainage are very traditional means of excluding water from buildings. When the exterior cladding system is brick veneer, these features are common practice and are very important to avoiding moisture problems. The owner would do well to look at the drawings for any brick veneer to make sure the air gap and waterproofing are included, and to ask the contractor how he will ensure that no mortar bridges the air gap, and how he will ensure that the water barrier which covers the sheathing is sealed.

With other cladding systems, the air gap and water barrier are not always standard. The more common contemporary practice for stucco, EIFS, clapboards and precast panels is to eliminate the air gap, squeeze and fasten all the layers together and rely on building paper or housewrap to keep the seepage water out of the sheathing.

And that practice works for many buildings and saves money in construction. But if there are holes in the housewrap (such as nails), or if the housewrap is not really effective in excluding water¹⁶, or if the housewrap loses its water-exclusion properties over time¹⁷, then any water leaks at the exterior become paths for moisture intrusion into the building.

An air gap and sealed water barrier greatly improve the ability of the exterior wall to exclude moisture. If the request comes after the architect has decided on a design without these features, the costs of adding them may be more expensive. But if the owner asks for an air gap and sealed water barrier at an early stage, there may be very little additional cost.

Interior wall finish which passes water vapor freely

The owner determines what will be used to decorate the interior walls. Common practice in many com-mercial buildings in the USA has been to use vinyl wall covering to decorate and protect the indoor surface of exterior walls which are constructed

using paper-faced gypsum wall board. In air conditioned build-ings in a hot and humid climate, this practice has been absolutely disastrous.^5,18,19

Time and time again, beginning in the early 1980’s, forensic investi-gations have identified impermeable vinyl wall covering as being the principal cause, or the most significant contributor, to mold growth in walls.

The problem is that when humid air leaks into the building behind the wall from outdoors, or when water leaks in around windows or

The problem is that when humid air leaks into the building behind the wall from outdoors, or when water leaks in around windows or