Of the many principles incorporated into green building, moisture management is key to succeeding in at least two: health (in terms of avoiding mold) and durability (in terms of avoiding premature deterioration of materials). And while the business of building homes gets more difficult and more complicated by the day, it’s often the fundamentals that we struggle with.
Like gravity, water works in simple, predictable ways. Just as we almost intuitively understand the laws of gravity, I believe we also understand the fundamental laws of how moisture works.
If you stop and think about it, you already know how to flash buildings and manage rain from getting into your walls. And you already understand that all cladding systems will leak and that penetrations of the building envelope —like windows, doors, and plumbing pipes—will transfer moisture to hidden, unintended places. It’s in actual practice that we fail.
The old-timers didn’t have caulk. They relied on shingling materials and flashing to direct the water away. They were careful not to trap moisture in the building envelope and to keep siding and wood trim off the ground. The moisture problems we’re encountering in today’s construction industry are not happening because water has begun behaving differently. We are simply forgetting the basics and are using caulk to cover our mistakes.
Back To Basics
Though the fundamental principles of moisture management have not changed, yesterday’s homes were definitely simpler and more forgiving. Today’s buildings are far more complex, and there are many more details that require your attention. Fortunately, some of the problems I see most often are also some of the easiest to fix. Here are the most common.
Exterior materials and finish grade. We’ll start at the bottom with a problem I have already mentioned: wood and masonry materials too close to grade. It may not be necessary to have the full 2-foot clearance used a century ago, but we sure could use 6 inches. Notice in Figure 1 how the siding is clearly separated from the soil. This practice should be used with faux stone applications and cement siding as well. Remember that whatever the cladding material is made of, there is usually a wood-based sheathing underneath. Holding these materials up and away from the soil will let them dry out, making them more durable and often improving their performance.
Deck flashings. Figure 2 shows another problematic area. When using deck ledgers, belly bands, or horizontal trimboards, we typically place “Z”-type flashing over the top to direct the water out. Unfortunately, we often forget to attach the top flange to the draining layer above. In many cases, the siding is then installed so it fits tightly onto the 90-degree metal flange. To make things worse, caulk is used to seal the contact point between the flashing and the siding.
This technique ignores the fact that the bottom of the siding is the escape route for water draining down the weather barrier. There are a couple easy solutions. First, specify “Z” flashing that has a 105-degree slope instead of a 90-degree slope. Also, ask for a wider vertical section to run up against the building. This lets enough material connect the housewrap to the flashing leg; 4 inches should be the minimum. I have seen builders in the Pacific Northwest order their 2X belly bands or deck ledger with a chamfer to let the flashing sit tight. And remember that this chamfer needs to be primed if it is not already treated.
Finally, hold the siding up a 1/2 inch above the flashing, and don’t caulk the joint. This makes for adequate drainage continuity, provides a capillary break, and allows some drying to occur. This same technique should be applied when using a vented rain screen system.
Pan flashings at doors and windows. A moisture management practice that seems to be catching on in some markets but is nonexistent in others is the use of a pan flashing, slope, and a back dam under windows and doors. This construction detail has been used for centuries. In Figure 3 you will see the pan flashing under a window on the roof of St. Peter’s Basilica in Rome. When the basilica was built, it was expected to last for at least a few centuries, so they employed this tried-and-true technique. Today, thanks to new materials and products, this process is far easier and can be done for a very small cost.
The detail in Figure 4 shows the method for preparing a window opening, installing the beveled sill or using shims, installing the DuPont Flex Wrap (this stuff works great), and then installing the window. Adding the back dam on the inside where the window frame or jamb connects to the sill ensures the window will have a long life, providing views and ventilation, and draining the rain to the outside. All windows will leak someday, some sooner than others, so preparing for the inevitable means improved durability.
I feel most builders consider these basic flashing techniques an additional project cost, but they should have been included in the initial price. When we frame a house, we don’t think twice about adding a few studs here and there. But properly flashing a window or door is seen as an extra cost.
Wall penetrations. Finally, I’d like to address the connection between wall penetrations and the drainage plane. Again, it’s about gravity.
Today’s homes include a wide variety of wall penetrations that range from plumbing and electrical lines, dryer vents, bath fan exhausts, and furnace and fireplace vents to light fixtures on a deck. All of these penetrations need to be integrated and layered into the drainage plane.
An easy solution is to use a flashing boot, as shown in Figure 5. Quickflash manufactures this product, which is available through the company Web site, www.quickflashproducts.com, or through many local plumbing and electrical suppliers.
Besides these common problem areas, I see a lot of room for improvement in designing and building overhangs, wall-to-roof intersections, chimney crickets, and installing head flashings over windows and doors. Because 90% of the failures we see relate to moisture control problems, you can’t afford to ignore the effect of water and moisture on your buildings.
Establish and clearly communicate acceptable standards of quality with regard to these details. Create a mock-up like the one in Figure 6 or display photos of correct execution on your jobsite for all workers and subs to see. Then enforce your quality, which may require holding back payment for work that does not meet your standards. Otherwise, you may reinforce poor performance, which you will pay for in the end.
Mark LaLiberte, president of Building Knowledge Inc., is a highly regarded green building consultant who helps builders nationwide understand and apply proper building science construction principles to improve their homes. www.buildingknowledge.com; www.laliberteonline.com.