by Patricia McDaniel
To some extent, houses have always leaked. But in recent years, using new materials like panel sheathing, housewrap, and flanged windows, we've tightened up construction to the point that houses no longer dry well. During the same period, there's been a decline in traditional methods of carpenter training. One result is that houses are going up without proper attention paid to building paper and flashings. Once these new tight houses leak and the insulated framing cavities get soaked, they're more likely to stay wet and rot than the airy, "breathable" houses of the past.
In more than 20 years of building, repairing, remodeling, and inspecting homes, I've found that even the most critical flashing details are often installed wrong or overlooked altogether. These errors are common to expensive custom homes and inexpensive modular houses alike. Often, it's evident that the carpenters made a conscientious effort, yet still the details are wrong.
Again and again, flashing and papering mistakes come back to one overriding principle: Get the layers right so that water flows out on top, not behind, the housewrap and flashing. A second important principle: Don't rely on the siding to stop the water. I tell customers that the purpose of siding is to look good, and that, yes, it sheds most of the water off the house. But it also lets water past it, so the backup system of housewrap and flashings must deflect that water and allow it to weep or drain back out from behind the siding before it soaks the sheathing and framing.
Materials of Choice
We use Tyvek and Typar interchangeably (my carpenters tend to prefer Typar because it's quieter and easier on the eyes.) Properly installed, we get good performance from either material.
The housewrap debate. There is some research that indicates that housewraps don't perform well when they're subject to persistent wetting. However, in my inspection work, I've never come upon a situation where there was rot behind housewrap that I couldn't trace to a water intrusion higher up - for example, a roof leak, a rip in the housewrap, a backwards lap, or a missing flashing.
I don't claim to have seen everything, but it's my strong opinion that if you get the details right, the brand of housewrap you use is not important. If you just can't bring yourself to trust housewrap, use tarpaper - it's been around for a long time and still performs well.
Eaves membrane. Any areas subject to a lot of moisture - splashback zones, areas below intersecting roofs that dump accumulated rainwater, or tall beachfront houses, for example - can benefit from extra layers of tarpaper or self-sticking eaves membrane (commonly referred to on the job site as "bituthene") in addition to the housewrap. Just make sure that wherever you use it, the top edge of the membrane or tarpaper is behind the housewrap above.
For the 36-inch-wide rolls of bituthene, we use one of several available brands, including Grace and Certainteed. For the narrow flashing rolls, we prefer to use Grace's Vycor. Although this stuff is very sticky, it adheres best to clean surfaces, and sticks poorly to dirty housewrap.
Get a brake. If you're serious about keeping water out of the houses you build, you should invest in a metal brake and learn to use it. With a brake and a roll of coil stock on hand, we can bend the necessary flashings as we go, and not have to rely on a roofer or metalworking sub who may or may not be at the site when we need it.
Don't rely on chemistry. Don't substitute the stickiness of bituthene for proper layering. The same goes for house-wrap tapes and caulks. In time, chemical bonds can break down, but properly lapped layers will not move.
Sequence Is the Culprit
The photos that follow illustrate common leak spots that occur from installing housewrap improperly. Some of these mistakes result from nothing more than carelessness. But a root cause of many problems is the construction sequence that's common on job sites today: The frame gets sheathed and wrapped, then everything else - doors, windows, decks, and flashings - gets put on top. Immediately you have a layering problem - unless you do something about it.
All too often I see sloppy and incomplete house-wrap installations; the housewrap is installed any which way, with no attention to proper layering.
Gable ends. For some reason, builders often think they can get away without wrapping the gable ends (photos A & B). This may be because they're thinking of the wrap only as an air infiltration barrier but not as a weather barrier, and figure there's no reason to worry about air leaking into the attic.
This is a mistake, because the gable is one of the most exposed parts of the house. Driving rain gets behind the siding, runs down the sheathing, and winds up behind the housewrap.
The problem often appears as drips along the top extension jamb of windows or sliders below. It may be hard to diagnose, because the leak may show up well below the point where the water runs behind the housewrap.
Band joist. Another problem area is the band joist. Walls may be sheathed and wrapped while they're lying on the deck. Then, when the wall is lifted, the band or mud sill remains unprotected (C & D). Splashback from below as well as water running down the housewrap from above can cause severe wetting. If you install wrap this way, make sure you go back and cover the band joist with a strip of housewrap, tucking it under the wrap above. Note that in photo D this would be difficult to do, because hold-down straps were installed over the wrap above and band joist below.