An Effective Moisture Barrier Around Windows Requires the Right Order of Installation

Originally published by: Energy VanguardNovember 14, 2014

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I see a lot of interesting stuff at construction sites and in people's homes. I also see stuff I never got to see because people send me photos. I like photos! Remember that ice chest someone had incorporated into a duct system? That was sent to me. So are the first two photos below.

The first one is from my friend Kristof Irwin in Austin. He snapped this pic at a home under construction because it so clearly illustrates a lesson that all builders have to learn: how to do things in the right order.

siding before window flashing kristof irwin

See anything missing there? Oops! They started installing the siding before the windows were in. Even worse, they didn't leave enough space to install the windows there. I actually got to see this house a few weeks later, and they'd had to go in and cut the siding back around the whole opening to make it work.

They may actually have done a decent job with the flashing details to keep water out, but they sure made things hard for themselves by not installing the windows before the siding.

The photo below shows the opposite side of the same problem. Instead of waiting too late to install the windows, they installed them too early. Then someone realized they'd forgotten to install their main liquid water control layer, the drainage plane. (Well, we can hope they didn't think of the house wrap as a vapor barrier, can't we?)

So they just ran the house wrap over the windows as if they were doing it normally. I imagine they came back and cut the house wrap around the windows and then taped it all up from the outside.

building enclosure house wrap windows wrong sequence 600

The problem with just taping it from the outside is that you're relying on two things: (i) The tape will be installed properly and last a long time, and (ii) no water will get behind the house wrap.

Tape can and does last, when you use the correct type and install it properly. But water has a tendency to get into all kinds of places it shouldn't oughta go. If you don't shingle the materials so that they shed water away from materials that are susceptible to moisture damage, you make it more likely that water will get in. Installing the house wrap after the windows means that it's impossible to shingle it properly.

Here's what you want your house wrap to do around windows.

house wrap window flashing moisture management

Notice that the house wrap comes over the flashing tape at the top of the window. The tape is good, but with the house wrap coming over the top like that, the chances of water getting in are greatly reduced. The Hammer & Hand Best Practices Manual I wrote about recently has great details on how to do this.

One of the biggest lessons I learned when I was building a house was how important it is to do things in the right order. At best, you get lucky and it doesn't matter. Sometimes, it means extra work but you can still end up where you want to be. Other times, it can spell trouble for the future because of the compromises made.

The builder of the home in that second photo certainly didn't want to pull all those windows out and then install the house wrap to be able to get the flashing details right. But that's really the only option there if they want to prevent the future homeowners from having to deal with the moisture problems caused by the mistake.

As painters in the 1930s found out, insulated buildings don't dry out so well. When water gets in, it stays in and wreaks more mischief. Doing things in the right order and understanding moisture management is not optional anymore.

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