60: Windows Sill Pan and Sill Dam

Now that the roof is on I need to get the outside OSB sheathing covered up and protected. It’s been out in the weather for five months and is beginning to show signs of ageing. So, it’s time for housewrap…pronto.

Back when I built the exterior walls and sized my window rough openings, I had not yet done any research on the “best” ways to install windows. I have studied lots of houses being built over the years, but I’d never seen anyone put in a sill pan or a sill dam for a window. I didn’t even know they existed until a YouTube video pointed out the need and the benefits for both; so I had to lengthen my rough openings by ¾ inch. Fortunately, I didn’t have to adjust the rough opening width.

What is a sill pan and sill dam you ask? After installing the housewrap (some call it the Tyvek or weather resistive barrier) many people then cut the housewrap at the windows, fold the flaps into the rough opening and staple. They then just slide the window frame in and screw/nail it in place. They are depending on the window’s nailing flanges to keep out any rain that gets behind the siding.

But sometimes, rain does get behind the house siding and then the nailing flange of the windows. It then runs down the sides of the rough opening (the correct term is the window jambs) and pools on the window sill. It rots the sill out, then proceeds to soak the insulation under the window, rendering it useless, not to mention providing the moisture needed for mold growth. So, your utility bills go up, you get sick house syndrome and then a big repair bill to tear it all out and rebuild it. You think I’m exagerrating? A friend of mine has a neighbor who is having eight windows rebuilt right now (at considerable expense) for this very reason.

So, the sill pan is a waterproof layer that catches any water that runs down the insides of the jambs and along with the sill dam harmlessly push that water back outside the house siding where it will do no harm. There are several ways to make a sill pan. You can buy then prebuilt, but they are expensive (and none of the local stores stock them, so you have to special order). Or, you can make them using window flashing material. The material I am using is a foil faced, butyl rubber adhesive backing material made by Tamko. The roll is 6” wide by 100 feet long and runs about $13 per roll. Sticky doesn’t even come close to describing how well butyl rubber adhesive sticks to everything (including your fingers). So, if you use it, plan well and in advance of how you’re going to maneuver it around to install it.