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Last Updated: , Created: Sunday, June 24th, 2001

What is a "drainage layer" for a window installation?

Janice from Thornhill, Ontario writes: I'm having some wood/fiberglass windows installed and would like to know the product you recommend as the moisture barrier between the outside capping and inside insulation.

Your website recommends installing such a barrier but no specific product is mentioned. Neither my installer or Marvin Windows has heard of such a moisture barrier. I also asked at both Home Depot and Revy and none of their sales people had heard about this extra layer to protect moisture getting into the walls through the outside window capping.

-------- Reply

We are talking about drainage layers. Basically we have discovered that all windows eventually leak. So to protect the house, we need to install something back inside the wall where it is protected from the weather, that will prevent any leaks from moving into the wall, and in fact, direct any water back to the outside where they can drain away. It is not a simple thing to define as there are many possible ways to accomplish this, and very few installers and manufacturers have come around to accepting that this is the single most important problem with windows (as they don't see it as a window problem, but a framing problem).

A complex but very through drainage layer would be an etafoam gasket put between the house wrap and the window frame with caulking over the top of it -- all before the siding went on the house. An acceptable alternative, often done during renovations, is to foam the entire cavity with polyurethane foam and then caulk over the outside surface of the foam. Whatever it is, it must be a protected second line of defense that effectively will direct penetrating leaks back out away from the wall.



Keywords: Installation, Drainage, Walls, Air Barriers, Windows, Water Penetration

Article 1269