Attack of the Dew Point!

Over the years we have seen a lot of window installations gone wrong. I remember a house where after only four years dangerous mold developed on the inside walls. When the builder opened the drywall, the studs were black! What had happened?  

The holes in the wall where water can penetrate into a house are typically the windows and doors. Even if water somehow manages to seep behind the roof flashing or enters the floor system through a badly installed balcony floor, in the end the water will always make it into the building through the window and door openings. Because we have a long track record of successfully installing hurricane rated windows and doors, we have been called to many failed installations to share our expertise. Here is what we found in the majority of cases:  

Windows (other than Henselstone) are sitting on metal sill pans. The sides and the top are typically connected to the membrane of the house (house wrap, tar paper stucco wrap) but on the bottom there is a gap with a few weep holes often caulked. The thought is that if water leaks through the typical American double hung or crank casement window, this sill pan can save the day because the water will drain out of instead of into the wall. So, let's look at this train of thought in detail: A builder buys substandard fenestration product which he expects to fail. The buying decision is almost always based on price. Then he custom-makes copper sill pans and installs them. The labor and material cost of these sill pans are lumped into the framing cost, making the windows look like a good deal, when in reality they are not. Since the windows were cheap and the expectation is that they will fail, no pressure is put on the manufacturer to improve his technology (like make a window that does not fail!).

Although sill manufacturers, stucco manufacturers, architects and engineers specify flashing pans to be on a slope, in reality they rarely ever are. Imagine setting a window on this pan. The slope will now point into the house instead of away from it.

Although sill manufacturers, stucco manufacturers, architects and engineers specify flashing pans to be on a slope, in reality they rarely ever are. Imagine setting a window on this pan. The slope will now point into the house instead of away from it.

The craziness continues. The metal sill is now an energy bridge between the outside and the inside climate of the house. Let's take a typical day in beautiful Charleston, SC. The outside is hot and humid, the inside cool and dry (air conditioning). The house wrap does its job. The house is tight. But wait, the sill pans. Here the energy conducting metal connects the outside with the inside climate and, guess what? Yes, it rains right in the middle of the wall. No problem, I have been told many times, the water will run out. True to a degree, if the weep holes are not clogged or caulked and the pan is actually on a slope (which it typically is not). Add to that the fact that the typical American window is not prefinished. The moisture does not drain out but is absorbed into the raw wood window frame. Ever wondered why on a nice hot day your double hung window sticks? Yup, the frames have swollen with the moisture, squeezing the sashes. Not only do they not operate well, the aluminum cladding (which does not expand with the wood frames) delaminates. Now rainwater can even enter into the miter gaps. Then, within a few years the sills rot out. The window has failed and yes, now you need your sill pan to prevent the worst. 

Note how these instructions from a major EFIS manufacturer say nothing about weep holes, slope, spacers to window frame or material of the sill pan.

Note how these instructions from a major EFIS manufacturer say nothing about weep holes, slope, spacers to window frame or material of the sill pan.

There are some real solutions to this issue: 1) Buy a window that will not leak. Yes, they exist! Check the design pressure ratings (which include a water resistance rating). A good window should have a design pressure rating of 60 psf or higher. Watch out for manufacturers who advertise a design pressure rating but exclude water penetration. 2) Check the construction of the window. A good window has interior and exterior seals with a drainage system in between. Metal cladding should be a separate, extruded profile, spaced from the wood (because of condensation), and welded in the miters (not glued). Miters in wood frames and sashes should be joined, not nailed or stapled. The product should be completely factory prefinished. 3) Avoid the use of sill pans. Your window installation should keep interior and exterior climates as separated as a regular wall does. If you have to use pans, use non-metal products. Insure that the pan is on a slope. Some products in the market place already have a built in slope. Keep the window frame from the pan with non metallic spacers. Seal the end dam to the window and keep the opening to the exterior properly drained. 4) Design you water proofing. Water does not only run downward. If you do not have a fenestration partner who knows his construction engineering, consult with a specialist on proper water proofing. Make that consultant take responsibility (i.e. issue a warranty for his design). Make sure you and your sub contractors are using the right materials. Tar paper is different from Tyvek. Stucco is different from shingles and brick. Use proper flashing and sealant materials. If you switch one piece of your water proofing design, others might have to be changed as well. Caulk is never a primary waterproofing material!!!