Why Windows May Fail

Mistakes in window installation can lead to drafts and/or water egress, or windows that do not work well. Correct flashing, fixing and gap sealing are extremely important. But it starts even before a window gets on site, with the correct installation of the glass into the sash, so the double/triple glazed glass units (IGU’s) do not fail.

Mistake 1) Using the wrong support blocks while glazing:

Wrongly positioned support blocks or blocks that are not load bearing may cause the window frame or sash to sag and deform. This could affect the seal, leading to condensation within the IGU. Support blocks must be made of as little heat-conducting material as possible. For optimal load transfer, the blocks are arranged in the corner areas or under the central pillars. The supporting blocks transfer the weight to the frame if placed correctly.

Mistake 2) Not keeping water out of IGU’s:

Where possible, drainage must be provided to the glazing rebate, especially windows glazed from the outside, so the IGU does not sit in water in the event of the seal failing, or directly on a wet window frame. This could also lead to a failure of the IGU, to water egress and condensation within the IGU, as well as rot and mould of the window frame. Sealing the IGU properly into the window frame has to be a priority.

Mistake 3) None or incorrect flashing:

Flashing is essential to achieve weatherproofing around windows and doors. It has to be installed so that water does not penetrate from the outer skin to the inner skin of the building envelope.;

Flashing must be installed from bottom to top, in the following order, so that each layer overlaps the one below:

1. Sill flashing

2. Jamb Flashing

3. Head Flashing

At each transition, an overlap has to direct water to the element below. If proper flashing, and drainage below the window or door are not provided for water to drain to the outside of the wall, water will be captured and result in damage.

Mistake 4) Wrongly fixing windows within the building envelope:

Windows must be anchored securely in the reveal and the window opening to be held firm. This area must be carefully sealed and insulated to prevent drafts, moisture and heat loss. Poorly insulated anchoring of the windows could lead to a loose window, water egress and drafts.

Mistake 5) Inappropriate gap filling around windows and doors:

In the past, the space between the window frame and the wall was just left without any sealing or gap filling, covered up with beading and/or architraves. This creates a thermal bridge. Construction foam is used to fill the joint between the window and the wall, to improve the sealing and reduce drafts.

For energy-efficient window installation, the joint is professionally sealed on the outside with expanding composite tape that completely fills the space. On the inside sealing foils can be used.

Essential materials for proper window installation should include:

Installation items available at Siegware

Images by Fensterbau Ratgeber, AGWA, and Paarhammer