This project is a window repair on a clerestory.

These windows were installed in the 1980’s, and show some of the classic mistakes of the era. I don’t see any bad intention in the install; really what was done shows that the person who did the work thought they were going the extra mile. And the work lasted pretty well- certainly helped by the very deep overhangs.

This shows the windows before starting work. The jamb and sills were installed, and then the trim installed over them. The windows are double-pane, insulated glass units (IGU’s), and were installed with a bead of sealant to the inside, and another bead of sealant on the outside, with the jambs acting as part of the primary waterproofing.

The white smeared over the trim is a silicone sealant that was installed by a handyman when the windows began leaking during a heavy rain and wind event.

There was no good way to tell if there is any hidden waterproofing before removing the trim. Water was getting through the windows and draining to the inside, leaving water stains and causing damage to the interior walls and trim.

When I began to remove the trim I could see more of the picture. Removing the trim was fairly easy once I cut through the sealant.

There was no Weather Resistant Barrier of any sort behind the trim, and I could see that water had been getting through to the sheathing for years- this is the darker wood at tho lower part of the photo, just above the white. The white material is a PVC roofing membrane, and the lack of flashing and water stain showed that any water getting through the exterior would drain under the roof membrane, potentially damaging the roof structure.

Fortunately, this roof had been put on only a year or so before this, and the water damage from the leaking windows did not extend far below the PVC.

Removing the jamb/sill assembly was quite difficult as the silicone sealant that was used had to be cut away from the glass before the jambs and sill could be removed. Several IGU’s cracked when pulling because some sealant hadn’t been cut all the way through.

Part of the problem with the install was that continuous bead of silicone used to essentially glue the wood to the glass; glass and wood are differential materials that expand and contract for different reasons and at different rates. Glass moves because of heat, expanding as it gets hotter, contracting as it cools. Wood moves primarily because of moisture, expanding when it’s wet, contracting as it dries, but is nearly unaffected by heat.

The perfect sealant joint is a very specific design, and this joint is the opposite of what is necessary to seal materials that move differently. What happened here is that the wood and the glass expanded and contracted at different rates, for different reasons, and because of the design of the joint, the sealant could not flex to allow that movement. Over time the bond to either the wood or the glass would break in small areas, allowing water to intrude. It worked for over 20 years here, although the lower windows had begun to fail a number of years earlier.

We chose to go a different route for waterproofing these windows. There were some constraints that limited what products could be used- being winter in the NW, the chance of rain and wind coming up at any time made using liquid applied products quite difficult. Tarping the structure would have made that possible, except that the house had a number of functions that made that an unattractive option. Also, we were also working on the siding below, making it possible that the tarps would have to be maintained for months.

What we chose to go with was a Butyl flashing tape that would allow the flashing to be wrapped into the house and up the interior piece of trim, creating a pan. This would ensure that any water that managed to get behind the trim would be directed by the flashing out onto the roof.

Once the flashing was installed glazing tape was installed to the inside of the trim. Glazing tape is a closed cell foam with an Acrylic adhesive on both sides. This glues the foam to the interior, glues to the glass, and the foam flexes, creating a secure seal around the window. The IGU are placed on rubber spacers, which protect the flashing from the glass and allows the glass some room for movement.

With the new design the trim was built and installed first in panels. These were installed with a drainage space between the walls and the trim to prevent capillary action or trapping water behind the trim. After the trim is installed, jambs were built with integral, sloping sills, and those were installed with glazing tape to seal them to the window. The jambs extend over the top of the trim, and the sills are extend beyond the plane of the trim with a 15 degree angle to drain water. One of the reasons for this is to create a system that drains most water, places joints where it is unlikely for water to be able to infiltrate, and any water that infiltrates will drain to the exterior with no reliance on sealants and sealant maintenance.