Bad Window Replacement:Top 10 Screw-ups:Bad Window Caulk (Part 2)

Bad Window Replacement: Top 10 Screw-ups: Bad Window Caulk (Part 2)

When good windows happen to bad installers who don’t know how to caulk
Part 2, continued from Part 1

#6. Bad window caulk: Joints too tight!

Caulk is supposed to be a permanently flexible material that can stretch and contract when the substrates move, such as when an aluminum window frame shrinks in cold weather. But here’s the rub: Caulk can only stretch 30% to 50% of its freely moving thickness. If you install a caulk joint between two surfaces that are 1/4″ from each other, the maximum movement the best silicone caulk will tolerate is +/- 50% of the gap, or 1/8″. If the service conditions cause that caulk to stretch 3/16″, it will fail.

Just about every door threshold I see beneath swing and sliding doors has a caulk joint size of 0″. No space at all! Even if the threshold was originally bedded in gobs of caulk, that caulk was then squeezed down to being paper thin during installation. The caulk will fail and water will enter the building under the window or door.

#7. Not sealing the trim!

Many window systems consist of the window itself plus a series of trim pieces that attach to the window. These pieces might be called mullion connectors, expanders, trim extenders or simply, trim. If these pieces are exposed to the exterior, they must be sealed to the window.

Clad wood windows often have an “accessory groove” around their edges that receives some sort of trim or mullion connector. It is a pandemic that these seem to rarely be sealed to the window. The capillary-sized space created between the metal pieces loves to inhale water.

How can you tell is a trim piece is sealed to the window? You should be able to see some caulk squeezing out of the space where trim and window meet. This exposed caulk is called “squeeze-out.”

#8. Not sealing mullions!

When the side of one window stacks onto another with the use of an “H” shaped piece called a mullion, incredibly these are often not sealed. Not incredibly, these mullions then drip water into the wall and damage wood, drywall and other finishes. Why are supposedly professional installers doing this? Seal the mullions or risk having a leaky job due to bad window cauling.

In this forensic disassembly we remove a leaking pair of windows to discover the problem: the installers did not use caulk to seal the h-shaped mullion (blue) to the mating surfaces on the adjacent windows (red dotted lines). This resulted in a puddle of water beneath the mullion on the stud framing (right). No caulk is even worse that bad window caulk.

#9. No second line of defense!

Here is where the installer and the managers who train and oversee them need to start using their heads. They should always ask: “What will happen when the first line of defense fails?”

The first line of defense will usually be the exterior caulk and any sealant covering fastener heads in the “wet area”. It doesn’t last forever. Where will the water go when these things fail?

Quality installers and window companies try to include a second line of defense to their installation design. Often this is an interior caulk joint that will prevent water that bypasses the first caulk joint from entering the walls. It might also be a flashing that has an upturned edge beneath the interior face of the window. If that flashing is end dammed (see #2 above), the system will tolerate failure of the exterior caulk and keep protecting the building from water damage. See below for an example from DuPont Tyvek. Full instructions: How to Install Tyvek

#10. Final Quality Check!

A good installer will not just stay on his side of the installer/manufacturer divide. A good installer realizes that a problem with the window product will reflect upon him and his work. A bit of window product QC will go a long way toward increasing customer satisfaction and reducing costly callbacks. Here are the quick checks that an installer should do to make sure that the overall installation, not just his work, is acceptable:

1. Fit and finish: Does everything look ok?
2. Operation: Does the window open, close and lock easily?
3. Tightness: Listen closely when closing the window. Do the street sounds dramatically reduce as they should?
4. Any bad or missing window caulk?
5. Visible sources of water leaks: Any holes, gaps or unsealed fasteners in the “wet areas”, especially in the sill.

Frequently Asked Questions

Why is caulking such a common source of installation failures?

Caulking fails frequently because installers might apply it to dirty surfaces, skip tooling, skip backer rod or squeeze it into joints that are too tight to accommodate movement. Since caulk is the primary barrier against water and air, any shortcut weakens the installation. Without proper joint width, surface preparation, tooling and backer rod caulk loses adhesion, cracks prematurely, and allows water to reach vulnerable parts of the wall assembly.

What happens when trim and mullions aren’t sealed correctly?

Trim pieces and mullions can act like miniature water conduits. If they aren’t sealed to the window, they create capillary gaps that can pull water straight into the wall cavity. Unsealed accessory grooves, H-mullions, and trim extenders can collect and direct water in unexpected ways, causing hidden damage that isn’t discovered until stains, rot, or interior leaks appear. Proper sealing isn’t just cosmetic; it’s essential to protecting the structure behind and beneath the window and cladding.

Why is a final quality check essential after every window installation?

A final quality check catches issues that might not be obvious during the work itself, such as loose fasteners, missing sealant, misaligned sashes, poor sound tightness, or visible leak pathways in the sill area. Without this final review, minor problems can turn into expensive callbacks or long-term failures that frustrate customers and damage the installer’s reputation. A thorough check ensures that both the product and the installation perform as intended before the job is considered complete.

How can I check my windows?

• To check operation, open, close and lock all sashes 3-5 times. It should work well.
  • To check alignment, visually look for parallel between sash and frame.
  • To check for tightness, open the sash and place a dollar bill in the gap between sash and frame. Close the sash and try to pull the bill out. The bill should not come out easily.
  • To check for air tightness, open the sash and listen for the ambient outside sounds. Then, while listening closely, close and lock the sash. The sound should be immediately squelched.
  • When windy, listen for whistling noises at all seams. You can even use a stethoscope without the end piece, just the tube.
  • If a draft is suspected, light a match and hold it at the suspect location. Does the draft cause the flame to move?
  • To check the sill for water tightness, tape over the weep holes and fill the sill with water. The water should stay in the sill and not leak through the frame into the area below.

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