Window leaks are the sneakiest water intrusion I see on jobs. The stain shows up under the window, or on the wall a foot away from it, and the owner swears the roof is fine. They're usually right — the roof usually is. The window frame itself is the leak, and the fix is almost never another bead of caulk on the outside.
Here's the thing most people get wrong: the caulk you can see around a window is not what keeps the water out. There are two seals on an RV window. The primary seal is the butyl tape sandwiched between the window frame and the sidewall — you can't see it, and it's doing all the real work. The visible caulk around the top and sides is just an assistant, steering runoff away so it never has to test the butyl. Once that tape has dried out, cracked, or pulled away, no amount of caulk smeared over the frame is going to hold.
Check the easy thing first
Before you pull anything apart, check the weep holes. Slider windows have tiny drain slots in the bottom of the frame so water that runs down the glass can get back out. When they clog up with dirt and sealant crumbs, water backs up into the track and spills inside — and it looks exactly like a seal failure. Run a piece of trimmer line through each hole and pour a little water in the track. If it drains, you just saved yourself two hours of work. If it still comes in, the seal is the problem and the window needs to come out.
Pull it and rebed — the real fix
Do it right and you'll never touch that window again. This is a two-person job on anything bigger than a small vent window — one of you inside on the screws, one outside holding the glass. Pick a warm, dry day with no rain in the forecast.
1. Pull the interior clamp ring. Remove every screw around the inside trim ring and take the ring off. Photograph the screw pattern first. You will not remember which screw went where.
2. Break the seal. With your helper supporting the window from outside, work a plastic scraper under the exterior flange and cut through the old butyl tape. Slow and steady — scratching the sidewall here is how you create a new problem while fixing the old one.
3. Clean both surfaces. Scrape the bulk of the old butyl off the flange and the wall, then wipe down with mineral spirits and follow with a denatured alcohol wipe. The new tape needs a clean, dry, oil-free surface to bond to. This is the most tedious part of the job and the one people rush. Don't.
4. Tape it. Run one continuous strip of Dicor butyl tape around the entire perimeter of the exterior flange, covering every screw hole, and lap it over itself at the seam with no gap. Don't stretch it — stretched butyl relaxes later and leaves a void right where you need seal.
5. Reinstall. Set the window in the opening right-side up — weep holes go on the bottom — and center it evenly in the opening. Your helper holds it from outside while you refit the clamp ring. Start with the corner screws, then work around snugging everything up before the final pass. You'll see the butyl start to ooze out around the frame — that squeeze-out means you've got full contact. Don't overtighten, or you'll warp the flange.
6. Finish the edges. Trim the oozed-out butyl with a plastic razor blade, then run a bead of Geocel Proflex RV along the top edge and a few inches down each side. Never caulk the bottom edge — any water that does get behind the flange needs somewhere to go. And never use silicone on an RV exterior: once it's cured, nothing sticks to it — not even more silicone — so the next repair becomes a nightmare.
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The mistakes that bring me repeat business
- Caulk-only patch jobs. Smearing new caulk over the frame without rebedding is a bandaid. It buys you a season, maybe, while the real seal keeps failing behind it.
- Silicone anywhere on the exterior. Just don't.
- Stretching the butyl tape so it looks neat. It retracts and leaves a gap.
- Overtightening the clamp-ring screws and warping the flange.
- Caulking the bottom of the window and trapping water in the frame.
- Skipping the hose test. Hit the window with the hose when you're done and check inside. Finding a drip now is a five-minute fix. Finding it after the trim is back on is a Saturday.
Pull, clean, rebed, finish the edges, test. It's a couple of hours of honest work per window, and it's permanent. Do it the shortcut way and you'll be doing it again next year — and the year after that. I'd rather fix it once.
Learn it once. Fix it right.
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