A brown ring shows up on the wall under a window, or the carpet by the dinette is damp after a night of rain. The word almost everyone reaches for next is “seal.” Sometimes that is right and the fix is a part that costs very little. Sometimes the seal is the only sound thing left and replacing it buys you one more season of the same stain.
SDG Doors & Windows builds custom RV and trailer windows to order in Waycross, Georgia, and stocks window seals, cranks and trims through parts and accessories. We sell both answers, so we have no reason to talk you into the expensive one. What follows is how to tell them apart before you spend anything.
An RV window seals in three separate places
“RV window seal” gets used as though there is one of them. There are at least three, they sit in different places, and they fail for different reasons.
Around the glass. The pane sits in the sash against a gasket or bead that holds it and keeps water out of the frame itself.
Where the frame meets the wall. The window’s outer flange lies against the sidewall of the RV, and that joint is sealed and clamped from the inside. This is the one that gets blamed least and leaks most, because nothing about it is visible from inside the rig.
At the moving joint. On a window that opens, the sash closes against a compression seal and the latch or crank is what pulls it tight. A seal here can be perfect and still leak, if the hardware no longer applies any pressure to it.
There is a fourth thing called a seal that is not a gasket at all: the sealed cavity inside a double-pane window. When that one fails you get fog or haze between the panes, and no gasket on the market fixes it. Our write-up on pane types in custom trailer windows covers how those units are put together.
The stain is not the entry point
Water runs before it stops. It enters at the highest failure, travels down whatever surface it finds, and appears wherever gravity and the wall cavity let it out. That is why a mark below the bottom of the window frequently has nothing to do with the bottom of the window.
Test it in order, from the bottom up, and you will find the real one:
- Dry everything inside and put down a towel or a sheet of paper where the water shows.
- Close and latch the window as you normally would.
- With a hose on low pressure, wet only the wall below the window for a couple of minutes. Have someone inside watching the dry paper.
- If nothing appears, move up to the bottom edge of the frame. Then the sides. Then the top edge. Then the glass and the sash.
- Stop at the first zone that produces water. That is your entry point, and everything above it is still unproven.
Working upward matters. Wet the whole window at once and you learn only that it leaks, which you already knew.
The diagnosis in one place
What the evidence points to, and what it means you are buying
Five findings from the hose test, and the part each one sends you toward.
Findings summarize the article below. Seals, cranks and trims listed as supplied on the SDG Doors & Windows parts and accessories page, read September 30, 2026.
When a seal is genuinely the answer
Replace the gasket, and only the gasket, when the frame is straight, the corners are closed, the glass is clear, the hardware still pulls the sash down firmly, and the rubber itself has gone hard, split, or taken a permanent flat set where it was compressed. A gasket that no longer springs back is not sealing anything, however good it looks.
This is the cheap outcome and it is a real one. Seals, cranks and trims are listed on our parts and accessories page as stocked inventory. We supply those components; we do not manufacture them. Tell us the window you have and what failed, and we will tell you whether we can match the part.
When the window is the answer
A new gasket cannot correct a frame that no longer sits flat. If the frame is bent, corroded through, racked out of square, or has opened at a corner joint, the sealing surface has moved and new rubber goes back against the same gap. Pressing fresh material into a joint that is no longer parallel is the most common way a leak gets repaired twice.
Treat it as a window purchase when any of these is true:
- A corner joint of the frame has separated, or you can see daylight along an edge with the window latched
- The frame is corroded through, dented, or has been pried during an earlier repair
- The sash no longer lines up with the frame, so the seal touches at some points and not others
- There is fog or moisture trapped between two panes
- The opening itself was never the right size for the window that was fitted to it
Our windows are custom-built rather than picked off a shelf, which matters here: if an earlier repair has enlarged or damaged the opening, the replacement can be made for the opening you actually have. The full measuring and ordering path is in how to choose, measure and replace trailer windows.
Sealant is not a substitute for a seal
The most common field repair for an RV window leak is a bead of sealant run around the outside of the frame. It works for a while. Then it does two things you will not like: it hides the evidence you need to diagnose the leak properly, and where it bridges a drain path it holds water against the wall instead of letting it out.
Windows are designed to shed water, not to be watertight at every point. Some frames are built to let water that gets past the glass run back out at the bottom. Sealing that route closed is how a small leak becomes a wall repair.
If sealing is the right job, three rules keep it honest:
- Seal the joint the design intends to be sealed — the flange against the wall — and leave the bottom drain path open.
- Remove the old material first. New sealant over failed sealant bonds to the failure, not to the wall.
- Match the product to the surface. Which sealant suits your sidewall, your roof line and your frame material is the sealant maker’s specification and your RV manufacturer’s, not ours. We do not supply sealants and we do not publish a compatibility chart.
Why the timing matters more than the part
The part is rarely the expensive half of an RV window leak. Water that gets into a wall goes to the framing, the insulation and the floor, and by the time it shows as a soft spot or a lifted panel the repair has stopped being about a window. A seal caught in a season is a component. The same leak left for two is a structural job at a shop.
That is the argument for running the hose test on a dry afternoon rather than waiting to see whether it happens again.
What we can supply
Our windows page lists single-pane and double-pane windows, tinted and clear options, and windows designed to resist moisture and maximize insulation, custom-built to unique dimensions and shipped nationwide. Glass is cut to fit, so a replacement pane for a frame you are keeping is a separate option from a complete window — both are covered in our guide to glass options for RV and trailer windows. If you are ordering a window anyway and want it darker, specify it at that point; the reasoning is in our piece on tinted glass versus film.
Seals, cranks and trims come from our parts and accessories inventory. Windows, glass, doors and screens are made in-house. That split is worth knowing before you call: for a gasket we are matching something to your existing window, and for a window we are building to your opening.
What a quote needs from you
Whether it turns out to be a seal or a window, send the same information and we can answer both:
- Photographs of the window from outside, from inside, and close on whatever failed
- Width and height of the opening, or of the existing frame, measured at more than one point
- Which zone produced water in the hose test
- Whether the window opens, and how — slider, crank, or fixed
- Single pane or double pane, and tinted or clear
- Whether you need a seal, a replacement pane, or a complete window
- How many, and whether they need to match each other
We do not publish prices, sizes or lead times on this site, so those come back on the quote rather than off a page. Send the list through our contact page and we will tell you which of the two you are actually buying.
Frequently Asked Questions
How do I know if it is the RV window seal or the window itself?
Latch the window and look at the frame. If the frame is straight, the corners are closed and the glass is clear, and only the rubber has gone hard or flat, it is the seal. If a corner joint has opened, the frame is bent or corroded, or the sash no longer meets the seal evenly, it is the window.
What does fog between the panes mean?
The sealed cavity inside a double-pane unit has failed and moisture is now inside it. A replacement gasket does not reach that and cannot fix it. You are looking at new glass or a new window.
Can I just run sealant around the outside of the window?
It will usually stop the drip for a season, and it can make the underlying problem worse by hiding the entry point and blocking the path water uses to drain back out. Seal the flange joint the design intends to be sealed, remove the old material first, and leave the bottom drain path open.
Does SDG Doors & Windows sell RV window seals?
Yes. Seals, cranks and trims are stocked and supplied through our parts and accessories inventory. They are components we supply rather than products we build. The windows, glass, doors and screens are what we manufacture.
The stain is below the window. Is the bottom of the window leaking?
Often not. Water enters high and runs down inside the wall before it appears. Wet the wall below the window first and work upward one zone at a time, and stop at the first zone that produces water.
Does a new window have to match the old opening exactly?
Our windows are built to the dimensions you give us rather than to a catalog size, so a replacement can be made for the opening as it is now, including one an earlier repair has changed. Send measurements taken at more than one point, because an opening that has moved is rarely square.