RV Window Leaking: The Seal Is Under the Flange

The seal is under the flange, not around the glass. Lippert's installation manual puts the tape on the outer edge of the window flange, to maximize the contact area between the tape and the wall, and it requires the sealant seam to overlap at the bottom of the window. Both are instructions to a builder, and both are the places a leak appears years later.

The leak causes the maker lists are all at the frame. Lippert names them in its own manual: an improper rough opening, protrusions, uneven surfaces or jagged edges that impede proper seating of the window; an undersized radius block, which creates a gap and causes an installation leak in the radius; and tape stretched across the window, which rolls or folds in the radius and leads to a leak there. None of them is the pane, the gasket or the latch.

One thing that looks like a leak and is not. Lippert's window replacement manual states that water between the panes exiting the weep holes is not a defect, and that the weep holes are designed for that purpose. Weep holes that run water on a wet day are doing their job.

Dometic specifies a flexible non-hardening compound for the outer frame of its wall windows, and names an example, SikaLastomer-710.

On silicone, the honest answer is that the makers do not agree with the internet. A blanket rule against silicone on an RV window is not supported by the window makers' documents. Lippert's RV technical sheets specify 100 percent silicone sealant for the top seal seam on an 8800 series window on a sloped surface, black silicone-based sealant for filling old latch screw holes, and DOWSIL 1199 silicone glazing sealant for repairing glazing that has separated at a corner. The only prohibition found anywhere is on a residential vinyl window instruction sheet, for a different product and a different material.

What that means in practice: match the sealant to the job rather than to a rule. A non-hardening bedding compound under the flange, a sealant the maker names around the flange edge, and silicone where Lippert names silicone. What is worth avoiding is a bedding that hardens, because a bedding that has become brittle is one of the ways the seal under a flange fails.

It is a clamp. An interior clamp ring screws into the exterior frame and the wall is held between them. Lippert's procedure is eight screws, corner first, partially tightened and then fully, with a stated reason for going in stages: driving the screws in all the way can strip or break the head.

The fastener is specified, and the maker says why it matters. Lippert's manual gives the screw as a number 8 by three sixteenths by one inch maximum, non-self-tapping, and its procedure is to tighten them partially first and then fully, with the reason stated: driving the screws in all the way can cause a stripped-out or broken screw head. A screw that is too long for the frame it is going into is the same class of mistake, and it is the one that damages the window rather than the fastener.

A latch that will not engage is usually alignment. Lippert's adjustment sheet for its 8700 series says to move the catch so the center of the catch lines up directly with the center of the rivet on the latch when closed. On the 8800 series exit latch, the base is held by two Phillips screws and, if the handle will not engage properly, the catch can be moved to make the connection.

An operator can be replaced rather than living with. Lippert's sheet for its 3000 series operator gives the procedure: drill out the rivets securing the operator to the frame with a three sixteenths bit, fit the replacement, and re-rivet it. A window that will not crank shut is a part rather than a frame, on the units that sheet covers.

And the closing rule for travel. Lippert's quick reference states that all windows should be fully closed before towing or moving a motorized RV, to prevent product damage, and gives the closing procedure for a knob-operated window: turn the knob counterclockwise until the window is flush to the frame, then continue until it will no longer turn.

Fogging is a seal failure, and it is the edge seal of the glass unit rather than anything at the wall. Lippert's insulated glass warranty describes it as material obstruction of vision from film accumulation on the interior glass surfaces, resulting exclusively from sealant failure of the glass edge seal, and covers it for ten years from the date of manufacture. That warranty text is written for site-built residential windows rather than RV windows.

What the makers publish for an RV window is thinner. No Lippert or Hehr RV window service document covers repairing a fogged dual-pane unit, and the RV window support pages carry no fogging or insulated-glass document at all. A maker of replacement windows states that fogging is typically due to seal failure, and that fog between the panes of a double-pane window can etch the glass and require replacement.

So the honest answer is that this is a replacement rather than a repair on the evidence the makers publish, and that the RV-specific warranty terms are not in the documents a reader can reach.

Lippert's windows are designed for vertical walls, and its manual says installation on a sloped wall can lead to water penetrating into the RV. That is a design statement rather than a fault, and it is the reason a window that leaks on a wall with any slope is worth looking at from outside before it is resealed.

The opening has a tolerance and a condition. Lippert gives the rough opening a tolerance of plus or minus one eighth of an inch, requires it to be square, level and plumb, and states that an improper rough opening, protrusions, uneven surfaces or jagged edges can impede proper seating of the window and lead to leaks. It also notes that an undersized radius block can create a gap and cause a leak at the radius, which is the corner of the opening rather than the window.

A window leak and a roof leak can look alike from inside, and the roof leak guide covers the other one. If the water is appearing at the wall rather than the window, the delamination guide explains what water in a wall does over time. And the pre-trip walkaround carries the window check that catches a seal before it leaks.