The following is for a Seitz / Dometic S4/S5 window but the procedure will be similar for other window types and even skylights. The same procedure is relevant for sealing caravan windows.

Leaking windows and sky lights can be a problem on motorhomes and campervans and are often the main source of damp problems. This same procedure is the same for sealing caravan windows as well.

We had a leak on one side window. Rain water was seeping in the top of the window and pooling inside the top of the frame then dripping down the inside of the window and then pooling in the bottom of the frame before dripping down the inside wall. Thankfully no damp had taken hold but it needed to be resealed.

First I needed to know what was leaking and what parts and sealant to buy. I removed the internal covers from the window frame to expose the fixing screws. The screws were carefully removed with my wife holding the frame from the outside just in case the outer frame dropped out. It became evident there was also sealant around the outer frame so it didn’t drop out! The inner frame also contains the blackout screen and fly screen so be careful these don’t fall out. It also gives you the chance to clean them.

Remove the decorative panels to reveal the securing screws

Remove the decorative panels to reveal the securing screws

The window was very carefully pushed out at all corners and eventually came out.

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There was a rubber seal around the edge of the window and the old sealant. I ordered a new rubber seal (cost around £10) and a tube of non-drying mastic (cost around £4). The window was re-inserted for a few days whilst the parts arrived. We sealed around the window with some tape and plastic sheeting just in case it rained.

The parts have arrived. The old seal and mastic needed to be removed. The rubber seal came off fairly easily but was super glued in on the corners so that needed cutting out carefully. The new seal comes in 5m lengths and I only needed 2.4m so I have enough spare for another window if needed. The new seal is not continuous so the join is placed at the bottom middle of the window.

I noticed there appeared to be a few large bubbles in the sealant which is where the water was probably coming in, so I need to ensure this is not replicated when replacing the sealant.

The old rubber seal is perished and needs replacing as well as the old sealant needs removing

The old rubber seal is perished and needs replacing as well as the old sealant needs removing

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The old mastic sealant needs to be removed. In most places it came off easily but a few places a blunt knife was needed. In my case one of our best cutlery knives was the ideal choice – to my wife’s horror!

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The old rubber seal and sealant has been removed and the surface cleaned. The channel where the rubber seal sits needed cleaning as well

The old sealant around the window cutout on the side of the van also needs completely removing and cleaning.

The new rubber seal was fitted and then the mastic sealant was pipped around the inner edge of the rubber seal making sure it was just a little proud of the rubber seal. You don’t want to put on too much sealant otherwise it may stop the frame from pushing tight against the wall.

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Frame with the new rubber seal and the new sealant

The non-setting mastic will grip the frame to the wall very well. Next refit the inner frame and screw up very tightly. Some excess sealant may squeeze out so clean this off quickly.

As the name suggests the non-setting mastic remains a sticky mastic and does not dry solid. But it does form a skin so it is not sticky to the touch after 48 hours. DO NOT use the sealants that dry hard such as Sikaflex 512. These hard sealants will certainly seal the window but will be extremely difficult to remove again should the seal break. Also do not use regular bathroom type silicone sealant. It may work in the short term but will not provide a lasting seal.

I also noted on the instructions that came with some of the sealants not to use or administer them in temperatures under 5°C. Although it was winter when I used the sealant I was lucky to have a warm few days.

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Parts needed:

1x Seitz S4 and S5 Windows Seal – Frame to Wall rubber seal.
1x Hodgson Mastic Non Setting caravan sealant – White
Alcohol or white spirit for clean off old sealant – see the instructions for the sealant.