The second set of leaks on this Renault Megan. We like to be thorough, and it would have been easy to stop looking for leaks at these leaky doors, but because it's a Renault we went straight for the bulkhead and found a leak as shown in part 1.
Leaky doors are common on all cars. There is a plastic sheet stuck to the door behind the door cars. it's usually glued in place with a bead of a very sticky adhesive. After about 12 years, there's a tendency for the plastic sheet to dry out and shrink, pulling at the adhesive which has also had its solvents evaporate making it less sticky and brittle. This allows water to pass this barrier and drip down the door card into the cabin.
Solution: We replace these plastic sheets with new ones. We generally don't buy the parts, as on most cars it really is just a sheet of clear plastic, much like a bin liner. Instead, we make our own out of a waterproof foam sheet, which is a little thicker and durable.
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Question: "Why are you putting those bars down the window? Aren't you just creating a leak where there wasn't one before?"
Answer: The outside of the window does have a rubber seal on it and this will deflect most of the water and prevent it from going inside the doors, but some water always gets past this. Most will run down to the bottom of the door where it can flow out through drainage holes. There's a plastic membrane which is another barrier preventing water getting inside the car, but this isn't a submarine quality waterproof seal. You often find that cars are designed in such a way as there are multiple weak barriers which deflect water away to a place where it can escape. Water leaks are typically caused by a number of factors, an outer rubber not as tight as it was, inner seals shrinking, and drainage holes blocked. Then, if you get a particularly heavy rain, they all get overwhelmed.