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  4. Door Membrane

Door Membrane

Polythene door membrane
Fig.1 - An old style polythene door membrane still found on many cars.

A door membrane might also be called a 'moisture barrier', 'water barrier' or 'splash guard', but we call them membranes because until recently almost all of them were little more than a polythene sheet attached to the inside of the door using a sticky adhesive.

As it is older cars that tend to leak, we can expect that cars with this style of barrier will be in use for many years to come.

The problem with this style of door membrane is that over time the polythene can shrink, which then pulls at the adhesive laterally, causing it to pull away completely, allowing water to pass.  Polythene membranes can also crack and split with age.

We have found they are often damaged during repairs and modification of the vehicle. Mechanics and technicians accessing the sound system, electric windows or locking mechanism may just cut holes in them to gain access, not realizing they serve a vital purpose of preventing water entering the car.

This video shows James checking a door for leaks. This is an example of what happens when door membranes leak.
Foam membrane
Fig.2 - A more modern style of soft foam membrane.
Broken door membrane
Fig.3 - This foam membrane has a split. It is likely somebody did this deliberately to gain access to the inside of the door.

More recently, moulded foam membranes have become more common. These are still glued in place with a sealant which can perish, however, the membrane its self is less likely to shrink or split.

An advantage of the polythene sheet type is that if they did need to be replaced, it was possible for technicians to make their own. Manufacturers might charge up to £50 pounds for what amounted to a sheet of plastic which only cost a few pence for us to make, thereby saving customers significant money.

The problem with the moulded type is that they are practically impossible to remove from the adhesive without destroying them, and they can be costly to replace. However, it is still possible to make many of them from foam sheeting by using the broken one as a template. Because the foam is soft, it will often mould its self around fittings and features of the inner door.

Metal door membrane.
Fig.4 - Metal door membrane.
Metal door membrane resealed in place.
Fig.5 - Metal door membrane resealed along with various fixtures, blanking plates and grommets.

Metal membranes have been used by Volkswagen for a long time and are becoming more common, along with hard plastic types.

Although these don't perish or shrink, and they are often screwed in place, any sealant or gaskets designed to waterproof them will still perish. 

It is a simple enough job to seal them back in place using a quality heatproof sealant, although this style comes with its own problems as various motors and electrical units are attached to them.

The main cause for any part of a door membrane to fail is heat. Your car can get very hot when left in the sun, which will cook all the solvents and plasticizers out of the adhesive. For this reason, you may find all of your door membranes start to leak at the same time. Or it may be on just the side of the car which is normally parked in the sun.  Either way, if one is leaking, we would recommend you have all fixed, as it's likely the others will leak soon. This could mean your car getting wet all over again. As drying the car can be a large part of the cost in restoring a rain damaged vehicle, it doesn't make sense to take the risk.

Written by . Last updated 24/10/2023 17:57