Moisture barrier in doors, yes or no?

[jmackro]

Well said Jay.

Well, I don't claim to be an expert - maybe I'm missing something.

I have to admit that I have always attached the moisture barrier to the door sheet metal, because that's the way the factory did it. And of course, whenever I have just completed my super neat installation, the door mechanism needs lubricating, the window lift jams, or something else forces me to rip off that nice moisture barrier. Maybe it would be better to just stick some self adhesive carpet runner or plastic shelf liner to the backside of the door panel, cut out the holes for the door handle and fasteners, and not worry about having to upset it the next time I need to get inside the door.

 

[Arde]

Perhaps the distinction here is adhered vs attached. The sheet metal barrier adheres through its entire perimeter so humidity cannot pass the barrier. The door panels, even with their own humidity protection, are attached at discrete points for mechanical support, but are not necessarily a tight barrier. I agree it is a pain to undo the barrier when doing door repairs, so at times I have left my barriers partially perforated. Cupertino is pretty dry, so I have not paid a price for that neglect...

 

[deQuincey]

As long as you have a sheet of plastic blocking any moisture inside the door from the panel, how does the panel know whether that plastic is adhered to the door or to the panel? Why wouldn't it serve equally well as a barrier if it was stuck to the panel?



I have used a few coatings of Thompsons Water Seal to make the fiberboard more water-resistant.

you elaborated the answer yourself, dear Jay

because the reason for the barrier is not only to protect the panel, but also close the door openings

if you use the plastic sheet in its original way you close the ways that communicate the outside with inside the cabin, not only water but air and smells, noise,…this affects the panel and more

@Arde have mentioned it, continuous vs discrete,

so it is good to apply a sealing coat to the panel, knowing that you are not sealing the door itself

hence to apply the plastic vapour barrier to the panel is not the best way to proceed because the plastic it is not performing at its best (not fulfilling its function), and because there are better ways to do it (seal the panel with a coat of latex or similar)

in Spanish they say “killing flies with cannon shots” o “matando moscas a cañonazos”…

i say also not fulfilling the requests

but you know there is always someone that likes it anyway

 

[mulberryworks]

As long as you have a sheet of plastic blocking any moisture inside the door from the panel, how does the panel know whether that plastic is adhered to the door or to the panel? Why wouldn't it serve equally well as a barrier if it was stuck to the panel?



I have used a few coatings of Thompsons Water Seal to make the fiberboard more water-resistant.

I think the example of our X3 will help here. I had to replace the broken plastic traveling dog for the rear window mechanism and didn't get the foam weather seal stuck back quite perfectly. Since our driveway was sloped quite a bit the rainwater that managed to pass by the window gasket dripped directly down onto the weather seal rather than down to the bottom of the door where it would drain out of the drain hole which is placed outside of the door gasket.
This resulted in water entering the cabin from behind the door panel and pooling below in the space between the body and the door since the door seal was doing its job of preventing water to pass either direction. This resulted in a splash on the ground when opening the door and finding a wet carpet.

The door's weather seal must be placed on the door to complete the weather sealing of the car. Sealing the back of the door panel is fine, but unless it's fastened to the door with many points and perhaps a compliant gasket, it's not going to keep outside things outside. And those fasteners are likely to compromise the seal of the door panel, defeating the objective of protecting it and the interior of the car.

I agree that it's annoying to have to remove the plastic to work on the inside of the door but there are many types of adhesives available these days. I'd think it's possible to find one that would remain flexible and sticky so that would allow reattachment of the plastic sheet at least a couple of times though that would increase the chances of making a bit of a mess.

 

[rsporsche]

the only thing possible could be the lack of a slight air gap between the plastic and the door panel and where / if any condensation might form. this comes into play where cold air meets warm air. so here's where it becomes really theoretical - if the a/c in a coupe was able to get cold enough for a long period of time, moist / very hot air in Florida, Louisiana, or other humid gulf coast states condensation could form. but since we know that no coupe has ever been found guilty of having a really cold a/c more than 12" away from the grille ... this ain't gonna be a problem

 

[jmackro]

because the reason for the barrier is not only to protect the panel, but also close the door openings

OK, I understand your reasons for adhering the barrier to the door. And note that I always have adhered the plastic to the door sheetmetal until this discussion got me thinking about it. Still, next time I have my door apart, I may decide to risk the hazards of the air, smells and noise that are outside the cabin. After all, I also own a convertible (and yes, it also has its moisture barriers stuck to the doors, though now I wonder why!).

 

[mulberryworks]

OK, I understand your reasons for adhering the barrier to the door. And note that I always have adhered the plastic to the door sheetmetal until this discussion got me thinking about it. Still, next time I have my door apart, I may decide to risk the hazards of the air, smells and noise that are outside the cabin. After all, I also own a convertible (and yes, it also has its moisture barriers stuck to the doors, though now I wonder why!).


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Nice. I first saw a Duetto 45 years ago driving down Broadway with a large wooden airplane propeller sticking up from behind the seats. The Duetto was rusty but I still thought it was oh so sexy.

 

[HB Chris]

I won’t be driving in pouring rain and I certainly don’t know where these supposed smells from inside the door could be coming from. There is always room for differing opinions vs. from an expert.