Purpose of transparent plastic sheet between door and door panel?

Wasn't sure where else to ask this.

I have a 1st gen. The sheet and the black goo holding it is worn down. What happens if I reinstall door panel without this sheet?

 

I'm pretty sure that it's meant to keep out moisture that makes it past the window seals from getting onto the back of the door panel. FWIW, companies have been doing this for a long time, my '70 VW bus had the same thing on the inside. I used a contractor's trash bag and new black goo on that project. I forget what the black goo was called but they had it at my local small hardware store in the same area as butyl tape, it came on a roll. Haven't seen it at a big box store, but I haven't looked for it there either.

 

I’ve always wondered the same thing. Maybe some combination of a moisture + sound barrier? But not a very substantial one in either case and it’s easily destroyed if you mess with it. So maybe it’s not a critical component?

We used a thick trash bag to replace it on the tailgate of a truck we had awhile back. And just stuck it on with the residual goop. It stuck well in some places, in other places not so much. We just rolled with it.

 

Moisture barrier. Definitely want to keep it.

 

I took all mine off when I did sound deadening and thermal insulation. Do you think you still need it after putting that stuff on?

 

Its to keep water from getting into the vehicle.
Its impossible to completely waterproof the window and some rain water is going to get between the weather stripping and window into the door, this sheet basically makes the door a contained structure so that the water can only go down and out the drains at the bottom of the door, without it there is a gaping hole the water can get into the car through.
It also helps as a noise barrier, I know it doesn't seem like much but if its missing you can really hear the air rushing by the door going above say 40mph.
On the point that its not very strong you have to remember nothing is pushing on it with any real force, there is a sheet metal door on one side and an entire vehicle on the other side, the most force it sees is water trickling down it so it doesn't need to be beefy.
I do not know if its needed after sound proofing the door because I have never bothered to look into doing that.
If you need to reseal it butle tape would work very well, a little permanent but definitely gives a strong seal.

 

It's called a vapor barrier keeps condensation out of the cab that is caused by moisture from the rain getting into the door hollow and has a marginal effect on sound quality from the speakers by sealing the void giving them better response. Also helps in a miniscule way to reduce road noise.

I would keep it or replace ot with higher quality audio grade equivalent. That stuff is usually greenish tinted transparent.

 

It keeps the door panel dry.