Do impermeable floor coverings contribute to floorboard rust?

I think I may have recognized a pattern over the past few years with some project vehicles.

In your experience, do vehicles with a full rubber floor mat (instead of carpet) tend to trap moisture under the carpet and contribute to rusty floorboards?

My sample size is limited, but I have a couple of trucks that were factory equipped with rubber floor mats and they exhibit rust from the inside/topside out.

This seems logical in my mind...that water trapped under an impermable floor mat could not evaporate and contribute to rust and other problems...but it has never been something I've seen passed around as "Common Knowledge" among other car folks.  

And so...the reason I'm posting this now is because I'm shopping for carpet for a couple of projects and "Mass Backing" on carpet seems to be highly regarded as a good thing for noise reduction, thermal insulation, AND being waterproof.  People extol the virtues of waterproof mass backing as keeping water AWAY from the floor.  But I see water getting under carpet as somewhat inevitable (I live in the midwest with  oppressive humidity...and I know we don't have the worst of it here).  To me it is logical that ability for water to eventually evaporate out of the vehicle would be important.  In building terms...it would be like the breathable house wrap.

I'm sure there are aspects of this I haven't overthough yet.

I'm interested to see what other folks thoughts and experience are.  

Yes, but only in older cars. 

In reply to mazdeuce - Seth :

Every car is an older car unless it gets destroyed in its youth. The worst thing I have ever seen is the jute mat many companies have used. It seems to breath, but the reality is that it holds an amazing amount of moisture against the steel, while being under the rubber in a couple of F150s I have to maintain. Say goodbye to the outboard seat belt mount.....The only real solution is frequent maintanence.

The jute underlay in 70s Datsuns greatly contributed to their rustage.  It was basically a sponge.

Thanks...it does make sense that the Jute is a good matrix/wick for the water to do more damage.

So.  If I were to replace carpet in a couple of project cars (with fully 23 years between their ages) would it be wise to opt for the breathability of a carpet WITHOUT the mass backing?  That's kinda what I'm thinking.  

I realize the real answer is to not allow moisture (or oxygen) to be allowed in in the first place.  But like I say...you really can't fight nature.  Water's gonna wat.

Up here in the Northland and other areas that use salt on the roads the salt/snow tracked in melted and went thru the old carpets and poorly designed floor mats causing rusted the floors. The new designed carpets have plastic/rubber backing and the floors stay rust free longer. Water leaks will still contribute to rusty floors and floor mats may speed up the rusting by blocking heat from HVAC and sun.

That's another good point.  We track the salt in on our feet.

FWIW, I have a 92 F250 with the vinyl floor, and its still perfect (not driven in northern winters). 

In the rainy, salt free pacific northwest. Rubber floor mat cars definitely rot the floors out at a greater rate than their carpeted brethren.  I still see it all the time here at the shop.

I used to get super excited when I would find a poverty spec late 70's VW Rabbit for sale and lifting the rubber mats always revealed unspeakable horrors that the carpet shod versions just didn't have.

In reply to Trent :

The ones where the factory held the carpet in by hammering nails right through the floorpan?  Those were laughably bad for rust.  I am amazed that those were the GOOD ones in the PNW.

OTOH that was the same company that sent Dashers to the US with saltwater in the cooling system to dodge some tariff or fee or something.  Saltwater, in a iron block / aluminum head engine.

The one old British car I have owned that didn't suffer rust on the inside of the floorboards was the Jensen Interceptor. The factory glued neoprene closed cell foam (wet suit material, rather expensive) to the painted metal by way of thermal insulation and no water could get through that to the metal.

ClemSparks said:

Water's gonna wat.

QFT

Back in the day,  cocomats became popular since they didn't trap moisture. Personally, I think they look cool, too. 

Yes. My 61 Corvair wagon was a life-long Colorado car. Not a speck of outer body rust. Floors were gone. Rubber carpet FTL.