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What did you put under your interior carpet?


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I'm wondering what you put down under your entire interior carpet -- sound deadening, some sort of under-carpet fabric?  Mine just had some tar coating in the foot wells with some felt like fabric over top.   


Insights and recommendations appreciated!


1968 BMW 2002

1998 M Roadster

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I put fatmat under everything. However if i were to do it again, i would do fatmat and a thin foam layer particularly on the transmission tunnel since some of the carpet surfaces feel very hard, not as soft as they should feel.


1976 BMW 2002 Chamonix. My first love.

1972 BMW 2002tii Polaris. My new side piece.

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I've never removed the tar sheets from my '02's floorboards, but when I had my carpets out for cleaning, etc, I padded underneath with old-fashioned waffle padding that was used under carpets in your house.  Not even sure if you can get waffle padding any more, but call a couple of carpet stores.  Often you can buy or they'll even give you leftover pieces--more than enough to do a car.  I liked it because besides being cushiony, it (1) didn't absorb water like foam padding and (2) the waffle pattern allowed air to circulate under the carpet so it would dry faster if it did get wet.  


I'd be leery of any kind of padding that would retain water--the old-style jute backing held water like a sponge, and would quickly rust your floors.  I found that out the hard way with my '59 Renault.  Fortunately I caught it before the floors rusted through--scraped and painted the floors with Rustoleum and used waffle padding under the rubber mats.  That was in 1966 and the floors haven't rusted since!



'69 Nevada sunroof-Wolfgang-bought new
'73 Sahara sunroof-Ludwig-since '78
'91 Brillantrot 318is sunroof-Georg Friederich 
Fiat Topolini (Benito & Luigi), Renault 4CVs (Anatole, Lucky Pierre, Brigette) & Kermit, the Bugeye Sprite

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Fatmat is one of many stick-on rubbery foil faced products.  Prices for these products tend to reflect the slickness of brochures as much as product quality.  All are kinda the same in that they dampen (deaden?) the sheet metal's vibrations.  "How much to use" and "where" are eternal questions, and the answers depend on your goals.  A sound box on wheels has different needs than a car you'd like to just make quieter. 


The things that I worried about when choosing mine were odor, thickness of material, and sticky-ness.  Odor?  Some of the available materials will make your car smell like a tar roof on an August afternoon in Texas. Butyl Rubber is reported to be odor-free in online info, so I chose a product that advertised it was 100% Butyl rubber.  Many name brand products don't mention what there goo is made of, beware....  Thickness?  I chose 80 mils as a good compromise for one layer (some people advocate using multiple layers).  My goal was general quieting, not the ultimate loud sound system.  Sticky-ness?  Everybody's product is the best, according to the advertisements.  I didn't worry about it except to NOT use it above the headliner- too many reports of various brands falling off due to heat.  You do need to make it adhere fully- clean surface to start with and as much pressure as possible with a small hand roller.  Over and over again.


As far as effectiveness of the material is concerned, there is no rocket science- it's just the addition of distributed weight over vibrating surfaces acting as a damper.  What pattern is embossed in the foil is unimportant.  What logo they print on the foil is unimportant.  The thickness of the foil is even unimportant, assuming it stays in place and contains the rubbery goo.  Presumably sheet lead with the right glue would be very effective.  And heavy.  And expensive. And dangerous if taken internally! 


I ended up covering the entire floor, panel behind and under the rear seats, the two rear side panels behind the ash trays, and a large rectangular area inside the doors.  Along the way I checked with the "thump" test- Cover an area on one side of the car first.  Bang on it with your fist to see if the frequency of vibration seems to have gone down compared with the same area on the other side.  Base on that highly scientific method, you don't have to cover every surface.  I ended up doing so anyway because I had enough material.  I bought my not-name-brand material on Ebay, so It was affordable.  72 sq. ft. was purchased, and I had about 2 Sq. Ft if less in small scraps when done.  One more installation point- carefully shaped small pieces work fine to fill in odd areas, just use the force of the roller to meld them with their neighbors.  Cover all joints with aluminum foil tape to keep the goo contained.  Buy a name brand tape and check the specs for temperature rating to make sure the adhesive will be OK.  If you can't find a spec, you need to assume that there isn't one and move on.....



Combinations of the weighty deadener material and other materials such as the waffle padding, horse hair, various densities of CLOSED CELL foam (no sponges, thank your) will tend to work on wider ranges or audio frequencies.  Again opinions are all over the internet, feel free to embrace any you'd like... Generally, the less dense the dampener, the higher the frequency it will be effective on, and the thicker the dampener the more effective it will be.


Esty may chime in, but too thick or too squishy under the carpet is no good in practical terms - installation of the carpet won't work right. 

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  One more thing I would not do is put Fat Mat in the doors. In my case I began getting tar or adhesive on my drivers window when I rolled the windows up or down.

 My car is a  modified 71 and I was looking to "add lightness" so I dispensed with a radio an deleted the rear seat . The biggest weight-saver though was removing the factory tar-based floor covering.It also gave me a better look at the condition of the floors.      

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