I can't tell you that I have experienced a difference with my van since I got it new, am not done with my conversion yet & haven't really had a chance to see how cold/warm the floor is. But, I'll tell you what I wanted to have done, what got done and what I think about insulating the grooves.
I took my van to a well known Sprinter shop in the Portland area for some upgrades in Dec. of last year. What I wanted done was to have a layer of thin foil backed closed cell foam (like R-3) put under the entire factory floor. I remember the owner saying that the foam would get squashed at the higher ridges...I said that was alright...I wanted it anyhow.
When I went to move my Espar heater, I had to cut out a section of the factory floor. I was very surprised to see that there was only the foil covered foam in the deep channels and only metallic tape on the ridges. So I decided to remove the metal strip covering the factory floor by the back doors to see if I could get a better look. What did I see? This:
So much for doing what your customer asks for and pays for!
The metallic faced, closed cell foam looks exactly like the stuff I put on some of the metal under my luan ceiling & wall panels. It's supposed to have an insulation factor of R-3 (but less if squashed):
http://www.homedepot.com/h_d1/N-5yc...splay?langId=-1&storeId=10051&catalogId=10053
It would have taken under 5 rolls of this stuff to do my van floor at a cost of $81.40. Even though I paid the guy a small fortune, apparently the metallic tape is a whole lot cheaper. But, the time & labor to cut little strips of foam and tape them down would seem to me to negate any cost savings on the materials. It seems to me that just filling the recessed channels is a waste of time
I don't know much about Fatmat, but their website says the sheets have a rubberized compound and they also have an aluminum liner. I didn't see an R-value listed. But, if you have installed this on your floor, it seems to me that if it is dense/thick enough, it would provide a thermal break between the metal van body & interior and it should also give you a reflective surface.
If you have any Fatmat left over, maybe you could do a little test. Like take a piece of it and wrap some of it around an empty can or cup. Fill the cup with hot water or ice water and then put your hand on the Fatmat part, then on a part that isn't covered. Is there a noticeable difference in temperature? If so, I wouldn't worry about adding anything else to the floor. If you do a test, I'd like to know if the Fatmat makes a difference. (I tested the duct insulation on my vans metal walls in winter; they were much colder without the duct insulation).
One other thing...It wasn't clear whether you are you putting 1/4" plywood over your existing wood floor or over the metal floor? (Hopefully you are putting it over the existing wood floor; otherwise, I think 1/4" is 50% too thin).
Kat