Other than aesthetics is there any functional purpose of having foam under coils. Doing DIY bed: 6" Quantum Edge Elite Bolsa Coils (leggett & pratt)(Arizona Premium Mattress).
Dont have the coils yet but think they will have a thin fabric over them (should keep them from skating around).
(1.) Would an inch or two of foam under coils do anything for functionality.
(2.) For whatever reason if I decide to go with some foam under coils is any particular density best or just anything works? The bed I am tearing apart has a 2" piece of very dense/hard plasticky foam.
Regards,
B)
Hi Fallguy007.
Welcome to our Mattress Forum. 
To steal some words directly from Phoenix…
There’s multiple reasons to use or not use a polyfoam base underneath a pocketed spring unit. Some manufactures will use a thin 1" base for the little extra solidity that it will provide beneath the pocketed coil spring unit. It will also help to “square things off” and can have a bit more of a finished look when the ticking surrounding the mattress is applied. Having the foam base can also help a bit with spreading out the load when placing the mattress on a surface using less than even supports or ones that are spaced out over a bit of a wider area. On some pocketed coil innerspring mattresses using a polyfoam edge reinforcement unit, having the bottom polyfoam can help with structural integrity by providing a surface upon which to glue the polyfoam edge system and create a “foam tub” into which the pocketed spring unit is placed. Some manufacturers eschew using a polyfoam base, as they have clients, especially with latex systems, who desire no polyfoam at all in their mattress. Using less polyfoam also helps reduce cost by a small amount. Others prefer the feel of their finished mattress without the polyfoam base. While there really is no specific “rhyme or reason”, these would be some common explanations.
If you create your own DIY mattress and desire to have a base piece of polyfoam, 1"-2" is common, in anywhere from a 1.5 – 2 lb range, with higher ILDs (mid 30s+). If you wish to avoid polyfoam, you could use the same thickness with a piece of firmer Dunlop latex, which would tend to be less “bouncy” than using Talalay.
I hope this helps!
NikkiTMU