Just saw a similar post on the Roland VS web site about foam for iso booths and trying to find cheaper alternatives to Sonix or Auralex.
Yes most foam rubber from "foams r us" is cheaper. There is a big reason it is cheaper - that is because it is not fire resistive. Typical foam rubber used for packing material or seat cushions etc will ignite easily, burn very rapidly and emit LARGE amounts of of hot toxic smoke.
I have 28 years in the Fire Department as my real job to attest to this.
You may ask why doesn't my seat cushion go up in flames when a cig is dropped into it. Part of the reason is it is coverd often by a semi flame reatrdent fabric but mainly because it is lying flat - not hung vertically on a wall or glued to a ceiling. The flame spread rate of this stuff in this configuration is amazing (and scary)
While from most fire code aspects you can do pretty much what you want in your own home - if you are a commercial entity or in an apartment with other dwellers you are potentially on the legal hook for damages resulting from an excessive fire involving this stuff. But that is for the leagal eagles to decipher. My concern is the safety of you folks out there.
If you can't afford fire retardant foam and insist on using other stuff at least "knock it down" after use so it is not standing up where it's flame spread is instantaneous.
Good foam will have a manufacturers UL Flame Spread rating - normally for wall and ceiling finishes it should have a Flame Spread Rating of 1 or 2 ( no greater)
If anone wants further info on this type of situation they can e mail me direct at
bradff@hotmail.com
Stay safe and happy recording
PS - I have asked a lot of questions on this board - am glad I can throw out an answer for once!!!!!