ventilation box materials

[donnam]

Is it okay to build a box for ventilation out of galvanized steel, foiled backed board or cement board? What is the safest and least expensive route to go? Thank you for any advice!
Donna

[MikeAurelius]

Use only non-flammable materials, like galvanized metal.

I STRONGLY DO NOT recommend the use of foil backed foam board. The foam is designed for use inside wall structures and is not designed to be exposed to open flame - it releases toxic (deadly) fumes when it burns. Additionally, you usually end up having to build a wood support frame to hold it, while galvanized can be pretty much self supporting if properly constructed.

Cement board is way too heavy and difficult to cut and shape.

[donnam]

I hoped galvanized was okay to use, but I wasn't sure whether or not it would give off any fumes if heated by the torch. Thanks!!
Donna