Whats the difference between Handi-foam and the foam I can buy at my local DIY store?
The foam from your local DIY store (which usually comes in a single can) is called one-component foam and has an open cell structure (similar to a sponge). It relies on moisture in the air to cure and generally takes around 24 hours to cure fully. It is not designed for insulating and will absorb moisture. This type of foam cannot be sprayed and is used for sealing gaps around window frames etc. Handi-foam is a sprayable two-component foam which cures chemically when the two components are mixed. It cures quickly in 5-10 minutes and forms a closed cell structure making it an excellent insulator, which is impervious to moisture.
