How to Sound proof a generator room?

Here's what you need, Accoustic Blankets...

http://www.allnoisecontrol.com/products/AcousticBl...

You need isolation of both high-frequency and low-frequency noise. High frequency is carried by air and you just need a good seal on the doors and windows. Low frequency may require mounting the generator to an isolation table (rubber feet) and surrounding it with walls having loosely attached padding (or as someone suggests, egg crate foam). The padding absorbs much of the low-freq that is rattling around inside the shed when the door is closed. Add more LOOSE layers as necessary.

I worked on a quiet room that had insulation 10 inches thick: pink fiberglass batts, in two separately supported layers -- on facing in and one facing out, so there was a small dead air space between them.

You can also get accoustic dampening plasterboards. Or build another internal wall with a 2 or 3 inch cavity to fill with sand. (though if you do, use brick ties and do it over 2 or 3 days or the weight of the sand will push the wall over!)

Apply egg crate foam about two inches thick to the walls, some people use it to make their mattress more comfortable.Great for soundproofing. Still will need to vent cupboard.

get a good Heavy duty fan turn on high over the generator