insulating silencer boxes without ductliner

I'm currently building silencer boxes for my HVAC. My boxes are almost identical to Sharward's in size and design. I found a local supplier that has Johns Manville Linacoustic RC, but it's only available in 100' lengths. And it runs about $250- 300. I'm thinking about getting water heater insulation instead and covering it with that landscaping material Dan Fitzpatrick speaks of in Keith's thread. What do you think?