A - The maximum performance of foam board insulation depends heavily on proper installation. Therefore, it's best to have a certified insulation installer do it. If you'd like to have it installed professionally, you should do the following:

• Obtain written cost estimates from several contractors for the R-value you need. Don't be surprised to find quoted prices for a given R-value installation to vary by more than a factor of two.
• Ask contractors about their air-sealing services and costs as well, if needed.

To evaluate batt installation, you can measure batt thickness and check for gaps between batts.

If you want to install it yourself, you should try to obtain instructions and safety precautions from the manufacturer. Carefully follow these instructions. You should also check your local building and fire codes.