The best way (and really the only way) to insulate the exterior of the roof is to install rigid foam on the top of the existing roof decking.
- Check what your local code minimum R-Value is and determine the foam thickness based from that.
- For a high performance home, it is recommended to increase the foam thickness to exceed the code minimum.
Your builder or green rater can help you with this decision. A simple cost analysis can be run on different thicknesses and calculate a payback on each option.
For more information:
Read our tips "Three Ways to Make Your Roof More Energy Efficient."