This is a great question, and quite often I find the aesthetic principles dictate a roofing material decision more than any other factor.It's great that you are considering the efficiency factor as well.
The energy efficiency of roofing materials is determined by two properties:
- Solar Reflective Index (SRI),which is a roofing material's ability to reflect the sun and stop the material from absorbing the sun's heat. Typically, the lighter the color the higher the Solar Reflective Index; however, specially colored roofing materials have been designed to have higher reflectivity as well.
- Thermal Emittance,which determines how much of the sun's heat is absorbed. A higher thermal emittance allows a roofing product to radiate any heat it has absorbed and not allow it to be absorbed into the building it is covering.
Most asphalt-based roofing products already have a thermal emittance of at least 80%, and most metal roofing products are coated to come close to that level. As a result, a comparison of two roofing products may be left to its Solar Reflective Index.
Solar Reflective Index
The best bet is to not make a generalization about which is more energy-efficient but rather have a look at two specific products and compare their Solar Reflective Index.
- The Energy Star program has taken the guesswork out of this decision.
- While you can have a look at the criteria here,you can also download the Material Safety Data Sheets for each product and see the SRI for yourself.
Or you can just look for the Energy Star label when purchasing roofing materials.
Tax credit for roofing
The Energy Star program transitions nicely into the second part of your question, as the only criteria for earning the 2011 tax credit for roofing is that the materials meet Energy Star criteria.
For 2011, the Tax Credit for Consumer Energy Efficiency provides a refundable credit for 10% of the cost of roofing materials, up to $500.
For a complete outline, see the Energy Star section on tax credits here.
For more information:
Get 3 more energy efficient roofing tips in our Know How article "Three Ways to Make Your Roof More Energy Efficient".