Reflective coatings - namely acrylic elastomeric coatings, with high reflectivity - can be a cost-effective way of reducing solar heat gains through the roof in warm and mixed climates.
They are easy to apply (typically with a paint roller) to existing roofing materials, and relatively inexpensive ($20 to $30 per gallon, for good quality products).
Reflective coatings can be a cost-effective way of reducing solar gains through the roof.
Used in conjunction with 1) shade, 2) attic and roof ventilation and 3) attic insulation they can reduce solar heat gains through to roof to a minimum. That's important in hot climates, or even in mixed climates.
They can also protect the roofing, extending its life and lowering maintenance expenses.
The downside of reflective coatings
If you are considering applying a reflective coating, make sure that it is compatible with your roofing.
Some manufacturers claim that their roof coatings are effective over asphalt shingles, but that's a very controversial issue. Most asphalt shingle manufacturers do not recommend their application and studies on this subject do not confirm such claims.
If you are interested in this issue, take a look at the ARMA Bulletin 227 PDF, with the title "Field Coating of Installed Asphalt Shingles", in the Publication section of Arma Technical Bulletins.
Some new studies are questioning the performance of reflective membranes, at least in colder climates; they may not provide the expected energy savings.
For a critical view on reflective roofing, see: White roofs, green myth?
Purchase and installation
When buying roof coatings, prefer Energy Star products, and make sure that the coating is compatible with your roofing.
Pay attention to the manufacturer’s instructions, especially those involving their application. Clean the roof using a presser washer or other effective cleaning process. You shouldn't apply the coating on dirty and greasy surfaces, or on rusted metal areas. The coating will not stick for long. Also be careful when cleaning the roof, to avoid damaging it…
Also repair the roof before applying any primer and installing the coating.
Surface preparation is critical.