One of the most common questions I get when I describe homes with insulated rooflines is, "What does that do to the shingles?" Some roofing companies have made a lot of noise about this topic, saying that if the shingles can't conduct heat downward into the attic, their lifetime will be greatly reduced.
What's the truth about this claim? How much does shingle temperature really rise if the insulation is right at the roofline and the attic isn't vented? Fortunately, it's easy to find out because the Florida Solar Energy Center (FSEC) has done the research.
In the summer of 2000, they studied this very issue. (You can download FSEC's review article about this topic, with this experiment described starting on page 24.) They looked at the shingle temperature for some houses that had insulation at the flat ceiling and a vented attic and for other houses that had insulation along the roofline. Here are the data:
As you can see, the difference isn't that great. The graph below shows the temperature difference throughout the day:
The biggest temperature difference was about 9 degrees Fahrenheit, which occurred around noon. The average temperature difference through the day was about 2 degrees F.
This FSEC study didn't look at shingle lifetime, but I think that adding 9 degrees to a temperature of 150 degrees won't make much difference. What kills shingles isn't so much the temperature. It's the ultraviolet radiation that they're subjected to all the time.
Here's a bit of anecdotal evidence for you. Southface has a building constructed with structural insulated panels that they built in 1996. The asphalt shingles have been on the insulated roofline for 14 years now. They also have a detached garage with a vented attic. There's no difference in appearance or performance of the two roofs.
Now, let me ask you ask you this - Do you know anyone who has ever collected on a shingle warranty? The answer is probably no. Even if it's yes, however, shingle warranties are structured so that you're not going to collect much if you do make a claim unless you have catastrophic failure in the first couple of years. An insulated roofline will have minimal effect, and certainly will not result in catastrophic failure.
Shingle photo by simplyrikkles, from flickr.com