Basketball great Michael Jordan famously told Parade magazine, “The minute you get away from fundamentals, the bottom can fall out of your game.”
The combined features of a roof and attic are arguably the most fundamental feature affecting home energy performance. And research shows that you can strategically (and affordably) boost the performance of your roof and attic assemblies.
Over the past few years, we’ve looked at many types of roofing, underlayments and roof assemblies. For example, we found that adding a single inch of rigid foam on Steve Easley’s roof deck in Scottsdale, Ariz., was found to lower the OSB temperature by 30-40 degrees F, making even the most extreme heat manageable.
But roof systems consist of many layers. Let’s start at the top and work downward, suggesting some affordable improvements.
Another affordable way to crush roof efficiency is by combining so-called “cool roofing” with low-cost radiant barriers. While we’d like to see metal or tile roofs on more homes, the reality is that cautious buyers may not pay for that upgrade. So, make the most of asphalt by specifying reflective products.
Cool roofing costs a little more than standard shingles. How do you upsell it to clients? Explain that not only will it last longer than conventional alternatives, in extreme heat conditions, it also protects other parts of the roof assembly from accelerated aging, and may allow the buyer to specify a smaller, more-efficient HVAC system. And that’s before we even talk about attic insulation.
Standard asphalt-based shingles (including fiberglass/asphalt composition) usually have service temperature tolerances up to around 180-200 degrees Fahrenheit (82-93 degrees Celsius). At those levels, they may begin to soften, lose granules and age rapidly. In part, this is because sustained high surface temps increase oil exudation (where asphaltic oils seep to the surface), leading to premature brittleness and cracking.
Past studies have measured peak roof temperatures for asphalt shingles in hot climates at about 170°F (76°C)—just brushing the edge of the “breakdown” zone. The rest of the roof, including OSB decking and traditional felt paper underlayment, has about the same tolerance for sustained heat before they, too, start to show negative effects.
These roof temperature estimates, however, are based on yesterday’s weather data: when hot days rarely exceeded the 90-to-100-degree range. The world is getting hotter, and that’s a problem for new and existing roofs.
Oak Ridge National Laboratory (ORNL) estimates that dark asphalt shingle roofs can run 50-70°F (28-39°C) hotter than the ambient air temperature under typical summer conditions. So if the ambient temperature is 90°F (32°C), shingles on a roof might reach 140-160°F (60-71°C). If temps hover around 120°F for several days, as they did in some places last year, shingles may hit 190°F or more, suffering permanent damage.
A study by the Lawrence Berkeley National Laboratory found that cool roofs can lower surface temperatures by up to 50°F (28°C) under peak conditions. In the context of rising temperature extremes, they offer roofing assemblies a “safety zone.”
First, they prevent shingles from reaching damaging temperatures. By maintaining surface temperatures closer to ambient air levels, they reduce thermal cycling, the repeated expansion and contraction of materials due to temperature changes. This, in turn, minimizes cracking and extends the functional life of shingles by several years.
Next, they protect traditional underlayments and roof decking. The materials beneath shingles, including some underlayments and roof decking, are susceptible to heat-related damage when temperatures hit extremes. For example, the resins in OSB begin to break down at about 200°F (93°C). It should be noted that some of the more modern underlayments can handle higher heat—up around 240°F—but felt paper may not.
Cool roofs clear a potential caveat about radiant barriers. Without reflective shingles, applying a radiant barrier on the underside of a roof could push roof assemblies into the red zone. By adding cool roofing, this affordable heat defense remains one of the best retrofit approaches to knocking down temperatures in attic spaces.
A simulated study of a residential attic in El Centro, California, by Oak Ridge National Laboratory, found that installing a radiant barrier in the attic of a home with a cool-colored shingle roof resulted in a 28.2 percent reduction in energy consumption compared to a conventional roof and attic assembly. This combination was found to be as effective as doubling the ceiling insulation from R-19 to R-38.
Once you’ve got the roof deck and radiant barrier right, adding insulation will push your roof assemblies way into the green. If you’re starting at a 28.2 percent reduction in heat gain due to cool roofing and radiant reflection, you should be able cut the home’s overall energy footprint by 50 percent or more once you have added insulation to the attic.
What’s the best way to insulate attics? Good arguments can be made for vented and unvented methods, and the IRC now includes provisions for both types (see Chapter 8 of the 2021 International Residential Code [IRC], and Chapter 11 [Energy Efficiency]). When you introduce the criteria of affordability, however, the needle tends to shift toward more conventional, vented methods. This usually means using blown-in materials such as mineral wool, fiberglass or cellulose.
When it comes to selecting one type of blown-in product over another, the good news for builders is that if properly applied, they all perform well. In fact, as I wrote about in detail recently, some of the negative rumors about cellulose are exaggerated or unsupported. It won’t turn to dust. Nor is it fire prone. There are minor differences in product density among the different materials, however, as shown in the table below.