Rigid Foam Insulation Can Armor Your Home Against Extreme Heat

As parts of the United States experience life-threatening temperatures, the first, most effective line of defense may be a thin layer of rigid foam between your home and the sun.

Unless you’re a building scientist, you probably don’t think about home insulation much. Or you didn’t think about it until temperatures started to hit triple digits in the United States for a week at a time. Now, along with purchasing air conditioners, you’re probably looking at upgrading your insulation, replacing some old windows, finishing a bedroom in the basement (where air stays cooler), or moving to a cooler climate.

The problem with the latter solution, however, is that with climate change in the equation, there’s no guarantee that anyplace will stay cool for long.

Rethink Insulation Products 

You can achieve some short-term relief at home at a relatively low cost, by adding a modest layer of rigid insulation to the exterior of your roof. This creates what’s known as a “thermal break.” Experts say that even an inch may be enough to dramatically reduce heat gain inside a home during the day.

Steve Easley, a building science consultant in Scottsdale, Ariz., is remodeling his own home with Green Builder, and dealing with never-before-seen hot temperatures, so he’s keen on cooling the conversation. He’s putting a stone-coated metal product over a roof that is covered with 1 inch of external rigid Thermax foam, aiming to bring indoor attic temperatures down to that of ambient outdoor air.

"I know some of the researchers who helped put together the standards for California's Title 24," Easley notes, "and during the years prior to that high performance standard, they did some tests. They found that roofs with just an inch or so of foam above the deck resulted in attic temperatures close to ambient outdoor temps."

rigid insulation

Insulating above the roof rafters and sheathing (this UK example had no additional layer of wood sheathing) can drastically reduce heat gain inside the attic or living space. Insulation can also be applied over an old asphalt roof. Note: Ecotherm is a UK brand. Credit: Martin Pettitt

“You’re trying to keep the heat from ever getting inside the home envelope,” Easley says. “By adding an insulating thermal barrier above the roof deck, any additional insulating you do below the roof deck offers optimal performance. Instead of just keeping the heat out, the roof assembly holds the cool, conditioned air indoors. Your HVAC doesn’t work as hard, and your energy bills go down.”

Roofs still need interior insulation, for optimal performance, of course, but as Easley notes, unvented attics, sealed with closed-cell spray foam, offer huge energy advantages. The key to succesful installation: careful sealing of ceilings and walls below the attic to keep out uncontrolled moisture

"Keep in mind, that most ducts running through an attic have only an R-8 insulation blanket at best," Easley adds. "Most are closer to R-2. That's a huge amount of surface space to run through a hot attic. By controlling ambient temperature in the attic, you can greatly reduce the amount of air conditioning (or heating in cold climates) that you need."

You can find a lively discussion about the best practices for installing rigid foam on the roof, prior to installation of the final roofing. When using this approach, you need to keep in mind certain limitations. 

Installation Caveat: Thermax and other polyiso-based products have a compressive strength of about 25 psi, so they need to be installed with care. For example, they mustn’t be crushed by contractor boots or placed in such a way that wind can get under them or cause them to shift. Some experts recommend cover boards , although a thoughtful roof design and installation may allow you to skip this step.

Battens: Yes or No?

You’ll occasionally hear some disagreement on contractor forums about whether the addition of battens beneath certain types of roof material such as metal or concrete tile. Proponents say that battens enhance a roof’s durability by creating a thermal gap that also disperses heat.

Battens won’t work under asphalt shingles, of course, but for most panel-type roof products, they’re a low-cost detail that will further reduce your roof’s potential heat gain. With roof temperatures regularly exceeding 150 degrees Fahrenheit in many parts of the United States, they keep roofing from making direct contact with surface underlayments such as self-stick membranes. 

These membranes have different characteristics with regard to heat and some may be damaged or degrade more quickly. Make sure the one you choose can handle a regular cycle of hot and cold.

Over the Top: A More Sustainable Re-Roof Scenario

So let’s say you own a home in the Pacific Northwest, and you’re baking under the latest heat “dome.” You have an asphalt roof that’s nailed right to your roof, dumping about 45000 BTU/ sq ft/ day into your living space. Tearing off that roof to put insulation on the plywood underneath and re-shingle might cost you another $15,000, not to mention the waste created when you tear off all that old asphalt. You’ll also lose many years of the existing shingle lifespan.

One option to consider is the addition of the rigid foam directly over the top of the old shingles, followed by wood battens. These battens will form a nailing grid to which you can attach standing seam metal roofing or cementitious roofing panels. If you angle the battens at a 45 degree angle toward the peak of the roof, they can also act as natural channels to transfer hot air out the top of the metal roof assembly, further reducing heat gain into the home.