Here’s a new trick for cooler attics, energy savings, and reduced heat gain in HVAC systems.
As summer approaches, we’re back to a common reality: Keeping comfortable costs money. More than 90 percent of all new U.S. homes have air conditioning, but in places where electric costs are high, summertime air conditioning bills can surpass those for heating, even in cold climates.
There’s a good reason: recycled warmth. For example, the amount of heat from the sun that strikes a 1,200-square-foot roof is about 250 British thermal units (BTUs) per square foot, or 300,000 BTUs per hour.
This absorbed heat eventually gets re-radiated into the attic space, causing attic temperatures to easily reach 130 to 140 degrees Fahrenheit. That heat gets conducted and sucked into leaky, poorly insulated ductwork. To make things even worse, a high-mass, heavy roof with concrete tile absorbs the sun’s heat all day long and re-radiates it into the attic well into the night, further driving up air conditioning costs.
One inch of R-6 polyisocyanurate sheet foam on top of a waterproofing membrane can lower the surface temperature of the attic side OSB by 30-40 degrees.
Why Do We Allow Building Practices to Be So Inefficient?
The usual reasons: lack of building science knowledge and money. For decades, most new homes have been built on slab foundations, resulting in heating and cooling equipment almost always ending up in the attic—the absolute worst place to put it.
Heat always flows from hot to cold, and the greater the temperature difference, the faster the flow. The temperature of air-conditioned air passing through ductwork is about 55 degrees; the temperature in the attic can easily be twice as much. That heat is conducted and leaks through the ductwork, and it heats the air you have already paid to cool. That is a tremendous temperature difference for your A/C to overcome.
Studies from the California Energy Commission found that the surface area of duct work for a typical home is about 40 percent of a home’s floor area. Some basic heat transfer math, for a 2,000-square-foot home, shows that attic heat conducted into R-2 insulated duct work is about 30,500 BTUs per hour, or more than 2.5 tons of air conditioning load.
Even with today’s code requirement R-8 duct work, the additional heat load is 8,000 BTUs per hour, or 0.67 tons. This is why smart energy experts want HVAC equipment and ducts out of the attic and into conditioned spaces. But finding room for HVAC units and ductwork in living spaces can be challenging and cost more.
Another option is non-vented or sealed attics where spray foam is applied to the underside of the roof deck. These attics are usually within 5 degrees of that of the living spaces, making this an efficient but more-expensive option.
A Less-Expensive Option to Reduce Attic Heat Gain Into Attic Ducts
In my own real-world tests, I used a less-expensive option that provides close to the same result. Applying 1-inch R-6 polyisocyanurate sheet foam (Dow Thermax) on top of a waterproofing membrane and OSB roof deck lowered the surface temperature of the attic side OSB in a vented attic by 30-40 degrees.
It’s always important to check with the roofing manufacturer that the system you select is compatible with this system. The California Title 24 energy code has had above-deck roof insulation as an option since 2016 for high-performance attics. I suspect 1 inch of extruded polystyrene R-5 Styrofoam would yield similar results.
The image below clearly shows the difference between the roof deck insulated to R-6 at left and no insulation on the right.
For my Revision House Scottsdale project, we installed 1 inch of Thermax over the OSB roof sheathing and roofing membrane in conjunction with a non-vented attic, creating R-54 insulation over the pitched roof areas. I expect that this will perform very well, significantly reducing my cooling loads and costs. I am also using a low-mass, metal, ventilated shingle roof which does not store much heat, thereby reducing continued heat gain into the attic after the sun goes down.