When Claire and Cavin Costello founded their Phoenix-based architecture firm more than 15 years ago, heat was always part of the picture.
“Living in the desert, it’s just a daily-life thing when it’s 110 degrees for a month,” said Cavin Costello, co-founder and principal architect of The Ranch Mine. “And you just have to deal with it, or else you’re running your air conditioning 24/7 and paying thousands of dollars a month.”
Arizona sits right along the United States-Mexico border and is, bluntly, very hot. The arid region contains bits and pieces of all four U.S. deserts, and, during the summer months, temperatures in its metropolitan hubs regularly hover above 100 degrees Fahrenheit. Arizona has gotten about 2 degrees warmer over the past century due to climate change, according to the U.S. Environmental Protection Agency, meaning that the state’s residents will face increasingly extreme warm-weather conditions — and cooling costs — in the years to come.
But there are ways to mitigate those cooling costs that an architect can bake into the very structure of a home, Costello said. Usually, the firm outlines the options for potential clients through the lens of three heat transfer methods: convection, conduction and radiation.
Often, homeowners think about convection, or heat moving through the air, when they’re considering high temperatures, Costello noted. Dealing with convection heat often means sealing a home’s building envelope, ensuring that wanted air (generally that’s the cool air during summer months) stays inside and that the unwanted air (the oppressive heat outside) doesn’t filter inside.
“The first thing that we want to do is we want to make airtight buildings that allow us to control where we're bringing in the air and what air is coming in,” Costello said, often through zip sheathing, tape and high-performance windows that offer “really tight seals.”
By orienting those windows intentionally, however, architects can optimize a home’s cross ventilation.
“Simply put, windows on two different sides of the build,” Costello explained. So, as temperatures cool at nightfall, homeowners can crack open their windows, funneling air through the building to flush out hot air and passively cool the home.
When it comes to conduction — or heat transferring through a material — mitigation tends to come from insulating. But the typical U.S. construction method of sandwiching insulation between a home’s wood studs is a big mistake, according to The Ranch Mine.
“The problem with that is that wood is a decent conductor,” Costello said. “So if you're only insulating between the studs, anything that hits the exterior of your building gets into that wood stud and comes through.”
In the architecture world, this transfer is called “thermal bridging,” and the design solution is cocooning a project in exterior insulation, which Costello likened to “putting a coat on your building.”
Radiation is Costello’s final method, one he said “people understand the least, but is actually the most impactful.” This is the temperature increase that accompanies sunlight, so it’s what warms your skin when you stand in a sunny patch and what raises the temperature inside a room when sunlight pushes through a window, flooding a home’s interior.
In the desert, a climate known for particularly grueling thermal radiation, architects can focus on shading and orienting a building to control when direct sunlight comes into the home, Costello specified. Reflective materials can also play a part, he noted, which is why rockets boast a shiny metal skin.
“If you can think of it in those three ways, that's the best way to sort of mitigate the heat into your house,” he said.
A ring-shaped home that operates ‘off peak’
For one project on the north of Phoenix Mountain Preserve, The Ranch Mine married these strategies for a 4,090-square-foot private home dubbed “O-asis.” The low-rise residence is white (“so it’s very reflective, which is great,” Costello said) and its windows are tucked under generous overhangs, strategies addressing thermal radiation. Its stucco facade hugs layers of exterior insulation, preventing thermal bridging, and the home’s ring-shaped form addresses convective heat using an ancient strategy for cooling: the courtyard.
“Courtyards have been used for thousands of years,” Costello explained. “You can essentially create like a microclimate in there that’s shaded the majority of the time.”
A rectangular pool behind the home helps, too: “it can add some humidity to the air, which in the desert cools it,” he noted.
With its passive strategies doing what they can, O-asis also has a solar array and Tesla batteries. The nearly 13-foot-tall home isn’t net-zero, but “it’s close,” Costello said.
As the sun beats down on Phoenix, the solar panels soak up energy and recharge its batteries, storing up enough power to keep the house “off peak,” according to Costello.
“A lot of the energy usage [in Phoenix] is 3 to 6 p.m., when the sun is at its hottest,” he explained. But once it gets to 2 p.m., O-asis is “running fully off of the battery, so they're not tapping into the system at all during that peak load; they're using all solar power throughout that time all the way into the night.”
‘We cool it with ideas’
But selling clients on passive design strategies that help keep both temperature and energy bills in check isn’t straightforward, Costello noted. Resilient building comes with a higher upfront cost that can be harder to stomach for features that pay off in the long term.
That’s one of the reasons The Ranch Mine started working on a Sedona project dubbed "Fox and Kit." Part design lab, part custom home, the low-energy-use house has all the passive cooling strategies that the firm might suggest for a client, plus a few extras.
“Our philosophy is: Before we can cool a home with machines — we still need air conditioning — we cool it with ideas,” Costello said.
For Fox and Kit, those ideas pair careful shading, orientation, and a multilayer reflective building envelope with “all the nice finishes and features that we want,” Costello said, such as an advanced lighting system and stacking glass doors.
The Ranch Mine is its own client for the home, but the firm is building it as a twist on a model home. Instead of using it for walk-throughs, The Ranch Mine plans to use the single-family project as a rental that allows potential clients to test out this high-efficiency mode of living.
For other high-dollar purchases, such as cars, a client can test drive the product before purchasing, Costello noted. But that’s not possible with custom homes; clients often have to decide based on photos or videos.
“You’re not able to live the experience of the difference of these places,” he said. “And so we thought, well, why don't we make a home that shows all the stuff and how it mitigates heat?”