You can call Lynn and David Horobin crazy for not building their family’s new home with air conditioning — after all, isn’t the globe warming?

And surely they know that Napa’s Alta Heights neighborhood, which climbs the city’s eastside hills, gets more than its fair share of the afternoon sun that blares through west-facing windows, turning many a home into a hotbox for hours every day.

But the couple stoutly defend their decision not to install a system many people would consider essential.   

“I’m sticking to my guns, because I know this house is not going to need air conditioning,” said David Horobin, an architect whose design for the “green” home has been taking shape on Montecito Boulevard since June.

For one thing, the Horobins’ home won’t have enough west-facing windows for afternoon sun to be able to heat up the house. They’re aiming their views to the east and south, while north-facing windows will carry cooling breezes through the home and out upper-level skylights in what the architect called a “thermal chimney.”

But the home’s first layer of defense against temperature extremes is on the ground floor — literally: A full-length crawl space underneath the house is insulated with nothing more than a sheet of plastic over the dirt.

“We’re literally connecting the thermal mass of the earth to the house,” the architect explained. “The thermal mass of the earth is always 55 to 60 degrees, so the earth is cooling down that crawlspace all the time.”

The crawlspace helps cool the first floor, while the house’s 6-inch-thick concrete walls, sandwiched in inches more of polystyrene foam, will absorb and trap environmental heat before it can raise the temperature inside.

According to Horobin, these walls have an “R value”— a measure of resistance to heat and cold, used to calculate the effectiveness of building insulation — of 60, making them some four times as energy-efficient as traditional house walls.

“Once this energy is in the concrete, the foam on the outside keeps the energy from going through,” said Horobin, explaining that concrete “loves to absorb energy.”

The concrete-inside-foam blocks he’s using to build the walls provide “as ideal a thermal sandwich as you can get,” he continued.

The roof system is also modular and sandwich-styled for energy efficiency: The Horobins’ roof will be assembled from a set of numbered panels called SIPS, for “structural insulated panel system.”

Each panel encloses 9 1/2 inches of expanded polystyrene foam inside two slices of a half-inch-thick material called “oriented strand board.” Also known as OSB, oriented strand board is made from shreds of recycled wood, and is most easily described as “the next generation of plywood,” in architect Horobin’s words.

Because of the sandwich system, the roof’s R value of 45 will again beat that of the traditional roof, which loses efficiency with traditional wood framing, Horobin said: “It ends up being twice the value of an R30 roof.

“Forever.”

The roof panels — which the Horobins are considering assembling on the ground, then lifting atop the home — will be topped with traditional-looking, recyclable roof tiles made from natural clay.

Horobin, who has earned a Green Building Professional Certificate from Sonoma State University, also designed the home’s roof overhangs to limit overexposure to the sun.

“We will get warmth coming through the windows in the winter, but the sun will be at a high enough angle in the summer that it will shade the windows,” he explained.

Energy-efficient, double-glazed windows — not “exceptional windows,” but “very good windows”— will also help cut down on what’s termed “solar gain,” Horobin said.

What Horobin calls the “envelope” of the house is designed not only for comfort during hot weather, but for retaining warmth in colder months.

Instead of a furnace, the family will heat its home with a water-to-water heat pump that transfers heat from a solar-powered swimming pool to a radiant heating system inside the house.

As in summer, the Horobins say their winter energy savings will start in the crawlspace, with its dirt floor covered only in moisture-retaining plastic.

“In the wintertime, that 55 to 60 degrees acts in our favor as well,” Horobin explained. “We need less energy to get up to 72 degrees.”

Lynn Horobin added, “And once we get it up to temperature, it’s not going to leach heat out of the walls.”

This is part four in an ongoing series about David Horobin’s construction of an envirnmentally responsible home in Alta Heights.