About an hour’s drive north from Boulder’s city center, a small cabin-like structure sits nestled tightly between three healthy Ponderosa pines. Its south face, with seven giant triple-pane windows, overlooks Rocky Mountain National Park and lets in rays of wintery sunshine. Andrew Michler, the building’s mastermind, architect and builder, calls this 1,200-square-foot space home. With unmistakable pride, he also claims it’s Colorado’s most energy-efficient house.

The building is in fact Colorado’s first internationally certified “passive house,” or, according to Michler, a house built according to a “globally-recognized building design technique that promises huge cuts in energy use.” By using super tight insulation and thick south-facing windows — trademarks of the passive house — Michler’s home relies almost entirely on its natural surroundings for its electricity, heat, cooling and water.

He doesn’t pay a gas bill; the sun does the work instead of oil. A 1,500-gallon rainwater catchment tank supplies all non-potable water. All of the electricity flowing through the sockets also comes from the sun. When it’s too hot inside, Michler simply opens the windows or blows air through a series of tubes that he installed underground, so as to let the earth cool down the air before diverting the streams inside. Altogether, Michler’s house consumes about 90 percent less energy than his neighbors.

“The passive house works surprisingly well in Colorado,” Michler says, due in part to the state’s 300-plus days of sunshine. However, it’s not just the sun that makes the off-the-grid house almost completely self-sustaining; its calculated structure and layout also play vital roles in its efficiency and success.

The wool-insulated walls — made of natural, durable, recyclable materials — are completely airtight, which allows Michler to precisely and reliably control the indoor air quality and temperature. The triple-pane windows are designed to maximize solar gain and minimize energy loss, so the sun can effectively heat the house, even in the dead of winter. This unseasonably warm snow season, Michler only employed an external heating unit once, when the internal temperature dropped below 62 degree Fahrenheit. (Outdoors it was minus 10).

Inside, Michler’s wife and two adopted stray cats, Serendipity and George, amble around the lightly colored wood living room. Inspired by Japanese tiny home architecture, Michler designed the whole house to be multi-functional, with movable, multi-use furniture, like the staircase with removable wooden storage boxes that can be used as tables or stools when guests are over.

“Most of the rooms flow together and can change use over time,” Michler says. “Depending on how many people there are, either two or 10 people, we can make everyone’s stay really comfortable.”

The passive house concept was born in the early ’70s by a group of experimental engineers at the University of Illinois. The 1973 OPEC Oil Embargo banning foreign petroleum exports to the U.S. (repealed by the Obama administration in 2016) had sent fuel prices skyrocketing and thrown household budgets in panic. Running the furnace harder and longer to heat drafty, cold houses was no longer a viable option. Thus, it fell upon structural changes in home building to combat the poor insulation and leaky windows and doors that made regulating warm indoor air nearly impossible.

The engineers at the University of Illinois pioneered a highly insulated building model they called the “Lo-Cal house” in 1976. The model, compared to the most-efficient design promoted by the Department of Energy at the time, projected saving 60 percent of energy consumption. A Canadian group of engineers caught on and rolled out an even more efficient “superinsulation house.” When solar energy entered the scene around the same time, William Shurcliff, a Nobel-prize winning physicist at Harvard, coined the term “passive house” to reflect the increasingly multifaceted nature of these high-performance buildings.

By the end of the ’80s, Shurcliff, who was as much concerned with helping household budgets as he was saving the planet from environmental disaster, summarized in the 1986 Energy Review what he considered the path of future building construction.

Passive houses should include five main principles, he wrote: thick insulation, airtight construction, prevention of moisture accumulation, steady fresh air supply and optimum window usage. When U.S. conservation initiatives waned in the ’90s, Wolfgang Feist, a German physicist, built upon Shurcliff’s work, further refining the modeling and product design while sticking to Shurcliff’s original five-pointed framework. These points became the cornerstones of the modern passive house.

In 2015, space heating was the leading cause of energy use in residential homes, according to the U.S. Energy Information Administration. Initiatives like Michler’s passive house and like-minded projects are pioneering the possibility of major energy reductions in everyday homes.

The airtight seal around the house, while necessary for regulating temperature, also creates a prime environment for mold and condensation, hence Shurcliff’s concerns about moisture control and air supply. To avoid these issues, Michler installed an air-exchange system that uses two fans to circulate air around the house. Essentially, one fan blows out stale, used air and another fan sucks in fresh air from outdoors. Both streams pass over a mechanism called a heat exchanger located under the house, which ensures the heat from the indoor air can effectively transfer to the new air with no energy lost.

However, activities like cooking in the airtight space can be problematic, according to Michler. For appliances that need high-density energy, like a stove and his back-up space heater, he uses a small amount of propane gas that he purchases when necessary. But food and gas particles can be released into the air and potentially linger. He teamed up with University of Colorado Boulder’s Indoor Air Quality and Health research lab to ensure he was creating a healthy, livable space.

Ryan Militello-Hourigan, a graduate student at CU Boulder working on the research team, visited the house last fall and conducted several experiments to test its air quality.

“We found that overall the house has very good air,” Militello-Hourigan says. “[Michler] was conscious of potential problems when he was building, and so he used healthy materials. The thing we did notice was that the overall air exchange rate was fairly low compared to conventional houses.”

Michler isn’t a formally-trained architect. Everything he knows is self-taught, accumulated over years of writing about and working in sustainable construction. In 2012, he finally decided he’d try and create for himself what he’d been eyeing in magazines and theory books for years. Only seven months after initially sitting down to plan his house, Michler broke ground.

“Our team was young, not age-wise but experience-wise,” Michler says. “There is tremendous pressure [when building a passive house]. You have to get everything just right. One or two things wrong can compromise the entire system.”

Michler spent most of the past three years building and refining his home, staying involved in every step of the process. He predicts his home cost about 10-15 percent more than a conventional home, but was somewhat offset because he already owned the land, and the house created a much smaller footprint.

Last year his house received its certification from the German Passivhaus Institute. Building upon his acquired knowledge and riding the stoke of his success, Michler founded Passive House Rocky Mountain, an organization that now trains and supports architects, builders or anyone interested in constructing and certifying their own passive house along the Front Range. He envisions the possibility of schools, community buildings and many more homes transitioning to passive design, creating healthier spaces for future generations.

“I’m looking forward to seeing how far we can push the envelope, especially in trying to make [buildings] more affordable and beautiful,” Michler says. “Shelter is as much about comfort as it is [about] energy performance.”