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An unassuming light-gray cottage that overlooks the railroad tracks in Shirley, Mass., is Dave Posluszny’s fort.

Once the sun hits the house and warms it up inside, the heat barely finds its way out. The house has no chimney or piping in the walls for warm air to escape through. The windows let in the light but do not open.

The four walls, stuffed with insulation a foot thick, keep heat from appliances and electronics like the refrigerator, TV and computer trapped in the 900-square-foot space.

If you wonder how that all adds up, just look at the thermometer on the kitchen countertop, Posluszny says.

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The house has no furnace, except for a small electric-heat baseboard in the crawl space that he turns on once in a while on cold nights. The house stays at 70 degrees even when a polar vortex is ravaging the region.

In this house, every square inch is calculated to reflect sunlight effectively and keep the air clean and flowing.

“Everything serves a purpose,” Posluszny says. “You can’t change one thing in the house without affecting the other.”

Posluszny built the house himself. The 31-year-old, who works as an engineer, always took on construction jobs while attending high school and college.

In 2012, after coming back from Afghanistan — his second tour of duty overseas as a member of the Army National Guard after an Iraq tour in 2005 — Posluszny decided it was time to use the knowledge and skills he gained in school and on the job to create a comfortable house for him and his wife.

Among the most noteworthy features of the house are the double walls all around it.

Posluszny built separate walls running inside the conventional exterior walls, making sure that they do not touch each other.

That’s because wood studs conduct energy, providing a path for heat to escape from inside, he says. He filled the gaps between those walls with cellulose insulation — the densest kind he could find.

The insulation he used is mostly made of clean, recycled paper — such as from overstocked books that had to be thrown away — mixed with borate, a mineral that also serves as an insecticide.

He also created a 4-foot space for insulation in the scissor truss to keep warm air rising toward the cathedral ceiling from escaping through the roof.

Energy audits show typical older New England homes have air leakage of 4,000 cubic feet per minute or more. Posluszny’s house has air leakage of 22 cubic feet per minute. He says one could suffocate in the house without the air exchanger that is constantly running.

The house has the living room in its center with a catwalk above to connect loft spaces on either side. The railings for the staircases, catwalk and the loft double as clothes dryers. But don’t worry about humidity, Posluszny says. The heat pump attached to the hot-water tank removes moisture from the air as the air passes its chill coil.

The tank is installed directly above the bathroom in the loft space to minimize the distance for the hot water to travel and limit heat loss.

Posluszny painted the ceiling with high-gross paint to take most advantage of the light coming through the windows.

“It reflects the light like a mirror,” he says. “The light bounces around here.”

All windowsills are deep because of the thick walls. They double as nightstands in the bedroom and as glass shelves in the kitchen.

With 4.5-kilowatt solar panels atop the roof that produce more energy than he could use during the day, his electricity bills are net zero at the end of each year. The house Posluszny razed to build the new one had an oil tank, a wood stove and a propane tank in addition to electricity.

“I knocked it down to one,” he says of energy sources.

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