From the outside, the Victorian cottage looks like many of the other early 1900s homes on the block in the Whittier neighborhood. Inside, handsome cherry-stained woodwork and a built-in buffet is classic “This Old House.”

But beneath the exterior siding is a thick layer of foam insulation that offers airtight energy efficiency. A geothermal system heats and cools the rooms. And, if you stand in the alley and scrutinize the roof, you might see an array of solar panels glinting in the sun.

Homeowners Stewart and Linda Herman have retrofitted their 1907 two-story into a “net zero” home that produces all the energy it uses. You can take a look at their retirement retreat and learn the details of their innovative rehab on the Minneapolis & St. Paul Home Tour April 29 and 30.

It’s the 30th year of the tour, which promotes city living and diverse walkable neighborhoods, from Jordan to Mounds Park. Owners will show off their new and improved kitchens and bathrooms, finished basements and attics, suite additions, historic restorations, and, like the Hermans, their strategies for going green.

Stewart and Linda, who were teaching at Concordia College in Moorhead, Minn., planned to move to Minneapolis once they retired. Instead of building new, they decided to buy an older home they could update, renovate and make as energy efficient as possible.

“We take climate change very seriously and believe in living sustainably,” said Stewart, who has written a book and articles on tapping new sources of energy. “And we wanted to do it on a typical small lot in an urban setting.”

In 2014, their daughter found the perfectly imperfect property in south Minneapolis: The two-story Victorian was in foreclosure, had water damage in the basement and had suffered an ill-fitting addition by a previous owner.

Still, it boasted a south-facing roof and was in a walkable area close to public transit, the Wedge Co-op and Lake of the Isles. The Hermans felt it was definitely worth saving.

For the planned renovation, they wanted a big bright kitchen, an owners’ suite with double closets, two offices for writing, as well as the sunroom getaway Linda had always dreamed of.

And finally, Stewart hoped to transform the 100-year-old dwelling into a net zero house, producing as much energy as it uses for heating, cooling, lights and appliances on an annual basis.

Initially, Linda wasn’t totally on board with going green.

“I had a misconception that net zero meant a cold dark house,” she admitted. “I wanted it to be attractive and comfortable.”

To explore the possibilities of a net zero remodel, the Hermans enlisted SALA architect Marc Sloot, who has experience in green and sustainable building. He crunched the numbers to determine if a net zero renovation would be feasible — and affordable — for an existing house on a small city lot.

“It’s not just about making it net zero,” said Sloot, “but also reusing the 1907 house and preserving some of the traditional details.”

Sloot designed a reconfigured floor plan with a new maple and birch staircase, a light-filled galley kitchen and updated electrical and plumbing. He also designed a small two-story bump out on the back of home to create space for a mudroom, an expanded owners’ suite and a sunroom. To make the addition seamless, Sloot followed the form of the original house, repeating architectural details, and adding a gable and Victorian art glass windows.

For the interior, Stewart went into recycling mode. He removed all the original wood trim, stacked it in a truck and drove it to his former home in Fargo, N.D., where he sanded and refinished, then later reinstalled the boards. Other recycled materials include 12 of the original doors and two stained glass windows.

“I drove back and forth from Fargo about 50 times since buying this house,” he said.

After two years of planning and construction, the Hermans moved into their home, which they named Bella Luce (“Beautiful Light”), last fall. (They’re in the process of applying for the highest level certification from green building programs LEED, GreenStar, Green Path and Living Building Challenge.)

For all involved, including Morrissey Builders, the renovation has been a success. “It’s a shining example of how this can be done in an established neighborhood on a tight lot,” said Sloot.

Linda is delighted “at the quality craftsmanship and how comfortable it is. There’s lots of nooks and crannies and old-house character.”

And Stewart is passionate to share the experience with others on the home tour.

“It’s only one house,” he said, “but we hope we’ve set off ripples and inspire others to do what they can to reduce their carbon footprint.”