Philadelphia-based developer Postgreen planted its roots a few years ago with a lofty mission: to build a LEED-Platinum house for just $100,000 in labor and materials. Its second project—a Passive House–certified home—was just as ambitious.

But the overall goal was simple: Learn and master the techniques and designs that will accomplish high-performance building at a price middle-class home buyers can afford, all the while promoting the value of architect-designed spaces. “We wanted to take all of that information and try to refine the house we want to build,” says Nic Darling, chief marketing officer. “Our long-term goal is to have a repeatable sort of product.”

One of the company’s most recent follow-ups, the three modern rowhouses that make up the Skinny Project, did just that.

The two-bedroom, one-bath LEED-Platinum homes, ranging from 1,300 to 1,400 square feet, get their names from the “Skinny” floor plan developed by Interface Studio to accommodate narrow 13-foot lots common to older urban neighborhoods such as those in Philly. Designed “to be built narrow but live wide,” the open floor plans position anything immobile along one wall, with the kitchen, bathroom, and mechanicals extending only 4 feet into the living space. Mobile closets allow homeowners to customize bedroom sizes and create flex space.

Post Green used SIPs for the 100K and Passive houses but, after dealing with the challenges of storing and maneuvering panelized systems in the tight infill environment, now opts for double-stud walls with advanced framing. Air sealing was conducted to Passive House requirements and wall cavities were insulated with blown-in cellulose to achieve R-33; the R-55 roofs also feature blown-in cellulose along with 2 to 5 inches of IsoBoard.

The airtight envelope helps cut down on HVAC capacity, and each unit employs two mini-split ductless units. A Shuco thermal solar system provides 70% to 80% of hot water needs, and all three buyers opted for the 3-kW photovoltaic array upgrade.

Other efficiency-boosting features for the homes, which achieved HERS scores from 23 to 26, include exterior solar shades, CFL lighting, and Energy Star–rated appliances. An electric coil added to the 95% efficient energy recovery ventilators provides supplemental heat for the ductless units on extremely cold days.

In addition to the ERVs, the team speced low-VOC and no-added-formaldehyde finishes and interior products.

Careful attention to the number of fixed versus operable windows, and using casement and awning units instead of double-hung, are one of the ways the developer battles moisture infiltration. The crew used Furring Master strips to create the vented rainscreen between the ZIP System exterior sheathing and the CertainTeed fiber-cement siding. Another change from previous projects, these 1/4-inch metal furring strips are reliably straight, Darling says, while their U-shape means only the edges are touching, providing for maximum drainage space.

Including solar upgrades, the homes sold for between $250,000 and $300,000, a figure that, in the Philadelphia market, achieves Postgreen’s mission to provide ultra-efficient homes at a rate their clientele—largely teachers and small-business owners—can afford.

“A lot of the problem people are having when trying to build green homes is that they’re not starting from scratch, they’re starting with existing houses that they have … so that adds 15% to the cost,” Darling says, noting that simply tacking on green features is costly. “If they took a step back and design from the ground up they may only add 5%.”

In addition, each project helps Postgreen refine processes to make their green homes more affordable. In fact, Darling says an improvement in process and planning is one of the biggest keys to increasing affordability. Design and documentation cut down significantly on waste.

The developer also saves money in how it markets the homes: What’s initially provided is a fairly basic house, keeping base costs lower, but online design tools make it easy for buyers to upgrade and customize. Finally, the team focuses on energy efficiency and health details, rather than on flashier products that might add expenses with little ROI, such as recycled materials.

Buyers are certainly intrigued. A startup company with just eight homes under its belt (all of which pre-sold), Postgreen now has more than 30 infill units in development. “We were fortunate to introduce a product that costs less than a lot of what people were looking at, and it certainly costs less in yearly maintenance and energy.”

Darling also attributes some of his team’s successes to its design-oriented approach. “We’re trying to provide a product to a group of people out there who like design but never imagined they could afford a house designed by an award-winning architect,” he says. “I would say that appeals to people as much as the sustainability elements.”

Katy Tomasulo is Deputy Editor for EcoHome.