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Green Building News

U.K. Builder Trying Again on Eco-Home Protoype

Stewart Milne Group’s first eco-home, Sigma, didn’t provide the airtightness and thermal resistance expected, but the firm is applying what it learned to Sigma II

First of its kind. Sigma, the eco-home prototype designed and built by U.K. based, Stewart Milne Group. A follow-up project, Sigma II, is being developed to improve on the efficiencies of the original.
Image Credit: Stewart Milne Group

Members of the family that had been recruited to live for two years in a prototype eco-home at the Building Research Establishment, in Watford, England, said they largely enjoyed living in the house and participating in the evaluation of its performance, according to the home’s builder, U.K.-based Stewart Milne Group.

The family was, in fact, sad to leave the home. But the harder issue to reckon with fell to Stewart Milne, whose data on the building’s performance showed that the building, called Sigma Home, did not operate as efficiently as expected. Citing an article by U.K.-based Building magazine, GBA noted in early June that performance data showed Sigma’s overall heat loss was about 40% worse than expected and its energy consumption was almost twice that of Passivhaus standards.

Lessons learned

The builder blamed deficiencies in several areas, including Sigma’s renewable-energy fixtures. The energy contribution from the home’s wind turbines, for example, was negligible (one turbine also fell from the roof, raising safety concerns). And the building’s PV system produced only about half the consumed energy – not enough to offset the cost of the system, according to the builder. The most critical issues were pegged to the thermal resistance of the shell and its airtightness.

“It is clear the answer lies in improving future build systems, with a focus on airtight solutions, heating and ventilation systems and taking a ‘heart and lungs’ approach to the installation of services at the concept stage,” Stewart Delgarno, director of product development at Stewart Milne, said at the time.

The builder has begun putting its Sigma analysis to work in a project called Sigma II, which will incorporate a number of strategies designed to make this second prototype home not only airtight, energy efficient, and more comfortable to live in, but also less expensive to produce.

The walls of the first house, for example, featured OSB facings separated by solid timber studs with fiber insulation between them. The builder found that each stud created a thermal bridge that contributed to heat loss, and attempted to remedy the problem by installing a reflective vapor-control barrier between the inner face of the wall and the drywall finish.

A manufacturing plan

Installation of the barrier, unfortunately, is relatively cumbersome and expensive. It also didn’t work as well as expected. For Sigma II, a recent Building story explains, the builder has decided to use a wall-panel system that features metal webs, rather than wooden studs, that create more of a thermal break and allow the builder to use less expensive blown-in fiber or expanded polystyrene beads in wall cavities. Rather than installing the vapor barrier over an entire panel set, the OSB joints will be taped.

Just as significant in terms of reducing cost and improving airtightness is Stewart Milne’s plan to prefabricate wall sections, which also will allow windows to be preinstalled using a flexible membrane bonded to the window frame and the panel wall. The walls are transported with the windows in a recessed position. Once a wall is in place onsite, the flexible material allows windows to be pushed out from the inside and properly aligned with whatever veneer will cover the outside of the wall.

The eventual goal is to develop processes for manufacturing building parts that can meet any of the top four most stringent ratings in the six-tiered Code for Sustainable Homes, Britain’s environmental impact rating system for residential construction.


  1. Dape | | #1

    Eco building
    It's clear there are many problems with this builder's Eco house. It gives the impression that it's not meant to be lived in. I would have thought the designer would have made the house more pleasing and child-friendly. Reminds me of the opening graphics to the seventies series 'The Prisoner.'

  2. Dape | | #2

    Eco building
    Looking at the house again, it seems to be that the shape is all wrong if you want to cut down on the heating. Surely the windows would have been better to be angled, giving a greenhouse effect with specialized blinds to control the effects of convection.

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