Which Building Will Win the UK Passivhaus Award for Small Projects?

The UK Passivhaus Trust awards are unusual in that the winners are chosen by popular vote by the members of the organization, which promotes the Passivhaus Standard in the United Kingdom. The trust is also unusual because it uses the German term Passivhaus rather than Passive House, which I think everyone using the tough standard of energy efficiency should—learn more about it at "What is Passive House?"—but that is another story.

We called the winner in the last awards in 2018, Juraj Mikurcik's Old Hollaway House, but that was easy; it is still one of my favorite Passivhaus home designs. This year, the competition in the small project category is not so straightforward, as they are three very different houses.

New Forest EnerPhit

New Forest EnerPhit is a renovation by Ruth Butler Architects of an existing house to the EnerPhit standard, which is a bit less restrictive than the new build Passivhaus standard. The house was originally rendered in plaster so putting insulation on the exterior didn't dramatically change its appearance, but it is so efficient now that a single electric radiator heats the whole house.

The owner is happy: "Our home is now using only a third of the energy and is much more comfortable, in all weathers. It is a pleasure to live in, the redesigned layout gives us more light and a better connection with the garden."

The house is a great demonstration of what we should be doing to millions of existing homes, renovating them to reduce demand for energy and then cutting off the gas so they sip a bit of electricity, with a little help from solar panels on the roof. (See all the technical data on performance here.)

Larch Corner Passivhaus

I described Mark Siddall's Larch Corner Passivhaus previously as a Passivhaus wooden wonder that shows how we should be thinking about carbon, noting "there are Passivhaus architects who design to hit the numbers but would insulate with baby seal fur if it did the job, not really caring about the sustainability of the materials used."

Siddall cares very much about the materials and has designed this house almost entirely out of cellulose. The walls are cross-laminated timber, the insulation is wood fiber, the exterior is Siberian larch. "The 12.6 tonnes of wood fibre dampens the amplitude of daily temperature swings and extends the time lag of solar heat reaching internal surfaces." Airtightness is a ridiculous 0.041 m3/hr/m2@50Pa, the tightest house in the U.K. and probably the 3rd tightest in the world, leakage is equivalent in area to a hole roughly the size of a nickel.

Larch Corner is designed to minimize embodied carbon, including the upfront carbon emissions that come from the manufacture of building materials. Building out of cellulose (or as I have called it, building out of sunshine) minimizes those emissions.

The house is a technical marvel, but I was always a bit troubled by the aesthetics, particularly the sawtooth facade. It turns out there were not a lot of choices; site restrictions determined the height and the shape of the land and setbacks are behind the sawtooth. Read more at UK Passivhaus Trust.

Devon Passivhaus

The planning process in England is often confusing. There are weird things like the country house exemption clause, Paragraph 79, which is described by RIBA as "a means of gaining approval for an exceptional one-off house on a site where refusal would normally be expected." McLean Quinlan Architects designed this house to the Passivhaus standard to make it exceptional, and it truly is, the house is an architectural stunner, among the most beautiful Passivhaus designs I have ever seen.

"The overall design is simple and clean. An elegant brick front complements the brickwork of the old garden wall and a discreet front door opening references the gate in the garden wall. Further along, an oriel window breaks through, hinting at what is behind. Elsewhere, external surfaces are dark render, designed to recede visually in deference to the surrounding garden. Tucked within, the house has a glass-roofed courtyard at its centre, a winter garden flooding light into the interior. Spaces are arranged around this central core so the building functions both as a home and a gallery for our clients, great collectors of ceramics and art, with spaces to display and curate."

The architects note that this was their first Passivhaus design but they "will now be applying Passivhaus principles to all their projects." Perhaps in the next one, they will think about some of the other issues that architects are concerned about these days, like embodied carbon. While the house is not insulated with baby seal fur, it is built out of a Structural Insulated Panel (SIP) made of stainless steel framing and six inches of polyurethane foam.

We have shown research demonstrating that over the life of the home, the upfront emissions from making PU foam are actually bigger than the operating emissions that the insulation saves. The architects say that the wall has a lower embodied carbon than bricks and mortar, but I am not convinced, and it is a dealbreaker for me. More at UK Passivhaus Trust.

And our vote goes to....

The New Forest EnerPhit is an important project, but I wonder if renovations shouldn't be in a separate category, it is very hard to compare them to new builds. It deserves an award of its own, but I did not think it should win this one.

I should declare a conflict of interest because I know Mark Siddall of the Larch Corner Passivhaus personally, meeting him a number of times at Passivhaus conferences, and have admired his work and thinking for years. I do not think my choice is biased by that, and can honestly say I was saved from the embarrassment of picking the Devon Passivhaus by a load of polyurethane foam.

Essentially, we have a carbon budget to stay under if we are not going to heat the planet more than 2.7 degrees Fahrenheit (1.5 degrees Celsius), and every molecule of carbon dioxide counts against it, which is why embodied carbon, or upfront carbon emissions, is the issue of our time, which has to be considered in every project. And Siddall's Larch Corner Passivhaus is a demonstration of how it's done. The Devon Passivhaus may be one of the most beautiful I have ever seen, but Larch Corner Passivhaus is the future of building.