“The most sustainable house is the one you don’t build” More retrofits being planned

17 April 2023 by Rachel Rose

New Zealanders have a vast job ahead retrofitting existing housing stock. Massively improving the thermal performance and energy efficiency of existing residential buildings is not optional if we are to meet our carbon emissions reduction commitments.

Work in progress by architect and Passive House designer Murray Robertson is highlighting the feasibility of deep retrofits to particular types of New Zealand housing. He is currently working through the retrofit strategy for one half of a 1940s duplex, a former state house, that was badly damaged by fire.

Photo: Murray Robertson

Murray bought the property with a plan to subdivide, build a new house and then tackle the retrofit. A destructive fire in September changed the order of work; now the first goal is to build back better. 

“The work that I have done so far, which I had reviewed by Sustainable Engineering, shows it’s very suitable for EnerPHit. It highlights how well-designed those 1940s state houses were,” says Murray. “I looked at EnerPhit on a stand-alone house and it was hard, I couldn’t get it to work on the modelling method. But this house is semi-detached and shares a party wall. Along with the two-storey cubic shape, it has a good form factor so it works easily with the modelling method.”

Initial modelling indicates EnerPHit certification could be achieved with just double glazing, possibly hitting full Passive House performance with the inclusion of triple glazing. The suspended timber floor has plenty of space underneath making it easy to insulate and the roof is steep, minimising the thermal bridging at the eaves.

The existing window layout won’t be changed. “The windows are really well optimised. It had good daylight inside but the window area isn’t too high,” Murray notes. He says other experienced designers have also found that old state houses are great candidates for retrofitting. “It really highlights how good the designs were.”

Because wall linings were damaged by both fire and water, this project will introduce the air control layer and service cavity on the inside of the house. This contrasts to the retrofit certified to the full Passive House standard this month, Till Cottage. The new windows will be locally made uPVC. Planning for ventilation is more challenging compared to a new build, says Murray, but also fun.

Murray has begun a blog series on the project that will chart progress and reflect on the history of state housing in New Zealand. He comments on the vital importance of improving the energy efficiency of our existing homes, saying “the most sustainable building is the one you don’t build,” but rather retrofit.

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