Cork retrofit blitzes new build NZEB standard

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Completed: February 2018
Budget: €280,000
Standard: Nearly zero energy
building (NZEB)
€40 per month gas bill

If you’re a passive house advocate in a position to do a deep retrofit but your budget is tight enough that you might have to make some fundamental compromises, what should they be?

Such compromises might come down to spending less on the heating system and other technological solutions and more on the fabric of your home – or vice versa.

Architect Loic Dehaye knew exactly what his priorities were when he finally got the keys to a three-bed semi in Ballinlough, on the southside of Cork city.

He’d had his eye on this 1930s-built house for two years and even missed an opportunity to buy it at one stage, but with a sale having fallen through no less than three times, he managed to snap it up on his second attempt.

The finished result is beautiful, inside and out.

It’s easy to see why he persevered. Its location, mere minutes away from both schools and work, would limit the family’s daily commuting times and help to reduce CO2 emissions. Its south-facing rear aspect, suitable for large sections of glazing, would ensure a bright open-plan living space that could open out to the garden and maximize solar gain.

And being a semi-detached house, enough space was available to extend out sideways yet still retain side access for bins and bikes etc. “I knew it had great potential,” says Dehaye.

“It also was derelict, giving me a good opportunity to go for a deep retrofit.”

As well as a deep retrofit, the €280,000 budget incorporated the building of a two-storey side and rear extension that would make the house into a four-bed with an open-plan kitchen and living area, a guest toilet, a storage room, a utility room and a converted attic space/play room.

Achieving airtightness in the existing structure was a challenge.

These two goals — retrofit and extend — were so closely intermeshed that Dehaye admits not being able to separate out what proportion of the budget went on the retrofit and what was eaten up by the extension. But having worked on several Enerphit projects for clients over the past number of years, a fabric-first approach to the whole project seemed appropriate.

“We decided to put the capital into the building structure by having all the elements and construction details to passive standard, and keep the mechanical to a simple, cost-effective, well-proven and low maintenance system,” he says.

Specifically, he opted for a condensing gas boiler (to an existing gas supply connection), a solar thermal hot water system, solar PV panels and an Aereco demand control ventilation system.

1 Architect Loic Dehaye had his eye on this 1930s-built house for two years and even missed an opportunity to buy it at one stage, but managed to snap it up on his second attempt; 2 & 3 as well as a deep retrofit, the €280,000 budget incorporated the building of a two-storey side and rear extension, a steel and timber frame structure built on site; 4 airtightness taping and membrane at joist ends; 5 the new timber frame walls were pump-filled with cellulose insulation from Cork manufacturer Ecocel through holes in the internal OSB layer; 6 thermally efficient, lightweight & insulated window sill from Youghal-based Passive Sills, certified by the Passive House Institute.

Although Dehaye is mindful of the environmental impact of opting for a fossil-fuel burning set-up, the system has been designed to be able to switch to a heat pump in future when budget allows.

Indeed, the decision to hold off on investing in a heat pump was made easier by some modelling Dehaye did with the DEAP software to compare the carbon emissions of an air-to-water system with the gas boiler and solar thermal, which found that the emissions were similar given the extent to which electricity production in Ireland still relies on fossil fuels.

“It means electricity production needs to continue to be cleaner and self-produced where possible to maximise the benefit of a heat pump,” he says.

So, the bulk of the budget went towards the fabric, including external insulation over the concrete blocks of the original dwelling, while the extension is a steel and timber frame structure built on site and insulated with cellulose insulation from Cork manufacturer Ecocel. Irish-sourced larch timber cladding was used for the front, and triple glazed alu-clad windows were fitted all round, with openable rooflights over the landing and kitchen.

This article was originally published in issue 30 of Passive House Plus magazine. Want immediate access to all back issues and exclusive extra content? Click here to subscribe for as little as €10, or click here to receive the next issue free of charge

Achieving airtightness in the existing structure was a challenge, however. “We were targeting one air change per hour but could not achieve it. For budget reasons and sustainability, we tried to keep as much partition and structure in place, which made it harder. We did achieve 1.9, which is very good for an existing structure but not enough to qualify the house for Enerphit.”

Dehaye also says that the installation of an Isover metal-stud acoustic insulation system to the party wall has been a big success.

“We completely forget we are living next to a family with three young kids in a semi-detached house,” he says.

The finished result is beautiful, inside and out, but the idea of delaying investment in heat pump technology might give some pause for thought. Indeed, this choice is not unlike the ones being made by many motorists interested in buying a new electric car but who have chosen to hold off until battery technology and ranges improve (and prices fall), and until further progress is made in greening electricity generation. This is especially true when considering the carbon intensity of grid electricity has dropped by over a third since 2010, that considerable further drops are inevitable within the lifespan of heat pumps installed today, and the potential of using heat pumps to help mop up wind energy generation at times when demand is otherwise low.

But as Dehaye puts it, the enhanced fabric in this now NZEB-compliant dwelling is there now for the next 40 or 50 years, while the heating system will have a likely lifespan of 15 years or so.

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