‘Net-Zero’ retrofit, Leeds

Sustainable building professional Nigel Banks has undertaken a radical retrofit of a 1850s house in Leeds as his family home, using triple glazed sliding sash windows, PERFORMANCE entrance doors and arched windows from 21°.

The family had originally wanted to build a newbuild Passivhaus but were unable to find a suitable plot nearby. Instead they found a Victorian detached house, which they decided to buy and undertake a ‘2050 retrofit’, enabling the family to be ‘net zero carbon’ and ‘net zero energy’. The home was refurbished over a period of four months in 2015 to near Passivhaus standards of insulation and airtightness to deliver year round comfort (thermal, acoustic and air quality). The aim was to have a warm, draught-free home with no cold surfaces and no internal condensation.

Nigel modelled the retrofit using a simplified version of PHPP (Passive House Planning Package) and the project is aiming to achieve AECB Silver with the space heating requirement of the building anticipated to be around 40 kWh/m²/year.

Fabric first

Walls

The walls of the house presented the greatest challenge (and opportunity for improvement) as they were responsible for 42% of the house’s original heat loss. External wall insulation was ruled out for aesthetics and conservation reasons, and there was no cavity, which only left internal wall insulation (IWI) options.
With IWI great care is needed to avoid interstitial condensation (water condensing behind the insulation on the now cold wall). Nigel explored three IWI options, eventually choosing Isover’s Optima ‘moisture open’ system, which has given the walls a U value of 0.185 W/m²K down from 1.6 W/m²K (ie reducing heat loss from the walls by nearly 90%). Insulated plasterboard was also used in the window reveals and partition walls to prevent thermal bridging and maintain continuity of insulation.

Windows and doors

The triple glazed sliding sash windows supplied and fitted by Green Building Store has an overall heat loss (or U value) of 1.0 W/m²K , making them around 5 times as insulating as the single glazed windows they replaced. PERFORMANCE entrance doors (Ud 0.9 W/m²K) and PERFORMANCE high performance windows were specified for certain aspects of the project (eg a fixed window behind original stained glass window as ‘secondary glazing’ and two arched windows in the attic rooms).

Floors & roof

Insulation measures were also undertaken on the floors and roof to ensure continuity of insulation and a complete thermal envelope

Net-Zero Energy

Once the fabric first measures were addressed, Nigel looked at ways to reduce energy use further. An MVHR system was needed, as airtightness levels had been increased significantly, to provide ventilation and heat recovery. The project also benefits from solar PV panels, LED lighting, wood burning stoves, an air source heat pump and a 20 kWh battery storage system. Since the completion of the retrofit, the house has been performing well and is expected to deliver close to net-zero energy use.