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1980s Retrofit

Insulation, Ventilation, Solar PV, Batteries and an Air Source Heat Pump
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Age/Period: 1980s
Type: 2 storey detached
Years in residence: 15
No. Bedrooms: 4
Wall type: Ashlar stone and cavity wall
Area: Batheaston

Key Features

Triple glazed windows and doors
Internal and Cavity Wall Insulation
Loft Insulation
Draught Proofing
Mechanical Ventilation with Heat Recovery (MVHR)
Air Source Heat Pump (ASHP)
Solar PV and batteries
Energy efficient appliances and lighting


Back in 2008 Linda Mallory and Av Bains had a chance to do some renovation on their new home before moving in.

A simple Google search on “how long do double glazing windows last?” led down a rabbit hole and the discovery of the Passivhaus Standard for buildings – buildings which use high levels of insulation and draught proofing to require remarkably little energy to heat, even if it’s -20oC outside. For people who were used to living in draughty, difficult to heat Victorian terrace houses it was a revelation, and the journey to retrofit their home to as close as practical to Passivhaus standards began.


Triple Glazing, ventilation, and insulation

The initial refurb happened in two stages. First the double-glazed windows on the  south part of the building were ditched in favour of triple glazed one. And, because it didn’t make sense to do that and then have trickle vents in them for ventilation, a mechanical heat recovery system was installed. This sucks out stale air through ducts in the ceilings and pumps in fresh air. The heat from the extracted air pre heats the incoming cold air via a heat exchanger. To see how effective it was, they turned off the MHVR in winter and the internal house temperature dropped by 3oC. Also, as the air for the house is sucked in from a single point, the incoming air  is filtered for pollen improving the air quality indoors.

Blown fibre cavity wall insulation was put in, as well as (what was considered then an excessive amount) 300mm of loft insulation. Warmcell was used because it was recycled, and its loose fill, so it completely fills in between and around joists, wiring and pipework and helps to ensure airtightness.

To reduce the energy loads LED bulbs were installed throughout their home. To  avoid comprising the work to achieve good air tightness, track lighting was chosen over individual recessed units to avoid lots of holes in the ceiling. Even the cooker extractor fan was chosen for its ability to recycle air internally without the need for a large extract duct to the outside.

The second phase of the refurbishment saw the remaining double-glazed windows replaced with triple glazed ones, 50mm internal wall insulation added to the north bedrooms, the radiators replaced with aluminium ones which allowed the boiler to run at a lower temperature. Special airtightness tape was used around the windows this time in an example of what was learnt from the first refurbishment being used for this one. This second phase was finished in 2012.

An air blower leakage test in 2014 gave a result of 3.5 ACH-1 @ 50 Pa, not bad for a retrofit and significantly better than the current standard required for newly built homes. Since then, the leaky wooden front door has been replaced with a modern one and the garden patio doors with triple glazed ones all with internal thumb turns, so no keyholes for the air to come through.

2019 saw the start of some upgrade works. First the MHRV unit was replaced with a better unit and the insulation around the ducting increased to reduced heat losses further. The loft insulation was increased to 400mm (current building regs are for 270mm) and special air tightness tape used to seal around the holes in the ceiling.

Solar PV and Batteries

Solar panels with batteries were installed in 2021. These reduced their electricity consumption by around 40%-50% as measured over the course of the first year.

Air Source Heat Pump

Finally in 2022 they decided to install an air source heat pump. As preparation for this, the cavity wall insulation was replaced with EPS Bonded Beads. This was because during the previous building work it was noticed that the blown fibre had slumped in quite a lot of places. EPS bonded beads are 20% more efficient than blown fibre.

The radiators were again replaced this time with high output ones that will be provide enough warmth at the low water temperatures associated with an air source heat pump.

The silicon seals around the windows were replaced to aid draught proofing, the individual thermostats were swapped for a whole house system that also takes into account the external air temperature, and the hot water tank with a much more insulated modern one.

The last part of the jigsaw has seen them move to an energy tariff from Octopus Energy that charges different rates at different times of the day. The solar panels charge up the batteries in the day and then the batteries power the house during the peak rate while the cheap middle of the night rate is used to heat up the water for the house.


Warmcel Insulation:
MVHR: Green Building Store (Harrogate), and Earthwise (Bristol)
Cavity wall attraction and refill: Arrow Insulation (Bristol)
Solar PV & Batteries: Infinity Renewables under B&NES Solar Together bulk purchase scheme
Windows: Nordan and Internorm
Doors: RK Doors Systems