This was a retrofit of a bungalow which wasn’t originally aiming for any certification, but once it was done we decided to apply for AECB Carbonlite Retrofit Standard certification. The bungalow had solid uninsulated floors, a cold loft and cavity walls that had been filled with foam many years earlier. The windows were all air-filled double-glazed units, and the heating system was a gas boiler.

A whole house retrofit done in one go, which was certified to the AECB Carbonlite Retrofit Standard.

Who was on the team?

The contractor for the job was Jon Day from Myzsa Group, who was responsible for coordinating most of the work. A heat pump was installed by Integrated Energy, and the MVHR ventilation system was designed by Luft MVHR. The fan test was done by Ritchie & Ritchie.

Space heating demand & carbon emissions (before and after according to PHPP)

The final space heating demand according to PHPP was 29.7 kWh/m².yr (representing an 86% reduction from baseline).

Fuel Use (before and after)

Insulation Improvements

Walls

The external walls (u-value 0.123 W/m²K) were insulated using 200mm of grey EPS in two staggered layers, with a flush connection to XPS which went down to the footings (which were not deep at all). The original pebble dash was rendered smooth first, before an adhesive fix of the first layer of boards, and a foam fix of the second layer, with thermally-broken fixings going through both layers. The EPS was rendered with Baumit SilkonTop and the XPS with Baumit MosaikTop. The original plan had been to only use 160mm, but due to a material change to 200mm on site, it then required the rafters to be extended at both the eaves and gables, and also the window aluminium overcills that had been ordered no longer fitted and had to be re-ordered. Care was taken to maintain insulation continuity – this required a reconfiguration of the garage roof and a couple of other external structures so that the insulation could pass behind.

Roof

The original cold loft was insulated at ceiling level. This was converted to a warm roof (u-value 0.139 W/m²K) that was insulated with wood fibre boards over the rafters, and with wood fibre flexible batts between / under rafters – under the rafters, the batts were held within homemade rafter extensions made with bits of ply attached to rafters with battens running along the bottom. Because of the extra weight of wood fibre in addition to the existing solar PV panels, we asked a structural engineer to come on site and check the loadings – he then recommended strengthening a couple of the purlins as a result.

Floor

The solid floor was uninsulated, and the clients were not in a position to dig up the floors and relay them. So we added 30mm of phenolic over the existing slab (u-value 0.565 W/m2K), followed by a floating OSB floor before original carpets were returned. Although less work than digging the floors up, doing this still required emptying the property of contents (and occupants) for several weeks.

Airtightness Improvements

The airtightness strategy was to rely on the existing wet plaster layer internally (on both external and partition walls) – the gable walls in what was the cold loft space were just unplastered concrete blocks, so these had to be plastered, and there was also the bit of wall connecting the loft with downstairs (behind the parallel ceiling joist) which had to be addressed as it was also unplastered – but because of the narrow space here, the gap was filled with a cement slurry instead.

The walls were then taped to the OSB floorboards at the bottom (the OSB being also taped at the joints), from masonry to the window frames (using plasterable tape before replastering reveals), and to the air membrane that was stapled under the roof rafter extensions. This was easy enough to do on the gable ends, but for the eaves it was more complicated due to the geometry of the junction – here we had to cut back some of the ceiling, then take some plaster off the wall and tape from here onto a strip of membrane that made a connection to the roof structure. The contractor came up with a solution using OSB boards cut round the ceiling joists as a point to tape onto at this point (and the same boards provided a good base for the second layer of flexible wood fibre under the rafters). We have written about this airtightness challenge in a Linkedin post.

Ventilation Improvements

A Zehnder MVHR ventilation unit has been installed on the gable wall within the warm loft, following a design by Luft MVHR. It has radial ducting connected from two plenum boxes – ducts run within the warm loft along the eaves and drop down to the rooms below. The external ducts run out the gable wall.

Door & Window Improvements

New windows and doors were supplied by 21 Degrees, and were installed within the external wall insulation layer to minimise thermal bridging – this was achieved for both doors and windows using brackets fixed to the outer leaf. For those brackets supporting the window on the cill, we had to take care that these brackets were cut shorter than the frame since the external insulation would not wrap round the frame here (as it did on the jambs and lintel) due to the aluminium overcill that was to be installed later. The average u-value for all glazed doors and windows was 0.96 W/m²K which included installation (i.e. thermal bridge heat losses at the perimeters).

Damp Improvements

Heating System Improvements

A 5kW heat pump (Mitsubishi Ecodan) was installed to replace the gas boiler – this is the smallest unit that Mitsubishi do, even though the actual heating load is calculated at 1.66 kW. Initial monitoring indicates it is operating with a COP of 4.4. The heat pump was sited on the garage roof, just outside the position of the hot water tank on the other side of the wall in the warm loft.

This was based on a design by Integrated Energy. There had been an initial design done by another firm, which had calculated that we would need an 8 kW heat pump – after discussion with them, a re-design knocked that down to 6 kW, but part of the overestimation was probably to do with not accounting for the future level of airtightness nor the ventilation system with heat recovery, but rather taking MCS default values (which assume a leaky house and opening windows for ventilation). Another couple of concerns with the design was the actual positioning of the heat pump unit which would have needed significant lengths of external pipework to reach the house, and also the presence of a buffer vessel which would have only increased inefficiency while not being required. All this made us decide to find another designer who could do a more efficient design (we would recommend anyone looking for a local designer, to check if they have Heat Geek training).

Appliances & Electrics Improvements

Water Improvements

Chimney Improvements

The original remaining chimney stack was removed when the insulation on the roof was being carried out.

Thermal Bridge Improvements

Thermal bridge modelling was carried out on 8 junctions to minimise heat loss (and check condensation risk where needed) – these were the window and door junctions including threshold, roof to wall junctions, internal partition wall to ground junction, and the external wall to floor junction (which is shown below as an example – it achieved -0.06 W/mK based on external measurements).