Retrofit Projects& case studies

This project started in 2023, when we helped to tank the basement. Subsequently we have made a retrofit plan based on PHPP to help get this house to reach the AECB Carbonlite Retrofit Standard, created a work sequence plan and helped to finalise the window schedule. Once the windows are decided (the client is considering high-performance sash windows), we will run thermal bridge modelling on the various junctions in the house.

The house is currently hard to heat well, meaning that some rooms are not really used as they should be – this is because of the single-glazed windows and air leakage, but also because walls on the ground floor are not insulated, while other walls, as well as the floor and roof, are insulated but insufficiently (e.g. walls and roofs have only 50mm mineral wool that was added internally by the previous owner). The plan is to (re-)insulate all the elements with sufficient amounts of wood fibre while improving airtightness, replace doors and windows, and install an MVHR system and heat pump. The basement joists are now drying out sufficiently for this insulation work to progress, but the kitchen has a crawl space underneath so we will wait and see when that is opened up as to what the strategy should be.

The particular challenge for this project is going to be ensuring adequate noise abatement through the party wall – the neighbouring house produces a lot of noise throughout the day and night, and this passes through the wall very well. What is clear that a lot of noise is coming directly through the floor joists which run into the party wall, and it seems likely that these connect with those from next door – we have written a LinkedIn post about this. An acoustic consultant (Atelier Crescendo) was asked to help us find the best possible solution, especially for the space-constrained walls next to the staircase – they have done a noise test, and after a potential contractor visited to discuss practicalities around the staircase and rehanging joists, we are now moving forward to refine the best strategy.

We were asked to visit a retrofit project that had already started, in order to advise on how to achieve a good level of airtightness and insulation using the correct products. The project included the retrofit of the existing house (which had been taken back to brick), and a new build extension at the rear (the walls of which were already built).

After a site visit, it was clear that there were things that needed to be rectified – for example, by looking along the window openings we could see that the newly built extension wall had insulation that was very poorly installed, with large gaps between the PIR boards and inner leaf (this will cause thermal bypass and a reduction in performance – an excellent read is this paper from the Passivhaus Trust). This is of course partly down to an inappropriate design from the architect (although it’s standard design, partial-fill cavity walls are very hard to build correctly!), and bizarrely after our visit the architect had visited and said that it conformed to building regulations. Conforming on paper or not, unfortunately for these walls (although some boards around windows that were more accessible have now been redone) it’s likely that they will not perform as designed.

For the existing house, we advised on u-values and materials for the walls, roof and floor, as well as discussing the windtightness and airtightness strategy. We will be carrying out some thermal bridge modelling when needed as the project progresses. The contractor on the job is enthusiastic to use new materials, so it will be nice to see how this job goes forward.

We were asked to help investigate why a semi-detached house that was retrofitted 8 years earlier was still cold and hard to heat. An air test (permeability of over 13 m³/m².hr) revealed some interesting results, and along with photos from the original retrofit and some site investigation, the reasons became clearer.

The earlier retrofit had done a fairly good job insulating the roofs and making them airtight (for both of the existing house and extension) – here we could see that correct membranes and tapes were used, but these were just taped onto the breeze block inner wall, which then only relied on dot & dab as the airtightness layer for the walls! Also in some places, the membrane had not been taped (e.g. to some window frames). In addition the wall within the intermediate floors was leaking a lot of air (this area is always unplastered in old houses), as well as a big gap in the wall above the front door where air was directly circulating to an outside porch roof. What was also clear was that the quality of the insulation in the extension walls was very poor – some parts had no insulation, others had it poorly installed and in inadequate amounts.

Currently the plan is to get an independent survey done of the cavity, then make a plan on how to improve the performance there. For the intermediate floor and porch areas, the plan is to use membranes and tapes to bridge these areas, but for the whole dot & dab wall we’re not yet sure – we are hatching a plan on what to do here that doesn’t involve taking it all off. The lesson from this project is that if you’re going to retrofit, do it right the first time!

We made a retrofit plan based on PHPP for this end terrace house, which included some thermal bridge modelling and creating a work sequence. This was for clients who were intending to do most of the work themselves, and who were also taking on the task of putting the window schedule together, as well as the MVHR design. The house had thick sandstone walls (some of which were below ground level), a solid floor and a cold loft.

The plan was to use wood fibre internally on the front elevation, and external wall insulation on the side and rear (which entailed planning for the roof line to be extended, and also to insulate below ground level). The eaves detail proved challenging to think through in terms of thermal and air/windtightness continuity, and also in terms of sequencing – here the existing roof rafters sit on top of the sandstone wall, so in order to maintain insulation continuity to the external wall insulation, some of the sandstone will have to be removed. Another tricky junction will be the window and door reveals – here because we have thick walls, we will need to be careful to check that the insulation is sufficient (the thicker the wall gets, the more work the insulation on the reveal has to do to keep the temperature above dewpoint) – thermal bridge modelling for this and a few other junctions will be done once the window schedule is finalised.

For this project, we did not (yet) carry out a whole house PHPP model, as the clients were not sure they could cope with a whole house retrofit. Therefore after an initial site visit, we advised on specific u-values and build-ups for a first phase, which was the retrofit of an existing first floor dormer room which was known to be very cold. This room had a flat roof, a timber wall at the back and part timber walls part masonry cavity walls on both sides – there was some mineral wool installed already, but not very much and without any windtightness protection. The flat roof deck also had signs of water ingress, so a decision was taken to re-roof it before starting. In addition, the entire timber structure seemed a bit flimsy to us, so we had a review done by a structural engineer who recommended strengthening work prior to starting. The existing windows were double-glazed (air filled) and were not sealing properly, so it was decided to replace the windows (but only with double-glazed argon filled) and insulate the wall and roof with wood fibre to achieve a good u-value (wood fibre is also excellent in roofs at keeping out summer heat).

For the walls, the strategy was to add a windtight layer first before installing flexible wood fibre between the timbers, then rigid T&G boards in front and underneath, followed by an airtightness membrane. There was a challenging area to make windtight and airtight, which was the connection through the floor void to the walls below (in phased retrofit, you have to ensure that these layers are not left flapping around which would allow air to enter the construction through the perimeters). For the roof, the ventilation pathway was maintained through the windtight membrane being pinned along the sides of the rafters with strips of ply. We helped advise the contractor (Reece Dalton) in terms of the BoQ and sourcing the material needed. Our work also included doing a thermal bridge model for the window to wall junction, and checking the window schedule and specification.

This is a full retrofit done in one go to EnerPHit (the Passivhaus retrofit standard) that is currently underway (2025) and being undertaken by DK Building. It is a very sensible retrofit insofar that it is not adding new elements (extension or loft conversion) because these are simply not needed. The house had solid walls, a suspended floor and cold loft, and we helped create a retrofit plan based on PHPP.

The house is going to have a heat pump with underfloor heating, and solar PV tiles for the roof, and of course – insulation for all the fabric. The plan for the walls was to go for internal wall insulation using wood fibre on the front elevation and then external insulation elsewhere (also using wood fibre but with a rainscreen cladding). It was decided to convert the suspended floor to a solid floor because the moisture content of the wood was somewhat above what would be considered safe. The plan for the loft was to keep it as a cold loft, but the roof at the eaves needed thought due to the rafters forming an uninsulated sloping corner of the first floor ceilings – here we needed not only to insulate this area but also have continuity of insulation over to the external wall insulation (which will entail removing some bricks later). This was one of the 19 junctions that we looked at for thermal bridge modelling. In addition, we created a work sequence plan, liaised with the ventilation designer (Luft MVHR) and helped pull together and finalise the window schedule (including advocating for custom frames for those windows that would be on the internally insulated wall). A Passivhaus certifier (Ecospheric) has been appointed and we are liaising with them as the Passivhaus Consultant on the project.

We created a retrofit plan to take an existing house to reach EnerPHit (which is the Passivhaus retrofit standard). The planned retrofit included an extension that annexed the existing separate garage, the drawings for which had already been produced by an architect – these needed some modification to ensure a thermally sensible structure (i.e. without dot & dab internally, partial-fill wall cavities, and thermal bridges). The existing house had masonry cavity walls for the most part (with a timber wall between the front bay windows), a suspended floor and a cold loft.

As the wood joists in the suspended floor were dry, the plan was to insulate between these joists in the existing house (which the clients intended to do themselves), and to create a solid floor for the extension. The walls for the existing house were going to have internal insulation, and for the extension the walls were planned as full-fill cavity walls. For the roofs, the plan was to retain the existing cold loft, but for the new extension roof to have wood fibre insulation over and within the rafters. We also carried out thermal bridge modelling (26 junctions in this case). The window schedule had been put together by the client, but we helped cross-check the quotations that came back (including advocating for custom frames for those windows that would be on the internally insulated wall). We also created a work sequence, liaised with the MVHR designer (Luft MVHR) as well as Integrated Energy who were doing the heat pump and solar PV design.

We helped an architect who was building his own Passivhaus to review the PHPP and thermal bridge models, and then to answer certain queries from the Passivhaus certifier (Max Fordham).

When we were brought in, there were already drawings from an architect for the retrofit of the existing house as well as plans for a new extension to the rear and side as well as a loft conversion – these we helped modify regarding the proposed construction build-ups. In the end the planned size of the extension was also reduced for cost reasons (with PHPP being modified for all iterations).

The retrofit involved insulating the floor, converting the cold loft to a warm roof (with dormers) that was insulated over rafters with wood fibre (which is excellent at keeping out summer heat compared to other insulations), and externally insulating the existing walls. The original walls had badly installed partial fill cavity insulation, which had to be retroactively filled completely because if that cavity had remained, it would have ended up allowing thermal bypass behind the external insulation layer (thermal bypass is where cooler air bypasses insulation, reducing its effectiveness). The extension walls were made in timber frame filled with mineral wool and with wood fibre boards on the outside.

In addition to PHPP, we carried out thermal bridge modelling, liaised with the MVHR designer (C&G Design Consultancy), produced the window schedule and hammered out the work sequence. As the retrofit proceeded, we responded to intermittent requests for advice from the contractor Sean Hugill, and helped do condensation risk analysis for the roof construction for building control.

This is a phased retrofit to EnerPHit level that is currently underway (construction started in 2024), and involves rebuilding a disused barn and linking it to the existing house, along with a rear ground floor extension to the existing house (phase 1 currently), and the retrofit of the existing house itself (phase 2). An architect had already produced plans for phase 1 when we were brought in, and these included typical standard details (such as unsuitable build-ups which did not mitigate moisture and overheating risk, as well as thermal bridging at junctions). The original plans also had a glass roof for the link building which would have made the building overheat in summer.

We therefore helped revise the specification for the different elements based on PHPP (which definitely included getting rid of that glass roof!). For the existing roof we had proposed wood fibre insulation fitted within rafter extensions fixed internally to the existing rafters, so we had a structural engineer check the roof structure was adequate for the extra weight of that plus the planned solar PV panels. We produced 39 thermal bridge models for the project – quite a few, but there were so many different junctions in this project! We also created the window schedule, which included some involved discussion with 21 Degrees (who were supplying the windows) and Case Construction (the contractor) regarding junctions, including a complicated intermediate floor junction between window panes in the link building. We helped to create a work sequence plan and liaised with the Luft MVHR who were doing the ventilation design. Now that a Passivhaus certifier (ZE Passivhaus Services Ltd) has been appointed, we are liaising with them as the Passivhaus Consultant on the project.

We created a retrofit plan to take an existing house to reach EnerPHit (which is the Passivhaus retrofit standard). The planned retrofit was to include an extension, the drawings for which had already been produced by an architect. We created the PHPP model, which was based on a mixture of internal and external wall insulation, retaining the cold loft and ensuring insulation continuity at pitch along the eaves, and replacement of the suspended floor with a solid floor (since the crawl space humidity and wood moisture content were above safe levels). We also carried out thermal bridge modelling (18 junctions in this case), put the window schedule together (which in this case required some windows on internally-insulated walls to have custom frames to allow enough insulation on the reveal to avoid condensation risk), created a work sequence and liaised with a solar PV designer. Once the spec had been finalised, we met together on site together with the clients and a couple of possible contractors to discuss taking the project forward.

We put a retrofit plan together for a split level bungalow with a mix of uninsulated suspended and solid floors, unfilled cavity walls and a metal roof under which we found 50mm of straw boards as insulation (just shows you never know what you’ll find during an assessment!). The house had an air heating system and was costing a fortune to heat properly.

The retrofit plan was based on PHPP analysis and thermal bridge modelling, which showed a possible 91% reduction in space heating demand from the baseline situation which was based on filling the cavity walls along with externally applied wood fibre (with rainscreen), insulating over and within the existing roof (also with wood fibre), and insulating floors (for the suspended floors, the wood had too high moisture content so we had a plan to insulate using recycled glass aggregate). We also helped pull together the window schedule, create a work sequence and liaised with the MVHR designer (Luft MVHR) for the ventilation system.