Florence Collier, founder of Humblebee, takes us step-by-step through the deep retrofit of her 1970s home. She explains the costs relating to each measure and the cumulative total as the project advances.

Interview with Florence Collier

Florence Collier, founder and owner of Humblebee is a chartered mechanical engineer and Passivhaus Designer. A large part of Humblebee’s work is around home retrofits, both modelling and carrying out retrofit assessments. Here Florence takes us through the retrofit of her 1970s house, giving us costs for each part of the project, talking through her decisions and showing us how the different aspects of the work fit together.

Front of house (before the retrofit work)

Reducing carbon emissions was the driver of the project

For Florence reducing carbon emissions was at the forefront of what she wanted to do, but she also had a desire for more community-based living. Cohousing would have been ideal, but she was aware of how much work and time goes into getting a scheme off the ground. When a property came up with a simple footprint, in a good location, that was in need of updating, she thought this was a perfect opportunity to retrofit to the EnerPHit standard, and put into practice everything she had learnt from the Passivhaus training course.

Back of house (before retrofit work)

The simple shape of the building made the retrofit easier

The house was a typical 1970s cavity wall brick construction with a ventilated suspended timber floor. Florence explains that it was basically a box, with a plastic portico stuck on at the front and a single glazed, brick porch at the back. It was clear that to do a proper retrofit both of these structures would need to be removed.

If you have skills and time, you can save money

As a professional in this field Florence was able to model the whole building, make decisions on materials and commission each piece of work as required. She did work with an architect on a few of the details and would advise anyone who doesn’t work in this industry to get a specialist on board to guide you through the process. The material and labour cost of the retrofit to date is just under £106,000, which doesn’t include Florence’s time and other items that could add an extra 10-15% to the total cost.

The plans for the project

All decisions tried to balance efficiency and embodied carbon

The plan was to get as close as possible to the EnerPHit standard, which is the Passivhaus standard for retrofit. This essentially targets a space heating demand of 25 kilowatt hours per square metre per year. Florence knew they couldn’t reduce the embodied carbon already in the building, but they could keep any new features as low carbon as possible and reduce the operational carbon. Moving off fossil fuels was the goal alongside creating a healthy home, free of mould and condensation and with fresh filtered air.

The first step is to get over any mental blocks that you’re having to starting this journey, because it’s so easy to say, well, I can’t do this unless I’ve done this.

Florence took on this step herself

Loft insulation was a small but important first step

When Florence found herself stuck at home during lockdown, it seemed like the ideal opportunity to get started. So they ordered in the materials for the loft insulation and then installed it themselves. Florence explains that although the loft insulation was less than 3% of the total retrofit cost it was an important first step, as between 8 – 10% of their heat loss was through the roof. They chose to use wood fibre and hemp, which aren’t the cheapest insulation products, but sustainable and nice materials to work with. Check out Florence’s blog to go into more detail about this step.

Material costs for loft insulation £2,700 for 56.5m²

The cavity wall was filled with EPS beads

Cavity wall insulation can be added independently of other retrofit works

Whilst waiting for other larger works to begin Florence was able to go ahead with the cavity wall insulation as it didn’t depend on other aspects of the retrofit. She found it was a straightforward step in terms of the material choice, not choosing a natural material in this instance because of the risk of drawing moisture from the ground. In a cavity wall, the cavity is there to conduct any trapped moisture away from the internal walls, so your insulation needs to be moisture repellent. They used EPS polystyrene beads, which have a decent thermal conductivity and contain some adhesive so the beads won’t pour out of your wall if you make a hole in the brickwork. Again, this was a relatively cheap step to take, and the effect was noticeable that winter. Check out Florence’s blog to go into more detail about this step.

Total cost of materials and installation £1,701 (approx. £15.50/ m²)

The new triple-glazed windows go in

Next came the replacement of the old PVC framed windows

The house had double glazed PVC framed windows that were coming to the end of their life. Florence wanted to avoid using plastic in their replacement and opted for timber windows instead, which also looked good aesthetically. They chose triple-glazed windows from 21 Degrees, but from their slightly cheaper range that were not Passivhaus certified. From the PHPP model these windows would still allow them to achieve their energy targets. An external powder coat finish on the windows extended their warranty and protects them from the weather.

Florence explains that a lot of the heat loss through windows is through the frame, so they simplified the number of panes and openable parts. Fixed windows have narrower frames, so it’s about trying to find the right balance between fixed and openable. Having fewer openable windows also helps with airtightness.

The windows sit in a plywood box

Florence was aware that installing their new windows was always going to be a challenge, as they needed to ensure both airtightness and insulation continuity to avoid thermal bridging. Installation ended up taking a day per room, with a plywood box built and fitted to each window before then lifting all of it into place. This was needed to bring the plane of the window in line with the planned external wall insulation. The box allowed the window to be pushed out into that layer, which could then be taped and sealed for airtightness. Check out Florence’s blog to go into more detail about this step.

Total cost of windows and installation £22,033

Florence putting on some external wall insulation

External insulation is like wrapping your house in a blanket

Florence explains that heat gets conducted out of your house wherever there is a junction or fixing structure that is not made of an insulating material. Wrapping the house in external wall insulation (EWI) is therefore an easier approach compared to trying to insulate internally. Florence chose wood fibre as a natural material with low embodied carbon, which would also be good to work with from a general health perspective. The brick construction of the existing house was built to be vapour permeable and wood fibre would maintain this breathability. A lime render was used on most of the house to create a weatherproof layer, with some timber cladding on the ground floor frontage to provide variety and break up the box shape.

Where the walls meet the floor can also create a thermal bridging issue, so Florence wanted to continue the wall insulation down to the sub floor. Wood fibre was used from the damp-proof course upwards and then below that they used a polystyrene plinth, that could be wrapped all the way round. A French drain type system with gravel was used to channel any water away from the building itself. Check out Florence’s blog to go into more detail about this step.

Total cost of materials and installation £26,100

The MVHR unit is in the cold loft

The siting of the MVHR unit was not ideal

Florence explains that she designed everything in parallel, making sure she could see the route for the ductwork before choosing windows. They were clear from the start that the MVHR unit would need to go into the loft. This means the main ducts are running from the unit along the loft floor into a couple of rises, with branches off to minimise the amount of distribution within the house itself. They have a couple of bulkheads dotted around and one or two ducts on display as a bit of a talking point. As they don’t have particularly high ceilings they weren’t going to lower any ceilings to accommodate the ducts and also didn’t want to go through floor joists on the first floor.

In general Florence would not advise putting the MVHR in the loft, particularly if it is a cold loft. It’s fine if you’ve insulated at rafter level, but if you’re trying to hit targets like EnerPHit, all heat loss counts and the amount of ductwork outside the thermal envelope, even insulated does contribute to the overall efficiency of the MVHR. They tried to minimise this by insulating all the ducts with 50mm foil-backed duct wrap. Check out Florence’s blog to go into more detail about this step.

Total cost of MVHR system and installation £ 9,880

Some external shading was built into the design

Shutters could be used more as a simple shading solution

Being half French, half Italian, Florence is used to shutters as a shading device. She sees them as an easy way to slash air conditioning loads in commercial buildings and to make domestic settings more comfortable. Florence was aware that her southwest facing bedrooms were going to overheat in the summer and the initial plan was for simple outward opening shutters, that you pull to and latch. Florence then realised that their windows were quite large with only a third of the window openable, so that type of shutter wouldn’t be possible. They quickly moved to the idea of sliding shutters, but it was still a challenge to find the right dimensions and the right sliders. The next issue was how to fit the solid wood frame to the external wall installation. The answer was to fit lintels above and below the windows made of a composite foam material, that would be less of a thermal bridge than timber.

The wood fibre insulation on the outside of the building has a good thermal mass so this also helps to keep the building cool in the summer. The MVHR can be put in ‘summer bypass’ mode, so when it is cooler inside than out, it can reduce the temperature of the incoming air. But the shutters have increased the comfort level and provide privacy, a sound barrier and security if you want to have the window open. Check out Florence’s blog to go into more detail about this step.

Total cost of shutters and installation £5,547

The suspended timber floor

The finished earth floor

Improving the thermal performance of their floor

Florence tells us that when she first started planning the floor there just seemed to be two options, insulate their suspended timber floor or replace it with concrete. The floor joists could be insulated with an airtight and windtight membrane but would still need to be ventilated which would compromise on the thermal efficiency. She was not keen on concrete because of the embodied carbon and saw it as a cold and hard finish. After hearing Jeffrey Hart talking about earth floors she spoke to another retrofit assessor who’d had an earth floor put in with underfloor heating. They had a deep void, so room to build up the layers needed for an earth finished floor.

They started with an insulating layer of recycled foam glass aggregate which is a lightweight, inert and hydrophobic material. This was put on a geotextile membrane to a depth of 450mm which gives you most of your insulation. Soil goes on top. Florence used MOT type one, which is compacted down to create a solid base. The earth floor finish is a clay, lime and sand mixture with added dust to give the pigment. This layer is only 20mm and is then oiled, waxed and buffed to create a smooth finish.

The original estimate was around £20,000, with most of the material cost going on the glass aggregate. However, they experienced a few delays, initially because the amount of aggregate needed had been miscalculated and so more had to be brought in. The MOT soil was then very wet, so extra time and large fans were needed to dry it. Some miscommunications over how the floor joined the interior wall led to more delays and this work had to be done retrospectively once the earth floor was finished. This all led to extra costs. Check out Florence’s blog to go into more detail about this step.

Total costs of materials and installation £30,318

Want to go in-depth on retrofit? Check out the Kinver story in The Hub.

Finally the electric hot water tank allows them to be independent of gas

Florence explains that they always planned to get off gas and had been keeping their old boiler going until they could get their new cylinder installed.

The hot water cylinder is a Mixergy tank, which Florence describes as an intelligent hot water tank. It has been installed in the loft and will have an insulated cupboard built around it. The tank heats from the top down, separating the cold from the hot, so you’ve always got an amount of hot water ready to use. It uses machine learning to heat water when needed and a PV diverter so any excess solar that is being generated goes to the immersion heater and gets stored as hot water.

Total cost hot water tank and installation £6,859

Extra heating will be from electric panel radiators when required

As the work progressed, radiators were removed from the house and once the water tank was working there was no further need for their combi boiler. The plan is to use electric panel radiators where extra heat is required. More heat may be required at the moment as the floor is still drying out. Florence also realised after a cold winter that the MVHR system hadn’t been on the right setting for full heat recovery.

Replacing plasterboard ceilings is one of the last things to do, but it’s a messy job

One key task remains which is the airtightness of the ceiling on the upper floor. Although they’ve insulated the loft, they’ve still got cracks in the plasterboard ceilings, and a leak in the condensate from the MVHR means they now have a hole in the bathroom ceiling that is just covered up with a piece of ply. They plan to take the plasterboard ceilings down, which is going to be a messy job. That will be the final stage and should reduce their heat losses to EnerPHit levels.

Are heat pumps enough or are fabric improvements needed first?

Florence says there is a lot of talk about heat pumps and whether you should do fabric improvements first or just swap your boiler out for a heat pump and be net zero ready. Florence designs heat pump systems and thinks they’re an incredible technology but feels that reducing it to either this or that option means you’re not taking into account the benefits that come from a well-executed retrofit. You might be able to get the numbers to stack up, but for Florence it is about more than just reducing the carbon emissions. A really important part of her retrofit has been about health, comfort and the general quality of life that has been achieved through the fabric improvements. Even temperatures throughout the house, no draughts, moisture or condensation and constant fresh air all add to the comfort levels. The MVHR helps asthma sufferers and allows you to dry clothes inside as the moisture is constantly being removed. Together these aspects are incredibly desireable.