Toby Cambray talks about the experience of working on his own retrofit project, and the lessons learnt in the process.
Interview with Toby Cambray
Toby Cambray co-founded Greengauge building & engineering consultancy in 2010, specialising in sustainable and low energy design. He is a certified Passivhaus Consultant and has completed a PhD on moisture in buildings.
In the previous episode we discussed moisture risk in buildings and how to assess it.
There’s no such thing as an ‘easy’ retrofit
There are always compromises, so how do you best juggle and manage those? Toby bought his house with the idea that it would be relatively straightforward to retrofit. There was nothing (e.g., listed building status) to prevent getting permission to make changes.
The house had deep eaves, making external insulation easier. It was a question of how to execute that while minimising thermal bridging, getting the windows in the insulation zone, and incorporating airtightness.
The original bay was a feature Toby decided to remove
Toby had lots of ideas of what he would like to do. He built an extension and did some minor remodelling first, before starting retrofit work. From a holistic point of view, it would have made sense if the extension had been planned to receive the same external insulation that was added to the house later. The building would have performed better too, as there is a thermal bridge where the extension’s cavity wall meets the new external wall insulation on the main property.
Living in the home first can provide useful information
It takes time living in a home to understand what’s best for how you use it.
There is also thinking about how the home might need to be used in future, and other considerations like whether you’re adding value.
The floor plan of Toby’s house already worked well, so there was little reason to change the layout. He considered taking out a wall with a chimney. That would have been a major structural challenge, so he prioritised keeping things simple and less expensive.
Watch an entire retrofit of this mid terrace home in The Hub.
A truss and fill insulation system can take different forms
There’s a lot of experience with truss and fill systems, especially within the AECB membership. A timber frame is fixed to the outside of the building, with a thin wood fibre board over it to create a cavity. Any blown insulation can then be installed to the cavity; Toby chose cellulose insulation.
There are different ways of doing the frame too. Toby used a Larsen truss with 50 x 50mm battens on the wall, then plywood gussets to support an outboard batten. You could equally do it with an engineered joist, or similar.
There are different ways of finishing the outside. Toby used a 25mm batten cavity with a render-carrying board, and then a render finish. It could also be timber clad. Rendering straight onto the wood fibre is possible but might need more careful thought when it comes to rain robustness.
A second layer of defence between the outside surface and the insulation is comforting, even if you’re not in a very exposed area for rain. If you have render direct on insulation and the render cracks, there’s a higher likelihood of water passing through to the inside.
Plywood window boxes were constructed to take the new windows
The first step was preparing the window openings
To ensure the new windows were in line with the cellulose insulation, Toby built plywood boxes and installed them in the opening so they cantilevered out. They were fixed through the face of the ply into the edge of the cavity wall masonry, and they allowed the new windows to project into the insulation zone.
The bay was removed
New studs for the insulation layer
The cement board going on
A cavity acts as a capillary break in the wall build-up
The cavity in the original wall build-up acted as the capillary break to stop moisture reaching the inside of the building. With the retrofitted wall build-up, Toby made sure the 25mm batten cavity behind the render-carrying board was ventilated and served that function instead.
Below-ground insulation links the floor to the external walls
Toby dug a trench around the building, installed moisture-resistant insulation, then backfilled it and used paving slabs to hold the insulation in place. The 200mm timber frame with the cellulose sits on top of that. The below-ground insulation finishes just above ground level to keep the timber clear of ground level. It required a lot of hand-digging and re-doing drains that are close to the house.
Planning ahead helped with airtightness in the roof
Toby removed the old insulation from the roof. Some leftover mineral wool from the extension went between the ceiling joists, followed by a vapour control membrane over the joists. He installed cellulose insulation over the membrane – it comes in compressed bales which you open and it expands into loose cellulose.
When installing the membrane, Toby left plenty of length on it and folded it back under the rafters. When the scaffolding was up and the end of the rafters exposed, he peeled and then slit the membrane so it could fold down between the rafters. It was then a case of taping that all around, first to the rafters and then down onto the existing render. It felt like it was going to be time consuming, but then proved to be okay once Toby got into it.
To achieve continuity of the roof insulation with the wall insulation, Toby was able to remove just a single row of tiles. He slipped in some insulation that the wall insulation could butt up to. Waterproofing it required some leadwork, which was an unexpected expense, but a nice way to finish it and robust so it won’t let moisture in.
Following established principles can help keep embodied carbon low
Toby hasn’t calculated upfront or embodied carbon. Instead, he has followed general principles that he has learnt from other projects, including using very little concrete or steel.
In terms of concrete, there are small amounts of mortar for repairs and holding the paving slabs in place for the below-ground insulation, plus the 12mm cement board carrying the render. Some stainless steel bars were required as part of a Helifix Helibeam system, which was used to replace a failed concrete lintel.
Retrofit can make it tricky to accommodate MVHR
In an ideal world, Toby would have chosen MVHR, but finding a good position for the unit is always difficult in a retrofit. Put it in the loft, and access for maintenance is harder. Put it in a room and you lose valuable utility space. Installing the ducts is also tricky and can involve a lot of compromise.
Toby chose an Aereco MEV system over MVHR. The system was installed at the time he built his extension, as this was the best time to get ducts running to and from the loft.
MEV is effectively the extract half of a full MVHR system. There’s one fan in the loft, with a duct to each wet room. Each living space has an inlet through the wall, and on each inlet and outlet is a humidity sensitive material that passively operates like a damper. When someone has a shower, the damper opens a bit more to help clear the elevated humidity. The fan in the middle has a pressure sensor which increases the fan speed when the terminal opens more.
Decentralised MVHR, with individual room units, could have been an option but Toby’s experience is that it tends to be relatively expensive to do a whole house that way.
Plan ahead for low carbon heating
Toby’s home still has a gas boiler, a decision he was comfortable with at the time. He will upgrade to an air source heat pump at some stage, and has the infrastructure and electrical supply in place where the hot water cylinder would probably be positioned.
The retrofit industry perhaps overlooks the DIY culture of the UK
Toby has travelled in parts of Europe and recognises the DIY culture we have in this country compared to other places. With motivation, and the right research and guidance (including the House Planning Help podcast!), a proficient DIYer could undertake a retrofit project.
But it’s also important to be aware that it’s a lot of work, with stress and long hours. Toby was working under floodlights until midnight the day before his cellulose insulation was installed, finishing off taping so there was a cavity ready for the insulation.
Standard details are a good starting point for DIY retrofitters
You might have to pay for the information, but it’s less expensive than engaging a consultancy service. If you have building experience and broadly know how a timber frame goes together, or how a masonry wall goes up, then a lot should be fairly obvious. It’s about thinking through the next steps as you work, and coming up with solutions for how X joins to Y.
Toby understands moisture risk and so designed a window detail himself that others thought was overkill. He was happy with it as a belt and braces approach to keep water away from the cellulose insulation. It’s a case of assessing risks and accepting when there is value in paying for advice. Structural calculations are one area that Toby was glad to have professional support.
Every project has unique technical challenges
Toby uses the example of external insulation. There are a host of ways to do it, but you can guarantee that any project will always throw up a detail that no-one has ever come across – or, if they have, that a solution is not well documented. Again, it’s taking time to think about how to do it well using the array of products that are available.
Budget for and prioritise the ‘big ticket’ items
The biggest expense on Toby’s project was new triple glazed windows. The next biggest cost was re-roofing the property, as the bituminous underlay needed replacing. The roofer was able to reuse the original tiles, and the roof work was carried out while scaffolding was up already, which helped. The cellulose insulation was a big undertaking, but not necessarily the biggest cost. Toby did a lot of work himself getting the timber frame ready.
If Toby tried to value the time he has put into the project then it would probably be one of the most expensive retrofits done recently. He has really come to respect the speed at which professional builders work. But he is also interested in the subject, gets enjoyment from doing the work, and can take lessons learnt back to his practice. All of that provides additional motivation.
Junction details and internal finishes take a lot of time
A big lesson for Toby was that lots of time, thought and effort goes into the junctions. The timber frame went up in a few days, which really looked like progress. On the other hand, working at the foundations, taping around the rafters, and sorting out the windows all took a long time.
A lot of functional and structural items don’t have to look nice, they don’t have to be finished. But since Toby has begun to work inside, time has been eaten up finishing window reveals, and sanding and painting so that the finish is to a standard he wants.
It’s hard to capture in words how much more comfortable a retrofit is
In conclusion, Toby observes that he knows the physics of why his home feels more comfortable, but he finds it difficult to express how different it feels when everything’s a more even temperature. You can only truly feel it by experiencing a well-insulated house. It’s a feeling that it just … works.
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