HPH259 : Constructing the masonry superstructure of Portree Passivhaus – with Mike Coe
Mike Coe has a progress report from Portree Passivhaus, which is now almost watertight. We pick up the story as the groundworks begin and look at some of the challenges as the superstructure has gone up.
Interview with Mike Coe
Mike Coe has been on the podcast a number of times. Our first interview with him was when he completed building the Cropthorne Autonomous House, where we imagined he might stay put for many years. However, with concerns over climate change, he and his partner Lizzie Stoodley decided to move further north to the Isle of Skye in Scotland.
This meant selling the autonomous house, which was not straightforward, and taking on a second project with well known architects Rural Design.
The sloping site was perfect for upside down living
Mike's previous house, the Cropthorne Autonomous House, had bedrooms downstairs, and the living space and kitchen upstairs. Mike prefers this layout and was keen to repeat it.
That's why one of the attractions of the site was its slope. The main entrance could be at the top of the house, taking you directly into the living space.
Rotten Rock provides a perfect base from which to build
Mike explains that whenever anyone builds on Skye they look for ‘rotten rock', which is a combination of the normal clay subsoil but with volcanic small rock particles mixed in. This makes a stable foundation that you can build straight onto.
Although they were digging into the hill, the rotten rock is almost solid enough to dispense with a retaining wall (but not quite!).
His first self build guided a lot of decisions
At the autonomous house the heavyweight structure of the building was used as a thermal store, which would build up heat during the summer and slowly give it back over the winter.
Hence Mike wanted to repeat what he knew worked well. Portree Passivhaus is a masonry building with a cavity wall.
This is unusual for Scotland which has a lot of timber frame buildings, that are quick to put up in the inclement weather.
Resilience was another key factor
The building's heavyweight structure will also make it a ‘fortress' for whatever weather is to come.
I think it’s inevitable that we’re going to get more extreme weather. We haven’t had that much of it in the UK yet, but there’s rarely a week when there isn’t some kind of extreme weather event somewhere in the world which makes it into the news.
The building sits on a tray of insulation
Concrete is poured into a tray of insulation to form the lower ground floor slab. The internal concrete block walls then get built onto the edges of that slab, so the internal part of the house is completely isolated.
The cavity wall is 375mm wide and uses TeploTies for low thermal transmissivity.
With a large open plan living area, the building requires a steel structure for extra support.
The first floor is a beam and block floor, and above this there will be a green roof.
We’ve got a very substantial roof structure in order to take the weight of the soil on which the grass will grow on the roof. But below that, sitting on top of the structural I-beams, there’s another layer of insulation.
There is no environmentally benign dwelling
Mike is well informed when it comes to climate change and believes that we, as a world, could be doing so much more to reduce carbon emissions. So why is he building with masonry when more sustainable materials exist?
There’s undoubtedly embodied energy in this house. How significant it is in terms of global carbon emissions compared to things that could be very, very easily avoided, I don’t know. In use, the house will have a tiny environmental footprint. It’s very energy efficient, it doesn’t have a heating system, it’ll have renewable energy systems as well.
Although it will be a number of years into the future, Mike says the house will pay back its CO2 debt. And for him it's not just about preventing climate change, this is a house that's ready to face the extreme weather events that have been caused by climate change.
When you look to the future it's not going to blow down. And the turf roof is part of the resilience strategy.
Mike gradually took on more responsibility
Having been very hands-on during his first self build, he had wanted to do less this time. However, he's found himself being drawn in!
Due to concerns over finances, he decided to save some money by installing the mechanical ventilation system and doing all the electrics himself.
But knowing the level of detail required on a project like this, he's also monitoring quality.
It’s all about during and post construction quality control. And trying to get across the message that the insulation has got to be kept clean and dry – difficult in this climate – as dry as possible and put in neatly without any gaps – I don’t know why but it’s just proved to be very tough.
So Mike has spent weekends pulling insulation out, cleaning it, putting it back again. He's also taken responsibility for some of the trickier details.
Mike spotted a thermal bridge across the cavity
With an architect and contractor who haven't built a Passivhaus before, Mike's experience has become extremely useful. During construction he noticed that the steel was creating a thermal bridge across the cavity.
The solution was to shorten the steels and put thermal break pads in. Using stainless steel bolts also helped reduce the transmission across the steel.
Delays have had an upside
Mike's contractor has had to start another project for a local hotel and this has meant a reduced workforce on Mike's site.
However, as pensioners, Mike and Lizzie have been able to squirrel away extra cash.
This gives Mike cautious optimism that he could get towards the end of the project… and even hire decorators (something they'd really like to do).
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