Kirkburton Passivhaus: Making Passivhaus cost-effective
After a series of radical retrofit projects, our building team are back on with building a newbuild Passivhaus project on the outskirts of Huddersfield. Our clients Peter and Susan had visited, and been inspired by, our first Passivhaus project at Denby Dale and were in a similar position having a suitably sized garden for building a house
Design & Build
Once again we’ve got together with architect and long-time collaborator Derrie O’Sullivan, as part of a Design & Build project led by us. Derrie has taken the project through to Planning and is joining us for monthly project meetings, helping us with any architectural and aesthetic issues that arise.
Different/ the same?
The project is once again going to be built using cavity wall masonry onstruction. with Yorkshire stone, which was decided early on by Derrie, the clients and the planners. I am happy with this as it’s easy for us and allows us to continually refine and tweak our cavity wall Passivhaus methodology.
Flat roof
This time, to meet planning requirements, the project needed to have a flat roof, to prevent blocking view of neighbouring houses, so that will, obviously, require different detailing.
Layout & design
The house is approximately about 120 m2, in a very simple rectangular shape. Because it is set down in the garden, there is a separate retaining wall, similar to Golcar. Like Golcar it is an ‘upside down’ house with a bridge across to the entrance door.
There is a large living/ dining kitchen area on the first floor as you come into the house across the bridge. The living room has an L-shaped balcony which acts as shading for some of the ground floor rooms, and will have a lovely view. The ground floor has 2 bedrooms, a bathroom and a garden room. The garden is a very important element for the clients.
Value engineering in PHPP
Our good friend Nick Grant has put a lot of work into promoting true Value Engineering. We’re going to try to value engineer this project tightly. At our Golcar Passivhaus project we overshot the Passivhaus criteria in a few ways in terms of energy efficiency and airtightness (eg following a change of specification of windows, having better PSI values at junctions, and with an airtightness of 0.25 rather than the 0.6 that we modelled in PHPP).
We need to be very careful when designing in PHPP. At the moment we’re at 14 kwh/m2/annum with a 0.4 air changes. We’re pitching the airtightness at 0.4 within PHPP because we’ve got a track record of achieving good airtightness and are confident we can do so again on this project.
For cost and aesthetic reasons we’re simplifying the window installation detailing which increases the thermal bridging and PSI values at that junction so we’ll need to design in some negative PSI values on other junctions where we gain more. The main challenge with this project is that it’s sheltered by the trees so solar gain is bad. We’re having to be very careful.
For the good of low energy construction we’ve got to make these buildings affordable.. keeping the costs down through value engineering. So our approach this time is more refined and sophisticated than at the Denby Dale and Golcar projects, building on our experience and knowledge and by careful design detailing at early stages.
Foundations
The foundation detailing is similar to Golcar and Denby Dale. We’re using the same principles of taking the insulation down to the strip foundations and using an aerated brick (Celcon thermal block) to reduce thermal bridging. BUT there is a big difference – the ground conditions are clay and the structural engineer has recommended using Cordek underneath the slab. This has meant we’ve had to change the order of things which has had a knock-on effect on our airtightness strategy. At our Golcar and Denby Dale projects the slab was on top of the insulation – the slab was ‘power floated’ – but in this one we’ve got more complications as the insulation is on top of the slab.
Foundations (from ground up)
- Cordek 95mm – to take up ground heave
- 200 mm reinforced concrete slab built into the inner leaf
- 200 mm Kingspan Insulation
- 75mm screed (floor finish made of sand and cement giving a smoother finish).
Insulation and materials used within walls
- Celcon aerated block is used on the inner leaf of the blockwork to reduce thermal bridging, with a damp proof course to keep the Celcon block dry
- Knauf 37 insulation is used in the cavity above ground
- XPS insulation below damp proof course
Airtightness strategy
Having to use the Cordek throws up questions about airtightness in the detailing. We’re taking the airtightness strategy below the insulation, which we don’t feel is a risk in terms of interstitial condensation because the ground temperature is never less than 10 degrees. I’m proposing to use Pro Clima’s Aerosana Viscon paint/ spray as the airtightness layer up the wall to the top of the slab, meeting the wet plaster. It will actually be on the cold side of the insulation so in theory warm moist air could condense. I’m pretty sure that it’s not going to be a problem as we’ve got all this detailing to prevent thermal bridging and keep the ground warm but we’re going to tape the top layer of the PIR insulation with Pro Clima Tescon Vana tape as a vapour control measure. Alternatively, we could have added in an airtightness membrane between the screed and the slab but the membrane would then be subject to many pressures by being trampled over so we’ve decided to go down the taping route instead. We’re looking closely at this detail to make sure it’s OK.. From a value engineering point of view this seems like the best strategy. This is another case of building on our the experience at previous projects. In the next blog we’ll look at the radical changes to window detailing we’re making and our strategies to prevent overheating at the project. Bill Butcher, Director, Green Building Store
