Passivhaus is a proven system of construction that originated in Germany and guarantees quality, comfort and energy efficiency. Having built and lived in a passivhaus before, we knew that we wanted this second house to be passivhaus too. Beyond that there were two choices to make: whether to build a house that's thermally massive or thermally light (put simply: masonry or timber-frame). The walls and floors of a thermally massive passivhaus are constructed from concrete, which acts as a thermal store, absorbing the heat from the sun during the warmer months and slowly giving it back during the cooler months. The timber frame of a thermally light passivhaus cannot store up the heat in the same way, but it takes much less time and energy to heat than a conventional house and won't lose the heat as quickly.
Our first build, The Autonomous House, was of thermally massive construction and we were very happy with how it performed over the five years that we lived there. The internal temperature remained fairly even (around 21C) for much of the year, only dropping to 17C in the coldest winter months, from mid Jan to Feb. However, with no draughts and cold spots, the house was comfortable even at 17C. We designed it using the Passivhaus Planning Package - complex software into which you enter all of the construction details, as well as data about the location's climate. It calculates how much energy will be needed to heat the house (the details can then be altered if necessary to end up with the desired low figure).
We want to use the same method of construction for this house. It adds an extra level of quality control before, during and after the build; it also adds to the initial cost, unfortunately, but we'll have the satisfaction of knowing that the house is designed and built well, and is energy-efficient and a comfortable temperature all year round.
The Portree Passivhaus will have a similar concrete core to our previous house, but this time the exterior will be clad in rubble stone, with key surfaces in stacked Caithness slate. We've chosen a turf roof, which should be less vulnerable to strong winds than slate or tile and will also help the house blend into its surroundings.
Our design brief to the architects contained the basic layout and our preferred method of construction, but we left the final 'look' of the house to them. We chose a local company, Rural Design (based in Portree), as we like what we've seen of their work, which shows great imaginative, contemporary design flair, balanced with sensitivity to the surroundings. We weren't disappointed - the designs they came back with were very impressive. Sadly, we had to reject the four-storey circular tower on practical and financial grounds, but we're very happy with this 'wedge-shaped' design, especially once we'd added a few tweaks, introduced more stonework and a curve or two.
As with the previous house, this one will have large triple-glazed windows, mainly on the south and west walls, to maximise solar gain. To guard against summer overheating, the main south-facing windows will feature a large, bronze-clad 'frame'.
Other features, typical of a passivhaus, such as meticulous airtightness detailing, elimination of thermal bridges (penetrations from indoors to outdoors), masses of insulation wrapping the entire structure, and installation of an efficient mechanical ventilation system with heat recovery, will keep the house at a comfortable temperature throughout the year and should result in running costs for the house being very low.
The internal layout will be 'upside-down', with bedrooms on the lower floor and living areas above. This arrangement worked very well in our previous house: heat rises, so the living areas tended to be warmer in winter and bedrooms cooler in summer; it also means that you make the most of the views.
The entrance can still be on this upper floor level because the house is built into a steeply sloping hillside. The main door will open into a porch and a hallway leading into the main living space.
The upper floor consists of a living, dining and kitchen area in one large open-plan room, with a separate bed/sitting room and en suite for elderly relatives/guests off the entrance hall.
Stairs on one side of the main living space will lead up to a mezzanine containing an office and a small sitting area, which should have good views out to the distant Cuillins; stairs on the opposite side will lead down to the bedrooms and utility room.