We've chosen to build a passivhaus - this is a proven system of construction that originated in Germany and guarantees quality, comfort and energy efficiency. Beyond that there were two choices: 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 as a conventional house does.
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 that temperature. 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 then 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 the new house - but this time we're going to apply for Passivhaus Certification. This 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 officially recognised as passivhaus - and we get a nice plaque for the wall....
The Portree Passivhaus will have a similar concrete core to our previous house, and will also have large triple-glazed windows, mainly on the south and west walls, for solar gain - with summer shading designed into the window 'framing' to avoid overheating. Externally it will be clad in rubble stone, with key surfaces in stacked Caithness slate. The main south-facing windows will be framed with bronze cladding. We've chosen a turf roof this time, which will help the house blend into the surroundings and will be less vulnerable to strong winds than slate or tile. Other features, typical of a passivhaus, such as meticulous airtightness detailing, masses of insulation and installation of an efficient mechanical ventilation system with heat recovery will all help to keep the house at a comfortable temperature throughout the year.
The basic layout and our preferred method of construction were in the design brief to the architects, but we left the final 'look' of the house to them. We chose 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. The designs they came back with were very impressive and after just a few tweaks, the introduction of more stonework and an added curve or two, we were happy to go ahead.
The internal layout will be 'upside-down', with bedrooms on the lower floor and living areas above. We were very happy with this arrangement in The Autonomous 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 slope of the site means that we will be able to enter the house on the upper living floor. This floor consists of a living, dining and kitchen area in one large open-plan space, with a separate bed/sitting room and en suite for elderly relatives/guests. Stairs on one side of the main living space will lead up to a mezzanine containing an office and a small sitting area; on the other side, stairs will lead down to the bedrooms and utility/services room.