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 were in no doubt that this was the method of construction to use. Beyond that there were two choices to make: whether it was to be 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 timber-frame house and won't lose the heat as quickly.
We chose masonry for our first build, The Autonomous House - partly because it was a construction method that local builders were familiar with. We were very happy with how it performed during the five years that we lived there. The internal temperature remained fairly even - around a comfortable 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 even comfortable at 17C. So, although timber-frame is a more common construction style here in the Highlands, we chose a masonry-built passivhaus once again.
A passivhaus is designed using the Passivhaus Planning Package - complex software into which you enter all of the construction details, as well as data about things like the orientation of the house on its site and the location's climate. The software then calculates how much energy will be needed to heat the house - and the details can be altered, if necessary, to end up with the desired low figure. We employed a passivhaus consultant to make these calculations for us, as it's important to get the figures right before building commences. The more complicated the design of the house, the more complex the airtightness detailing becomes - which is why passivhaus designs are often quite simple. Our design presented a few challenges, but with the passivhaus consultant's help, and knowledge we'd gained from our previous build, we were able to come up with a workable design.
We didn't have a firm idea of how we wanted Portree Passivhaus to look. Our design brief to the architects consisted of a basic layout, numbers of rooms 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 liked what we'd seen of their work - imaginative, contemporary designs, balanced with sensitivity to the surroundings.
The designs they came back to us with were very impressive, including a four-storey circular tower, which we loved but had to reject on practical and financial grounds. Instead, we chose this 'wedge-shaped' design, adding a curve or two, the buttress on the west-facing side, and the outer layer of stone to help it blend in more with the stone walls of the surrounding croft land. We also chose to have a turf roof, which is less vulnerable to strong winds than slate or tile, and also helps the house blend in.
As with the previous house, this one has triple-glazed windows, the largest of which are mainly on the south and west walls, to maximise solar gain. To guard against summer overheating - a problem that many houses with large areas of glazing face - the main south-facing windows feature a large, bronze-clad 'frame', which shades the house from the intense heat of the midday sun during the summer months.
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 (MVHR), keep the house at a comfortable temperature throughout the year - despite there being no installed heating system.
It's important to ventilate a highly insulated airtight house, so the MVHR unit is key. Fresh air enters the house through an external air-intake tower, passing through a section of ground tube which slightly cools the air on its way in. The fresh air enters the MVHR unit in the utility room and is fed into living areas and bedrooms via a system of ducts installed throughout the house. Stale air is extracted from the kitchen and bathrooms and then passed through a heat exchanger in the MVHR unit. This takes any heat from the outgoing air and feeds it into the incoming air stream.
The internal layout is 'upside-down', with bedrooms on the lower floor and living areas above. This arrangement had worked very well in our previous house, so we didn't hesitate to repeat it here. Heat rises, so it makes sense to have the living areas, where you spend more time, on the higher levels and the bedrooms below where it's cooler. Although, the mechanical ventilation system keeps temperatures pretty even throughout.
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.