PASSIVE HOUSING – GENERAL DESIGN PRINCIPLES
Passive housing, zero energy housing, sustainable housing, green homes - these may all be slightly different by definition, but architecturally the fundamental basics are pretty much the same. Consumer awareness has become heightened around the concept of a passive home, and there is more support durning the design process for these considerations.
Masterspec asked architect Jennifer Hanson from A Studio Architects, co-designer of the Zero Energy house in Auckland's Point Chevalier, to explain some of the guiding design considerations, most of which are inherent in the architectural design process.
“First and foremost it starts with the site, and context” explained Jennifer. “Good site and contextual analysis will help to orientate the house in such a way that passive solar gain can be maximised. This often needs to be balanced with views, privacy, prevailing winds and other site specific considerations”.
Design efficiency. A compact design will reduce space heating requirements, reduce the amount of material used in the build, and reduce the overall house footprint. According to Jennifer “a thoughtful, well considered, small space can feel spacious without necessarily being large”
Orientation. The orientation of the interior layout of the house is equally as important as the site orientation. “Living spaces should follow the sun, and lesser used rooms, such as bathrooms and storage areas, can act as buffers on the south side of the house”.
Sun Interaction. Consider how the sun will interact with the house. “How will the sun get in, how will it be kept out, how is it held in the interior space, how can heat gain be removed when it's not wanted and how best can it be captured to heat water and photovoltaics”. Key to these considerations are windows, insulation, thermal mass, ventilation techniques and your chosen heating system.
Insulation. This is a key line of defence against heat loss. “The NZBC contains minimum insulation standards - clients need to be aware that these values are not absolute targets, and higher insulation values will result in a more comfortable home. Also be aware that heat is lost through thermal bridging in the framing. If you can minimise points where there is no insulation between wall cladding and wall lining, the insulation will be more efficient, and heat loss will be reduced. If using pre-nail frames, ask to review the set out drawings, and remove any unnecessary doubling up of studs, that may have resulted from the computer's standard set out of framing and how it relates to window and door openings. This will help reduce thermal bridging”.
“Heat rises, so good insulation in the ceiling is key. Again reduce thermal bridging where possible, and ensure that the roof can still breath”.
At the opposite end of the house, a concrete slab can lose heat both through its base and its edges, and both should be considered when insulating the slab. “The slab can act as a good thermal mass, if it is finished with a solid material or left as exposed concrete. We want the heat to be released to the inside of the house, not to escape outside”.
Reducing heat loss. In winter, once the natural heat from the sun has been captured, don't let it go! “Gaps and windows are the biggest sources of heat loss. Wrap up the building well during construction, tap gaps and aim for air tight construction. Windows are great for letting in sun and light, but consider the size of the window in relation to its orientation. A higher percentage of glazing on the north wall and a low percentage of glazing on the south wall will logically position the windows where they can be of most use to the building thermally. Also consider the heat loss that the window will cause. Window furnishings will help in this regard, but so will the specification of the window itself. Consider thermally broken frames, timber instead of aluminium, double or triple glazing, and argon gas filled units”.
Heating and ventilation systems. Do your research, advises Jennifer, or engage a specialist. “Specialists can provide a wealth of knowledge in solar hot water and photovoltaic systems. In this Zero Energy project, the team at Solar City worked with us to develop an efficient roof scape of tech-savvy solar systems, the engineering of which is well beyond our understanding. We helped to integrate the systems into the architecture of the house”.
Material selection. Reducing toxins used during the build will result in a healthier home. “If a natural alternative is available for a synthetic building material, it is worth considering. When choosing between products” says Jennifer “consider the life of that product...what is in it, how far did it travel to get here, what happens to it at the end of the building life cycle”?
Landscape. The location and type of new trees on the property should be carefully considered with respect to the shading effects on the house and outdoor living areas, both to provide some respite in summer in outdoor areas, but to also reduce shading the house in winter, and keeping solar receivers unshaded at all times.
Finally, establish and check performance criteria. Some performance targets require an in-depth knowledge of thermal engineering to achieve. “We would never attempt to meet a clients specific target without advice from a more scientificly-inclined mind. There are a number of companies across NZ with engineers who can help guide you through the design process and run calculations to check the performance of the house along the way. In our recent experience with the zero energy house, Jo Woods from ECubed kept things in check, with a super close eye, as it was also to be her own home”.
Images courtesy of A Design Studio Limited.