Putting design decisions together

The sequenceFree decisions first

Every design decision limits the ones after it. The order below runs from free to expensive. It's also the order in which they're hardest to change later.

A house that gets steps 1 to 4 right needs less from steps 5 to 8. A house that gets them wrong can only buy its way out with dearer glass, more insulation and bigger equipment, and even then the west bedroom will still overheat.

The last row matters too. Equipment and solar are the cheapest way to lift the Whole of Home score, and that's why they're tempting. But they wear out and get replaced. The walls, windows and orientation are there for the life of the house.

ShapeLess skin, less heat loss

Heat escapes through the outside surface of a house: roof, walls and floor. The less surface there is for each square metre of floor, the less heat there is to lose.

Compare two 200 m² houses with 2.7 m ceilings:

The two-storey house has about a quarter less surface to lose heat through. It also needs a smaller slab and roof, which are expensive. Every extra corner, wing and step in a plan adds wall.

There is a trade-off. A house elongated east to west gets more north-facing wall for winter sun. In Canberra the best plans are usually compact and moderately elongated, with the long side facing north. They aren't sprawling, and they aren't perfectly square.

In my experience of rating Canberra houses, an exceptionally well-planned house can reach seven stars with ordinary windows and standard insulation. It needs a small footprint, a short external wall for its floor area, and rooms and windows facing the right way. In practice, though, that house is a simple rectangular box. The more interesting the design, with more corners, more glass and more articulation, the harder the walls, roof and windows have to work to make up the difference.

InteractionsEvery decision changes another

No single decision makes a good house, and most help only when the decisions around them are right:

• More north glass brings more winter sun. It only pays off if the eave is sized for it, the glass is good enough not to lose the heat overnight, and there's thermal mass in the room to store it.
• Thermal mass evens out temperatures in a sunny room. In a shaded room it just takes longer to heat.
• Insulation pays off steeply at first, then levels off. Past a point, an extra R-value buys almost nothing, while sealing gaps or fixing a thermal bridge might.
• Airtightness stops draughts and heat loss, but a well-sealed house needs designed ventilation to stay dry and healthy.
• A covered terrace improves summer comfort outside, and can take the winter sun from the living room behind it.
• High ceilings feel generous, but in a heated room the warm air collects above your head.

This is why the energy rating is most useful as a design tool. Each option can be tested against the others while the design can still change.

AirtightnessThe hole in the bucket

Airtightness is measured by pressurising the house with a fan in the front door, called a blower door test, and counting how many times the air inside is replaced each hour at 50 pascals of pressure (ACH50).

A house at 15 ACH50 is leaking warm air through hundreds of small gaps: downlights, skirtings, window frames, plumbing penetrations and the joint between wall and ceiling. Insulating a leaky house is like filling a bucket with a hole in it.

Sealing is cheap at construction and expensive afterwards. It needs a continuous air barrier shown on the drawings, care at every penetration, and ideally a blower door test before the plasterboard goes on, while leaks can still be fixed. A well-sealed house then needs planned ventilation: exhaust fans for moisture, openable windows, and for very tight houses, heat-recovery ventilation.

FurtherBeyond seven stars

Seven stars is a legal minimum, not a target. Some clients want to go further:

• Eight to ten stars. A NatHERS rating can go to ten. In Canberra, going from seven to eight stars cuts the allowed heating and cooling load by about 30%, and ten stars allows almost none.
• Passive House. This international standard targets a combined heating and cooling demand of about 15 kWh/m² a year, airtightness of 0.6 ACH50 or better, windows around U 0.8, and heat-recovery ventilation. It is certified independently, including a blower door test on the finished building. NSW accepts Passive House certification as a way to meet the BASIX thermal requirement in some locations, and the ACT's version of NCC 2025 adds a Passive House pathway too. Its calculations don't convert directly to NatHERS stars.

Both cost more to build. Both also produce houses that stay comfortable with very little heating, which in Canberra's winters is a different experience to live in, not just a smaller bill.

DeliveryGetting the design built

A design only performs if it's built as drawn. The weak points are predictable:

• Documents that don't agree. The plans, window schedule, insulation specification, energy certificate and, in NSW, the BASIX commitments must all describe the same house. Any conflict will be resolved on site, usually in favour of the cheaper option.
• Substitutions. A window, insulation batt or wrap that looks the same can perform very differently. Every change should be checked against the energy certificate.
• Insulation gaps. Batts that are compressed, cut short or missing around downlights and framing can lose a large share of their value. Inspect before the linings go on.
• Air leaks. These are invisible once the plasterboard is up. A blower door test at lock-up stage catches them while they're still easy to fix.

Builder selection and site supervision sit outside a standard architectural engagement, but the drawings can make good construction easier. Clear details, a full window schedule and an air barrier shown on the plans leave less to guesswork.