You've saved hard and you're about to build. You want a home that's comfortable year round and doesn't cost a fortune to cool. Fancy appliances won't get you there. The building envelope will, and specifically four control layers: water, air, vapour and thermal.

Get them right and the house stays dry, comfortable and cheap to run. Get them wrong and you get leaks, draughts, mould and big power bills. Here's what each layer does and what to ask your builder.

1. The water control layer

What it does: keeps rain out of the structure.

What it's made of: roof sheeting or tiles, sarking, flashings, gutters and downpipes, the wall wrap behind the cladding, and correctly flashed windows and doors. Stormwater drainage then carries the water away from the house.

What good looks like:

  • Wide eaves to keep rain and sun off the walls. Learn more about why eaves matter.
  • Wall wrap lapped the right way (upper over lower, like roof tiles) and taped at penetrations.
  • Windows flashed at the head and sill, not just sealed with silicone.
  • Ideally, a drained and vented cavity (a "rain screen") behind cladding: battens that leave a gap, with vents at the bottom and top and insect mesh. Any water that gets past the cladding drains and dries out, and paint lasts longer because the cladding isn't sitting wet.
  • Downpipes connected to stormwater, and ground graded away from the slab.

2. The air control layer

What it does: stops uncontrolled air leaks through the building.

This is a continuous seal around the house: wall wrap, tapes, and seals around windows, doors, downlights and every pipe or cable that passes through the wall or ceiling. Leaky homes let hot, humid air in during summer and push your cooling out. You can't insulate your way around air leaks.

Air sealing and ventilation are different things. The air control layer stops the leaks. Ventilation then brings fresh air in on purpose, where and when you want it, using exhaust fans, openable windows or a mechanical ventilation system. The building science saying is "build tight, ventilate right".

Mechanical ventilation (HRV and ERV)

A heat recovery ventilator (HRV) or energy recovery ventilator (ERV) supplies filtered fresh air and exhausts stale air, recovering much of the heating or cooling energy as it goes.

  • An HRV transfers heat only.
  • An ERV transfers heat and some moisture. In humid climates like coastal Queensland, an ERV helps keep humid outdoor air from adding to your indoor moisture load.

Benefits include removing pollutants, carbon dioxide and excess moisture, and providing fresh air without opening windows on a 35-degree day. Check and clean filters regularly, keep intakes and exhausts clear, and have the system serviced periodically.

More reading:

3. The vapour control layer

What it does: manages water vapour so it doesn't condense inside walls and roofs, where it causes mould and rot.

Water vapour from showers, cooking and breathing moves through materials, generally from warm, humid air towards cooler, drier air. The trick is to stop it condensing on a cold surface inside the wall, and to let any moisture that does get in dry out.

This is where Queensland is different. Much of the vapour control advice online comes from cold climates, where you put a plastic vapour barrier on the inside of the wall. In a warm, humid climate with air conditioning, that can trap condensation in the frame, because the humid air is outside and the cool surface is inside. In most of Queensland, use a vapour-permeable wall wrap on the outside of the frame, and avoid plastic vapour barriers on the inside unless a building scientist has designed the wall for it.

NCC 2022 added condensation management requirements, including exhaust fans that discharge outdoors rather than into the roof space.

Ways to manage moisture:

  1. Use a vapour-permeable wall wrap, lapped and taped properly.
  2. Use a drained and vented cavity behind the cladding so walls can dry.
  3. Duct exhaust fans in bathrooms, laundries and over the cooktop to the outside, and use them.
  4. Choose materials that aren't damaged by moisture, like moisture-resistant plasterboard in wet areas and insulation such as rock wool.
  5. Keep indoor relative humidity roughly between 40 and 60%. In a Queensland summer, that usually means running the air conditioner on dry mode or using a dehumidifier.

More reading: Understanding and managing condensation in buildings.

4. The thermal control layer

What it does: slows the flow of heat into and out of the house.

What it's made of: insulation in the roof, walls and (for raised floors) the subfloor, roof sarking or blankets, glazing, shading and orientation.

Under NCC 2022, most new homes need a 7-star NatHERS rating, which sets the insulation and glazing your builder must provide. That's the minimum. Things that make a real difference:

  • Insulation fitted properly. Compare insulation by its R-value, not the material. Then check it's installed without gaps, compression or missing sections. A batt squashed behind a pipe or missing around a downlight loses much of its value.
  • Roof and ceiling. In Queensland, most heat comes in through the roof. A light-coloured roof, sarking or an insulation blanket under the roof, ceiling insulation and roof space ventilation all help. We prefer fixed roof vents to whirlybirds, which are cheap but tend to fail after a few years.
  • Windows and shading. Glazing is often the weakest part of the envelope. Size and orient windows carefully, shade them with eaves, awnings or screens, and consider better glass where the sun hits.
  • Orientation and thermal mass. Good passive design (living areas facing north, shaded in summer) reduces how hard the air conditioner has to work. Thermal mass, like an exposed slab, stores heat and evens out temperatures when it's shaded and insulated correctly.
  • Efficient cooling. A correctly sized, efficient reverse-cycle air conditioner with good controls. A well-built envelope means you can use a smaller system.

More reading: Why new homes are still energy sieves and why building science isn't always scientific for project builders.

Questions to ask your builder

Water

  • How are windows and doors flashed at the head, sill and jambs?
  • Is there a drained and vented cavity behind the cladding?
  • How wide are the eaves?

Air

  • What wall wrap do you use, and do you tape the joins and penetrations?
  • How do you seal around windows, doors, downlights and services?
  • Do you offer blower door testing to measure air leakage?

Vapour

  • Is the wall wrap vapour permeable? Which product and class?
  • Do all exhaust fans discharge outside?
  • Will you install any vapour barriers, and if so, why and where?

Thermal

  • Can I see the NatHERS certificate and stamped plans, and what drove the rating?
  • What R-values are in the roof, walls and floor, and how do you check installation?
  • What glazing and shading are specified?

On site, our C7 Frame and C10 Rough In checklists help you check wall wrap, flashings, seals and insulation before they're covered up.

A builder who understands these four layers will be happy to answer these questions. One who changes the subject is telling you something too.