Basement Finishing near you

Part 10.8.1 of the Housing Provisions requires that where a pliable building membrane is installed in an external wall it complies with AS 4200.1, and that it is positioned on the exterior side of the primary insulation layer. That ordering is the whole point: the membrane is there to keep bulk water and wind out while still letting the wall breathe inward, not to wrap the insulation in a bag.

The permeance figures then vary by where you live. For material on the exterior side of the primary insulation, the Code sets a minimum vapour permeance of 0.143 micrograms per newton second in climate zones 4 and 5, and 1.14 micrograms per newton second in climate zones 6 to 8. In plain terms, the colder the zone the more the outer layer has to breathe. A product specified for a Brisbane job is not automatically the right product for a Hobart or Canberra one, and the zone is the first thing to establish with the supplier.

Insulation choice follows from that. What matters is not only the R-value on the bag but whether the assembly as a whole can dry, whether the insulation will sit hard against a cold surface, and whether the installer understands which face of the batt or board the membrane belongs on.

Condensation management in the Code is not only about walls. Part 10.8.2 requires that exhaust from a kitchen, kitchen range hood, bathroom, sanitary compartment or laundry discharges directly, or through a shaft or duct, to outdoor air. An exhaust fan terminating in a roof space or a subfloor is not compliant and never was, and it is one of the most common defects found in retrofitted lower levels where the easy route was upward rather than outward.

The Code also sets minimum flow rates for those fans: 25 litres per second for a bathroom, and 40 litres per second for a kitchen or laundry. If the finished lower level includes a bathroom or a laundry, those numbers are the specification you should see on the quote rather than a fan chosen on looks.

Where the work extends into a roof space in climate zones 6 to 8, Part 10.8.3 requires a ventilated space of not less than 20 millimetres above the primary insulation layer, with openings distributed as set out in the Code's table. That matters in a two-storey conversion where the lower level work continues upward into the roof.

A lower level is the part of the house that is hardest to air out, because it usually has the fewest openable windows and the least incidental draught. Use the fans that were installed, run them long enough to clear the moisture rather than switching them off with the light, and treat a persistent musty smell as information rather than a nuisance.

Keep the surface water picture under review as well. Garden beds built up against a wall, a downpipe that has been redirected by a landscaper, or a new paved area that now sheds water back towards the house will all show up inside the finished room eventually. Checking that once a year takes ten minutes and protects a substantial investment in linings and flooring.

A storage or workshop fit-out and a habitable finish are different jobs with different budgets, and it is worth being honest about which one you want. A storage fit-out can be lightly framed, unlined or lined with a robust sheet, ventilated simply, and left with a sealed slab underfoot. It tolerates a small amount of seasonal dampness because nothing in it is precious.

A habitable finish has to satisfy the room height, ventilation and condensation provisions, carries insulation and finished surfaces that are unforgiving of moisture, and usually brings a bathroom or a kitchenette with it. Each of those turns a tolerant space into an intolerant one. If the moisture position is uncertain, the sensible sequence is to fit out simply, live with the space for a full cycle of seasons, and finish it properly once you know how it behaves.