Insulation

The basicsWhat an R-value means

An R-value measures resistance to heat flow. Higher is better, and R4 resists heat twice as well as R2.

Two R-values get confused:

• Product R-value is the number printed on the batt or board.
• Total R-value is the whole roof, wall or floor: the insulation plus the lining, cladding, air spaces and the surface films of air on each side, less the losses through framing. Building codes set total R-values.

Insulation comes in two broad kinds. Bulk insulation (batts, blankets, boards and loose fill) traps still air and slows heat flow in both directions. Reflective insulation (foil sheets and foil-faced products) reflects radiant heat, but only works with a still air space beside the shiny face, and it loses performance if dust settles on it. Most good roofs and walls combine the two.

More insulation always helps a little, but the gains shrink as R-values rise. Past a point, sealing gaps or fixing thermal bridges does more than another layer of batts.

The minimumWhat the code asks for in Canberra

Under the NCC's elemental (deemed-to-satisfy) rules, a house in Canberra (NCC climate zone 7) needs roughly:

Most new houses comply through a NatHERS rating instead. The rating sets its own insulation levels to reach seven stars, often well above these figures, particularly in ceilings. From many ratings across many types of houses in the ACT, I typically find that seven stars needs R2.5 in the walls and R5 or R6 in the ceilings. Some of these rules still apply on that path too, including the thermal breaks and compensating for downlights.

The code minimum is a floor. Ceilings are the cheapest place to go well beyond it, because the roof space is easy to fill at construction and hard to reach later.

GapsWhat 1% costs

The NCC includes a table showing how much extra insulation is needed to make up for parts of a ceiling left uninsulated, around downlights, exhaust fans and flues. It shows how harsh small gaps are:

Leaving 1% of a ceiling bare costs as much as removing more than R1 of insulation from the rest of it. Beyond about 2.5%, no amount of extra insulation makes up the loss.

Downlights are the usual culprit. Older fittings need a clear space around them, and a ceiling full of downlights with 50 mm clearances can leave over half a percent uninsulated before anything else is counted. The fix is simple: specify downlights rated IC-4, which can be covered directly with insulation, and seal them against air leakage. Fewer downlights, or pendant and wall lights instead, help too.

Thermal bridgesHeat takes the shortcut

Wherever something more conductive than insulation passes through the insulated layer, heat finds a shortcut around it. These are thermal bridges:

• Studs and joists. Timber framing takes up a real share of every wall, and it insulates far less than the batts between it.
• Steel framing conducts heat many times better than timber. That's why the NCC requires a thermal break of at least R0.2 between steel frames and the cladding or roofing fixed to them.
• Slab edges exposed to outside air lose heat straight out of the floor.
• Window and door frames, lintels and steel beams that run from inside to outside.

The cure is a continuous layer of insulation outside the frame, or thermal breaks where the bridge can't be avoided. Detailing these junctions is drawing work, done before construction, not something a builder can fix on site.

FloorsThe forgotten surface

In a Canberra winter the ground a few metres down stays relatively mild, but the edge of a slab is exposed to frosty air. Much of the heat a slab loses goes out through that edge.

• Slab on ground: insulate the edge, and ideally underneath. If the slab has in-slab heating, edge insulation is essential, or the heating warms the garden.
• Waffle-pod slabs sit on polystyrene pods, which provide some insulation underneath, but the edge beam still needs attention.
• Suspended timber or steel floors: insulate between or below the joists, and seal the floor against draughts from the subfloor.
• Floors over garages or open undercrofts need the same insulation as an external wall.

On siteInstallation, checking and moisture

Insulation only performs if it's installed properly:

• Batts must fill the cavity without being compressed. A squashed batt has a lower R-value than the label says.
• No gaps at the edges, around pipes and wiring, or where the trades have been through after it was laid.
• Installed to the Australian Standard, AS 3999 for bulk insulation, with the required clearances to flues and fittings that aren't rated for coverage.
• Photographed before the linings go on. Once the plasterboard is up, nobody can see what's behind it.

Insulation also changes how a building handles moisture. A well-insulated wall or roof has a colder outer side, where moisture can condense if it gets in. Insulation, membranes and ventilation have to be designed together. See Condensation and building breathability for the wall and roof requirements in Canberra.