Cold-climate design for an ACT granny flat.
Canberra winters get to -7°C overnight. A granny flat designed for Brisbane or even Melbourne suburban conditions UNDERperforms here. Here’s the design framework we use for every ACT build.
The six design pillars.
1. Building envelope insulation.
- Walls: R5.0 minimum (we default to R5.5 with rigid foam thermal break)
- Ceiling: R6.0 minimum (we default to R6.5 with thermal break at trusses)
- Floor (suspended): R3.5 minimum, slab edge R1.0
- Slab on ground: R1.0 perimeter, R0.5 underfloor (often skipped — we do it)
2. Glazing.
- Double-glazed throughout (Argon-filled IGUs, low-E coating on inner pane)
- U-value ≤ 2.0 W/m²K (mainland double-glazed standard is 2.6)
- Solar Heat Gain Coefficient (SHGC) ≥ 0.65 on north windows (capture winter sun)
- SHGC ≤ 0.35 on west/east windows (block summer afternoon sun)
- Frames: thermally-broken aluminium or uPVC
3. Solar orientation.
- Living areas oriented to north (within 30°)
- Eaves 600–750mm to provide summer shading while allowing winter sun
- South-facing windows minimised (heat loss only, no gain)
- West-facing windows shaded or smaller (summer afternoon heat)
4. Thermal mass.
- Concrete slab on ground (exposed, polished or tiled) acts as thermal store
- Internal brick or block walls on north side amplify thermal mass
- Avoid carpet over slab in living areas — defeats the purpose
5. Heating system.
- Reverse-cycle inverter AC (premium: Daikin URURU, Mitsubishi GE) — primary heating, electric, $200–$400/yr running
- OR in-slab hydronic heating (Rinnai gas or Daikin Altherma heat pump) — more premium, slower response, more even heat
- NOT old-style gas heater or radiant electric — expensive to run
6. Air-tightness.
- Continuous vapour barrier in walls (Procheck or Hydroshield)
- Sealed penetrations at every service point
- Compressible-strip seals at door and window frames
- Target: 5 ACH₀₅ or better (Australian average is 15)
- Mechanical ventilation with heat recovery (HRV) if very tight
What this costs vs base spec.
For a 55m² 1-bedroom granny flat, the cold-climate envelope adds:
- Double-glazed thermal-break windows: +$4,200 (vs single-glazed)
- R5.5 wall + R6.5 ceiling insulation: +$2,600
- Slab edge insulation: +$1,400
- Air-tightness detailing + vapour barriers: +$1,800
- Premium reverse-cycle AC (or hydronic): +$3,800
- Eaves design (deeper overhangs for solar shading): +$1,200
- Total cold-climate premium: $15,000
Running cost saving: typical $1,800–$2,400/yr in heating costs versus base-spec build. Payback ~7–8 years on the upgrades. Plus a tenant who stays longer because the place is genuinely comfortable in winter.
Why some “Canberra” quotes are dangerous.
Mainland Australian builders frequently quote ACT projects using their NSW or Brisbane base spec, then upgrade only what’s minimum to pass EER 6-star inspection. Result:
- Single-glazed where double-glazed is needed for actual comfort
- R3.5 walls instead of R5.0+
- No slab edge insulation
- Cheap reverse-cycle AC undersized for ACT loads
- No air-tightness detailing
The build passes EER 6-star on paper but real-world comfort is poor. Tenant ends up running the AC heater 18 hours/day in July at $3K/yr electricity bill. We see this often when we’re called to retrofit insulation or upgrade glazing 2–3 years into a tenancy.
Where we work.
Cold-climate spec, every build.
Double-glazed, R5.5 walls, thermal-break frames, air-tight envelope. Standard.