Retrofit · Thermal
Feb 2026
Walk into an unrenovated Queenslander at the end of February and the back rooms feel like the inside of a kiln. Walk into the same house in July and the front bedroom needs an extra blanket. Single-glazed sash windows, uninsulated weatherboard walls, an under-floor open to the breeze, a steel roof above an uninsulated ceiling — the building is, by 2026 standards, a thermal sieve. It's also, almost certainly, the most charming house on your street. The job is to keep what works and fix what doesn't.
How bad is "bad" — baseline numbers
An untouched pre-1947 timber cottage in Brisbane typically rates between 1 and 3 stars on the NatHERS scale. The new-build minimum since 1 May 2024 is 7 stars. The gap is huge, but most of it is closable — and the closing moves are well-understood, well-priced, and mostly non-destructive.
The priority order — what gives you the most gain per dollar
Resist the urge to start with windows. They're the most visible thermal weak point in the house, and they're rarely the highest-ROI fix. The priority list, in order:
Ceiling insulation. R5.0 batts in the roof space — typically $40–$60 per square metre installed, fully retrofitted on a typical 120 m² Queenslander for $5,000–$8,000. Up to 35% of unwanted heat gain comes through an uninsulated ceiling; this single move can bump a 2-star house to 4 stars by itself. Add a solar-reflective roof coating ($3,000–$6,000) and you're approaching 5 stars on ceiling alone.
Underfloor insulation. The elevated Queenslander has a fully accessible under-floor — a renovator's gift. R2.0–R2.5 polyester or rockwool batts between joists, secured with batten or strap; $4,000–$8,000 retrofitted. Particularly effective in Brisbane because the underfloor space stays markedly cooler than ambient in summer and slightly warmer in winter — insulating captures that. Add reflective foil to the underside if the budget allows.
Air-sealing. The cheapest performance you'll ever buy. Brisbane Queenslanders typically leak at 20+ ACH at 50 Pa pressure; modern compliant builds target 5. Most of the leakage is around skirtings, under doors, at the chimney throat, at penetrations, and at the verandah door seals. A thoughtful weekend of caulking, weatherseal strips, and chimney capping costs $300–$1,500 and can buy a half-star. Done by a contractor with a blower door test, expect $2,500–$5,000 and a full star.
External wall insulation — only when you're replacing linings anyway. The Queenslander stud cavity is typically 90 mm deep, which fits R2.0 batts. Adding insulation behind a wall lining that's already off (because you're doing the kitchen or bathroom anyway) costs $20–$40 per square metre. Trying to insulate a wall that's currently intact requires removing and reinstating one face of lining, and is expensive enough that it's only worth it if you're committed to the room renovation regardless. Don't blow loose-fill cellulose into a closed stud cavity in a Queenslander — moisture management in lightweight timber walls makes this risky.
Glazing. The instinct is to do this first; we put it last because the dollar-per-star ratio is bad. Replacing sash windows with thermally-broken double-glazed equivalents costs $1,500–$3,000 per window unit installed — a 12-window cottage is $20,000–$36,000. The thermal gain is genuine but smaller than the ceiling fix, and the character overlay constrains your options. If glazing is the final move once you've done 1–4, prioritise west and east elevations (where solar heat gain is worst) and retain originals on south and street-facing facades.
What to skip
A few common "energy retrofit" products that don't earn their place:
Reflective foil paints internally. Marketed aggressively, marginal in practice. The physics doesn't work the way the brochures suggest in subtropical conditions.
"Whole-house fans" through the ceiling in an uninsulated roof. You're just moving hot roof-space air. Insulate the ceiling first.
Single-pane sash replacement with single-pane aluminium. A surprising number of "renovators" make this swap because aluminium is cheaper. Thermally, you've gone backwards (aluminium frame conducts heat; sash timber doesn't), and you've lost character credit. The compliant move if you must replace is thermally-broken aluminium or treated timber.
Concrete slab "thermal mass" additions to the floor. Adds embodied carbon, doesn't fit the lightweight Queenslander's logic, and often performs worse in humid conditions than the existing timber floor with insulation underneath.
The character overlay constraint
Almost everything in the retrofit priority list is non-character-controlled. Ceiling, underfloor, internal wall insulation, air sealing, and reflective roof coating sit either inside the building envelope or above the eaves where they don't affect streetscape character. The only retrofit that triggers the Design Overlay Code is window replacement, and even there, internal secondary glazing (a discrete inner pane added to the existing sash) is usually allowed without DA because it's not visible from the street.
The full retrofit — what it costs, what it gains
A thorough thermal retrofit on a 150 m² Queenslander, done at the same time as a kitchen-and-bathroom renovation that opens internal linings:
Ceiling insulation + solar-reflective roof: $10,000–$14,000
Underfloor insulation: $5,000–$8,000
Wall insulation (during reno): $6,000–$12,000
Air sealing + blower door verification: $3,000–$5,000
Glazing upgrade (priority elevations): $15,000–$25,000
Total: $39,000–$64,000 for a typical end-state of 5–6 NatHERS stars on a house that started at 2. Annual cooling/heating energy drops roughly 60%; lived utility cost drops $1,000–$1,800 a year at 2026 retail electricity rates. Add a 6.6 kW solar system ($5,500–$8,500 after STCs) and a heat-pump hot water unit ($4,500–$7,500 installed), and you're looking at a Queenslander that costs $700–$1,500 a year to run.
The five-year horizon
Two trends to watch:
Mandatory disclosure of energy ratings at sale. Already in force in the ACT, on the federal agenda. When it lands in Queensland, the unrenovated Queenslander at 2 stars will trade at a discount to the retrofitted one at 6 — the price spread already exists informally, disclosure will make it explicit.
Targeted state retrofit subsidies. The federal Trajectory for Low Energy Buildings includes a stream addressing existing buildings; state programs typically follow. Watch for ceiling-insulation and heat-pump rebates over the next 24 months.
The thermal retrofit on an old Queenslander is among the best ROI residential investments in Brisbane right now — for comfort, for utility cost, for resale, and for insurance pricing. The character of the house is unaffected; the experience of living in it is transformed.
If you'd like us to put a priority list together for your specific house, get in touch.
By Ashton Genrich — Building Design — Open, QBCC 15387939