Trombe Wall Economics — BRANZ Sustainable Construction Fact Sheet (2023)

Source: BRANZ (Building Research Association of New Zealand), Sustainable Construction Fact Sheet #3, v3, January 2023 Data basis: Thermal simulation (SUNREL) of four actual New Zealand houses with Trombe walls, plus user interviews (2011/2012)

Key Economic Findings

• Benefit-cost ratio: 2.1 average across four walls (5% discount rate over 20 years)
• Simple payback period: 5.4 years average
• Maintenance cost: Effectively zero — all four homeowners reported no ongoing costs; occasional exterior repainting and cobweb cleaning only
• User pricing note: In all cases surveyed, the Trombe wall was not separately priced, as it was integrated with the rest of the house — indicating cost is embedded in masonry wall construction, not a bolt-on premium

Thermal Performance

• Average winter space-heating reduction: 35% (temperature set point 20°C, 7pm–11pm)
• Regional variation (NZ): Auckland 45%, Wellington 21%, Christchurch 20%, Invercargill 18%
• Comfort improvement: 10–15% more time in 18–25°C comfort band versus homes without Trombe wall

Construction Details

• Typical wall: ~200mm thick concrete (poured, tilt slab, or grouted block), dark-coloured exterior, single or double glazing, 15–50mm air gap
• Orientation: Must be within ±30° of true north (southern hemisphere) for reasonable benefit
• Thermal conductivity: Heat conducts through concrete at ~25mm/hour (8-hour delay for 200mm wall)
• Minimum sizing ratio: Trombe wall volume to living area volume = 1:80 for concrete-based materials
• Alternative materials: Rammed earth stores ~25% more heat than concrete per volume; adobe stores considerably less

Design Guidance

• Computer modelling shows marginal benefit from double vs single glazing; high SHGC (≥0.7) matters more
• Summer shading is critical to prevent overheating — fixed shading preferred over movable
• Interior vents no longer recommended (older references suggested them); unvented walls heat up more during day for nighttime release
• Interior surface must not be covered with plasterboard or insulation; conductive finishes only (shotcrete, cement plaster, tile, paint)

User Satisfaction

• All four NZ users recommended the technology
• Key praise: quality of radiative heat (“not stuffy like artificial heating”), low maintenance, seasonal self-adjustment
• One wall described as behaving “exactly the same as it did 40 odd years ago”

Limitations

• No absolute dollar figures for installed cost per square foot/metre (walls were integrated into house construction)
• New Zealand climate and construction market — may not directly transfer to US costs
• Sample size of four houses is small
• Economic analysis assumes specific energy prices and discount rate (details in supporting conference paper by Jaques)