Passive Cooling & Heating Estimator (Wall Materials)
Estimate thermal mass, breathability, and seasonal comfort for any wall material, thickness, climate zone, and orientation. Eco-friendly home design.
A good all-round choice for temperate climate, balancing winter heat retention with summer comfort (thermal-mass rating 4/5, high). The wall is highly breathable (rating 5/5), which affects how moisture and indoor humidity move through the assembly. South orientation is favourable for winter passive heating.
How this works
Thermal resistance R is wall thickness divided by the material's thermal conductivity k. Thermal mass and breathability are derived from the same material via short lookups:
k = THERMAL_CONDUCTIVITY[wallMaterial] (W/m·K)
R = wallThickness / k (m²·K/W)
thermalMassRating = clamp(round(5 − k × 2), 1, 5)
breathabilityRating ∈ [1..5] from a per-material lookup
seasonalComfort = narrative(climate, orientation, ratings)Lower k means a better insulator, which lifts the thermal-mass rating. Earth-based and natural-fibre walls (cob, adobe, rammed earth, timber, bamboo) score 5 on breathability; concrete scores 1; clay brick and AAC sit in the middle.
Worked example
A 0.30 m cob wall (k ≈ 0.55) facing south in a temperate climate:
- R = 0.30 / 0.55 ≈ 0.55 m²·K/W
- Thermal mass rating =
round(5 − 0.55 × 2) = 4 / 5 - Breathability = 5 / 5 (cob is highly vapour-permeable)
- Seasonal verdict: a good all-round choice in temperate climate; south orientation favours winter passive heating
Switching to a 0.30 m AAC wall lifts R to ≈ 1.67 — three times the resistance — but drops breathability to 3, which changes the right interior finish strategy.
Sources
- ASHRAE Handbook of Fundamentals — Heat, Air and Moisture Control in Building Assemblies
- ISO 10456:2007 — Building materials and products — Hygrothermal properties
FAQ
What does the thermal mass rating measure?
The thermal mass rating (1 to 5) is a comfort-oriented summary of how well the wall material resists conductive heat flow. It is derived from the material's published thermal conductivity (k, in W/m·K): lower k means better insulation, which scores higher. AAC blocks, bamboo, and timber score 5; cob, adobe, and rammed earth score 4; clay brick and concrete blocks score around 3; stone and dense concrete score 1 or 2. A high rating is ideal for climates with large day-night temperature swings.
What does the breathability rating measure?
Breathability (1 to 5) is a vapour-permeability proxy. Walls that breathe — earth materials, timber, bamboo, lime-mortared brick — let interior moisture move outwards rather than condensing inside the wall. Modern cement-rendered walls and concrete trap moisture. A 5 means the wall buffers indoor humidity well and is the right substrate for vernacular finishes like lime plaster. A 1 indicates that any decorative finish must include a separate vapour barrier strategy.
How is R-value calculated for the chosen wall?
R-value is wall thickness divided by thermal conductivity: R = thickness / k, in m²·K/W. A 0.30 m cob wall (k ≈ 0.55) gives R ≈ 0.55, while a 0.30 m AAC block wall (k ≈ 0.18) gives R ≈ 1.67 — three times more resistance for the same thickness. Energy codes typically demand R ≥ 1.5 for walls; vernacular materials often need to be paired with a separate insulation layer to meet the target.
Why does the calculator give different verdicts for the same wall in different climates?
Thermal mass behaves differently across climates. In hot-dry climates, mass is a strong asset — it absorbs heat during the day and releases it overnight, smoothing 15–20°C diurnal swings. In hot-humid climates, mass can trap heat overnight and you need cross-ventilation to release it. In cold climates, mass is useful only when paired with daytime solar gain, otherwise it just slows the wall down to outdoor temperature. The verdict text reflects these context-specific tradeoffs.
Why does orientation matter for passive design?
In the northern hemisphere, south-facing walls receive winter sun (when the sun is low) and need to capture and store that gain — high thermal mass with a glazed verandah is the textbook strategy. West-facing walls get the most afternoon summer sun and are the most prone to overheating, so external shading is essential. North-facing walls stay the coolest and most thermally stable, suitable for bedrooms in hot climates. East-facing walls get gentle morning sun and rarely overheat.
Are the conductivity values accurate for vernacular materials?
The published k-values for cob, adobe, and rammed earth come from regional soil tests and ASHRAE composite figures and vary by ±20% depending on density, moisture content, and local soil composition. A wetter cob wall with more straw will have a lower k than the table figure; a drier, denser rammed-earth wall will have a higher k. Treat the calculator as a comparison tool between options, not an absolute energy-code submittal.