By Mick O'Brien · carpenter & builder · sheds-to-homes, cabins and things that stay up in a storm
“I poured three tonnes of concrete in a NSW winter and watched the sun fail to warm it for weeks.”
— Mick. It'll still be standing in fifty years.
ArticlesCalculatorsSite PlannerShopLet's cut the crap. You're off-grid in Australia. Your house is either baking in summer or freezing in winter. Thermal mass isn't optional. It's the difference between a livable home and a death trap. Here's what actually works.
Let's cut the crap. You're off-grid in Australia. Your house is either baking in summer or freezing in winter. Thermal mass isn't optional. It's the difference between a livable home and a death trap. Here's what actually works.
| Material | Heat Capacity (J/kg·°C) | Real-World Aussie Impact |
|---|---|---|
| Water | 4,184 | Best for slow, even heat. But you need 1,000L to match 1m³ of concrete. Heavy. Prone to leaks. Only use in sealed tanks (not walls). Cost: $150-$300 for a 1,000L tank. Waste of time if you're not building a water wall. |
| Stone (Basalt/Granite) | 790-880 | Slow to heat up. Holds heat for days. But heavy as hell. Requires structural engineering. Cost: $120-$250/m³ (bulk stone). Don't use sandstone - it crumbles in our dry heat. |
| Concrete | 880 | Best all-rounder. Heats up fast, holds heat well. Easy to work with. Cost: $120-$180/m³ (ready-mix). This is your default. Use it everywhere. |
Don't waste money on "thermal mass" in the wrong places. Concrete floors under north-facing windows? Yes. Concrete walls facing west? No. West sun is brutal. Use timber or insulation there. Your money's better spent on a good roof vent.
Thickness matters. 100mm concrete = 10% of the heat capacity of 200mm. Minimum 150mm for floors. Less = useless. Don't skimp.
Water is a gimmick. Unless you're building a water wall (and you're not), it's a headache. We've seen off-grid homes with leaking water tanks costing $5k in repairs. Concrete is cheaper, safer, and works better.
Stone is for show. It looks nice but it's 3x heavier than concrete. You'll need extra foundations. Only use if you're mining it on-site (and even then, it's a pain). Concrete is 40% cheaper and faster.
Concrete (Ready-Mix): Boral, Cemex, local suppliers. $120-$180/m³ (3m³ minimum order). Tip: Order 10% extra. You'll need it.
Stone (Basalt/Granite): Local quarries (e.g., Rock & Gravel). $120-$250/m³. Ask for "dust-free" - sandstone is a no-go.
Water Tanks (If you must): Tuff Tank, Aquarius. $150-$300 for 1,000L. Don't use plastic - it cracks in our sun. Steel is better but costs $500+.
Concrete. It's cheap, it's reliable, and it works. Stone is a luxury. Water is a headache. Stop overcomplicating it. Pour 150mm of concrete under north-facing windows. That's your thermal mass. Everything else is noise.
Worth a watch: 5 Things About the Rocket Mass Heater · Fouch Family Off Grid
While many off-grid projects are achievable as DIY, certain situations require licensed professionals:
Always check local regulations and obtain necessary permits before commencing work.