This article provides a comprehensive look at underslab insulation, explaining why it matters, how it works, when it is needed and what the National Construction Code (NCC) requires. Whether or not you insulate your concrete slab can have a critical impact on the comfort and energy efficiency of your new home, and it cannot be changed – we encourage you to get it right!
Introduction: A Quick Story
Members of the Powerhaus team recently stayed in an underground home in White Cliffs, located in north-west NSW, in the middle of winter. After camping in frosty conditions, they were surprised by the warmth of the underground home, which stayed at a comfortable 22°C year-round without any heating or cooling. This led us to ask: why was this home so comfortable? The answers we found lead to a few easy rules of thumb around building underground homes and, by extension, houses on concrete slabs, and if they should be insulated from ground temperatures.
The science of underslab insulation
Temperature dynamics underground
At a depth of 3 meters, the ground temperature can be approximated using the average yearly surface temperature (Baggs, 1983). In White Cliffs, despite air temperatures ranging wildly from 0°C to 45°C, the temperature at 3m depth stays relatively close to the annual average surface temperature of around 22°C.
This is an approximation only and depends on multiple factors, including soil type, condition, vegetation and depth. However as a generalisation, it can help you think about how the outside climate might help or hinder the comfort of a new house build.
In cooler regions like Canberra and Melbourne, ground temperatures range from 12°C to 15°C. If you connect your slab to the ground to the ground in these areas, your house is going to be quite cold, all of the time.
Human comfort range
The optimal human comfort range used for your NatHERS star rating is approximately 18-24°C. This range can vary based on factors like humidity, breeze, and physical activity, as well as personal preference and what you’re used to. Interestingly, Passive house uses a slightly higher temperature range for designing comfortable homes of 20-25°C.
If we look at the underground temperature of White Cliffs – 22°C is in the goldilocks zone. The underground temperatures of Canberra and Melbourne, not so much.
Guiding principle #1: Optimal human temperature range for indoor temperatures is 18–24°C.
How your concrete slab will regulate temperature and increase thermal efficiency
Concrete slabs have high thermal mass, meaning they can absorb heat from the environment during the day and release it slowly at night. When a concrete slab is connected to the ground without insulation, it utilises the stable temperature of the earth to moderate indoor temperatures. Meaning, it will continually draw on that temperature to moderate the indoor temperature.
When you insulate your concrete slab, you have more control over its role in heating or cooling and prevent it from using the ground temperature to regulate. Insulation prevents the external temperature of the air, or in this case, the ground, impacting the internal temperature of the house. You can heat the insulated slab (preferably by sunlight) and it will remain warm over a long period, warming your home like a hot water bottle. The opposite is also true.
Is underslab insulation right in your climate?
Guiding principle #2: When the average ground temperature is outside the optimal human temperature range, always insulate your slab.
Using data from Australia’s Bureau of Meteorology, we dug out the average temperatures for the year across Australia, to estimate what the underground temperature would be and if it fits within the optimal human temperature range (yes, our team are geeky like that). We’re using this as a proxy to estimate if the ground temperature will help your home stay comfortable.
**This should not be used as a proxy for thermal modelling. If you are building a new home or a habitable dwelling, we always recommend advice from a professional thermal performance assessor. Get a quote to work with our amazing team here.
Cool climates like Melbourne, Adelaide and Canberra benefit from insulating under concrete slabs
In cooler climates, insulation is crucial to prevent constant cold temperatures in underground homes. Insulation allows for controlled internal temperatures using sunlight and shading without losing warm air to the surrounding soil.
The answer is easy: insulation. To prevent your home from being cold all the time, you will need to insulate the underground home from the soil temperatures. Insulation will mean that you will still have stable internal temperatures, but you’ll be able to control them using sunlight and shading – without losing all your warm air to the soil around you.
Mild climates don’t benefit from slab insulation
In mild climates, where average ground temperatures align closely with the optimal human comfort range, coupling the slab to the ground can be beneficial. This natural thermal mass can help stabilise indoor temperatures, reducing the need for additional heating or cooling.
Homes between Coffs Harbour and Cairns on the East Coast, and Perth to Carnarvon on the West Coast, can comfortably connect to soil temperatures. Notably, underground towns like Coober Pedy and White Cliffs fall within this range.
The impact of climate change
One final note on underground temperatures – they are slowly warming. A long-term Australian study show a 0.5°C rise per decade since the 1950s. If this trend continues, more southern regions of WA, SA, and NSW will become suitable for ground-connected slabs in the next 40 years.
What is the National Construction Code requirement for insulating your concrete floor?
Where the floor is connected to the ground and not a waffle-pod slab, the NCC says:
- In climate zones 6 & 7 you need insulation around the slab edge and under the slab with a minimum R value of 0.64, unless you’re using a waffle-pod slab (NCC 2022 13.2.6 (5a))
- In climate zone 8 you need a minimum of R1 slab edge and R2 underneath the slab, unless you’re using a waffle-pod slab (NCC 2022 13.2.6 (5b))
- If you have an in-slab heating system, no matter what climate zone, you need slab-edge insulation of R1 or greater. (Not relevant if the system is just in a bathroom) (NCC 2022 13.2.6 (4))
Recommendations
- Climate matters: Your local climate determines whether you should insulate your slab or connect it to the ground.
- Insulate when required: Properly insulating your slab is essential for maintaining comfort and energy efficiency in cooler climates, and in hot ones too.
Curious about whether you’re getting your slab insulation right? Get a quote from our team for energy efficiency modelling, and gain the confidence that your home will be as comfortable and energy-efficient as possible. Let’s get it right, together.
