Earth Coupled Thermal Battery Design for Buildings
Large thermal mass is a good passive strategy employed to save energy and maintain comfort. Thermal
mass helps stabilize temperatures of a building. It also helps to keep surfaces at uniform temperature,
which creates a better feeling of comfort. A thermally massive structure does not drop much in
temperature during thermostat setback periods, an effect that is measured by the thermal time constant.
A building with high thermal time constant will take longer to cool down if the heating system shuts off
or, conversely, longer to heat up if the A/C stops. It also reduces cycling of heating and cooling systems
plus energy storage strategies can be better employed. For a given high efficiency house, the time
constant can increase fivefold if the slab is earth coupled, ie from 20 hours to over 100 hours (Newell,
ASHRAE Journal March 2011).
In this way, thermal mass acts as a “thermal battery” with the amount of energy put in (stored) a
material as it heats equals the energy released as the material cools. The amount of heat that can be
stored is governed by the Specific Heat Capacity (Cp). There are many materials that can be used for
heat storage, but one of the more common thermally massive building materials is concrete. Concrete
has a specific heat capacity (Cp) of 0.88 kJ/kg·K), meaning that 1 kg of concrete stores 0.88 kJ of energy
per degree celsius of temperature rise.
However, the largest thermal battery available to a building is mostly ignored - that is the earth beneath
its foundation. The specific heat of organic soil (loam) is actually double that of concrete, at 1.9 kJ/kg·K
(See Table 1).
Table 1 - Density, specific heat, and volumetric heat capacity of common soil components at
20°C and 0.101 MPa (van Wijk & de Vries 1963)
To use soil as a thermal battery, we need to connect the building with the soil. Some designs use water
run-around loops to transfer building heat to the ground and vice versa. Because the fluid used is
commonly brine (glycol), the technique is called a brine loop (photo 1).
Photo 1 - A typical residential brine loop requires 300’ of PEX buried at 6 ft depth
However, a simpler option is direct conduction heat transfer between the earth and an uninsulated slab
on grade. Care must be taken to ensure the foundation is frost-protected. Fig 2 shows typical isotherms
around an uninsulated slab-on-grade. Isotherms are lines of equal temperature. As can be seen,
freezing temperatures in the soil can reach below the footing level and cause frost heaving.
Fig 2 – Typical isotherms around an uninsulated slab-on-grade
Regulatory Context
Regulatory requirements regarding sub-slab insulation vary. While SB-12 requires R10 for shallow
residential slabs, other juridictions and best practice guidelines do not have this requirement. ASCE 32,
which is concerned with frost protection, does not call for sub-slab insulation, only perimeter insulation.
Residential Requirements of the 2009 International Energy Conservation Code requires a minimum R-10
perimeter insulation (R15 if slab is heated), but the Code does not require sub-slab insulation. For
heated commercial slabs, perimeter insulation of R 7.5 to R-20 is required depending on climate zone
but, once again, there is no requirement for insulation under the field of the slab. In Canada CSA B214– 07 requires no sub-slab insulation, allowing for equivalent R-5 based on soil heat loss resistance
between underside of slab and top of water table.
First Step – Slab Frost Protection
Whether or not sub-slab insulation is employed, it is always necessary to protect the slab from frost
heave. The common method is to extend foundations below the frost line (Fig 3).
Fig 3 – Typical frost wall foundation
The actual depth of frost penetration depends on many factors, including variations in the outside air
temperature, the inside temperature, soil type, soil moisture content, the location of the ground water
table, the height of the slab above grade and the type of ground surface cover. Many of these conditions
are site-dependent, which means that the amount of insulation required will vary from site to site. For
southern Ontario, a frost depth of 4ft is typically assumed.
However, in all locations in Canada, insulation can be used to reduce the depth of the frost line so that
shallow construction can be used. Frost-protected shallow foundations (FPSF) have been in common
usage for more than 35 years. In Scandinavia, more than a million FPSF have been built with very
successful results. Interestingly, the design guides used in these countries are largely based on Canadian
research.
Standard ASCE 32 covers the design of frost-protected shallow foundations (FPSF) in areas subject to
seasonal ground freezing. The standard includes easy-to-follow steps with reference to design tables,
climate maps, and other necessary data to furnish a complete frost-protection design. The guideline
suggests minimum vertical insulation on the exterior of the foundation wall, grade beam or thickened slab
edge. It also specifies depth and R-value for insulated wings (or “skirt”) extending horizontally from the
foundation perimeter, to reduce the depth of frost penetration (Figures 4 & 5).
Fig 4 –Frost-protected foundation stem wall (Source CMHC)
Fig 5 shows typical isotherms around a frost-protected slab-on-grade. The horizontal insulation keeps
the temperature of the soil below the footing and slab above freezing to prevent frost heave.
Fig 5 – Frost-protected, uninsulated slab-on-grade (Source CMHC)
Using the above methods leaves us to safely couple a slab to the earth without risk of damage from frost
heave.
Next Step – Earth-Coupling
The isotherms shown in Fig 5 loop from outside wall to outside wall as shown is Fig 6. In this way, heat
loss to the cold exterior air is minimized which prevents frost penetration but also ensures that heat is
retained in a defined bubble of ground mass beneath the building footprint. The heat in the ground does
not travel beyond this bubble unless there is a high water table to strip heat away. This is an equilibrium
situation which may take a few years to reach, during which time higher heat transfer to the ground will
take place until ground is fully loaded thermally and heat transfer beyond the bubble approaches zero.
The bubble expands and contracts seasonally as building heat is added to the soil in summer or absorbed
in winter.
Fig 6 – Retained heat under a frost-protected, uninsulated slab-on-grade (winter)
Source: Bean, R, 2010
The ground equilibrates with the building temperature regardless of insulation. This happens whether or
not the slab is heated (ie in-floor radiant heating). Fig 7 shows little difference between insulated and
uninsulated slab in terms of heat transfer from a heated basement slab. This effect is similar for a slab
on grade with frost-protected foundation, heat is contained beneath the structure.
Fig 7 – Retained heat under a insulated and uninsulated basement slab (winter)
Source: Bean, R, 2010
Rather than thinking of this as a heat loss issue, we need to see it as a two-way heat transfer. In the
summer the earth coupled slab and ground's thermal mass absorbs excess heat to help cool the building
so that air conditioning systems will run less often. This heat is then available for winter use. Seasonal
heat transfer more or less balances seasonally. This effect can be seen using energy modeling which
predicts higher cooling load and demand as more sub-slab insulation is added.
To prove the benefits of earth coupling, US firm Build Equinox used modeling to determine that their
super-insulated Equinox House in cold climate Central Illinois (ASHRAE zone 5a) would not benefit from
sub-slab insulation. Monitoring results following construction proved this prediction to be correct. Fig 8
shows that the uninsulated slab requires less energy in the summer due to free cooling to the ground,
while winter energy use is slightly higher than the insulated slab. Annual energy use is about the same
for uninsulated versus insulated slab. Since peak electrical demand is in the summer, earth coupling is
favourable to utilities in terms of peak shaving.
Source: Newell, ASHRAE Journal Feb 2011, “Ground Heat Transfer”
Instead of a using concrete slab, a cheaper option is to omit the concrete slab and, instead, rely on direct
conduction heat transfer between the earth and building. In this case, the floor would be a timber or
joist construction, with a gap between floor and earthen floor (Fig 8). An air/vapour barrier is required.
Care must be taken to ensure the foundation stem wall is frost-protected to minimum 24” below grade.
Fig 7 – Frost-protected stem wall timber or joist floor construction, with a gap between floor and earthen floor
(Drawing – Local Impact Design, 2018)
What about Comfort?
Earth coupling effectively increases the thermal mass of a building, providing "inertia" against
temperature fluctuations which can cause discomfort. Temperatures equalize with the house and stay
steady - ground temperature below the basement slab or wood floor remains within a couple of degrees
of the wall temperature (Newell, ASHRAE Journal Feb 2011).
The real issue from a comfort perspective is that concrete has a high thermal conductivity, about 10
times higher than wood, so concrete feels colder to our feet. Adding wood or carpet layers would
improve comfort without significantly reducing the radiant heat transfer or thermal storage benefits of an
earth-coupled floor.
Conclusion
Is ground-coupled foundation design always the answer? The answer is yes, so long as there isn’t a high
moving water table to strip heat away. Also, in cold climates, the foundation must be frost-protected,
either using a traditional frost foundation wall or a frost-protected shallow foundation (FPSF). If these
conditions are met, a ground-coupled house will provide all the benefits of a thermally massive building –
comfortable temperature regulation (high thermal time constant) with energy savings from seasonal
thermal storage.
References:
1.
Bean, R, 2010 “Should you fully insulate under concrete slabs on or below grade?”
https://www.healthyheating.com/Radiant_heating_designs/insulating-underslabs.htm
2.
Newell, ASHRAE Journal March 2011, “Thermal Mass Design”
3.
Newell, ASHRAE Journal Feb 2011, “Ground Heat Transfer”
4.
Deru, M.P., A.T. Kirkpatrick. 2001. “Ground-Coupled Heat and Moisture Transfer from Buildings; Part 1:
Analysis and Modeling.” National Renewable Energy Laboratory.
5.
Deru, M.P., A.T. Kirkpatrick. 2001. “Ground-Coupled Heat and Moisture Transfer from Buildings; Part 2:
Application.” National Renewable Energy Laboratory.