Aerogels australia whitepaper field testing

BACKGROUND Aerogels Australia Aerogel field testing
Underfloor hydronic heating systems is a popular form of heating in most of Europe,
providing thermal comfort by directly heating the in ground concrete slab of a building. The
application of these systems in Europe is always specified by the manufacturers with
insulation under the heated slab as well as at the edge of the slab (Rehau, 2009 and Tiemme,
2009). This specification is due to the heat loss to the ground and resulting increase in energy
usage of the heating system.
Underfloor hydronic heating systems are gaining popularity in Australia, and the current
practice is not to insulate the slab. It has always been assumed that due to the milder climates
in Australia compared to Europe the application of insulation is not necessary. This report
provides an analysis of the impact on the energy usage of underfloor heating systems and the
energy efficiency of a building with an insulated slab.
THERMAL ANALYSIS
Underfloor hydronic heating systems deliver water at approximately 35 oC which is returned
to the heating system at 25 oC leaving the average temperature of the centre of the slab around
30 oC. Based on the hydronic system being 70 mm below the floor, this arrangement achieves
a floor surface temperature of 26.5 oC.
For sandy, clay or loam type soils subject to winter conditions a soil thermal conductivity of
around 1.5 W/mK can be expected (ASHRAE, 2005). Around the Adelaide plains and Victor
Harbour region during the winter period, the subsurface soil temperature is approximately
13.5 oC at 1 m depth (Baggs, 1983). This temperature is unaffected by daily variations in
surface temperatures and therefore unlikely to be affected by the heated slab. According to
AIRAH (2007), the R value of the indoor air film at the floor is 0.11 m2K/W for heat flow up,
and it is generally excepted that the room temperature for winter is 20 oC.
Under ideal steady state conditions, where there is no change in temperatures, it can be
determined that the heat flow into the building is 59 W/m2 of of floor area, whereas the heat flow
into the ground is 25 W/m2 of floor area. This indicates that the heat lost into the ground
represents 30% of the heat that is provided by the underfloor heating system.
This analysis has not considered the thermal mass or capacity of the ground. Including this
factor will increase the heat lost to the ground. For example over a 24 hour period the soil
under the slab may on average change in temperature by 1 oC. Such a change in temperature
rise of 1 m of soil below the ground equates to an additional heat flow of 10 W/m2, increasing
the heat loss to 37% of the total heat provided by the heating system. This heat is always lost
as the ground temperatures are always below the temperature of the heated slab. This occurs
due to the heat within the ground dissipating laterally through the ground which is
subsequently lost to the surface.
The analysis conducted also ignores the heat that is lost laterally from the slab to the ground.
This heat loss is significant as although the edge of the slab is a smaller area than the floor
area, it is more exposed to the ground surface.
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MEASUREMENTS
Preliminary measurements have been made of the impact of slab insulation at the site office
of the Beyond Development, a sustainable housing development at Victor Harbour in South
Australia.
Approximately half the house has no underfloor insulation, and the other half is insulated with
10 mm of aerogel with an R value of 0.74 m2K/W. Each side was separately heated. The
operation of the building involves the uninsulated half being used as a public display during
the day resulting in windows being open during the day. The other half is used as offices and
windows were left closed. The slab is charged during the night, and allowed to cool during
the day. During a two week period in the middle of Spring, measurements were taken of the
energy needed to maintain each section of slab.
Preliminary results showed that, on a per unit of floor area, the uninsulated section required
3.8 times more energy than the insulated section. Much of this additional heat flow can be
attributed to the open windows, however given that this only occurred for 8 hours during the
day it is estimated to increase the heat loss by no more than a factor of 2. This results in the
energy used by the uninsulated section being 1.9 times that of the insulated section. Further
measurements will be undertaken to confirm this result.
IMPACT ON BUILDING THERMAL EFFICIENCY
In temperate climates, residential buildings demand both heating and cooling. In regions such
as the Adelaide plains, heating represents approximately 50% to 66% of the total heating and
cooling demand (Belusko et al, 2008). Regions such as southern Victoria and southern part
of South Australia, including Victor Harbour, predominantly require heating.
Insulating a building under the slab, even without an underfloor heating system, will always
improve the energy efficiency of the building during the heating season, as temperatures
below the slab are lower than room temperatures. In periods of cooling, insulation below the
slab can result in an increased cooling demand in a building. This outcome is only a concern
for lightweight buildings without any internal thermal mass, and with poor night time
ventilation. In regions such as the Adelaide plains, based on building modelling conducted at
the University of SA, even for well insulated buildings, the increase in cooling demand is
often outweighed by the reduction in heating demand, resulting in either an overall reduction
in total heating and cooling demand of up to 3% or an increase of up to 3%. For this reason
underslab insulation is generally not considered in these regions. In predominantly heating
regions, such as Victor Harbour, any increase in cooling demand for these types of buildings
would have a negligible effect on the total heating and cooling demand from that building and
be outweighed by the benefits of underslab insulation during the heating season. In these
regions, underslab insulation will increase the overall energy efficiency of the building.
Underfloor heating often requires less energy to use than other forms of heating due to the
improved levels of comfort as well as being able to utilise solar heating systems and heat
pump systems at greater efficiencies. It is clear that without insulation, the efficiency of this
method of heating seriously deteriorates, and may negate the inherent benefits of underfloor
hydronic heating.
Consequently, for energy efficient underfloor heating, insulation under the slab is critical.
The most negative consequence of this approach will occur in regions such as Adelaide in
lightweight buildings with poor night time ventilation, resulting in a very small increase in
total heating and cooling energy demand. This increase is outweighed by the increase in
energy usage that would occur in underfloor heating systems which are uninsulated.
CONCLUSIONS
Due to the higher temperatures experienced in heated slabs in underfloor hydronic systems,
consideration of insulation is warranted. A heat flow analysis has shown that the heat lost to
the ground is at least 30%. Preliminary experimental results at Victor Harbour in South
Australia have shown a 47% loss of energy to the ground. In conclusion, it is clear that
insulation for underfloor heating systems is a critical requirement to maintaining the energy
efficiency of these systems.
Without underfloor heating systems, the application of under slab insulation in mixed heating
and cooling climates such as Adelaide, will most likely have no effect with the benefits in
winter being outweighed by the increase in energy consumption in summer. For
predominantly heating only climates, such as Victor Harbour, under slab insulation in a
building will result in a net reduction in the total heating and cooling energy demand.
Insulating underfloor heating systems in predominantly cooling climates represents a
significant energy efficient measure. Insulating underfloor heating systems in mixed heating
and cooling climates such as Adelaide will result in an overall lower heating and cooling
energy usage by the building.
REFERENCES
AIRAH (2007). AIRAH Handbook, Australian Institute of Refrigeration, Air-conditioning and Heating Inc.,
Melbourne, Australia.
ASHRAE (2005), ASHRAE Handbook – Fundamentals, American Society of Heating, Refrigerating and Air-
Conditioning Engineers Inc., Atlanta, USA.
Belusko M., Boland J. and White D. (2008), Impact of variation in behaviour pattern and weather data on
building energy performance, In Proceedings of the 3rd International Solar Cities Congress, Adelaide, Australia.
Baggs S. (1983) ‘Remote Prediction of Ground Temperature in Australian Soils and
Mapping its Distribution’, Solar Energy, 30, 351-366.
REHAU (Accessed 2009), ‘Installation instructions of underfloor hydronic heating systems’,
URL:http://www.rehau.co.uk/building.solutions/underfloor.heating/introduction/introduction.shtml, REHAU
Ltd.
Tiemme (Accessed 2009), ‘Installation instructions of underfloor hydronic heating systems’, URL:
http://www.tiemme.com/Home.pag, Tiemme Raccorderie S.p.a.
Aerogels Australia www.aerogel.com.au info@aerogel.com.au