Insulated homes with carbon-neutral precast concrete flooring

Concrete, Apr 2008 by Pickard, George

Rising energy prices, supply shortages and increased concerns over carbon dioxide and other greenhouse gas emissions make energy efficiency a high-priority issue for everyone. Prospective homeowners are more focused than ever on the cost-effective investment of their money in thermally efficient homes.

Conventional masonry houses incorporating precast concrete floors at ground floor and upper floors can, by taking advantage of their thermal mass, save significant amounts of energy over their lifetime compared with timber-framed housing. A recent independent study, carried out by Arup Research and Development, is the most comprehensive study to date to examine embodied and operational CO2 emissions. Over the life of a home, the operational CO2 emissions of a house have far more of an environmental impact than the embodied carbon dioxide of the materials used to build it. Some 50% of the UK's emissions are produced by the energy used to heat, cool and light buildings, making it essential that energy consumption over a building's lifetime be taken into account when evaluating construction materials.

In Arup's analysis of energy consumption and CO2 emissions of timber homes and concrete homes, it was found that the latter has the lowest total energy consumption and CO2 emissions. One reason for this is that thermal mass in concrete homes reduces the need for air conditioning in summer months and, by capturing solar gains, also reduces the consumption of winter heating fuel. Indeed, the winter savings alone from using concrete offset its slightly higher level of embodied CO2 compared with lightweight forms of construction in as little as 11 years. At today's Building Research Establishment (BRE) durability target of 120 years, the concrete/masonry specification outperforms timber for over a century!

Summer savings are also significant, as concrete acts as a thermal store, absorbing heat during the day (so keeping the building cool) and releasing it at night. This moderation of peak temperatures is called 'fabric energy storage' or FES. Compared with air conditioning, FES can reduce CO2 emissions by up to 50%, since around 90% of the environmental impact from buildings comes from heating or cooling and only about 10% is from the energy used to produce the building fabric.

"Timber houses with timber floors may have less embodied CO2 than those built from concrete but concrete homes win the carbon battle hands down," says Jeff Dyson, head of housing solutions, The Concrete Centre.

The Government's consultation document sets out ways to cut CO2 emissions by 30% and make all new buildings carbon neutral by 2016. Following the EU's Energy Performance of Buildings Directive, which requires the energy performance of buildings to be calculated in accordance with a national Standard, significant changes to Part L of the Building Regulations were made in April 2006. As far as the flooring element for dwellings is concerned, the insulation value on all floors has changed from 0.25W/m^sup 2^K to a more demanding 0.22W/m^sup 2^K.

"Concrete's thermal efficiency and adaptability means it is not only well placed to deliver the Government's carbon emission reductions but it can do so in a sustainable way," says Andrew Minson, director of technical services, The Concrete Centre.

So how do we actually achieve this? Almost everyone remembers their science lessons at school, which included practical experiments using thermos flasks to keep contents cool or hot. However, it's not practical or economic to live in a thermos flask! Instead, the next best thing is to use the insulation properties of air, which forms the basis of all insulation materials such as polystyrene and polyurethane. In particular, expanded polystyrene (EPS) offers excellent insulation in a lightweight, easily worked form, at the same time minimising the use of the raw material, a by-product of the petroleum industry.

Members of the PFF have been working on improving the thermal efficiency of their floors for over 30 years, combining the benefits of EPS with the thermal mass of the concrete beams and concrete screed. As precast floors can in most cases be specified at no extra cost, this is a clear case of simple specification equalling designed sustainable standards without pushing the boundaries.

Recognising the synergy of high-strength structural concrete prestressed beams and the excellent thermal resistance values provided by EPS, members can provide a range of solutions. Excellent insulation - with U-values of 0.20W/m^sup 2^K - is achievable with these systems. The design of the EPS infill panels minimises cold bridging at the floor beam, and the EPS panel does not degrade during its lifespan.

The environmental integrity of EPS is excellent:

* zero ODP (ozone depletion potential)

* zero GWP (global warming potential)

* BRE Ecopoints rating from 0.043

* CFC, HCFC and HFC free

* contains recycled content

* recyclable

* BRE Green Guide summary A rating.

In addition, the environmental integrity of precast concrete flooring systems is further enhanced, as for several years members of the PFF have been using high-carbon steel containing a high proportion of recycled metal as well as recycling aggregates and reducing waste on-site.