Timber homes might have less embodied CO2 than than those buit from concrete. But new research shows that over their lifetime, concrete homes win the carbon battle hands down. By Jeff Dyson of The Concrete Centre

New research has revealed that conventional masonry houses that take advantage of their inherent thermal mass can save a significant amount of energy over their lifetime compared with lightweight timber-frame housing. The independent research, carried out by Arup Research + 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 environmental impact than the embodied CO2 of the materials used to build it. Some 50% of the UK's CO2 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.

This study, A Lifecycle Analysis Examining the Role of Thermal Mass, compared lightweight timber homes with medium-weight and heavyweight masonry and concrete homes and found that the latter have the lowest total energy consumption and CO2 emissions. It demonstrates that the thermal mass in masonry homes reduces the need for air-conditioning in summer and, by capturing solar gains, also reduces the consumption of winter heating fuel. The winter savings alone offset the slightly higher level of embodied CO2 in a masonry house in as little as 11 years.

The use of thermal mass to capture solar gains on winter days is not a complex process. To make the most of the effect, glazing should be orientated to the south and there should be adequate thermal mass in the floors and walls to capture and store heat from the low winter sun. It can be used in standard house designs, thereby providing savings in heating fuel and CO2 emissions.

Straightforward guidance on passive solar design is available on the Carbon Trust's website, www.thecarbontrust.co.uk.

Study methods

The research is based on the study of a two-bedroom semi-detached house in the South-east of England, typical of the type of "starter home" envisaged by the government in areas such as the Thames Gateway. Analysis of lifecycle CO2 emissions was carried out on four weights of construction: light, medium, medium-heavy and heavy. The lightweight class was a timber-frame home with timber floors, exterior brick and internal plasterboard finish. The medium-weight house was the same but with brick and block cavity walls. The medium-heavyweight house had a precast concrete first floor and ground-floor partitions of medium-weight concrete blocks with a plasterboard finish. The heavyweight class had the highest thermal mass, with heavyweight blocks used for the external walls and internal partitions, and precast concrete for the first floor and loft floor.

Occupancy was assumed to be continuous, with a family of two adults, one at home during the day with a pre-school child. In all house types, gas-fired central heating with radiators was assumed with a set point of 19°C for the bedrooms, 21°C for the living room and 22°C for the bathroom. In the summer, three operating modes were considered: natural ventilation, conventional air-conditioning and mixed-mode (air-conditioning and natural ventilation).

In line with guidance from CIBSE, a house was judged to have overheated if 1% of the occupied hours in the living room were over 28°C, or 26°C in one or more of the bedrooms. The research also included the caveat that this must occur in at least three in five consecutive years before it was assumed that air-conditioning would be installed. From the outset, all the houses had solar shading and an appropriate ventilation strategy to help mitigate the effects of climate change.

Weather data representative of the climate of suburban London was used as the basis of the lifecycle analysis. The data covered the 20 years between 1976 and 1995 and was repeated in sequence to cover the period between 2001 and 2100. This was modified using a continuously ramped morphing factor that took account of the UKCIP02 medium-high scenario for climate change.

The lightweight home was found to need air-conditioning by 2021. This compared with 2041 for the medium-weight home and 2061 for the medium-heavy and heavyweight homes. This means that timber homes being built today could become uncomfortably warm in as little as 15 years.

Over the 60-year lifecycle of the lightweight home, the increased reliance on air-conditioning and inability to capture winter solar gains will negate its lower embodied CO2. The medium-weight masonry home was calculated to have about 1.25 tonnes more embodied CO2 than the equivalent timber house, yet over a 60-year period the timber framed home was found to emit 9-15 tonnes more CO2. The findings show that masonry and concrete construction provides the best long-term sustainable building option.

The results for housing are of relevance to other sectors, particularly offices, where keeping cool is a big design challenge. Here, adequate ventilation, solar shading and use of thermal mass are the three accepted methods of avoiding over-heating. The high cooling loads associated with offices enable energy savings to be realised if thermal mass and night ventilation are used to minimise the need for air-conditioning. This leads to a reduction in operational CO2 emissions, and offsets any CO2 embodied in the building's fabric.

Concrete: the year-round solution

The need for air-conditioning in homes can be exacerbated by the drive to reduce heat loss in winter through increased air tightness and insulation. These energy-saving measures can actually lead to a greater risk of overheating during the summer, as heat cannot escape so readily. The application of thermal mass can help alleviate this problem and it is therefore important that energy efficiency and thermal characteristics are not evaluated simply in terms of winter performance. A more holistic approach would consider how the building functions on a year-round basis, along with the recognition that thermal mass works alongside insulation and airtight construction to provide an optimal design solution. Modern masonry construction offers this combination and can be insulated to the same standard as any other form of construction to provide a warm and energy-efficient home.

The claim is sometimes made that lightweight construction is the most energy efficient form of building. This is too simplistic and reflects past standards in design. To evaluate energy in use properly we must now consider:

  • Significantly improved construction standards
  • The impact of climate change
  • The potential for passive heating and cooling techniques
  • Year-round analysis

When these factors are considered collectively, modern masonry and concrete construction provide the lowest overall CO2 emissions during the life of a house.