Monday, 31 October 2011

Critical Review of a paper on CO2 emissions from housing

Hacker, J.N., De Saulles, T.P., Minston, A.J. and Holmes, M.J. (2008), “Embodied and operational carbon dioxide emissions from housing: A case study on the effects of thermal mass and climate change”, Energy in Buildings, Vol. 40, pp. 375-384 [click on link for full paper online]

The paper in short: 
The paper examined focuses on the evaluation of CO2-emission quantities of four building types during construction and its operational lifetime requirements. It takes into account, both the embodied energy in building materials and associated carbon dioxide releases as well as CO2 emissions released during the buildings operational use in order to maintain thermal comfort.

The study evaluates the comparison of a 2-bedroom semi detached home build with different ‘weights’ of thermal mass. The examined building is a two-storey building located in the south-east of England (total floor area 65m2).  The house was assumed to be occupied by a family of two adults and one child. The following four building configurations are compared:

1. Lightweight
(Ext. walls: timber frame with plasterboard finish (inner leaf), insulation, ventilated cavity, brickwork cladding; Int.partitions: timber stud with plasterboard finish; Ceilings: timber with plasterboard and chipboard floor finish; Ground floor: solid concrete/screed; Roof: timber & tiles; Flooring: carpet throughout, with exception of linoleum in bathrooms and kitchen

2. Mediumweight
(As lightweight but with ext. walls of mediumweight concrete block with plasterboard (inner leaf), insulation, ventilated cavity, brickwork cladding)

3. Medium-heavyweight
(As mediumweight but with ground floor ceiling of pre-cast concrete floor units and internal partitions of mediumweight concrete block with plasterboard finish)

4. Heavyweight
(Ext. walls: heavyweight concrete block with fair-faced finish (inner leaf), insulation, ventilated cavity, brickwork cladding; Int. partitions: heavyweight concrete block, fair-faced; Ground and first floor ceilings: pre-cast concrete units; Flooring: carpet on first floor, with exception of linoleum in bathroom, stone tiles throughout ground floor) 

Floor plan:

Critical review:

The examined paper has considerable flaws in its evaluation and comparison of the four different building configurations. As a result a falsified and misleading interpretation of the CO2 emissions performance is circulated, leaving the impression of being part of an unbiased and sound academic research.  In fact the research behind this paper is conducted by the ‘British Cement Association’ and ‘The Concrete Centre’, both part of the ‘Concrete Society’ an organisation “…encouraging the use and development of concrete”  

During the 100-year modelling simulation a rise of 7°C in peak summer temperatures was assumed for the latter part of this century. Both periods for heating and cooling demand  were excessively exaggerated for light- and medium weight buildings. While CO2 emissions from heating were accounted for on a 24-h heating mode for 9 month a year, the air-conditioning unit seems to operate from June - September. Both heating and cooling appliances were operating with a set point temperature 24/7; no exceptions were made in regard to the occupancy patterns. The research is also missing to state, where carbon emission quantities for the calculation of heating and cooling equipment are derived from.

Taking into account a 100-year life span of a building might represent the life span of the structural components but rarely of windows, doors, internal finishes, insulation and technical appliances or occupancy behaviour. Fuel mix and energy supply source will change and the assumptions stated are already non-compliant with current standards, concerning boiler efficiency and U-values for ext. walls (TGD L). Air tightness, one of the most important factors in thermal performance is not mentioned at all.

The environmental cost calculation (tCO2/yr) of running air-conditioning equipment is based on 79 years for the lightweight, 59 years for mediumweight, but only 39 years for the medium-heavyweight and heavyweight option. Figures are based on beneficial effects of energy storage with increasing thermal mass. Alternative measures for lightweight options would include the instalment of sun protection equipment and shading devices or better quality windows in order to prevent excessive heat gains and in turn greater energy consumption from cooling equipment. Regarding the fact, that the south facing window area presents 21% of the total surface area, excessive heat gains during summer month might actually require greater cooling requirements for heavyweight construction especially as solar heat gains will be retained in the fabric much longer. Against the papers assumptions, cooling loads for a heavy weight construction could in this instance be much higher and require much longer application. A simple, unbiased and scientific approved method for comparison of energy performance is the Building Energy Rating Certificate (BER-CERT).

When calculating embodied carbon dioxide emissions (ECO2) most of the ECO2 intensities are assumed or estimated and are not based on concrete facts. As sources of energy are changing  towards renewables a more appropriate method would be the comparison of embodied energy instead of  ECO2 emissions. In addition, identical materials can have different ECO2 intensities depending on the way of manufacture ( from reclaimed material & production efficiencies) and history of transport (resource origin - cradle to site). The following graph shows cumulative energy requirements for domestic single housing over a 100-year period: 

In general it should be said that the CO2 emissions from housing are less dependent on the structural materials being used, rather on insulation levels, air tightness and proper design evaluation (orientation, heat gains, exposure and services employed, see also: Passive House Concept) and behavioural patterns in using energy efficient appliances. Even if the thermal mass can delay the processes of overheating and capture heat energy for longer periods of time, good thermal performance is not only achieved by choosing heavyweight building materials as this paper might suggest.

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