Life-cycle environmental impacts of a standard house and three log house cases: A comparison of a typical Finnish house and three ecological log house designs with alternative external wall thicknesses

This publication presents the calculation results for the life-cycle environmental impacts of a typical Finnish wood framed house, called 'standard house'. The calculation results are also presented for three alternative log house, with extensive use of wood and log products in structures. The log house cases vary only in their external wall log thickness. The results take into account the emissions from material acquisition, production and transportations, as well the emissions from construction phase. The lifetime emissions are considered in terms of materials for repairs and renovations, and emissions from operational energy use over a life-cycle of 50 years. Also, the energy use for demolition and removal of demolition waste from site is included in the assessment. The results for the material production show that the greenhouse gas emissions of log houses are 40% lower than those of the standard house. Total GHG emissions for standard house are 25 tonnes (in terms of CO2-equ), whereas the emissions for log house scenarios are 15 tonnes. The structures of log house cases store 3.8 to 4.2 times the carbon of the standard house in their structures. The standard house stores some 14 tonnes of carbon dioxide, whereas the figures for log houses are 53 to 58 tonnes. According to the results, the energy content of the structures of log house cases is 2.6 to 2.9 that of the standard house. Energy content of standard house is 270 GJ, and the values for log houses vary from 720 GJ to 800 GJ. The mass of standard house is 88 tonnes, while the total mass of log house cases varies from 70 to 74 tonnes. When the total lifetime emissions from material-related sources are considered, the results show that the greenhouse gas emissions of log houses are some 33% lower than those of the standard house. Total emissions for standard house are 39 tonnes (in terms of CO2-equ emissions), whereas the emissions for log house scenarios are some 26 tonnes. The total material need over the 50-year lifetime of standard house is 106 tonnes, while the material need of log house cases vary from 81 to 85 tonnes. The operational energy use of the log house cases of this publication is higher than that of the standard house, due to differences in space-heating energy needs. This is caused by differences in U-values of external walls. This results in a 10 to 19% higher carbon footprint of log houses, when no crediting for stored carbon or bio-energy are made. If the carbon storage is credited in the calculation of the carbon footprint, the differences between standard house and log houses diminish to a level of 0 to 10%.When both bioenergy and stored carbon are considered, the carbon footprint for standard house is 167 tonnes and for log house 200, 168 tonnes (CO2-equ). For log houses 243 and 270, the figures are 150 tonnes (CO2-equ), and 139 tonnes (CO2-equ), respectively. The results show that when both bio-energy of side-streams and structures and carbon credits are taken into account, the carbon footprint of log house 200 is at the same level as the standard house. The carbon footprint of the log houses 243 and 270 are 10 and 16% lower than that of the standard house. This publication also studies the so called total energy consumption figures, as defined in Finnish building regulations. The figure for standard house is 166 kWh/m2, and for the log houses 184 to 194 kWh/m2, when calculation is done as stated in regulations. Two alternative calculations of theoretical nature are also made. Firstly, the bio-energy related to wood-based structures and their side-streams is taken into account by assuming this bio energy could be used to replace heating energy use in the building. This is done by assigning the energy in wood-based structures an energy-type factor of 0.5, as in the Finnish building regulations. The results show that with these assumptions, the total energy figure for standard house is 164 kWh/m2. For log houses, the figures vary from 176 to 187 kWh/m2. Secondly, bio-energy is taken into account by assuming it to be completely emission free bio-energy, thus using a value of zero for its energy type specific factor. The results show that the total energy figure for the standard house is 159 kWh/m2. For log houses, the figures vary from 156 to 169 kWh/m2.