Log House – A Blueprint for Future Energy Efficient Buildings?

Abstract

Log houses are the oldest way to build homes in Northern Europe. Still today, almost all the buildings in many rural villages in Finland, Estonia and other countries are log houses. They are a very important part of identity in the built environment. In today's building industry, however, log houses stand mainly for the vacation homes, for the nostalgia in us. In the discussion on energy efficient buildings, however, the log house hardly plays any role at all. But there is no reason why log houses wouldn’t work as sustainable buildings. Wood is an ideal material for energy efficiency, has a negative carbon footprint and is widely available as local material. Therefore, this log house project in a village near Tampere was designed to study the potential of the log house in both modern, sustainable architecture and energy efficiency. The log house is built with a double log wall. This way, it is possible to reach passive house level insulation with massive wood walls because the double log wall eliminates the usual thermal bridges in a wood construction as both walls are load bearing. The biggest challenge is bringing the natural properties of a log house – the settling of the wood and lacking air tightness in the joints when the wood dries out – together with the air tightness as the key element for energy efficiency and building physics. To reach a reliably airtight envelope, a new method was engineered for building the log walls: The inner wall was built first. After that, a vapour barrier was wrapped around it, and only then the outer wall was raised. This way, bringing the vapour barrier to its place was not only easy, but also very accurate and safe – the only seam in the vapour barrier was bringing together the start and end point. Another challenge is the building physics as the façade can’t be ventilated. The insulation lies directly behind the outer wall. WUFI-analysis shows, however, that the wall construction is still on the safe side with the moisture. The airtight envelope is an important factor, as is the adaptive vapour barrier that is very tight at low relative air humidity and up to 100 times more open at a higher air humidity. The project shows that a log house is a truly sustainable building with a low carbon footprint, airtight building envelope and a safe building physics. Together with the long tradition in Northern Europe they are worth a deeper view in the field of sustainable building.