I took the photograph above a couple of years ago it is called ‘Solar versus Oil’. It is I feel a good pictorial representation of the dilemma I have faced concerning natural and petrochemical materials for my project.
I have decided after a lot of thought to stay with the idea of constructing the property from as higher degree of natural materials as possible. That is not to say there is no place for man made synthetic materials in the build there will be some parts for which they are better suited or more cost effective.
Thermal mass and breathability were key properties I wanted to include in the design. The traditional approach used to be to infill oak frames. However for greater energy efficiency it is better to encapsulate the oak frame in an external building envelope. I still wanted my oak frame to be the main structural element in the building. Some building envelopes can stand up by themselves without an oak frame. Others share structural loads with the frame. Indeed Carpenter Oak and Roderick James Architects have some fantastic designs that blend a very contemporary oak frame with structurally insulated panels. I like these designs very much they were a source of great inspiration but my personal preference has led me to a more traditional route with the frame as the main structural element surrounded by a breathable natural envelope.
I hope to use a material called Hempcrete for the walls and possibly the roof. Hempcrete is made by mixing shriv, the woody core of industrial hemp plant, with a lime binder. The resultant material is cast around a softwood frame forming monolithic breathable walls with some quite amazing properties.
Hempcrete is an environmentally friendly building material and locks up carbon dioxide. It is likely to last hundreds of years further enhancing it’s green credentials. It provides a unique combination of thermal inertia and thermal mass resulting in very slow internal temperature changes. High levels of air tightness can be achieved and it has high performance fire resistance. It is a very simple construction method and roughly comparable in cost terms to other building methods.
Hempcrete can be formed around a load bearing studwork frame but in my case the oak frame will be doing this work thus there will be a simpler non-load bearing frame to form the hempcrete around. The relatively lightweight hempcrete I hope will work well with the oak frame and may help reduce foundation specification requirements and cost.
One particularly interesting thought is that u-values describe energy transfer through a material using a steady state model. Hempcrete can provide better performance than it’s u-value might suggest when there is a significant difference in internal and external temperatures. The blend of thermal inertia and thermal mass in hempcrete helps to provide excellent dampening of external temperature changes. Aircrete and solid wood also have similar properties. Though a hempcrete monolithic structure also avoids layers, membranes, gaps, joints and cavities. It also has little to cause cold bridges particularly in wall structures.
The package for building regulations approval and construction drawings is underway. However the roof design is an area of debate and may be heavily influenced by budgetary constraints.
Choosing the right building materials and construction methods for my house has been a big project in itself though very interesting. The only trouble is I keep forgetting friends and family may not find insulation as interesting as I do!!