Long term solution to carbon storage with oak heat battery houses.
The start of 2020 seems to be a good time to envisage the future. What will housing look like in 2100? How will we build low impact, warm houses that also act as a carbon store? We have understood super insulation combined with thermal mass and passive design is key to achieving warmth through the winter and “coolth” in the summer but how can you do this and deliver low impact and a carbon store? Here is a vision of how we might achieve these things….
I hope we will tackle the climate crisis swiftly and decisively this year and over the coming years. Part of this will include a massive tree planting campaign which will absorb carbon, create wildlife habitats and oddly a dilemma. For the carbon absorbed by these trees to be truly removed from the atmosphere the wood will need to be kept rather than let rot back into the ground at the end of the tree’s life. I have been considering how in the future we might achieve this and solve some of the housing need questions raised above. In the following discussion I’m going to assume the benefit of high internal thermal mass is taken for read as we have covered this in depth in other articles.
I was quite surprised recently to discover that the specific heat of wood can be more than that of concrete. This could mean we can substitute high density wood for example oak for concrete in our high thermal mass housing design. (Light bulb moment!) The wood would need to be grown and in doing so would absorb carbon from the atmosphere. (And create wonderful wildlife habitats.) By using this wood in the construction of houses we would be locking up the carbon for many extra years as houses should last a very long time.
What would a high thermal wood mass, super insulated house look like? The penalty of using wood instead of concrete is that the density of oak is about a third of that of concrete. This means more of it would have to be used inside the insulated envelope to store the same amount of energy. However, because it has a higher specific heat than that of concrete the net result would be that about double the volume would be required. The sacrifice here would possibly be lower internal floor space however this effect could be mitigated by using more wood in the floors and ceilings. In effect the house could be very similar to the Hockerton Houses but with a slightly smaller floor – area may be 6% less.Of course, the practicalities of building in oak rather than concrete would be quite different and the material supply chain would take a long time to become sustainable. The benefits of substituting oak for concrete would be enormous though.
To finish let me emphasise that I am not suggesting building timber frame houses out of oak with insulation within the walls as this would not be able to store heat. A heat battery for a house needs to have internal mass surrounded by insulation on the outside of the building envelope. The configuration I am proposing is a thick oak structure with insulation outside this with no cold bridges of oak or any other material across the insulation layer.
Some of the background detail: Specific heat is basically a measure of how much heat energy a material can contain. The density is how much of a material you can fit into a certain space. The heat figure ranges I saw for concrete were 840 J/kg·K to 1800 J/kg·K (Kodur, Properties of Concrete at Elevated Temperatures, 2014) and for wood the range was 1300 J/kg·K to 2500 J/kg·K with oak being 2400 J/kg·K (EngineeringToolbox, n.d.). This makes oak a third better than the best capacity concrete. Obviously the density of these materials plays a role as well so for completeness a high density concrete might be 2300 kg/m3 (Guo, n.d.) The density density of oak varies but typically English Brown Oak is 740 kg/m3 (EngineeringToolbox, n.d.). So, comparing concrete and oak by volume, one cubic meter of concrete could store for each degree of temperature rise 4.1MJ and wood 1.8MJ. (The arithmetic 2300 x 1800 = 4.1x 106 and 740 x 2400 = 1.8 x 106). Our explanation of how heat battery works can be found three videos down. A pine building product of cross laminated timber (CLT) is available and is well understood. CLT has the ability to store heat if configured correctly but is less dense than oak.
Incidentally I would encourage you to become a member of the Woodland Trust to help support tree planting initiatives. HHP is a member of the charter branch network. Hands up here my daughter now works there! Hockerton Housing Project has become a tree charter group and is focusing on planting trees where it can. Come and see what we have done on one of our Sustainable Living Tours of the project.
I will be discussing how sustainable houses are delivered in Westminster on the 29th January. Please come and join the event. Other speakers include:
· Lord Best, Social Housing Leader, House of Lords
· James Harris MA MSC, Policy and Networks Manager, Royal Town Planning Institute
· Barry Goodchild, Professor of Housing and Urban Planning, Sheffield Hallam University
· Anthony Probert, Programme Manager, Bioregional
· Stewart Clements, Director, Heating and Hotwater Industry Council (HHIC)
· Dr Steffie Broer, Director, Bright Green Futures
· Rene Sommer Lindsay, Urban Designer and Strategic Advisor, R|S|L|ENT
· Simon Tilley, Director, Hockerton Housing Projects
· Emma Fletcher, Chair, Swaffham Prior Community Land Trust
· Mikhail Riches Architects
Thanks for listening your comments would be welcome!
Mr S Tilley, CEng MEng MIMechE
Director, Hockerton Housing Project Trading Ltd
NOTES on the Climate Crisis:
National Geographic: Sea level rise, explained:
BBC: Australia bushfires north of Sydney ‘too big to put out’:
BBC: Hundreds of temperature records broken over summer:
38 Degrees: DEFRA consultation on Environmental Principles and Governance after the United Kingdom leaves the European Union: Summary report of responses from 80,826 members of the public collected by 38 Degrees:
EngineeringToolbox. (n.d.). https://www.engineeringtoolbox.com/specific-heat-capacity-d_391.html.
EngineeringToolbox. (n.d.). https://www.engineeringtoolbox.com/wood-density-d_40.html.
Guo, Z. (n.d.). https://www.sciencedirect.com/topics/engineering/concrete-density.
Kodur, V. (n.d.).
Kodur, V. (2014). Properties of Concrete at Elevated Temperatures. International Scholarly Research Notices, 2014, 468510. Retrieved 1 2, 2020, from https://hindawi.com/journals/isrn/2014/468510