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.

Our HEAT BATTERY is explained in a new video click here. Can oak be used as a thermal store? Click here Oak as a thermal battery

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!

Simon Tilley

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

Date posted: January 9, 2020 | Author: | 8 Comments »

Categories: Uncategorized

8 responses to “Oak heat battery houses”

  1. HHP says:

    I read your recent email about the high specific heat of oak – very interesting.

    There is also the carbon ‘cost’ of creating concrete, as opposed to the carbon ‘income’ of growing oak, in terms of net CO2 in the atmosphere, which should favour the use of oak.
    However, I suspect that oak wood, in the sizes and volumes needed, is currently more expensive than concrete.

    I was puzzled by your remark about not letting trees rot to truly store the carbon they have absorbed during life. My study of soils and ‘regenerative’ or ‘conservation’ agriculture suggest that the carbon from rotting wood does get fed back to the soil, either directly, as it decomposes, or indirectly through the flora and fauna which consume it, and eventually perish, and feed the soil.

  2. HHP says:

    Thanks for the comment – interesting!

    Do you have something published or a reference for the carbon storage in soil from trees after they have died? This is really important to know specifically ie how much in kg per weight of wood. It would certainly make oak carbon storage less attractive financially. My thought is that in the future we will be paying for carbon released into the atmosphere and be able to sell carbon sequestration by the tonne. If its stored anyway in the soil the value of the oak as a carbon store will not be as high.

    In a high mass oak house the oak will not just be valued as a building material but also as a carbon store which will hopefully have a financial value too. (If the wood naturally is stored in the earth after it rots the net effect of keeping it is wood in terms of carbon is reduced. (Not sure how the carbon offsetting people take this sort of thing into account!)

    Any figures on this would be much appreciated.

  3. HHP says:

    High thermal mass may be good in a consistently cool climate but how would that help during a prolonged period of hot weather which is becoming more likely ? Oak frame would only ever be a niche market for people who could spend at least £500,000 on a house incl. land purchase. The more urgent issue is how to finance energy efficiency retro-fit in the existing housing stock.

  4. HHP says:

    I agree the more urgent issue is how to retrofit existing housing stock there is a lot to do!

    The purpose of the thought on Oak was to stimulate discussion on limiting swings in temperature internally in houses with a thermal store and create a carbon sink.

    In terms of overheating Oak wood work much like our concrete at HHP in that it acts as a buffer absorbing excess heat and releasing it again when the temperature drops. So thermal mass limits overheating as much as it supplies heat when required. This is one of the beauties of high thermal mass design in that it tackles both heating demand and cooling requirements in a simple passive way.

    The cost of building in Oak might be expensive but I believe in the future we will be looking at ways of storing the wood that trees produce . If the trees are let to rot into the ground then the carbon will be released again . So the question arises how do we look after this wood . It seems to me that the best way to use it would be to do something useful with it. I.e. build houses. The economics of this would be very guided by our thoughts on the cost of carbon and limiting climate change . I believe in the future this will be key . I believe we will have to come to a situation where sequestering carbon is part of normal economic life and makes financial sense. We are not there yet!!
    Thanks for your feedback.

  5. HHP says:

    An interesting article Simon. I think there may be a lot of people out there in the world who are somewhat ahead of you. What you are describing is CLT construction. (If you discount log cabins 🙂

    Cross laminated timber is widely available and widely used, even in relatively backwards Britain. See for example:


    In regards to specific heat, the most relevant property to look at is the volumetric specific heat, rather than the more usual -by-mass version.

    It’s also worth considering that clay, or earth more generally, is also pretty low-impact, cheap and has good thermal properties. In fact I think the designers of Hockerton well understood that point. It’s a lot easier to acquire a lot of earth than it is a lot of extra timber.
    Rammed earth, clay bricks or even clay plaster are very useful.

    The trick with wood is to overcome some of its other properties, such as flammability. Yes it forms char and that protects it so its structural properties are better than steel in many ways, but surface spread of flame needs to be addressed.

  6. HHP says:

    I’m with you in that I think the oak would have to be processed to become a usable building material as CLT is. The critical point I was proposing was that the 2 functions of thermal store and insulation are split. So my idea is to ignore the insulating properties of the word and just using it for its thermal storage capacity. The extra outer layer of insulation would stop the thermal store from losing its heat to the outside world. This is very similar to HHP construction. Do you know if any clt constructed buildings have used this configuration?

    I like the idea of using natural materials in building wherever possible. And straw bales have their place and I’m sure your house is very comfortable. From a physics point of view straw bales do work comparatively well as insulation but their thermal storage capacity is limited . Here are some figures I’ve just dugout . Bale specific heat capacitance, c = 232.1 J/kg·K (Although can be more.) Density, p = 190 Kg/m3, oak c being 2400 J/kg·K The density of oak typically 740 kg/m3 So oak has a capacity of about 10 times as much and is 4 times as dense so per unit volume oak can store 40 times that of straw bales. (https://web.ornl.gov/sci/buildings/conf-archive/1998%20B7%20papers/063_Arumi_Noe.pdf)

    It would be good to discuss how CLT could be used as a thermal storage medium with a insulated layer on the external surface….. may be this could be a low density wood?? . Thanks again for your feedback.

  7. Ella says:

    Hi, what would you suggest for a model of this for commune homes?
    I desperately want to be completely self sustainable on some land, the one thing I really want to combat is the heating in homes. I live in Kansas, so the winters dont get super bad, but they can be difficult to deal with at times.
    Thank you for your time, and hope to get some insight on my new life~

    • HHP says:

      Hi Ella. We have a LIVE zoom tour of the project this Saturday at 2pm 16th May. The link is on the front of the website. These houses have no heating and are always warm in the winter and cool in the summers. An ideal way to live and no more costly than a conventional build.

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