Hempcrete buildings – thermal performance and costs

 What can I expect from hempcrete in terms of its thermal performance?

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The thermal properties of hempcrete are extraordinary. It provides insulation because of pockets of air trapped within the material; both in the spaces between particles of hemp shiv, and in microscopic pores in the hemp shiv itself. Compared to synthetic insulation materials, hempcrete is a less-effective insulator, For example 240mm thickness of hempcrete meets current UK building regulations for insulation (compared to around 80 mm of polyurethane insulation board). However because hempcrete is cheaper than synthetic insulation materials, and is used to form a monolithic wall, the usual thickness of hempcrete applied in a new building is 300-400mm for a wall.

The typical thermal conductivity of hempcrete is typically 0.06 to 0.07 W/mK. U-values for hempcrete vary depending on the thickness, the type of binder used, the exact specification, application techniques and the skill of the contractor, however a typical u-value (for a 350mm thick hempcrete wall) is 0.17 W/m2K.

Hempcrete also provides thermal mass, as well as insulation, due to density of the lime binder once it has set. This means that hempcrete is able to store heat within the fabric of the material itself (like a giant storage heater). In this respect it is unlike lightweight insulations which only store heat within the air trapped inside the material. The ability of hempcrete to store heat in this way as well as insulating has two important advantages.

Firstly, it allows for natural ventilation of the building. Whereas modern highly insulated buildings rely on airtightness alone to trap the air within the lightweight insulation layer (leading to the use of closed windows – to preserve the heat – with trickle vents and mechanical heat recovery systems to ensure indoor air quality), hempcrete stores heat (from the sun, or from internal heating) in the thermal mass of its walls, to be released slowly as the building cools down. This means that in a hempcrete home you can open the window if it gets a bit stuffy; safe in the knowledge that all your expensive heat is not going to flow straight out and be lost.

Secondly the slow speed at which a typical hempcrete wall stores and releases heat has the effect of “buffering” natural changes in internal and external temperatures, e.g. night and day temperature fluctuations, so that with very little heating or cooling a constant internal temperature is maintained. Hempcrete keeps your home cool in the summer and warm in the winter, with a dramatic reduction in fuel bills.

Hempcrete is unique in this mixture of insulation and thermal mass. It can be thought of (in terms of natural materials) as halfway between straw bale (which has loads of insulation, but very little thermal mass) and cob (which has loads of thermal mass and very little insulation). The combination of the two gives hempcrete an extraordinary thermal performance which works dynamically to maintain comfortable indoor temperatures with very little need for mechanical heating, cooling or ventilation. During in-situ testing, hempcrete consistently out-performs expectations based on laboratory tests and computer modelling.

How much does building with hempcrete cost?

Building with hempcrete should cost no more than building with conventional materials, however it will not be significantly cheaper either.

As with any material, costs vary depending on the complexity of the design, the exact specification, and a number of other factors. Broadly speaking though, the cost of building with hempcrete should be around the same as building with a conventional “brick and block cavity” construction including a high-spec insulation in the cavity. In certain situations hempcrete offers a distinct advantage and can allow considerable savings in the build cost. For example when the ground is not suitable for a strip foundation, a hempcrete building (due to hempcrete’s lower density) can be built on a cheaper raft foundation, avoiding the extra cost (both financial and environmental) of a massive concrete slab.

Remember that this comparison is looking only at the construction costs. The extra thermal performance which hempcrete provides brings significant extra savings over the lifetime of the building, as well as improved health of the buildings occupants.

 

Leave A Comment

  1. Toby Gascoigne

    Hello,

    I’m having a structure built in my garden. It will consist of a concrete (or hempcrete?), pad as the floor with two walls (I am building off an existing shed which forms the third wall. These walls will be rendered and painted.
    My question… can I do all of this just using hempcrete?

    Thanks

    Toby

  2. Jacob Pratel

    I am building a tiny house on wheels, can hempcrete be used as a insulation at the same thickness of the traditional insulation? Also where can I source the materials to mix and pour it on my own?

  3. Shan

    What is the embodied energy and carbon of hemp construction materials, excluding sequestration?

  4. Anders

    Is hemp Crete suitable for insulating a steel boat hull or will the thermal mass properties keep the inside air in the boat “basement” as cold as the water outside?

  5. Nick Jacobs

    How does one getr building control approval for the use of hempcrete?
    Do they only recognise one of the proprietary binders?

    1. UK Hempcrete

      Hi Nick

      Building Controls in the UK universally accept the use of hempcrete as it has been on their list of pre-approved materials for so long (since Lime Technology had their Tradical Binder LABC approved many years ago). In our experience in the cast of wet cast on site hempcrete all properly manufactured and tested binders are accepted for new build – even if they come with an EN / CE certification (rather than a BBA one). A properly manufactured and tested binder will come with a full technical data sheet giving data on Thermal performance, Fire resistance, Acoustic performance, Specific Heat capacity, phase shift times etc. so on provision of this data, building controls are normally very happy indeed. On a side note (seeing your email) I wouldn’t advise the use of wet cast on site hempcrete for commercial scale projects, we would normally use a dry system (e.g. pre-cast hempcrete blocks) for this. Do get in touch if you would like further information.

      Kind regards
      Alex

    1. UK Hempcrete

      Hi Joseph,

      In theory yes, but that’s a long way into the future.. In terms of costs, I don’t think it would be reduced because of the cost of building a big enough printer to manufacture on such a large scale, plus the cost of transporting it over the country etc. Interesting idea though.

      Kind regards
      Alex

  6. pino lazerat

    HOW much does it costs? at least give me a material quote. im sick of this sales tactic of not giving the price.
    time is the most valuable thing in the world, stop wasting mine and please give me a material quote per stretch of wall.

    1. UK Hempcrete

      Hello

      Thanks for your comment.

      This is not a sales tactic, hempcrete costs are genuinely not something that is easy to list online because there are so many different ways of detailing hempcrete within a building. Any mention of a “per m2” cost would be potentially very misleading because somebody reading it doesn’t know the thickness of the layer or the application (i.e. new build, solid wall insulation, roof insulation, floor slab etc.). They also may not realise if it refers material-supply-only, or supply-and-fit price, what things are included with either option, and what elements this replaces in conventional construction (for cost comparison purposes).

      We note that you haven’t yet contacted us to ask for a quote via the contact form at: https://www.ukhempcrete.com/services/material-supply/ , but if you fill in that form, or telephone us direct on +44 (0)1629 343 143, our team will happily provide you with a fully costed quote either for hempcrete materials supply-only, or hempcrete construction services.

      Our time is also valuable, but we are happy to spend some of it doing this for you!

      Kind Regards
      Alex

  7. Elizabeth Conway

    Hi, if one built a straw bale house, would a Hempcrete spreadable mix be a good product with which to cover the bales? Thank you.

    1. UK Hempcrete

      Hi Elizabeth,

      Do you mean as a finish? If so then no, we would recommend using a lime or clay plaster/render.

      Kind regards
      Beca

  8. Mark

    Could you use hempcrete to build a full house, ie foundation, the walls, the render? Without using conventional slab, steel or wood frame. I realise it’s theoretical as you don’t know the ground on which it would sit etc but in your professional opinion, would it be possible?

    1. UK Hempcrete

      Hi Mark,

      You can indeed use hempcrete for all of these uses, though only in conjunction with other materials. For example, hempcrete walls always need a structural frame inside, as hempcrete is a non-load bearing material. A floor slab is completely possible, but hempcrete insulation alone is not sufficient for a floor. It has to be combined with other vapour-permeable materials to create a ‘breathing’ floor, which prevents moisture from building up in the ground immediately below the building. We usually cast the hempcrete floor layer on top of a free-draining, insulating sub-base layer. We also tend to use a higher-density mix than the mix used for walls, for additional structural strength.

      We hope this helps. If you have any more questions, please do not hesitate to contact us at info@ukhempcrete.com

      Kind regards
      Beca

    1. UK Hempcrete

      Hi Alvin,

      You can order hempcrete materials by emailing us at office/info@ukhempcrete.com or calling us on 01629 343143.

      Kind regards

      Beca

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