Design,  Hempcrete,  Research & Resources

Hempcrete & Woodfibre – Looking at performance (including U-values)

“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.”

Useful comparison of natural insulation materials. For Hempcrete – claims a U-Value of 0.19W/m²K at 300mm thickness

Woodfibre board :
100mm fibreboard : 0.52 W/m2K
125mm fibreboard : 0.42 W/m2K
150mm fibreboard : 0.36 W/m2K
175 mm fibreboard : 0.31 W/m2K
200mm fibreboard : 0.27 W/m2K (good enough for walls)
225mm fibreboard : 0.24 W/m2K
250mm fibreboard : 0.22 W/m2K
275mm fibreboard : 0.20 W/m2K
300mm fibreboard : 0.18 W/m2K (good enough for pitched or flat warm roof)
350mm fibreboard : 0.16 W/m2K

Isoccyanurate board :
25mm : 0.70 W/m2K
50mm : 0.37 W/m2K
75mm : 0.25 W/m2K (good enough for walls)
100mm : 0.19 W/m2K
125mm : 0.16 W/m2K (good enough for pitched or flat warm roof)

Hempcrete (according to UK Hempcrete) :
300mm thickness – 0.2 W/m2K
350mm thickness – 0.17 W/m2K
400mm thickness – 0.15 W/m2K (Passivhaus standard for walls)

Brick wall with Hempcrete cast internally

225mm uninsulated solid brick wall : 2.70 W/m2K
350mm hempcrete wall : 0.17 W/m2K
100mm hempcrete wall : 0.7 W/m2K

Thermal performance of solid brick walls are often underestimated. I performed some calculations, based upon these conservative thermal conductivity figures of 0.77 W/mK (outer brick of 110mm), 0.56 W/mK (inner brick of 110mm) and 0.06 W/mK (hempcrete), using this calculator, gave the following U-values :

0.47 W/m2K : 220mm solid wall & 100mm Hempcrete

0.342 W/m2K : 220mm solid wall & 150mm Hempcrete

0.27 W/m2K : 220mm solid wall & 200mm Hempcrete (meets current U-value requirements) (420mm total)

0.515W/m2K : 110mm half brick thick wall & 100mm Hempcrete (210mm total)

0.365 W/m2K : 110mm half brick thick wall & 150mm Hempcrete (260mm total)

0.284 W/m2K : 110mm half brick thick wall & 200mm Hempcrete (310mm total)

0.233 W/m2K : 110mm half brick thick wall & 250mm Hempcrete (meets current U-value requirements) (360mm total)

In reality, these may perform better than expected due to the underestimation of solid wall thermal performance.

UK U-value Requirements (as of August 2020)

Approved Document L – Building regulation in England setting standards for the energy performance of new and existing buildings

Approved Document L1B Regarding extension of a dwelling in the UK. Required U-Values (as of August 2020) Paragraph 5.2, Table 2 :

0.28 W/(m2.K)2 : Wall

0.16 W/(m2.K)2 : Pitched roof – insulation at ceiling level

0.18 W/(m2.K)2 : Pitched roof – insulation at rafter level

0.18 W/(m2.K)2 : Flat roof or roof with integral insulation

Real world performance of Hempcrete

It is regularly claimed that in-situ, Hempcrete seems to consistently outperform expectations based upon computer modelling and laboratory testing. It has a good balance of thermal mass and insulation. In contrast, straw bale has plenty of insulation but very little thermal mass, and cob has plenty of thermal mass but very little insulation. Hempcrete therefore has a good ‘buffering’ effect upon the internal temperature of a building.

It is possible to build or retrofit with hempcrete in a way that eliminates thermal bridging from fixings. In retrofitting internally, we utilise twisted wooden pegs to temporarily affix the shuttering and provide a supportive structure for the hempcrete slab once it has set.

Additionally, there are some theories around the manner in which hempcrete may be outperforming simulations based upon the way that it manages and interacts with humidity, as explained below.

Heat generating (and absorbing) phase changes

“Heat generating (and absorbing) phase changes: if you are familiar with evacuated solar tubes, this is a good parallel to describe what also happens within hempcrete walls. An evacuated tube absorbs heat from the sun, heating up the distilled fluid within a closed copper tube. The fluid changes phase from fluid to vapour = absorbing heat. The vapour moves to the top of the tube and ‘dumps’ the heat in the copper manifold, which is cooler – a phase change from vapour to liquid (releasing energy). This is similar to what happens within a hempcrete wall. I used to work for Thermomax/Kingspan.

In a similar way, during seasonal periods of high relative humidity, water vapour is absorbed by the hempcrete wall, some of which condenses to form liquid water within the hemp shiv pores, until it can hold no more water and a percentage of the air within the wall is replaced by water. This replacement, due to the properties of water and air, means that because water is more thermally conductive, and has a higher heat capacity to store heat, the thermal behaviour of the wall has changed.

As relative humidity drops, the wall will release the water and become less thermally conductive. In addition, as moisture goes through the state change from water vapour to a liquid water and back, it will give off or absorb energy, which will also affect the temperature and insulating properties of the material.

This means that hempcrete passively regulates temperature and humidity, and in doing so changes dynamically in ratio between thermal mass and insulative properties. It acts as a thermal mass by buffering and stabilising winter and summer temperatures, while also managing humidity levels and minimising the build up of condensation.”

Christina Goodvin on International Hempcrete Builders Facebook Group

Control of moisture

The way in which hempcrete remains vapour permeable, whilst buffering peaks of humidity, makes it uniquely suited to a victorian house with a solid-wall construction and a history of damp problems. The walls of such a house benefit from remaining ‘breathable’ so that any moisture can move through the wall structure in vapour form as it was originally designed to do.

Lime plaster and other vapour permeable materials are essential to conservation of old buildings, and in our own experience we have found that where modern impervious materials such as cement, concrete and gypsum have been used – problems with damp, blown plaster/render and damage to internal decor have increased dramatically. This is particularly noticeable where the old materials meet the new, hence my avoidance of mainstream materials more suited to new-build houses, when building an extension on this house.

Hempcrete can be cast in situ as a form of internal wall insulation, filling all voids to reduce the risk of interstitial condensation without the need for complex, fiddly and awkward to detail vapour barriers which can fail entirely if even a small mistake is made.

It can also be cast as a contiguous layer, up and around the roof structure to create a warm roof, without any breaks or joins where a weakness could occur. This simplifies detailing immensely.

SPAB explains that it is key to avoid impermeable materials that hinder evaporation in older buildings that used a ‘breathable’ construction method.


Thermal Properties of Hempcrete, a Case Study – Jere Komsi

Sustainability of solid brick walls with retrofitted external hemplime insulation.
Richard Griffiths and Steve Goodhew

Insulating solid wall houses using plant based materials – Judith Thornton

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