Jason our mechanical and energy engineer has recently been taking some time off from work to renovate and insulate his own cob cottage.  Having already plastered internally using traditional methods and lime based plasters it was now time to apply what was learnt to the external facade.   In its existing condition the cottage had cement render which needed to be replaced due to cracking and movement.   This presented an ideal opportunity to also apply some form of external wall insulation system.  Having researched various different ways of insulating cob walls ranging from hemp products to EPS systems to wood fibres Jason deciding on using a lime based insulating render system containing perlites.  A render system called Unilit 20 with a  thermal conductivity of 0.066 W/m.K.   Whilst  not as good as some modern insulants this is still a significant improvement over applying nothing.   A balance had to be found between maintaining the aesthetics of the cottage and cob, vapour permeability and cost.    In addition the render system  has extraction qualities which promotes the drying out of the humidity and moisture in the walls which in turn ensures the walls stay warmer for longer.

Cob Cottage with cement render

existing cement render condition
monitoring instruments
Prior to the work taking place SPAB (Society for Protection of Ancient Buildings) decided to use the cottage as part of a research project they were conducting.  This  research project, which is part funded by the Dartmoor National Park Authority’s Sustainable Development Fund is concerned with investigating the thermal performance of traditionally built walls and what can be done to improve them.  

The SPAB Building Performance Survey, prior to the works taking place, looked at:
  • Fabric heat loss through the U-value measurement of wall elements both in the form of in-situ and calculated U-values.
  • Air infiltration through air permeability testing and thermographic survey.
  • Moisture, room and wall moisture including wall surface and interstitial moisture behaviour.
  • Indoor air conditions and comfort levels via the measurement of CO2.
  • Interior temperature and relative humidity.

Once the insulating measures have been carried out SPAB will return this winter to carry out the tests again.  In addition to the external insulating render it is intended to dig out the ground floors and insulate them, increase the roof insulation and make the cottage more air tight.

Extract from SPAB interim report (draft)

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cement render removal
lovely bit of cob
now with skat coat
first coat of insulating render