Literally Rendering a 1930ies House Thermally More Efficient

It takes energy to keep our houses warm on a cold day, most houses at least. This heating energy is determined by the difference of inside and outside temperature, the thermal conductivity of its floors, walls and roofs as well as any incidental internal heat sources such as appliances and people (100W/person). While Improving the Thermal Performance of UK 1930s Semi Detached Houses covers a wider range of measures to address the issue, this story focuses on one aspect that, until now, ran under “future improvements”: replacing the pebble dash with insulating render to literally render a 1930ies house thermally more efficient.

Status Quo

A not negligible portion of the UK housing stock was built in the 1930ies, in various shapes and styles, and with parts of the external walls rendered in pebble dash or rough cast. This popular type of coating started in the 1920ies, partly to cover up poor brickwork, and continued to be used up to the 1970ies; it offers one major advantage: it does not need much maintenance. Unfortunately, its thermal performance is poor¹.

Solid walls with pebble dash

Houses built before the 1930ies, and partly in the 1930ies like our house, are still built with solid brick walls and do not benefit from cavity walls. One of these solid brick walls, the front elevation, is a pebble-dashed. Together with the internal plaster, this wall is about 270mm thick and shows the rather poor thermal performance:

Bay window with pebble dash

Another common feature of 1930ies or similar houses are bay windows. The wall of the window bay on the first floor is commonly rather thin: it uses a wooden framework and solid bricks toppled onto their side; the outside is pebble-dashed. Together with the internal plaster, this wall is only about 100mm thick and shows a terrible thermal performance:

Render replacement

The solution realised here was a compromise between keeping with the general appearance of a 1930ies house and an improvement of the thermal performance of some of its outside walls. This was achieved by:

  • completely removing the existing pebble dash and base coat in order to expose the underlying brick work (without damaging the bricks),
  • applying breathable insulating render² of 25–35mm thickness and a breathable, coloured finishing coat³, carried out by a local company⁴.

The improved thermal performance is as follows.

Solid walls with insulating render

The U-value is less than half, at about 47%. On a cold winter day with -10ºC outside and a cosy 21ºC inside, the heat loss would be only about 30W/m² instead of 60W/m², a reduction of 30W/m².

Bay window with insulating render

The U-value is one third, at about 34%. Again, on a cold winter day with -10ºC outside and 21ºC inside, the heat loss would be only about 35W/m² instead of 104W/m², a reduction of 68W/m².

Performance Improvement Specifics

The total wall surface area of our house is about 70m². The surface areas of the pebbled dashed solid wall and first floor window bay are about 6m² and 3m², respectively. Given the corresponding new U-values above, the reduction would be about 180W or 4.3kWh/day for the solid wall and 206W or 4.9kWh/day for the window bay, both given a temperature difference of 31ºC, with -10ºC outside and 21ºC inside.

In summary, for our house the additional insulation on about 9m² wall surface area instead of pebble dash (12% of the total wall surface) is expected to yield a reduction in heat loss of up to ~400W. This corresponds to about a 7% reduction with regards to a baseline heating power consumption of 6kW for the worst case scenario mentioned above and in the preceding article. These 400W are equivalent to 9.6kWh/day which would save about £1/day at the current gas prices; note, that is only for covering 12% of the external wall. I reckon energy savings to be about 50% if the entire house was covered with insulating render.


There are various possible responses to climate change and rising energy costs, in particular if that energy comes in the form of natural gas. A simple, low tech and very cost-effective way to save energy can be achieved by improving the thermal performance of houses through more insulation: energy not lost does not have to be put in in order to maintain the internal temperature. The addition to the general insulation of the house shown in this story is the replacement of pebble dash with insulating render; this measure both improves the thermal performance and maintains the typical appearance of a 1930ies house.


  1. Approved Document L — Conservation of Fuel and Power stipulates 0.16 W/m²K for external walls of new builds.
  2. For the insulating render ProofTherm was used, made from expanded perlite, produced and sold by ProofShield.
  3. For the top coat, Finish-WP, a lime based and highly breathable yet weatherproof product by LimeGreen was applied.
  4. The rendering was carried out by Renovation Plastering who delivered a sterling service.

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Peter Wurmsdobler

Peter Wurmsdobler


Works on the technological foundations of autonomous vehicles at Five, UK. Interested in personal mobility, renewable energy and regenerative agriculture.