When my parents, in their home on the top of a hill in South Wales, decided that it was time to do something about that cold room on the end of the house it was time to call in the experts. Fortunately, I’d been working at Greengauge for a while by then so reckoned I knew some people who might be able to back up my recommendation of getting some external wall insulation installed. As the only return so far my parents had seen on their investment in my university education being a self-mixed concrete shed floor, I was keen to dig into the numbers to quantify the impact the insulation would have.

The property is a 3-bed bungalow built in the 1960s. The plan of the building is roughly in the shape of a capital T (not the greatest of form factors) and the cold room in question is a rectangular room at the bottom of the T. At the very bottom of the T, a garage is joined to the wall providing some shelter to the short wall of the rectangle; the other two long walls are exposed to the elements. These two walls are the focus of the following calculations.


The main driving force behind taking action on the room was that the engineered wooden floor was starting to appear damp at the joins between planks. The first thought was that the moisture was being drawn up through the floor construction to the surface of the wood. However, after a bit of thought we started to rule this out. The house is located on the top of a hill in area where the ground is known to be notoriously dry. In addition to that, the internal floor level is around 400mm above external ground level. We assumed that to achieve this height, the floor is a beam and block suspended floor, further confirmed by the presence of air bricks in the external wall below internal floor level. Without a direct connection to the floor other than the cavity walls, ‘rising damp’ is looking unlikely. Externally a DPC can be seen in the cavity walls preventing damp rising through the brick work, and if this was the case the floor should appear damper around the edges, this was not the case, it was a fairly uniform spread of damp. The final straw against the case of rising damp was on lifting the wooden planks it was clear the top side of the planks were damper than the underside, confirmed by a wood moisture meter. A quick survey with a thermal imaging camera didn’t show any significant thermal bridges, suggesting no specific error in the design, just uniform heat loss.

This led us to the conclusion that the moisture was coming from the air and condensing on the cold surface of the floor. During a cold spell we left a mercury thermometer on the floor and this ended up with a minimum reading of around 110C. Examination of the psychrometric chart shows that for an internal temperature of 180C and internal humidity of 65% the dew point would be around 110C. With a bathroom and shower down the hallway a higher humidity at times was always likely, so it looked like we had found our culprit! Now to find the solution.

To reduce the risk of moisture condensing on the internal surfaces we either needed to increase the surface temperatures or reduce the moisture in the air through increased ventilation. Knowing that the fabric u-values were likely to be very poor we opted to first improving those to increase the internal surface temperatures.

To maintain a higher surface temperature on the floor, the straightforward answer seemed to be to add internal floor insulation. Unfortunately adding any significant thickness of insulation would increase the floor height and result in a step to the rest of the house and would require adjusting the door height. Therefore we decided to remain with the 4mm of foil back of insulation already in situ to maintain consistent floor levels in the house. An extra layer of insulation had already been added in the attic, so the best way of reducing the heat loss from the room and maintain the internal surfaces at a higher temperature would be wall insulation.

External wall insulation instead of internal insulation is often preferable for a number of reasons:

  • No loss of internal wall area, so thicker insulation can be added.
  • Less risk of interstitial condensation in the wall structure by keeping the wall warm. Internal insulation will reduce the wall temperature.
  • It can help to cover any existing thermal bridges by effectively wrapping the building in a blanket.
  • Provides a new coat of render to re-waterproof the walls
  • An important benefit of external wall insulation was that it would insulate the walls below internal floor level reducing the heat loss through the perimeter of the floor.

External Wall Insulation – EWI

It was assumed that the existing wall construction was a thin uninsulated cavity wall with a U-value of 1.61 W/m2.K. This would be in line to meet the minimum building regulations of 1970.  An addition of 90mm of EPS insulation (50mm XPS below DPC) and a new coat of render would bring the U-value of the wall down to 0.29 W/m2.K an 82% reduction compared to the initial cavity wall. The annual savings from the improved walls compared below:

 Cavity WallInternal and External Insulation
Wall Cross Section
U-Value (W/m2.K)1.610.29
Internal Surface Temperature (0C)13.817.2
Energy Saving (kWh/a)2855
Litres of Oil saved276
Financial Saving (£/a)£143
Operational CO2699

* Initial assumptions for calculations listed at the end.

Figure 1 – Front wall before EWIFigure 2  – EWI being installed on the back wall. EPS attached mechanically with white plastic fittings. Best practice would be to insulate window reveals. However, this would have added additional complexity and we were limited by existing window design and how they opened.Figure 3  – Front wall after EWI. Drainpipes and lights attached to insulation with wide diameter wall screws.

Financial Payback

An extensive payback calculation would require assumptions on a number of factors such as future fluctuations in the price of oil and how much the money could earn invested in a savings account. However, as a rough calculation the price of external wall insulation is £100m2 . It is worth noting that a new coat of silicone render (required anyway for this property) would be in the region of £50 m2 . For the 27m2 considered in this calculation the EWI comes to £2700, therefore a payback time of ~19 years (halved if you consider the render would be done anyway) at £143 a year saving on heating. This is well within the predicted lifespan of the EPS and render system, likely to be a minimum of 50 years. As a long-term investment, it is likely that the insulation will break even if not return a profit, however it’s certainly not a financial investment to get excited about.

Whilst a payback calculation is interesting, it is arguably not that relevant, there are many alterations made to properties without thought of financial payback. For example, a new coat of render or an extension. In a similar light this EWI should give us an extra useable, comfortable room in the house without a damp floor and associated health risks, the benefits of which are far harder to quantify. This by far was the main motivation and benefit of the project. Anecdotally we all noticed an immediate improvement in the comfort of the room, due to the increased surface temperatures.

With changing attitudes to comfort and climate change this may well increase the value of the house down the line as well. A study carried out by the Department of Energy and Climate in 2013 revealed that making energy saving improvements to your property could on average increase its value by 14%. Whilst this increase will vary from house to house it shows that people are paying an interest in energy efficiency and this is a trend that I can only imagine continuing. https://www.gov.uk/government/news/energy-saving-measures-boost-house-prices


In addition to the financial and comfort considerations, the carbon impact should be evaluated. Carbon literacy is something as an industry we need to improve. 699 kg of CO2 is fairly meaningless to most people, it needs to be equated to something understandable. Firstly, this is a saving in operational carbon, carbon released (or not released in this case) due to the operation of the building, through heating etc. The other side of the story is the embodied carbon produced in the manufacture of the product, from extracting the raw material to processing it and delivering it to site. For EPS, www.greenspec.co.uk (using the Inventory of Carbon and Energy v2.0) estimate the embodied carbon is ~15kgCO2  per m2 of wall area. So for our 27m2 , that results in 405kg of embodied CO2. Using PH Ribbon this estimates the embodied carbon of the EPS and render to total ~210 kgCO2. Compared against the operational savings, the insulation will pay back for its embodied carbon in the region of the first 6 months to a year of its 50 year lifespan. Over 50 years it could save somewhere in the region of 20,000 kgCO2, equivalent to ~7.5 return flights from London to Hong Kong or driving 50,000 miles at 31mpg. Unlike the financial payback calculation, the CO2 payback is incredibly interesting and relevant, it shows that if this was applied to every house in country we could make a sizeable impact in reducing the carbon footprint of the country without a long payback period and minimizing the ‘carbon burb’. (an increase in carbon emitted in the production of ‘green’ solutions.)

Next steps

As mentioned earlier, EWI may not be the whole solution to a damp room. It is possible that as this room is at the end of a hallway, limited ventilation may still result in an issue of poor internal air quality (IAQ) and moisture build up. The next step would look at an individual mechanical extract ventilation system (MEV) for that room, activated on a humidity control sensor. However, from the age of the build it is likely that the fabric airtightness is quite poor and there is sufficient air exchange with the outside air to maintain good IAQ and moisture levels. We will monitor the room throughout the winter months, recording temperature and humidity to establish if MEV is required.


In conclusion, whilst financially this may not be the most suitable course of action for every individual, for a government committed to reaching net zero by 2050 it seems like funding a large-scale external insulation programme would be a no brainer. Long term financially the government will not be excessively impacted especially as it currently pays out for winter fuel allowances, occupants would get more comfortable and healthier homes and the reduction in heating demand would go a long way to making net zero by 2050 achievable.

Initial Assumptions for calculations:

  • Exposed wall area = 27m2
  • Existing U-value = 1.61 W/m2.K
  • Heating degree days = 1893 (Heating degree hours kKh = 80)
  • ~10.35 kWh per litre of Kerosene Oil
  • ~50p per litre of oil (pre Covid price)
  • 0.245 kgC02/kWh Oil
  •  Internal surface temperature calculated from an internal air temperature of 180C and an external of -20C

Written by Jack Preece, Building Physicist

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