103K: Bungalow mould condensation considering insulation

This recently purchased semi-detached bungalow was suffering from mould and condensation despite some thermal insulation. The primary issue was poor bathroom ventilation. The buyer was considering insulation, which is a good idea. He obtained a quote for £8,000 – I encourage the buyer to ensure that there is no areas of poor insulation. However, it won’t overcome the poor ventilation.

Surveyor Tips:

  1. Insulation is not an alternative to ventilation.
  2. Bungalows are particularly vulnerable to condensation and mould because of the increased surface area of external walls relative as a proportion of internal living space.

Root cause

Excess vapour causing condensation and mould – from insufficient ventilation No bathroom or kitchen extractor fans. The heating and insulation made the problem worse.


There is dampness on external walls and mould growth.

Condensation could be seen under double glazed windows.

There were high damp meter readings on external walls.

Condensation forms when the wall temperature drops below the dew point of 100%RH.

Condensation was visible under the carpet underlay.

Solid floors lose heat increasing the risk of condensation.

Condensation was forming on the inside of window panes.

Mould grows where relative humidity exceeds 85%RH for 6+ hours. Excessive humidity results from insufficient ventilation, poor air circulation and a cold surface. See surveyor.tips/mould. Trapped humid air is liable to form mould on cold surfaces.

Despite the high levels of humidity and visible signs of mould, some of the walls were dry.

Mould grows best in the “Goldilocks zone” between 85%RH and 100%RH. Mould needs oxygen. Too much condensation can suffocate mould stopping growth (temporarily).

The front bedroom is dry-lined which provides a certain amount of insulation.

Around the corners of the dry lining there is heat loss, where condensation and mould is at risk. The primary cause of mould and condensation is insufficient ventilation especially from the bathroom.

In the master bedroom there is a section of mould growing just to the left of the radiator.

You can clearly see a cold section of wall.

I believe that heat loss is a major component to the problems in this room. 

I’ve been wonder if there is a cavity wall and if so whether there is cavity wall insulation. Cavity wall insulation has a bad name because the material inside can become damp, such as from penetrating damp, causing increased heat loss. However, given that condensation is forming on the double glazing, I conclude that ventilation is the primary issue.

There are no obvious signs of significant cracking or damage to external render.

I think the external insulation would be money well spent. The main consideration is ensuring that you minimise thermal bridging, remove possible source of rainwater ingress, and ventilate as if you didn’t have insulation.

There are potential cracks in seals around the windows that should be inspected and filled before insulating.

I noted that condensation was forming on the cold-water pipe.

Cold water enters the property 8°C in winter. You should insulate around cold-water pipes.

To reduce humidity ventilate the bathroom and kitchen and dry clothes in a vented room or outdoors. 

The thermostat is centrally located. I understand that the boiler is on for a large part of the day.

This confirms my suspicion that ventilation is the key component. The secondary cause of high relative humidity is low temperature relative to the source of humidity, see surveyor.tips/humidity.

At the time of the survey that was a 6° difference between the datalogger and external wall.

In the graph below, from the datalogger I have adjusted for this temperature difference.


Thank you for sending data from the datalogger.

I have amalgamated the November and December log with the January and February log, starting at the same point in time, the vertical lines represent a period of one week.

The dotted line is November and December. The continous line is s January and February. 

On the face of it you have normal levels of relative humidity. But what is missing from this data is the temperature difference between where the datalogger is placed and the external wall. Taking the difference in the two above laser thermometer readings of 6°C, I have recalculated the surface relative humidity applying the August-Roche-Magnus formula: 100*𝑒(𝑐𝑏(𝑇𝐷−𝑇)(𝑐+𝑇)(𝑐+𝑇𝐷))  [c d are constants, TD = dew point and T = temperature). 
See surveyor.tips/datalogger-examples.

The Jan to Feb is split into four equal weeks:

  • Week 1) Sitting room
  • Week 2) Second bedroom
  • Week 3) Hallway
  • Week 4) Kitchen

The week when the humidity is lowest is the third week when the datalogger was in the hallway. I think this property is acting more or less uniformly, with similar if not identical vapour pressure, the only change being temperature, and corresponding relative humidity.  

I think the temperature difference between the hallway was another 2°C or so degrees warmer than the kitchen.

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