CASE STUDY No: 1 PDF Print E-mail

‘Condensation with a twist’
An interesting case where a combination of cavity wall insulation, blocked ventilation and the life style of the occupants of the property resulted in condensation forming within the cavity wall and damp walls.

Instructions: Investigate damp along base of left flank wall.

The property is a 1930s semi detached residence which has been in my client’s family since new.  It is constructed of 11 inch cavity brickwork,  the external face of which is smooth rendered, painted and in good condition. My client informed me this is how the property was finished when it was built. At the time of my inspection it was rented to tenants.

In December 2006 cavity wall insulation was installed and in 2008 central heating was installed. Up until the cavity wall insulation was installed there was no visible evidence of any damp problems in the lower sections of the external walls. Since the cavity wall insulation was been installed the lower section of the left flank wall had visually become more damp, the surface plaster was degrading and the wallpaper falling away in places. This was worse on the section of wall adjacent to the detached garage next to the flank wall.

My client had received three estimates from damp treatment specialists each of whom proposed to install a new damp proof course together with associated re-plastering for prices ranging between £1400 and £1800 + VAT. My client was concerned that the problem only appeared since the introduction of the cavity wall insulation and central heating and thought there may be a link. I was therefore invited to carry out an examination.

I raised a floor board adjacent to the left flank wall and located the presence of the original physical DPC which was still flexible and in good condition. Sub floor ventilation was via external air vents which vented into the cavity and then indirectly via regular 70 mm openings along the base of the inner skin of brick work just above oversite level.

With the client’s permission I removed a section of skirting board along the left flank wall. Using a masonry chainsaw I removed a brick exposed behind the skirting (Fig: 1). This exposed the cavity wall insulation which was damp to touch (Fig: 2). I removed the insulation exposed by the brick removal and it was physically wet, especially near to the outer skin of brickwork. Water could be squeezed out of it (Fig: 3 & 4).


Within the cavity there was some accumulation of mortar droppings at the base from the original construction. An endoscope examination revealed that the insulation material was dry 700 mm above external ground level

I had to consider why the insulation material was wet at low level and wetter nearer to the outer skin of brickwork. Rain penetration was considered and dismissed as there were no damp patches at higher levels, the external render was in good condition and the cavity insulation was dry above 700 mm.

I established that four people occupied the property and there was evidence that they dried their washed clothes on the radiators which were only installed in 2008. This bad practice introduces considerable amounts of water vapour into the atmosphere within the property significantly increasing vapour pressure internally.

As with most walls in residential properties the temperature of the wall fabric declines nearer to the outside. As the higher water vapour pressure inside passes through the wall on its way to the lower vapour pressure outside the temperature within the insulation material and especially that of the inner face of the external brickwork is significantly lower as the insulation material is doing what insulation is meant to do – insulate. Also the temperature of a wall at low level is always cooler by virtue of the fact that it is built into the ground - a form of 'thermal bridge'.

On the day of my visit the internal temperature was 15ºC and the relative humidity was 53.5% which equates to a dew point temperature (the temperature at which water vapour reverts form its gas state to a liquid) of 5.4ºC. Wall surface temperatures within the room ranged between 10.4ºC at low level and 17.2ºC at high level. The surface temperature of the inner face of the outer skin of brickwork at low level was 7.2ºC – only 1.8ºC above internal dew point temperature.

I had established that the cavity wall insulation had blocked the indirect sub floor ventilation and ventilation within the cavity. Since the introduction of the central heating in 2008 the tenants had been drying their clothes on the radiators adding considerable water vapour to the internal atmosphere. As this water vapour passed naturally through the wall the temperature gradient of the wall declined and, subject to conditions, dew point was reached causing condensation to occur within the insulation and on the internal face of the outer skin of brickwork - a form of interstitial condensation. This trickled down, accumulated in the mortar droppings at the base of the cavity some of which was above DPC level. In time, especially during winter, the debris became wet and this was transferred to the inner skin of brickwork causing the visible damp problems. Previously the mortar droppings were relatively dry because the cavity was ventilated.

The cavity wall insulation company removed the bottom 750 mm of cavity wall insulation and debris from within the cavity wall. The internal wall surface was made good and the wall is already noticeably drier. The tenants were told not to dry their clothes on the radiators and to ventilate the property. No DPC work or replastering was required and having deducted my fee the client saved over £1100 and did not have to decant the tenants.


Back to case studies

Contact Peter: Email - - Telephone: 015242 71794