Damp membranes cure all?
The above video is a site view of a house with some damp staining. Our surveyor is on site because the house is up for sale and damp has been mentioned in the homebuyer report. A damp roof membrane has been applied at low level to wall which were damp in the past.
Without removing the plaster and the membrane the exact cause is difficult to determine. However our surveyor takes the opportunity to talk through the problems, which can happen when damp membranes are used to cover a damp wall, without identifying and rectifying the source of damp.
Rising damp does appear to be present in the wall. However, the lower 1M has been dry-lined with a membrane behind it. The wall is solid brick and only 9″ thick. However, the external leaf is of a very dense brick, which has a glazed finish. One of the controls acting on rising damp in all walls is the rate of evaporation. Most of this occurs to the external leaf, due to wind and sunshine. However, some also happens inside the walls. The membrane is a vapour check and has reduced evaporation. It is not uncommon to see dampness rise a little higher than in the past when this is done.
Of course it may be that the person who installed the dry-lining didn’t extend it high enough and used the erroneous 1M limit of rising damp as a guide. This would leave a hygroscopic ‘salt band’ visible above, looking very much like continued rising damp.
The lesson here is that the source of water should be controlled where possible. If damp membranes are used it is good practice to take the membrane to ceiling height, rather than 1m up a wall. If this is not practical then a minimum of 1M past the last extent of damp is a minimum. This is further than would be recommended if a renovating plaster or cement render where used, simply because these products are at least a little permeable to water vapour, in comparison to the common damp membranes used by damp specialists.
The video highlights a problem where damp membranes have been used. However, use properly these systems can be extremely useful, especially where salt damp is found. Our surveyors know all of the various methods to solve dampness problems, including membranes, Dampcourse installation and condensation issues too.