What are the possible risks from interstitial condensation when adding internal insulation?

There are potential problems when adding a thick internal layer (up to building regulations standards) of bringing the 'dew point' too far back in the wall. It may be possible to cite technical reasons to building control to justify a layer that is slightly below building control standards, but minimises the risk of interstitial condensation.

Types of wood fibre board for internal insulation have a mineral layer in the middle, which in intended to encourage water to condense, and then disperse back into the building - rather than continuing into the external wall.

Ranyl Rhydwen researched this issue and wrote in Clean Slate issue 77 about the "...commonly advocated method of dry-lining with a vapour membrane. There are serious concerns that this method will lead to moisture build-up behind the membrane for several reasons, including the membrane becoming less than 100% intact. Dry-lining is very difficult to detail, and accidental puncture common; even a 1mm hole can make the membrane lose functionality. Moisture build-up in the wall will eventually result in rot and loss of the insulation function. This possibility needs further investigation as the implications are far reaching."

The following is from the research paper 'Dry-lining versus hemp and lime' (Rhydwen, Butler & others):

"The standard approach to internal thermal renovation is to use dry-lining and improve air tightness, however there are concerns regarding an increased risk of internal condensation as air tightness improves and interstitial wall moisture ingress, either due to membrane failure or liquid water surface diffusion. These risks are of particular concern with regards to maintaining the integrity of heritage buildings in the UK, many of which fall in to the solid, stone walled category."

"...the model for DL shows a slightly upward to constant trend in moisture of the stone wall adjacent to the air gap,whereas in the hemp/lime model in the third year there is a marked dip in the moisture content in the stone wall. This implies that the dry lining model tends to retain moisture in the wall, whereas the hemp lime tends to draw moisture out."

"Dry Lining causes moisture to accumulate in the wall and space behind it which will lead to rot, internal structural damage and increased frost damage. Hemp renders are not only insulating but also dry out the external wall and handle moisture well suggestive that they reduce the risk of rot, mould and frost damage overtime."

Click here for more about using breathable materials and avoiding problems of interstitial condensation.

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