While most people are aware of the ‘energy performance gap’, another equally challenging problem for the housing retrofit sector is often overlooked: the moisture performance gap.
From ‘The Moisture Performance Gap’ By Kristina Smith & Andrew Simmonds. Kristina Smith is an associate at Facta www.facta.co.uk
Moisture problems plague many UK buildings resulting in a mixture of rot, mould and poor indoor air quality. Often these problems are hidden, ignored or unrecognised.
Designers, specifiers and installers need to understand the ‘hygrothermal’ performance of existing buildings: how liquid water, water vapour, air and heat interact with each other and what the consequences are.

Insulated lining boards had been fixed on battens against an external wall. Black mould was found growing in the narrow space behind, feeding off the wet wallpaper.
These photographs (left) were taken after a poorly designed and installed 1980s thermal upgrade (with internal wall insulation) was taken apart.
The moisture could have come from any or all of at least three sources during different seasons – rain penetrating via cracks in the masonry or porous bricks, and soaking through the wall, water vapour driven in by sun shining on a wet wall (creating a stagnant, warm humid environment behind the linings), or humid indoor air leaking behind the insulation and condensing on the cold, unventilated wall surface underneath. The wallpaper left in place provides ample nutrients for the growing mould.
There are now a number of reports of damp-related problems appearing in retrofits carried out under government-promoted schemes. Although the reasons are not always cut and dried, such reports are of growing concern.

Lack of repair and maintenance has historically characterised many UK properties, and piecemeal thermal upgrades or DIY ‘improvements’ and ‘makeovers’ (as distinct from the recent concept of ‘whole house retrofit’) have been carried out to UK homes with little or no research into the impact on occupants’ health. Particularly worrying are the unseen but very real health risks from mould spore contamination of indoor air resulting from hidden mould growth, behind wall linings or in and around crawlspaces and basements. Such problems can be costly in terms of either underperformance or liability/insurance claims. They can take many years to come to light, by which time it can be very disruptive and expensive to put right. It is probable that the scale of indoor air quality problems related to unseen mould growth in existing homes combined with poor ventilation and the consequent impact on respiratory health is significantly underestimated.

This extract was taken from the AECB CarbonLite Retrofit Course
5.2 Moisture in Buildings – The Moisture Performance Gap 

The full lesson covers topics such as:

1. Is the building too wet?
2. Where does the moisture come from?
3. Moisture transport (also ‘transfer’) mechanisms
4. Material properties that influence moisture movement