Why should I carry out a moisture analysis on IWI installations?
Many older solid wall properties, particularly those constructed before the 1920s, were built using materials that allow moisture vapour to move through the construction. In an uninsulated property, heat from inside the property and from the sun helps to dry the wall. Once insulation is fitted, however, the heat loss from the inside will be greatly reduced. This can allow vapour within the construction to condense interstitially (between layers) on colder, less permeable surfaces. This can lead to serious issues such as rot on timber or freeze thaw problems in more susceptible masonry.
On the other hand, some of these homes have been altered over time, with the addition of measures such as external renders to prevent driving rain from entering the property and causing damp. This means that the constructions are now effectively vapour closed. In these cases, it is important to prevent excessive internal moisture from entering the construction and, again, becoming trapped.
It is therefore crucial to consider these risks carefully and develop a clear strategy to deal with them. This includes considering whether to use moisture open or moisture closed insulation.
What is a moisture open internal wall insulation system?
Moisture open internal wall insulation systems use permeable insulation, such as wood fibre insulation, which can allow water vapour to move more freely through the wall construction (up to a point, depending upon the buffering capabilities of the materials used and the level of moisture present), preventing them from becoming saturated and allowing the construction to dry easier. The downside is that they still rely on heat escaping from the property to help evaporate moisture within the wall. This means that walls must typically be insulated to a lower level than might be preferred. These systems can also be unsuitable where the walls are likely to be exposed to severe weather conditions such as driving rain.
What is a moisture closed internal wall insulation system?
A moisture closed insulation system works on the principle of occlusion. This aims to limit moisture entering the wall construction in the first place, reducing the risk of moisture becoming trapped. The external leaf is designed to prevent driving rain entering the construction. A vapour control layer should also be installed internally to prevent condensation forming behind the insulation layer. Appropriate condensation risk analysis should then be undertaken to determine whether a construction in a particular location can handle any residual interstitial condensation that might form.
Products such as Kooltherm K118 Insulated Plasterboard provide insulation, vapour control and drylining in a single board. Additional measures can be implemented to limit driving rain or other moisture entering the construction externally. As moisture closed internal wall insulation systems are not reliant on internal heat to warm walls, more thermally efficient insulation specifications can be used.
When adopting a ‘vapour closed’ approach, all junctions must be carefully designed to ensure the vapour control layer and insulation run continuously across the walls and at junctions. It’s also worth considering that vapour barriers should typically be inside the structure in a heated building and be outside the structure on a cooled building, which will affect the choice of approach when insulating a building.
Should internal wall insulation be vapour open or vapour closed?
There is no single "best" solution. The correct approach depends on the property's construction, condition, exposure, ventilation, moisture load and how the building is used and heated. A full assessment and development of a plan for improvement is crucial. Whichever approach is adopted, managing ventilation and indoor humidity is equally important to ensure the building continues to perform as intended.
The Department for Business Energy and Industrial Strategy (BEIS) has issued a guide to best practice to support project teams. This is supported by a clarification note which makes it clear that moisture closed internal wall insulation systems may be appropriate based on appropriate analysis in accordance with BS 5250: 2021 (Management of moisture in buildings. Code of practice).
This guide reiterates that the choice of IWI system should be based on a robust retrofit assessment and a good understanding of the moisture risks. The retrofit assessment should be carried out following the process laid out in PAS 2035: 2023 (Retrofitting dwellings for improved energy efficiency. Specification and guidance) and should ensure that “the lowest risk designs for internal wall insulation are consistent with the existing moisture strategy of the building”.
