When choosing an Internal Wall Insulation (IWI) solution, a key consideration is whether the system should be moisture open or moisture closed. The most appropriate solution will depend on the moisture strategy in the existing property. For this reason, it is essential to carry out a thorough moisture analysis as part of a whole building assessment of the property.
Learn more about what to consider before installing internal wall insulation.
Why does moisture need to be carefully considered in IWI applications?
Many solid wall properties feature permeable wall constructions. These allow moisture to slowly pass through them. 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 moisture to continue to pass in and out of the wall up to a point, depending upon the buffering capabilities of the materials used and the level of moisture present. This prevents them from becoming saturated and allows 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 ensures that moisture doesn’t get trapped within the wall construction. 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. Condensation risk analysis can be used to then determine whether a construction in a particular location can handle any residual interstitial condensation that might form.
Products such as Kooltherm K17 Insulated Plasterboard and Kooltherm K18 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.
Should internal wall insulation be vapour open or vapour closed?
This depends on the specifics of a project, including its use, location, construction, condition, significance, ventilation, moisture load and level of heating. A full assessment and development of a plan for improvement is crucial. This can be produced under the process contained within S.R. 54:2014/A2 2022 (Code of Practice for the energy efficient retrofit of dwellings) in Republic of Ireland or PAS 2035: 2023 (Retrofitting dwellings for improved energy efficiency. Specification and guidance) for properties in Northern Ireland.
In all cases, a careful moisture risk assessment should be carried out where IWI solutions are considered.
What should be considered in a moisture risk assessment?
The whole-building approach to understanding moisture risk can be broken down into four key principles.
1. Understand the context of the building and the building project and ensure compatibility of the design with this context.
2. Ensure coherence in approach and detailing.
3. Build-in extra capacity in the design and construction phase for uncertainties in the build process and future challenges.
4. Ensure that caution is taken in the use, maintenance and after-care phase where there are ongoing requirements of care and uncertainty of outcomes.
Particular care is needed if a property is determined to be at increased risk from wind driven rain. This is because this moisture may be absorbed into the masonry and can cause problems if it becomes trapped there.
A Driving Rain Index map is provided within Figure D.1 of S.R. 54:2014/A2 2022 in the Republic of Ireland whilst a map of exposure zones for driving rain can also be viewed in the relevant NHBC guidelines for Northern Ireland. It is important to be aware, however, that local factors can significantly affect the risk for a specific property. For example, buildings in elevated positions, or at the edge of estates may be a greater risk from driving rain, whilst those in more sheltered surroundings may have a lower exposure risk than the maps may suggest.
These local factors should be considered by a suitably qualified professional when assessing risk for a property. Further analysis of the structure may also be carried out following the methods in EN ISO 13788: 2012 or EN 15026:2022.
