Water or moisture ingress can significantly increase the thermal conductivity of some insulants. At about 1% moisture content by volume, the thermal conductivity of certain fibrous materials rises steeply by about 85%. This can lead to a significantly higher heat transfer through the insulation layer in applications such as perimeter or roofs.
Due to its closed cell structure, rigid polyurethane insulation is barely affected by water or moisture: it does not absorb or transport water, i.e. there is no capillary action, and so normal moisture in buildings does not lead to any increase in thermal conductivity.
Water vapour diffusion cannot cause increased moisture levels in rigid polyurethane foam insulation boards unless these have not been properly installed from a structural point of view, for example where vapour barriers are lacking, or due to air pockets or faulty seals.
The ability of PUR/PIR insulation to resist water and to quickly revert to former performance after exposure to excess moisture makes it a preferred choice for flood resilient building. It is recommended both for insurance purposes and in government guidance documents as part of the increasing need for flood management strategies1.
Only 2% water content in certain fibrous materials decreases the R value by 30%.
| Coating | Water vapour diffusion resistance: (Z) | Water vapour diffusion resistance factor (μ) |
|---|---|---|
| Methal, thickness > 50 μm | - | ∞ |
| Embossed aluminum 40 μm | - | > 89900 |
| Waterproof plywood | 21 m2/hPa | 148 |
| Mineral fiber tarred | 4.9 m2/hPa | 33 |
| Saturated mineral fiber | 8.0 m2/hPa | 56 |
| Paper or paperboard, felts | 13 m2/hPa | 87 |
| No coating (spray application, skin surface) |
- | 30 - 50 |
