When insulation absorbs water, its thermal conductivity can increase and its insulating performance can fall. Retained moisture may also affect the fit of flexible insulation and expose surrounding wall components to prolonged dampness. KlasseROCK® stone mineral wool is engineered to resist water absorption, helping reduce these risks.
Insulation is usually compared using thermal conductivity, fire classification and acoustic performance. But what happens when it encounters water? This is an important question for rainscreen façades and external walls, where insulation may be exposed to rain during installation or incidental moisture during the building’s service life.
To make the difference visible, we exposed KlasseROCK® Rainscreen Insulation and a glass mineral wool sample to water under the same demonstration conditions. The results highlight why moisture response should form part of every insulation specification.
Why does water absorption affect insulation performance?
Fibrous insulation works by trapping air within its structure. Because still air is a poor conductor of heat, these air pockets help slow heat transfer through the building envelope. When water enters and fills those spaces, the material behaves differently. Water conducts heat more readily than trapped air, so increasing moisture content can increase thermal conductivity.
Put simply: The more water insulation holds, the less heat it may hold back.
This does not necessarily mean the insulation is permanently damaged. Some products can recover their original thermal performance once they are completely dry. However, performance may remain reduced while moisture is present, and drying can create practical complications for the wider wall construction.
How does KlasseROCK® stone mineral wool insulation perform when exposed to water?
KlasseROCK® is manufactured from stone mineral wool and engineered with water-repellent properties. Its composition and water-repellent additive help prevent water from being readily drawn into and retained by the insulation. This provides several practical advantages:
1. Thermal performance
KlasseROCK® is engineered with water-repellent properties that help prevent water from entering and occupying the air-filled structure of the insulation. By limiting water absorption, it reduces the risk of increased thermal conductivity and helps protect the product’s intended thermal performance.
2. Maintaining shape, stability and fit
KlasseROCK® is supplied as a strong, rigid stone mineral wool board. Its construction helps it maintain its shape and integrity, reducing the risk of sagging, compression or movement. This dimensional stability helps preserve a consistent insulation layer and reduces the likelihood of gaps forming within the building envelope.
3. Reducing moisture held against the external wall
Because KlasseROCK® is designed to resist water absorption, less moisture is retained within the insulation and held against sheathing boards, membranes, timber, metal fixings and other wall components. When installed within a correctly designed ventilated rainscreen, the cavity also allows incidental moisture to dry naturally.
4. Helping protect surrounding materials
KlasseROCK® is inorganic, rot-proof and does not support the growth of mould, fungi or bacteria. Its resistance to water absorption also helps reduce the amount of moisture retained close to neighbouring materials. This can lower the risk of conditions associated with timber decay, mould growth and corrosion, although correct wall detailing and moisture management remain essential.
5. Reducing drying and recovery requirements
By resisting water uptake, KlasseROCK® reduces the amount of moisture that may need to leave the insulation following incidental exposure. This helps limit the risk of prolonged drying periods and moisture-related performance loss.
6. Reducing remedial work, delays and additional cost
Greater resistance to water absorption can reduce the likelihood of insulation requiring controlled drying, removal or replacement following incidental rain exposure. This can help protect installation quality, reduce the risk of remedial work and keep the project moving.
KlasseROCK Stone Mineral Wool:
What are the risks when glass mineral wool absorbs water?
Glass mineral wool products are not all identical. Some modern products include water-repellent treatments, so performance should always be assessed using the technical data for the exact product being specified. However, if a glass mineral wool product absorbs and retains water, several risks may arise.
1. Reduced thermal performance
When water replaces trapped air within the insulation, thermal conductivity can increase and thermal resistance can fall. The insulation may recover after drying, provided it has not been contaminated, compressed or physically damaged. Until it is completely dry, however, its thermal performance will be compromised.
2. Sagging, compression or loss of fit
Absorbed water adds weight. For most glass mineral wool insulation, this will contribute to sagging, compression or movement. If insulation pulls away from the supporting construction, gaps can form in the thermal layer. Even relatively small gaps may affect the continuity of the building envelope.
3. Moisture retained close to the external wall
Water held by insulation can remain close to sheathing boards, membranes, timber, metal fixings and other components. Moisture retained in one part of the wall may influence the condition and durability of neighbouring materials.
4. Mould, rot or corrosion in adjacent materials
Glass fibres themselves are inorganic. They do not rot and do not provide a food source for mould. The concern is the moisture that may be retained around them. Persistent dampness can create conditions that contribute to mould growth on dust or organic materials, timber decay and corrosion of susceptible metal components.
5. Longer drying and recovery
A wet glass mineral wool product may require time and suitable conditions to dry completely. Its performance will remain affected during this period. The project team will need to determine whether the insulation has dried sufficiently or whether replacement is necessary.
6. Remedial work, delays and additional cost
Wet insulation may require inspection, controlled drying or replacement. This can result in additional labour, material costs and delays before the wall can be closed. Preventing significant water absorption is therefore relevant to both long-term building performance and short-term project delivery.
Glass Mineral Wool:
What is the declared water absorption of KlasseROCK®?
The KlasseROCK® technical data sheet declares:
- Short-term water absorption: ≤1 kg/m², tested to EN 1609
- Long-term water absorption: ≤3 kg/m², tested to EN 12087
- Declared thermal conductivity: 0.035 W/mK
- Water-vapour diffusion resistance coefficient: 1
- Reaction to fire: Euroclass A1
- Manufacturing standard: EN 13162
These are product-specific values established using recognised test methods. They provide a more reliable basis for specification than assumptions about stone wool or glass mineral wool as broad material categories.
Why consider KlasseROCK® for rainscreen projects?
Choosing insulation is not simply about performance under ideal, dry conditions. It is about how reliably the product can support the building envelope when conditions are less predictable.
KlasseROCK® combines:
- Water-repellent performance
- Low declared water absorption
- A thermal conductivity of 0.035 W/mK
- Rigid dimensional stability
- Euroclass A1 non-combustibility
- Acoustic performance
- BBA accreditation
- A range of densities and thicknesses
For external walls and rainscreen façades, this combination makes KlasseROCK® a strong option where thermal, moisture and fire performance must all be considered.
Need technical advice or product information? Contact the Klasse team or explore our KlasseROCK® Rainscreen Insulation.
