Building thermal insulation · Stone wool
ROCKWOOL Thermalrock S
- Thermal conductivity (λ)
- 0.034–0.036 W/m·K
- Sound absorption (NRC)
- 0.90–1.05
Moisture in insulation — how do water absorption, vapour sorption and water repellency differ?
In short
The three terms deal with water in different forms. Water absorption is how much *liquid water* insulation takes up when immersed or wetted. Water vapour sorption is how much *moisture from the air* it takes up. Water repellency (hydrophobic treatment) means the fibres are treated so that water beads and runs off instead of soaking in. Wet insulation insulates less because water replaces the still air. The common misunderstanding: water-repellent insulation still lets water vapour through, so cold work still needs a vapour barrier.
Three values that get confused
| Term | Form of water | How it is tested, in principle | Example standards |
|---|---|---|---|
| --- | --- | --- | --- |
| Water absorption | Liquid: rain, leaks | The specimen is placed or partly immersed in water for a set time and the weight gain is measured | EN 1609, EN 13472, ASTM C1763 |
| Water vapour sorption | Vapour in humid air | The specimen is kept in a high-humidity atmosphere and the weight gain is measured | ASTM C1104 |
| Water repellency | Liquid touching the fibre surface | A property given by an agent mixed into the fibres; its effect shows in the water-absorption value | — |
| Vapour permeability | Vapour moving through the material | The rate at which vapour passes through a material or facing | ASTM E96 |
Notes:
- Units differ. Some tests report kilograms per square metre, others per cent by volume or by mass. Figures from different tests cannot be compared directly
- EN 1609 is a short-term water-absorption test by partial immersion, intended to simulate about 24 hours of rain during construction. It has been replaced by EN ISO 29767, but many product documents still cite the old name
- ASTM C1104 applies only to unfaced mineral-fibre insulation, so the result is for the insulation itself, not the facing
Why does wet insulation insulate less?
Insulation works through still air trapped in small spaces. When water replaces the air, heat travels through water, which conducts heat much better than still air. A study of glass-fibre pipe insulation submerged in water found the wet thermal conductivity to be tens of times higher than the dry value, returning close to the original once fully dry.
Beyond heat, insulation that stays wet brings other problems:
- Extra weight; the insulation slumps or falls out of place
- Rusting of the metal underneath. See corrosion under insulation
- Wet dust and dirt give mould somewhere to grow
- Stained ceilings and dripping water
Fibrous vs closed-cell insulation
| Fibrous (stone wool, glass wool, polyester fibre) | Closed-cell (rubber foam, PE foam) | |
|---|---|---|
| --- | --- | --- |
| Structure | Connected voids; air and vapour pass through | Separate closed cells; vapour passes with difficulty |
| Liquid water | Water-repellent grades shed water, but immersion or pressure still gets water in | Low water absorption as long as the skin and joints are intact |
| Water vapour | Vapour passes through, so cold work needs a separate vapour barrier | The material resists vapour itself, but joints must be fully sealed |
| Drying | Can dry out if air can move | Moisture that has got in leaves slowly |
Neither is better for every job. Hot work and work that needs to dry out generally suit fibrous insulation; cold work that must keep vapour out generally suits closed-cell insulation, or fibrous insulation with a complete vapour-barrier facing. See closed-cell vs fibrous insulation.
"Water-repellent" does not mean "waterproof"
Stone-wool manufacturers explain that a water-repellent agent is mixed with the fibres during production and that stone wool does not draw in and hold moisture from the air (it is non-hygroscopic), while remaining permeable to vapour. This helps when insulation is caught by rain during installation or by occasional drips, but it has limits:
- If it is submerged, exposed to running water, or compressed while wet, water can still enter the voids
- Repellency does not stop vapour, so it does not prevent condensation inside insulation on cold work
- Facings, tape and outer cladding are still the first line of defence against water
What to do when insulation gets wet
- Wetted by clean water and able to dry fully — stone-wool manufacturers state that once thoroughly dried the insulation keeps its original performance, provided there is no physical or chemical degradation
- Flood or dirty water, or slumped out of shape — the North American insulation manufacturers' association recommends replacement, because the contaminants are unknown and there is no assurance it will really dry
- Enclosed insulation — under foil, under metal cladding or in a closed wall — dries very slowly. Find and fix the source of the water; do not wait for it to dry by itself
- Store insulation dry and covered before installation, and let it dry before closing up the work
Where is a vapour barrier still needed?
In hot-humid air, vapour always moves from the warm humid side towards a cold surface. The following work therefore needs a continuous vapour barrier on the outside of the insulation, even when the insulation is water-repellent:
- Chilled-water pipes, refrigerant pipes and cold-air ducts — see why AC pipes drip
- Tanks and equipment colder than the surrounding air
- Walls and ceilings of cold rooms and heavily cooled spaces
The principles are in what is a vapour barrier? and dew point and condensation.
Examples of published values
The values below come from each product's manufacturer document and from different tests, so use them to see what data a product has, not to rank products.
- General-purpose stone-wool slab: ROCKWOOL Thermalrock S states moisture absorption of less than 0.04% by volume to ASTM C1104
- Stone-wool pipe section for industry: ROCKWOOL ProRox PS 960 SA uses a water-repellent binder (WR-Tech) to reduce the risk of corrosion under insulation, and states water absorption of 0.2 kg/m² or less to EN 13472, also after 24 hours of pre-heating at 250 °C
- Glass-wool pipe section: SCG PCI states moisture sorption of 0.2% by volume to ASTM C1104
- Closed-cell rubber insulation for roofs: AERO-ROOF states water absorption of 5% by mass or less to ASTM C1763
Sources
- ASTM International — ASTM C1104/C1104M Standard Test Method for Determining the Water Vapor Sorption of Unfaced Mineral Fiber Insulation (scope)
- Genorma — EN 1609:2013 Thermal insulating products for building applications: Determination of short term water absorption by partial immersion (scope, status and replacement)
- ROCKWOOL Group — Moisture repellence (water repellency and non-hygroscopic behaviour)
- ROCKWOOL Asia — Frequently Asked Questions (vapour permeability and drying out)
- US DOE Office of Scientific and Technical Information — Performance of fibrous glass pipe insulation subjected to underground water attack (wet and dried thermal conductivity)
- Building Science Corporation — BSD-106: Understanding Vapor Barriers (keeping water out and letting it out)
- Insulation Institute (NAIMA) — General FAQs (moisture, mould and when insulation needs replacing)
- GetBestInsulations — verified product data for ROCKWOOL Thermalrock S, ProRox PS 960 SA, SCG PCI and AERO-ROOF (based on the manufacturer documents listed on each product page)
Related products
Categories and example products for this topic
- ROCKWOOL Thermalrock S — Stone-wool slab; moisture absorption under 0.04% by volume to ASTM C1104
- ROCKWOOL ProRox PS — Pipe section with water-repellent WR-Tech binder; water absorption ≤ 0.2 kg/m² (EN 13472)
- SCG PCI / PCIF — Glass-wool pipe section; moisture sorption 0.2% by volume to ASTM C1104
- AERO-ROOF — Closed-cell rubber insulation; water absorption ≤ 5% by mass to ASTM C1763
Industrial and high-temperature insulation · Stone wool
ROCKWOOL ProRox PS
- Max. temp.
- 650 °C
- Density
- 120–150 kg/m³
- Thermal conductivity (λ)
- 0.037–0.038 W/m·K (50 °C)
Pipe insulation · Glass wool
SCG PCI / PCIF
- Pipe size
- ½–18 in
- Thickness
- 25–50 mm
- Service temp.
- 10 – 540 °C
Building thermal insulation · Elastomeric foam
Aeroflex AERO-ROOF
- Thickness
- 10–25 mm
- Thermal conductivity (λ)
- 0.035 W/m·K
- Service temp.
- 0 – 100 °C