Hydrostatic head tells you how much water pressure a fabric holds back before water comes through. It is measured to EN 20811 and quoted as a height of water column. Water-vapour permeability tells you how much moisture vapour passes through the fabric in 24 hours, commonly measured to ASTM E96 and quoted in g/m²/24 h. A hospital mattress cover needs both: a high hydrostatic head so liquids stay out, and real vapour permeability so the patient does not lie on an airtight surface.
What is hydrostatic head?
In the EN 20811 test, the fabric is clamped under a column of water whose pressure rises at a steady rate. The result is the pressure at which water appears on the other side of the fabric at three points, expressed as a height of water. The higher the figure, the more waterproof the fabric. Libacce's PU barrier fabric holds at least 2 m of water column, roughly 20 kPa.
What does that number mean in use?
Body weight, movement and a hand pressing on a wet surface all push liquid against the fabric. A higher hydrostatic head gives more margin before liquid is forced through. Just as important is whether the value holds up after the washing and disinfection the cover will see, so ask for results after laundering where they exist.
What is water-vapour permeability?
ASTM E96 measures the rate at which water vapour passes through a material. In its cup methods, the fabric seals a cup containing either water or a desiccant, and the change in weight over time gives the transmission rate in g/m²/24 h. The higher the figure, the more breathable the fabric. Libacce's PU barrier fabric reaches 500 g/m²/24 h or more.
Can you compare any two breathability figures?
No. ASTM E96 has several procedures, with water or desiccant and different temperatures and humidity, and other standards measure breathability differently: ISO 15496 also gives a rate in g/m²/24 h, while ISO 11092 gives a resistance (Ret), where lower means more breathable. Two numbers are only comparable when they were measured by the same method under the same conditions. Ask for the method, not just the number.
Why does a hospital cover need both values?
A solid plastic film can have an excellent hydrostatic head and almost no vapour permeability. Liquids stay out, but so does the patient's moisture, which collects between skin and surface. A breathable PU membrane blocks liquid water while letting vapour pass. Our article PU or PVC? compares the two materials.
What else is worth reading on the data sheet?
- Weight and thickness: ours is 150 g/m² ±15 (BS EN ISO 12127) and 0.30 mm ±0.1 (ISO 2286-3) for the reference construction.
- Tensile strength: how the fabric copes with load; warp ≥ 300 N, weft ≥ 210 N (ISO 1421).
- Elongation at break: how far it stretches; warp 62 %, weft 95 %, which is what lets a cover move with the patient.
- Tear strength: resistance to snags; warp ≥ 28 N, weft ≥ 27 N (ISO 4674-1).
- Peel adhesion: how well the coating stays bonded to the knit; ≥ 35 N/50 mm (ISO 2411).
- Hygiene and safety: antibacterial (ISO 22196) and antifungal (AATCC 30) finishes, BS 7175 Crib 5 flame retardancy, OEKO-TEX® Standard 100 Class 1.
Four questions for your supplier
- Which standard and method was each value measured with?
- Is the figure a minimum, a typical value or a single result?
- Was it measured on new fabric or after washing?
- Can you see the test report?
Download the full specification from our quality page, see it in use in our PU hospital mattress cover, or request a quote.