Vapour Management and Conversions

The vapour resistance of a material refers to its resistance to the passage of water vapour. This measurement considers the material's thickness and is typically expressed in MNs/g (MegaNewton seconds per gram). It is important to note that vapour resistance can only be quoted for a specific material thickness.

If the quantity is measured in MNs/gm (with a small "m" at the end), it indicates vapour resistivity. Vapour resistivity represents an intrinsic property of the material that is not influenced by the thickness and requires a different analysis approach.

The µ-value (mu-value) of a material, also known as the water vapour resistance factor, compares the material's reluctance to the passage of water vapour relative to air. The µ-value is a material characteristic and is dimensionless, lacking any units. However, when incorporating the µ-value into a specific construction, it must be multiplied by the material's thickness to account for its impact on vapour resistance.

In summary, the vapour resistance of a material measures its resistance to water vapour, considering the material's thickness. It is typically measured in MNs/g. The µ-value represents the material's relative resistance to water vapour compared to air and is a dimensionless property that requires multiplication by the material's thickness for construction applications.

Vapour resistance (MNs/g) to a µ-value

To convert the vapour resistance to the µ-value, you can follow these steps using an example:

  1. Multiply the vapour resistance by 0.2 gm/MNs. This value represents the typical vapour permeability of still air in the UK and converts the vapour resistance to the "equivalent air layer thickness."

  2. Divide the result from Step 1 by the thickness in meters. This calculation will give you the µ-value.

Example: For a material with a vapour resistance of 10,000 MN.s/g and a thickness of 100 mm (0.1 m): µ-value = (10,000 MN.s/g × 0.2 g.m/MN.s) ÷ 0.1 m = 20,000.

Convert a µ-value to a vapour resistance (MNs/g)

To convert a µ-value to a vapour resistance in MNs/g, you can follow these steps using an example:

  1. Multiply the µ-value by the thickness in metres. This will give you the "equivalent air layer thickness" in meters.

  2. Divide the result from Step 1 by 0.2 g.m/MN.s. This value represents the typical vapour permeability of still air in the UK.

Example: For a material with a µ-value of 4000 and a thickness of 3 mm (0.003 m): vapour resistance = (4000 × 0.003 m) ÷ 0.2 g.m/MN.s = 60 MN.s/g.

The equivalent air layer thickness (represented as sd) is a measure of the reluctance of a material to allow water vapour to pass through. It is expressed in meters and can only be specified for a specific material thickness.

The equivalent air layer thickness serves as an intermediate step in the conversions mentioned earlier. Suppose you must convert between an equivalent air layer thickness and a µ-value. In that case, you can utilize the corresponding step from the previous conversions that involve dividing or multiplying by the material's thickness.

Convert an equivalent air layer thickness to a µ-value

To convert the equivalent air layer thickness (sd) to a µ-value, you can divide by the thickness in meters.

Example: For a material with an equivalent air layer thickness (sd) of 2,000 m and a thickness of 100 mm (0.1 m): µ-value = 2,000 m ÷ 0.1 m = 20,000. It's important to note that the µ-value is a dimensionless quantity with no units.

Convert a µ-value to an equivalent air layer thickness

To convert a µ-value to the equivalent air layer thickness (sd), you can multiply the µ-value by the thickness in meters.

Example: For a material with a µ-value of 4000 and a thickness of 3 mm (0.003 m): sd = 4000 × 0.003 = 12 m.

Vapour resistivity is another term you might encounter, similar to vapour resistance. It represents the property of the bulk material and is typically measured in MNs/gm (MegaNewton seconds per gram-metre).

When converting between vapour resistivity and µ-value, you don't need to consider the material's thickness. Since both properties relate to the bulk material, you can use the relevant conversion step from the earlier explanation.

Convert a vapour resistivity (MNs/gm) to a µ-value

To convert vapour resistivity to a µ-value, you can multiply the vapour resistivity by 0.2 gm/MNs, a typical value for the vapour permeability of still air in the UK.

Example: If a material has a vapour resistivity of 1,000 MNs/gm: µ-value = 1,000 MNs/gm × 0.2 gm/MNs = 200.

Convert a µ-value to a vapour resistivity (MNs/gm)

To convert a µ-value to vapour resistivity, you can divide the µ-value by 0.2 gm/MNs, a typical value for the vapour permeability of still air in the UK.

Example: If a material has a µ-value of 400: vapour resistivity = 400 ÷ 0.2 gm/MNs = 2,000 MNs/gm.

Convert from US perms to a µ-value

Let's assume a 50mm thick material with a water vapour transmittance of 1.2 US perms, and we want to convert it into a can be converted to µ-value

Step 1. 1.2 x 0.0572 (multiplication factor to convert to g/(MNs)) = 0.06864 g/(MNs). Step 2. We then reciprocate the value to get MNs/g. 1/0.06864 = Vapour resistance 14.57 MNs/g. Step 3. Multiply 14.57 MNs/g with the vapour permeability of air to get the equivalent air layer thickness: Sd = 14.57 x 0.2 = 2.92 m. Step 4. Dividing the Sd by the material thickness of 50mm. µ-value = 2.92 ÷ 0.05 = 58.27 µ-value.

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