vapour resistances

8/12/2019 Vapour Resistances

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Reference: BS 5250: 2002 Annex E of 6

Vapour resistancesand µ-values

The vapour resistance of a material is a measure of the material’s reluctance to let watervapour pass through. The vapour resistance takes into account the material’s thickness, so can

only be quoted for a particular thickness of material. It is usually measured in !s"g #$ega-!ewton seconds per gram%&. If the quantity is measured in !s"gm #notice the small $m% atthe end& then it’s actually a vapour resistivity and should be dealt with as e'plained on thene't page.

The µ -value #$mu-value%& of a material is also known as its $water vapour resistance factor% . Itis a measure of the material’s relative reluctance to let water vapour pass through, and ismeasured in comparison to the properties of air. The µ-value is a property of the bulk material

and needs to be multiplied by the material’s thickness when used in a particular construction.(ecause the µ-value is a relative quantity, it is )ust e'pressed as a number #it has no units&.

To convert a vapour resistance (MNs/g) to a µµµµ-value

*. ultiply by +. gm"!s#this is a typical value in the for the vapour permeability of still air, andconverts the vapour resistance to the $equivalent air layer thickness%&

. /ivide by thickness in metres#this gives the µ-value&

Example:

0or a material with1vapour resistance 2 *+,+++ !.s"g and thickness 2 *++ mm

3e have1µ-value 2 *+,+++ !.s"g × +. g.m"!.s ÷ +.* m 2 +,+++

To convert a µµµµ-value to a vapour resistance (MNs/g)

*. ultiply by thickness in metres#this gives the $equivalent air layer thickness% in m&

. /ivide by +. g.m"!.s#this is a typical value in the for the vapour permeability of still air&

Example:

0or a material with1µ-value 2 4+++ and thickness 2 5 mm

3e have1vapour resistance 2 4+++ × +.++5 m ÷ +. g.m"!.s 2 6+ !.s"g




7quivalent air layer thickness #sd &

8ou might see the reluctance of a material to let water vapour pass through e'pressed as an

$equivalent air layer thickness%, which is usually represented as sd . 9s its name suggests, the

equivalent air layer thickness is measured in metres. :ike vapour resistance, it can only bequoted for a particular thickness of a material.

The equivalent air layer thickness was used as an intermediate step in the conversions givenon the previous page. ;o, if you want to convert between an equivalent air layer thickness anda µ-value, you can )ust use the relevant step from the earlier conversions #the one dividing or

multiplying by the thickness of the material&1

To convert an equivalent air layer thickness to a µµµµ-value

/ivide by thickness in metres

Example:

0or a material with1sd 2 ,+++m and thickness 2 *++ mm

3e have1µ-value 2 ,+++ m ÷ +.* m 2 +,+++

To convert a µµµµ-value to an equivalent air layer thickness

ultiply by thickness in metres

Example:

0or a material with1µ-value 2 4+++ and thickness 2 5 mm

3e have1

sd 2 4+++ × +.++5 2 * m

(Rememer that!

sd is the symol "or equivalent

air layer thickness

! an# that µµµµ-values have no

units$)




Vapour resistivity

8ou might also see the vapour resistivity of a material quoted. This is similar to the vapourresistance, but is a property of the bulk material and is usually measured in !s"gm #$ega-!ewton seconds per gram-metre%&.

(ecause vapour resistivity and µ-value are both properties of the bulk material, there is no

need to consider the thickness of the material when converting between them. 9gain, you )ustneed to use the relevant step from the conversions on the first page1

To convert a vapour resistivity (MNs/gm) to a µµµµ-value

ultiply by +. gm"!s#this is a typical value in the for the vapour permeability of still air&

Example:

0or a material with1vapour resistivity 2 *,+++ !s"gm

3e have1µ-value 2 *,+++ !s"gm × +. gm"!s 2 ++

To convert a µµµµ-value to a vapour resistivity (MNs/gm)

/ivide by +. gm"!s#this is a typical value in the for the vapour permeability of still air&

Example:

0or a material with1µ-value 2 4++

3e have1

vapour resistivity 2 4++ ÷ +. gm"!s 2 ,+++ !s"gm




*This value of +.gm"!s is used by convention in the . <utside the

, local values for atmospheric temperature and pressure can be used

to calculate the vapour permeability of still air from basic principles.

Tying it all together

The previous three pages have discussed several related quantities, and it can be trickykeeping them straight in your mind.

=ere’s a summary of the quantities discussed>

Name %ymol &nits ' property o"

Vapour resistance #no common symbol& !s"g specified thickness

Vapour resistivity #no common symbol& !s"gm bulk material

7quivalent air thickness layer sd m specified thickness

3ater vapour resistance factor#$µ-value%& µ #no unit& bulk material

> and the relationships between them>

apourresistivity

(measure# inMNs/gm)

apourresistance

(measure# inMNs/g)

Equivalent airlayer thickness

s#

(measure# in m)

µµµµ-value

(no unit)

Tip1 these conversions become

easier to understand if you lookat the units of measurementinvolved in each case.

2 vapour permeability of still

air #+. gm"!s&?

t 2 thickness of material #m&

iv$ y t

Mult$ y t

Mult$ y

iv$ y

iv$ y t

Mult$ y t

Mult$ y

iv$ y




The value of +. is used by convention in the .#;ee the note on the previous page.&

3hat about perms@

9n associated unit is the $perm%. This is a measure of permeance, or water vapour

transmission. :ike vapour resistance, it is a measure associated with a given thickness ofmaterial.

To be able to use this measurement in the (uild/esk software, you need to use amultiplication factor to convert it to relevant ;I unitsA then you must reciprocate the result.#Beciprocating is necessary because the perm is a measure of how easy it is for water vapourto pass through a material, not how difficult it is.& The multiplication factor will depend on

whether the measurement being converted is a ; perm or a metric perm. The multiplicationfactors are shown in the table below. <n some product data sheets you might see an ;Iequivalent to the perm, so this is included in the table.

&nitsMultiplication "actor to

convert to g/(MNs)

&% perm grains"#ftChCin=g& +.+DE

Metric perm g"#mC4hCmm=g& +.+F6G

%* equivalent ng s-* m- Ha-* +.++*

9fter converting to g"#!s&, the result must be reciprocated to give vapour resistance in

!s"g.

onverting this result to other useful measurements #such as µ-value& can be done using the

procedures summarised on the previous page.

+orke# example!

9 D+mm thick sample of material has a water vapour transmittance of ,$ ; perms.

onvert to g"#!s&>

,$ ' +.+DE 2 .$.01 g"#!s& .

Beciprocate>

*".$.01 2 ,1$23 !s"g

;o, this D+mm sample has a vapour resistance of 2 ,1$23 !s"g .

ultiply by the vapour permeability of air to give the equivalent air layer thickness>

;d 2 ,1$23 ' +. 2 $4 m .

/ivide by material thickness to give the µ-value>

µ-value 2 $4 J +.+D 2 DG.E .




www.builddesk.co.uk

In the (uild/esk software

3hen entering a material’s reluctance to let water vapour pass through, you have to enter thevalue as a µ-value.

3hen creating or editing a material, you are presented with a form where you specify thematerial’s properties. The relevant part of the form is shown below1

8ou can see that the(uild/esk software actually

asks for two µ-values, a

minimum value and ama'imum value. If you knowthat a particular material

behaves differently underdifferent conditions #the µ-

value varies with airhumidity, for e'ample& youcan specify the requiredrange of µ-values in the two

fields.

sually, however, only oneµ-value will be available for aparticular material. In thiscase, )ust enter the samevalue in both the µ-min and

µ-ma' fields.

3hat do you do if you don’t know a material’s µ-value@

9 good place to start is (; 7! *D4 #$Building

materials and products - Hygrothermal properties -Tabulated design values%& which provides µ-values formany standard materials. These materials and their µ-

values are provided in a standard catalog in the(uild/esk software.

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