Water Vapour Diffusion Resistance

[Nick Laslett]

Went down a bit of a rabbit hole today due to this thread:

 

For my roof I’m using ThermoRoof boards from Thermohouse. These are 250mm thick EPS, Nepor beads, Class II density. I wondered how airtight these were and if I needed to take further actions. This led to reading about US Perms, one measure for water vapour permeability. Now I have discovered Sd-values, the EU equivalent. I’m not actually sure how water vapour permeability relates to air tightness. 

Sd-Values are explained really well at this website:

 

https://blog.siga.swiss/en/what-is-an-sd-value/

 

The purpose of this topic was to capture some of this information. Hopefully it is of some use. 

 

Water vapour diffusion resistance expresses how strongly a building material hinders diffusion migration of water vapour molecules and is expressed in the form water vapor diffusion resistance factor µ number. The larger the µ number, the more vapor-tight the building material is. 

 

If the water vapor diffusion resistance factor is multiplied by the thickness of the component then the air layer thickness value is obtained, this is the Sd-value, e.g. 1µ x 1 metre = Sd-value 1m. EPS has a µ of 60, a 40mm thick piece of EPS has the following Sd-value: 60µ x 0.04m=Sd-value 2.4m

 

The diffusion capability of the membrane is represented by its Sd-value. The Sd-value is the equivalent thickness of air, measured in metres, with the same diffusion resistance as the membrane. The higher the value the greater the resistance to the passage of water vapour.

 

Indoor air normally contains more humidity than outside air. Because of diffusion, moist air from inside will seek to move through the thermal envelope to the outside. This is controlled by using a vapour control layer.  

 

The rule of the thumb is that the inside (airtight) membrane has to be 10 times more vapour-tight than the outside (windtight) membrane for the construction to be safe. 

 

A collection of µ values from DIN EN ISO 10456

https://second.wiki/wiki/wasserdampfdiffusionswiderstand

 

Concrete = 130

Brick = 16

Plasterboard = 10

Plaster = 20

EPS = 60

OSB = 50

MDF = 20

 

These are all at 1 metre thickness, multiply by thickness to get Sd-value.

 

Some Sd-values, where possible taken from product technical details:

Pavatex DB 3.5 airtight membrane - Sd-value = 3.5m

Pro Clima Intello Plus - Sd-value = 7.5m

Gerband SD Control Airtight Membrane - Sd-value = 2.3m

Siga Majpell 25 - Sd-value = 25m (the number in product name indicates this)

Tyvek AirGuard Reflective Vapour Barrier Membrane - Sd-value = 2000m

Aluminium foil - Sd-value = 200m

Plasterboard 12.5mm - Sd-value = 0.16m

 

 

Other websites used as reference.

 

https://www.siga.swiss/gb_en/knowledge/building-solutions/thermal-insulation-the-ultimate-guide-to-a-safe-thermal-envelope

Another website with example calculations for Sd-value 

 

https://www.giasxps.ro/index.php/en/electronic-library-polystyrene/77-xps-eps-comparison

 

[A_L]

25 minutes ago, Nick Laslett said:

I’m not actually sure how water vapour permeability relates to air tightness. 

 

 Air tightness relates to bulk air movement, vapour permeability relates to a physical/chemical process which allows water vapour to transfer through a material which has no bulk air permeability 

 

You might also find the attached document useful. It has a section at the end on Perm conversions

 

 

Vapour_Resistances includes perm conversions.pdf