Accurately modelling a foil faced breather membrane & its impact on the rainscreen void

[Joseph Little]

Hi

I'm looking at a closed panel timber frame buildup with breather membrane clad to outer sheathing board with ventilated void and cement board rainscseen beyond. Manufacturers state that the breather membrane is a flexible micro-perforated, multi layer laminated, low emissivity breather membrane sheet comprising of a white non-woven Polypropylene (PP) spun-bound fibre core, co-extruded low density polyethylene (LDPE) layer and a reflective aluminium foil surface that is thermally bonded together". Thickness is listed as 0.00048 m, MNs/g as 0.55. This gives a mu-value of 229.2. Datasheet states that foil changes thermal resistivity of cavity it faces to 0.77 m2K/W.

Q.#1: How do I model this in WUFI Pro?

Q.#2: Under BS6926 timber frame cavities should be highly ventilated, and in such cavities the U-value of any layer outside the outer sheathing board should be disregarded. Am I right in saying that this foil therefore has a vepour resisting function but no valuable thermal function?

Vielen Dank.

[Manfred Kehrer]

Hello Joe,

Q.#1: How do I model this in WUFI Pro?

Pick the PE foil from the database and change thickness and mue value to what you have stated above.
Normally we recommend a minimum thickness of 1 mm for all layers to ensure that at least three caculation elements exist within the membrane. If there are only two I would double the thickness and half the mue value.
In case the model shows

Am I right in saying that this foil therefore has a vepour resisting function but no valuable thermal function?

yes