Ventilated air space

[fostertom]

Help Q&A 19 Ventilated curtain wall (also roof tile spaces etc) offers 3 alternatives;

1) model the roof tile space as air layer from Generic Materials, with or without 'additional moisture capacity', but Info Text says "If an air-change source is placed in the air layer, the parameters remain those of an unventilated air layer".

My first question - is the air-change source simply ignored by WUFI and this 1) remains a closed air space?

2) model the roof tile space as external air

3) intermediate - model the roof tile space as a "material", with air-change source.

My second question - what kind of "material" is this 3), and where found in the database?

[Christian Bludau]

Hello Tom,

the air change model is a simplified model, there are no cfd-calculations. So the construction remains a "closed airspace", but the moisture and temperature is mixed up in an ideal way with the conditions at the given boundary condition.

There are two ways to simulate a roof, just leave away the tiles, the heat transfer coefficient was gained below the tiles on the weather membrane. That you can do in most cases, as the tiles are never air tight and the influence of them is mainly a offset of the temperature. We did measurements below tiles without airgap, only battens and there was always an air movement there... In this case calculate without tiles and use the absorptivity value of the tiles for the surface.
If you want to calculate with the tiles, use something like bricks or concrete. Maybe tighten them up as they usually only show a little water uptake...

Regards,
Christian

[fostertom]

OK, got it.

So the 'new' airspaces 'without additional moisture source' reduces moisture storage function from 47 to a realistic (as if 20C) 17kg/m3. And adding an air change source mixes as if there was actual air flow.

But still, despite the above measures, we are warned:
"the parameters of the air remain those of an unventilated air layer; they are not automatically adapted to take into account the air change rate produced by the air change source."
Is this just an academic warning, which can be noted and ignored (!) or are there other adjustments that we still need to make to 'the parameters of the air'?

In Air Layers> Moisture Storage Function, last para, confirm it means that neither 'new' nor 'old' model PH changed by temp; not that 'new' does it better than 'old'?

[Christian Bludau]

Hello Tom,
in every air layer you will get a kind of moving air forced by the gradient of temperature. In the air layers in the database, there is an effective value for µ and lambda included. Thats also the reason why you have to insert different air layers for different thickness. Depending on the size, opening, whatever you may get higher influences of eg. convection. Thats can not be considered in WUFI. A simplified way to consider that is the air change source. The warning was thought for the air layer without an source.
Please also see the materials info-text.
Christian