Air Leakage Assumption

[PallaviMantha]

Can you please tell me if there is any basic assumption for air exchange through the wall in WUFI, or does it assume no air leaks at all? I am wondering how it assumes the moisture gets in and out of the wall. Is it only through diffusion, or does WUFI assume a certain amount of air leakage?

Thanks

[Christian Bludau]

Hello Pallavi,
there are no assumptions for air exchange in the standard calculation in WUFI, only diffusion and suction is taken into account.
But we have two models included you could use for that:
1) Air exchange source for an air layer: Here the air is mixed with a boundary condition by volume, depending on an entered exchange rate or course.
2) Infiltration model: This model calculates depending on an entered tightness of the building envelop the moisture which could form at the position of the source in the construction by convecting air (at the moment only avaliable in WUFI PRO)
For more information please also see the WUFI help files.
Christian

[PallaviMantha]

Hi Christian,

Thank you for the reply.
To confirm, the infiltration model (based on the tightness of the building envelope) is the input available on the ASHRAE 160 screen of WUFI pro version, correct?
I could not locate this input on any other screen.

Thank you,
Pallavi

[Daniel]

The air infiltration model you can find under
-> Assembly/Monitors
-> sources / sinks
-> new moisture source
-> air infiltration model ibp

Here you can make the settings for airtightness, stack hight etc.

best regards
Daniel

[Axelarris]

In the Air Infiltration IBP model dialogue, is the "stack height" the height of the building, or the height to the neutral plane?

[Christian Bludau]

Hi Axelarris,
that is the hight of the connected indoor air volume. The neutral plane is in the "middle" of the height.

Christian

Stack height in WUFI infiltration model

stack_height.png (38.9 KiB) Viewed 19610 times

[Axelarris]

Thank you, Christian. One more question then: I've been told that the IBP model moisture source should be applied on all layers to the interior of the air barrier, but should not be applied exterior of the air barrier. (an example would be an assembly where the air barrier is a middle layer, such as a REMOTE wall with taped plywood sheathing acting as air barrier in between fiberglass batt and 6" of EPS on the outside). This seems counter-intuitive to me, since there would not be any exfiltration if it was a perfect air barrier, so there must be some exfiltration occurring in layers exterior of the air barrier.

So, should the IBP exfiltration model moisture source be applied to all layers in the assembly, connecting interior air to exterior air? Thank you.

[Christian Bludau]

Dear Axelarris,
the infiltration source is mainly thought to be used for an infiltration trough the first air-tight layer (from inside). This can cause condensation of water before the next air-tight layer.

Moisture leakage due to pressure differences

moisture_leakage.png (60.47 KiB) Viewed 19603 times

So the moisture source should placed inside the construction close to the next air-tight layer. I use 5 to 10 mils in the wooden sheathing if existing, otherwise in the mineral wool or material which is connected to the air-tight-layer (usually the roofing felt).

Here is an example of a flat roof with additional insulation on the top:

Infiltration source in flat roof

inf_source.png (6.37 KiB) Viewed 19603 times

For more information please also see "handling of typical constructions" in our tutorial section:
http://www.wufi.de/frame_en_tutorials.html

Christian

[Axelarris]

Thank you for the link, Christian. Can you please explain why the moisture source is placed only at the interior face of the sheathing? Why isn't it also placed in all layers interior of the assumed condensing surface (for example, the fiberglass batt)? Wouldn't the fiberglass also receive the "leaked" air from the interior?

[Daniel]

Hi Axelarris,

the exterior sheating is the most critical case, at is has the lowest temperature und thus the highest amount of condensate. However you can also spread the source over the whoe area and homogeniously distribute it over sheating and mineral fibre - the amount will be the same, as it takes the coldest point of the chosen aerea.

If you put a second source at the more interior part you would deposit the moisture twice.

So from our experience best and most realistic approach is to put the source in the exterior sheathing.

best regards
Daniel