IBP Air Infiltration Model

[Bonilauri]

Hello,
I'm using WUFI Pro 6.1 to analyze assemblies in hot, humid climates such as Florida and Queensland (Australia).

We are working with assemblies where the airtight layer is a breathable external membrane (sd = 0.5m).

The internal environment is provided with cooling and dehumidification, so the vapor flow is from the outside in.

I'm trying to set up the WUFI model to include possible condensation due to air infiltration from the outside, to condensate against the internal gypsum board. I set up the IBP Air Infiltration Model to Class C, in the internal gypsum board. However, even if I set the internal temperature at 5˚C (not the design temperature, I was just testing!), I see no condensation happening due to air leak.

Is the IBP Air Infiltration Model set up to only consider condensation through convection of internal air?

Thank you!

Enrico

[Manfred Kehrer]

Hello Enrico,
the IBP infiltration model should be renamed to "IBP exfiltration model" as this considers only indoor moisture convection to the outside of the insulation in cold climate zones.
If you want to model vapor from the outside convecting to and condensing on the inner side of the insulation, you should probably use an air exchange rate.

[Bonilauri]

Hello Manfred,
Thank you for the advice!

I'm going to test it, and see what happens.

Cheers,

Enrico

[Bonilauri]

If I wanted to simulate the equivalent infiltration of outside air of a Class A assembly (q50 = 1 m3/m2h), but using the air exchange rate, how would that translated into a 1/h value?

Thank you

[Manfred Kehrer]

First you would need to specify what typical pressure difference to assume from outside to inside for your building enclosure. Next thing is to reduce your q50 to that pressure. Be reminded that this is typical not a linear relationship, but as you probably don't have an flow exponent, you might assume a linear relationship. Then you can calculate the ACH according to:

Bild1.jpg (25.42 KiB) Viewed 8888 times

[Bonilauri]

Thank you!

[Axelarris]

First you would need to specify what typical pressure difference to assume from outside to inside for your building enclosure. Next thing is to reduce your q50 to that pressure. Be reminded that this is typical not a linear relationship, but as you probably don't have an flow exponent, you might assume a linear relationship. Then you can calculate the ACH according to:
Bild1.jpg

Since the air change source can only be applied to the entire layer, do you recommend breaking up the layer into two separate smaller layers of the same material (one has the air change source and one doesn't)? Or do you add a 5mm air space to exchange exterior air with?