Questions regarding for Infiltration Model moisture source

[CarlyMay]

I have several questions regarding the "Infiltration Model IBP" moisture source. Have read the help file but still have questions I would like addressed so that I may feel more comfortable and confident in this tool.

1. Can this be used to account for the effects of both infiltration and exfiltration? The input requests the mechanical ventilation OVERpressure and this would be true of buildings operated under positive interior pressure. However, what about buildings with negative interior pressure? Would this moisture source have the ability to accurately account for buildings operated under negative pressure such as laboratories? I see that the models will RUN with a negative value input for the mechanical ventilation overpressure (no errors) but the result are the same as with the same positive value. Per the help menu literature, the moisture source is defined as a function of the resulting air flow through the component which was in turn calculated as a function of the ABSOLUTE VALUE of the overall pressure difference. Wouldn't this mean that building operated under a net negative pressure are not accurately accounted for with this. (NOTE: The source type is called "infiltration model" but the infiltration there must refer to "into the component" not to air that is infiltrating from the exterior of the building to the interior of the building?)

2. Convective versus Diffusive Moisture Migration: I see in the help menu that the moisture source is a function of air flow through the component. I understand that this is an estimation used to account for moisture transport via convection. However, would there not also be an increase in (at least the rate of) the vapor transport via diffusion when there is either a positive of negative mechanical pressure induced in addition to just the vapor pressure gradient created an accounted for by the differences in interior/exterior temp/rh? If a building is operated under positive interior pressures this wouldn't this create a greater vapor drive towards the exterior as compared to a building operating under neutral pressure, especially in the winter, regardless of whether or not actual convective air flow is permitted to occur? On the flip side, wouldn't a building operating under negative pressure create a stronger vapor drive towards the interior in the summer as compared to a building under neutral pressure? Is this moisture source intended to only account for vapor transport via convection not any additional transport as a result of increase differential pressure? Based on the equations, I am assuming this only accounts for convection.

3. Deposit site of Source: I have reviewed the paper at the link below. I believe in that paper they have applied this moisture source to the underside of the OSB and identified that at the first convective condensation layer. Where is the appropriate location for applying this moisture source and what is the rationale behind selecting the location?

http://www.brikbase.org/sites/default/f ... uenzel.pdf

4. It seems like the big unknown is the air flow through the component, which the help literature claims is 1/15th of the total building air leakage based on 'empirical determinations of infiltrated amounts of water." Is this referring to liquid water infiltration from the exterior surface of the building? If so, are there studies that show that air movements into and out of the building follow the same relationship as water?

Thank you in advance for helping me understand more about this moisture source.

Carly

[Axelarris]

Someone please answer these excellent questions. I am still not sure why the moisture source should be located only at the assumed condensing layer. It seems like the layers to the interior of the condensing surface are also receiving that vapor-laden air, so shouldn't it be placed in those interior layers as well?

[Daniel]

First some clarifciations concerning the terminology and aim of the model:

- Infiltration in the context of the IBP model means: air flow in winter from the indoor climate into the building component with condensation on the cold side (condensation pane) of the assembly.
- The model in the current stage is not suitable to calculate infiltration from the outside in case of warm, humid climate and air conditions at the inside!
- The model only introduces moisture when the temperature at the chosen condensation pane falls below the dew point of the indoor air. If that is not the case, the model doesn't introduces any moisture.

To your questions:

1) No - only infiltration in the meaning explained above. In that case overpressure at the interior side increases the air flow through the component, as it furchter increases the indoor overpressure, while negative pressurfe would reduce the air flow through the component.
I guess in your case the temperature doesn't fall below the indoor air dew point - therefore no condensation occurs and you get the same result with positive and negative pressure. Infiltartion from the outside at the momemtn is not considered at all and cannot be calculated with the model (the pressure equation in the current online help version does not contain the influence of mechanical ventilation, becaus it was added later - we will change this).

2) The model only considers the convective moisture entry due to overpressure at the insidse. In parallel there is also an increase of vapor diffusion - however, this increase remains on a very low and negliable level - therefore it is not considered in the model. Again no infiltration from the outside into the component can be considered at the moment.

3) If condsation occurs this happens normally at the interface between an air permeable and an air tight material layer. The condensation will be asorbed by the materil which provides liquid transport. So you have to evaluate the assembly and choose the most critical position, where you assume that the air flow can get to. We normally put the source in the first 5 mm of the water absorping layer which is in conctact to the assumed condensation pane (for example the inner 5 mm of an OSB outside of am mineral wool insulation layer)

4) Yes - the big unkown is the air flow which deposits moisture in the assembly. The factor of 1/15th describes on an empirical basis the ration between the total envelope permebility and the air flow through the regular cross section of a single component which really leads to condensation and humidification of the assembly. There is no intended correlation with rain water leakeages from the outside! I don't know any investigations dealing with the question, if rain water and infiltration/exfiltration follow the same pathes...

Daniel