[EnergyPlus_Support] Doubt about Airflow Network and leakage through the envelope

[<m.aldegheri@xxxxxxxxx>]

Dear all,

I have a question about the calculation of the air mass flow leakage through the envelope.

My building have:
-a natural ventilation system (windows are opened with a control system)
-a mechanical ventilation system which provides a define airflow rate (ach/h)
-infiltration through the envelope discontinuity (I don't have particular vents or holes on the building, but I imagine that I have anyway a leakage because of the envelope is not completely airtight and there could be some cracks in wall joints etc..., please correct me if I'm wrong...!!!)

I have many doubt about the infiltration calculations.
I'm creating my model with Design Builder and (because of some aspects I will not report here now) I have to use the "natural ventilation calculated" in the model option.

Under this option, referring to the airtightness of the building, DB let me choose a level of airtightness (from poor to excellent). Each level is associated with a defined couple of value ("C" and "n") which are used to model the airflow mass according to the equation Q=C(dP)^n (reported in the manual) and there is a characteristic couple for each type of internal or external surface of the building. 
The online tutorial explains that for new buildings the level "good" can be chosen.
Exporting the .idf end reading it with E+ I notice that this procedure creates these objects:
-AirflowNetwork:Multizone:Surface
-AirflowNetwork:Multizone:ReferenceCrackConditions
-AirflowNetwork:Multizone:Surface:Crack

Reading the I/O reference (or engineering reference I don't remember) manuals the air flow mass is calculated by
Q=(Crack Factor)*Ct*Cq*(dP)^n
where:
-Crack factor is not specified in the manual
-Ct is a corrective factor by temperature
-Cq is the air mass flow coefficient in kg/s-Pa^n @1 Pa

My question is: how can I be sure about the correct adoption of C and n value in Db and its use in E+? The choice of the "good" level of airtightness seems to be blind action.
I try to explain better, I know that the the best way to evaluate infiltrations is through a blower door test, but the building I'm analyzing is under construction so this way is excluded. I have been suggested to check the parameters used in E+ with the ones reported in EN13465:2004. I red this document but I couldn't fine the clear definition of the parameters I'm looking for.

First of all, is my reasoning correct?
Am I calculating the loss through infiltration correctly?
Is this way is correct, how can I have a confirm of the correct adoption of the coefficient used in E+?

Can anyone help me? Maybe there is anything that I didn't understand?

Thank you all,
Marco

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