I am doing a passive house model, the required airtightness is less than 0.6 ACH at 50 pa difference. In Opensutdio, in space infiltration design flow rate rates, i can set 0.6 air changes per hour but the question is how to set 50 Pascal pressure difference?
You will need to convert the 0.6ACH50 value to a value at "natural pressure" conditions.
There are several ways to do this, of varying speed and accuracy.
What is typically done in residential calculations is to use a correction factor, known as an "n-factor" or "LBL factor" (after Lawerence Berkeley Lab, where it was developed) to convert your air change rate at 50 pascals (ACH50) to an air change rate at natural pressure (ACHnat).
ACHnat = ACH50/n
The n-factor comes from a series of corrections, based on climate zone (temperature influence), the number of stories (stack effect influence), and how exposed the building is (wind influence). Here is a table for looking up the n-factor for U.S. locations:
If you would like a more rigorous approach, you can derive the factor for your application by doing an hourly calculation with local weather data, similar to how the n-factors were determined above, or do a full airflow-network model in CONTAM or related pressure-network software. Consult the ASHRAE Handbook of Fundamentals chapter on Infiltration and Ventilation for equations on residential infiltration (16.24 in ASHRAE HOF 2017).
Thanks for your comment! So if the n-factor is 10, then the air infiltration rate is 0.06 ACHnat?
That is correct. If you have multiple spaces in the model, I would suggest calculating the total infiltration rate (cfm or m3/s), then dividing that by the exterior facade area (walls+roof) and assigning that in your model as a flow rate per exterior facade area. That way, infiltration is modeled in spaces proportional to their exterior surface area.
Matt - what is the source for your table of n-Factors above? Thanks!
This image is from an appendix in a WAPTAC reference manual.
The values are originally from work out of LBNL: Max H. Sherman, Estimation of infiltration from leakage and climate indicators, In Energy and Buildings, Volume 10, Issue 1, 1987, Pages 81-86, ISSN 0378-7788, https://doi.org/10.1016/0378-7788(87)90008-9.(paywall)
@mdahlhausen Thanks for the comprehensive response. Any chance you could please update the link for appendix in a WAPTAC reference manual will be greatly appreciated.
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