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Vlad Pezel
Principal, eMod Studio
Asked a question last year

Vn50 calculation question...in determining the volume, I'd like to clarify whether I can go all the way to the airtight layer, even though the layer is a VCL membrane behind the drywall and a service cavity. Initially following the general instructions on: https://passipedia.org/planning/building_services/ventilation/differentiation_between_vv_and_vn50_values#calculation_methodology - I went to the inner wall surface (plaster on blueboard). But following: https://passiv.de/downloads/Building_Certification_Guide/#p=58, it says at the bottom of the image that the volume goes to the airtight layer. What is the way? Thanks!

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Vlad Pezel
Principal, eMod Studio

I found out soon after (from the PHI Certifier) that the Vn50 volume is calculated to the interior finish surface. It is effectively TFA surface extruded upwards to the ceiling. If the ceiling is sloped than the Vn50 volume includes the sloped ceiling volume. There are few other intricacies in the examples and exceptions that need to be followed. All covered in https://passipedia.org/planning/building_services/ventilation/differentiation_between_vv_and_vn50_values#calculation_methodology24