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Differentiation between Vv and Vn50 values

The present document is meant to clarify the two different volume measurements required in PHPP calculations, which are listed and briefly defined on the following table:

Vv Vn50
Reference volume of the ventilation system Value of air volume that will be heated or cooled for calculating the n50 leakage value.
is used to calculate the air exchange rate is used for the calculation of n50
Rough calculation:
Calculated by multiplying the TFA times an average room height for the entire building.
In the case of residential buildings, a standard height of 2.50 m should be considered, regardless of the actual height.
Exact caltulation:
All rooms of the building (including the stairs + elevator) are calculated separately by multiplying the net area times the clear room height.

The next sections provide a detailed description of both values and their calculation.

Vv – Air reference volume of the ventilation system

This value, is calculated by multiplying the treated floor area by the average room height. It is thus rather a reference value than an exact depiction of the volume within the building. For residential buildings on PHPP, it serves to calculate the ventilation heat losses and is only relevant to determine the minimal air change rate of 0.3 1/h, independent of occupation, necessary to ensure an appropriate air quality. For non-residential buildings, the value is merely informative for the calculation of the air exchange rate and does not really influence the calculation of ventilation heat losses.

The air volume Vv, is the energy reference area (Treated Floor Area, ATFA) multiplied by the room height. A standard residential room height of 2.5 m is used for simplification purposes. This is a fixed value since the amount of emissions unrelated to occupants depend on the living space and hardly increase with larger room height. Therefore, for greater room heights in residential buildings it is recommended to not enter a larger value, since this would tend to result in an excessive exchanged air volume and an indoor humidity level during the winter that may be too low. For non-residential buildings, the mean value of all room heights should be entered here.

The air volume Vv is calculated automatically by PHPP based on the TFA calculation by the user and the entered room height.

For further reference, please see the current PHPP manual (PHPP 9 manual: section 21.2.9).

Vn50 – Net air volume for pressurisation test

The air volume Vn50 within the heated building envelope is the volume that will be used for calculating the n50 air leakage value. It is supposed to indicate the actual volume of air that will be heated or cooled within the building and should be calculated separately for each room. The calculation should be carried out in accordance with the EN 13829 standard and must be clearly documented. The total air volume within the thermal envelope should be taken into account.

Calculation methodology

The base area of each room should be multiplied by the average clear height of the room. Overall assumptions for determining the internal volume of the enclosed space (gross volume) by means of a reduction factor are not permissible.
The base area to be used for this calculation differs from the treated floor area (TFA) which is normally used. The TFA follows certain rules and applies certain percentages for considering areas from the energy use point of view. The base area for consideration the Vn50 value considers all net areas within the building, as can be seen in the example below (Figure). Irrespective of the degree of completion of the building during the airtightness test, the dimensions used should always be those of the finished building (e.g. where there is no screed).

As mentioned before, the Vn50 value is supposed to depict the actual volume of air that will be heated or cooled. According to the norm, air volumes above suspended ceilings do NOT count towards the total air volume. This is a means of simplification of the calculation and it is so regardless of whether the ceiling already exists, is tightly connected with the wall, or has different types of holes (“acoustic ceiling”). The reduction of the volume by layers of plaster does not have to be taken into account. From a practical perspective, this approach makes it possible to determine the air volume in existing buildings even without plans.

A full and comprehensible room-by-room calculation of the volume must be provided for each airtightness measurement. Auxiliary calculations may also be necessary; these must also be provided. For example, in the case of inclined roofs, the air volume is determined by means of triangular or prism calculations.

Visible rafters, beams, plasterboard encasings, wall-mounted installations (if these do not have the same height as the room) and the like are not taken into account; in the calculation they are not deducted for simplification. They are thus treated as if they did not exist (= air space).

The volume of window reveals is not taken into account either (measured only up to the inner wall surface). The same applies in the case of doors and openings. Please note that these stipulations are for the purpose of simplification. Any deviations occurring as a result are usually small.

Moreover, the volume of a stairwell is part of the air volume and is fully taken into account. Therefore, the base floor area of the stairwell is simply multiplied by the clear height of the storey. The volume for flights of stairs and intermediate landings does not have to be subtracted.

The air volume of elevator shafts and of any other kind of shafts inside the thermal envelope is part of the Vn50 and is fully taken into account. The volume of the elevator cabin as well as of pipes, ducts etc. themselves counts as part of the Vn50 in a simplified way and is not deducted. The base area of the shaft can thus be multiplied with the clear height of the storey.


Single family house - Calculation of TFA, VV and Vn50

The area counted in TFA is highlighted blue and yellow. The area to be considered for the Vn50 calculation is highlighted green.
Sections of example calculation project: The area to be considered for the Vn50 calculation is highlighted green.

A more detailed explanation can be found in the following sources:

Building Certification Guide

Airtightness measurements in Passive Houses

Airtightness - how and why?

planning/building_services/ventilation/differentiation_between_vv_and_vn50_values.txt · Last modified: 2022/03/01 09:25 by