Characterisation of the hygric inertia of a room for a reliable prediction of interior humidity variations
Creators: name: Roels, Staf
name: Janssen, Hans
name: Roels, Staf
Publisher: name: 8th Symposium on Building Physics in the Nordic Countries
Year: 2008

description: In existing building simulation tools two methods can typically be found to account for the moisture buffer effect of porous materials as claddings, furniture, etc. when solving the moisture balance of a building zone.  In the first method, often called effective capacitance model, the buffer effect of adsorptive materials is solved within the moisture balance of the room by correcting the moisture capacity of the indoor air.  This is a very easy method, but most of the time no guidelines are found for the correction factor.  The second model, mostly referred to as buffer storage model, calculates the moisture storage in the sorptive material separately.  This can be done in a simplified way, as in the effective moisture penetration depth models, or in a more precise way, really solving the moisture storage and flow in the material with a finite element or control volume method.  Though more precise, the buffer storage model has the disadvantage that it is hardly applicable on furnishing, often the most important buffer capacity in the room.  The present paper proposes an easy but reliable characterisation of the hygric inertia of a building zone, taking into account claddings, furnishing and even books etc. resulting in a reliable prediction of the interior humidity variations.
In the first part of the present study, the hygric inertia of a room (HIR) is characterised as a function of the moisture buffer potential (MBP) of the finishing materials and furniture in the room. The MBP is derived similar to the prescriptions in the Nordtest-protocol [1], but to account for the different time schedules of moisture loads, a weighted average between a short term and long term moisture buffer potential value as proposed in the companion paper [2] is used.  The validity of the proposed expression is studied numeri-cally, by simulating the dynamic response of a room with combinations of different cladding materials and furnishing.  It is shown that the hygric inertia of an entire room can be determined from its different contrib-uting components, independent of the boundary conditions considered.  An appreciable unique relationship between the dampening of the interior RH-variations and the HIR-value of the building zone is found.
The second part of the paper focuses on the quantitative prediction of moisture buffering in building zones.  Two methodologies are applied.  In the first, the HIR-model is implemented into the moisture balance for a building zone by adding the hygric inertia of the room to the moisture capacity of the zone air.  This corre-sponds to the simple moisture capacitance model, but now a reliable prediction of the effective moisture capacitance of the zone can be deduced.  The second methodology uses the buffer storage model.  By taking into account the short term and long term HIR-value, the hygric inertia of the whole room can be transformed into an equivalent moisture storage layer as implemented in the TRNSYS and Clim2000-codes.  Numerical simulations show that both methodologies can be used as simple tools to assess the dampening of indoor humidity variations but that the second methodology gives a more reliable prediction of the RH-course with time.

[1]	Rode C, Peuhkuri R, Time B, Svennberg K, Ojanen T.  Moisture buffer value of building materials.  Journal of ASTM International 2007: 4(5).
[2]	Janssen H, Roels S.  The dependable characterisation of the moisture buffer potential of interior clad-dings.  Paper submitted for Nordic Symposium on Building Physics 2008.