Detailed heat, air and moisture transport modelling in cavity walls ::::::: M. Van Belleghem, M. De Paepe M. Steeman L. De Backer & A.

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1 ::::::: Detailed heat, air and moisture transport modelling in cavity walls M. Van Belleghem, M. De Paepe M. Steeman L. De Backer & A. Janssens Ghent University CESBP 2013

2 Introduction: cavity walls Building envelopes exposed to environmental conditions 2/18

3 Introduction: cavity walls Building envelopes exposed to environmental conditions North-western climates with dominating wind-driven rain Cavity walls : good tolerance against wind-driven rain Act as capillary break preventing moisture to reach inner wall Ventilated cavity helps remove moisture from inner leaf 3/18

4 Durability issues Frost damage Rain penetration Mould development mould growth frost damage 4/18

5 Combined transport mechanisms HEAT BALANCE MOISTURE BALANCE 5/18

6 Aim Implication of simplifications on HAM transport in cavity wall validity? saturated cavity wall, dried by convection Coupeld HAM CFD tool CFD model for air transport in cavity Heat, Air and Moisture model for transport in porous materials 1D, 2D or full 3D modelling possible Comparison with commercial HAM model 6/18

7 Coupled CFD-HAM tool continuity at boundary HAM T s Y s T CFD s temperature Y s vapour mass fraction q q heat transfer porous material g g moisture transfer air Department of Flow, Heat and Combustion Mechanics 7/18

8 Coupled CFD-HAM tool coupling procedure T s, Y s q = h (T s -T ref ) g = h m (Y s -Y ref ) Y ref T ref porous material air h, h m Department of Flow, Heat and Combustion Mechanics 8/18

9 Case study: brick veneer / wood stud wall 9/18

10 Case study: brick veneer / wood stud wall west facing façade brick cavity wood fibre board insulation gypsum board 10/18

11 Case study: boundary conditions typical summer day exterior brick cavity wood fibre board insulation gypsum board interior T = 21 C RH = 50% air 11/18

12 Case study: initial conditions Brick veneer: saturated with water brick cavity wood fibre board insulation gypsum board Wood fibre board: saturated Constant wind pressure + stack effect Study drying of wall in time (time step 30sec) 12/18

13 Case study: velocity in cavity 13/18

14 Case study: velocity in cavity Ventilation in cavity is constantly fluctuating (stack effect) No symmetrical distribution in time and space Varying transfer coefficients! not taken into account in simplified models 14/18

15 Case study: temperature 15/18

16 Case study: cavity wall moisture content 16/18

17 Comparison with commercial software WUFI Maximum relative difference of predicted moisture content Brick 9,1% WFB 23,6% drying of wet wood fibre board (case 3) simplifications not always justified! 17/18

18 Conclusions New model allows detailed study of complex HAM transfer mechanisms in cavity walls Overestimation of drying/moistening rates by simplified model Drying of outer brick veneer leaf: mainly determined by outside conditions and mainly dries out to the outside Drying of inside leaf: cavity ventilation of mayor importance 18/18

19 ::::::: Detailed heat, air and moisture transport modelling in cavity walls M. Van Belleghem, M. De Paepe M. Steeman L. De Backer & A. Janssens

20 Questions? 20/18

21 Interaction between air and porous material Drying rate depends on: Air temperature porous material air Air humidity Convection Department of Flow, Heat and Combustion Mechanics 21/18