Decisive Use of Building Materials: Hygrothermal Analysis

1. Department of Built Environment Engineering, AUT Auckland University of Technology, New Zealand, marcela.brauner@aut.ac.nz Marcela BRAUNER, Ali GHAFFARIANHOSEINI, Nicola NAISMITH, John TOOKEY Decisive Use of Building Materials Based on Hygrothermal Analysis

2. Introduction • Housing in New Zealand is affected by high relative humidity. • Evaluation of hygrothermal processes of building constructions is not common. • Little, or no building physics has been taught to architectural students at the universities in New Zealand. • The consequence: even newly built houses are often not performing well. • This research utilises a combined model of design using a combination of experimental and simulation design. • The study compares the measured data with data from a hygrothermal simulation using WUFI Plus (Fraunhofer Institute, Germany). 1285

3. In-field experiment goals • Monitor indoor humidity in two houses with different room settings/scenarios measured in one-hour step for consecutive 5 days (120 steps in total) for each scenario while introducing water vapour (3.28 l/24h) into the room to simulate occupancy. • Compare relative humidity (RH) in the same room (size, position in the house, cardinal direction) by two houses. Where the only difference between them is that the C-house has traditional NZ exterior wall construction and the T-house has airtightness membrane and installation cavity on exterior walls. • Compare RH levels while introducing new materials into the room. • Compare the data with hygrothermal simulation results for the same scenarios. • Test the hypothesis: ‘The first material layer on the interior side of walls has a significant influence on the hygrothermal performance of the building.’ 1285

4. Test houses C-house T-house 1285

5. Test houses floor plan 1285

6. Tested scenarios • Existing plasterboard lining (unpainted) on the walls and ceiling without any additional humidification. • Existing plasterboard lining (unpainted) on the walls and ceiling with additional humidification. • Three sheets of MgO (magnesium oxide) board additionally installed (fixed with stainless steel screws) to some of the walls in the room. The area of the MgO board is 8.28m² which covers 18.9% of the wall area in the room. • Natural earth plaster in a thickness of 2 mm approximately is applied to MgO board sheets as a wall finish. All residual walls remain unpainted. 1285

7. Results Relative humidity levels by humidification and different materials added (as measured): • The maximum reached RH level in the TH by each scenario is higher than in the CH although the initial RH levels in the TH are lower than in the CH. • To eliminate the influence of exterior RH the study implements analysis of covariance (ANCOVA). 1285

8. Results as measured C-house T-house 1285

9. Results by elimination of exterior RH Estimated marginal means of inside RH by the elimination of exterior RH in: T-house C-house 1285

10. Hygrothermal simulation results T-house C-house 1285

11. Comparison between measurements and simulation • The differences are higher in the C-house. • All differences in the standard deviation and mean values are within the range of 5 % error from the measured values. • Possible reasons: • Estimated air change rate between the modelled zones • Estimated moisture dependent material properties • Missing data for diffuse solar radiation (available only global radiation) 1285

12. Analysis • The level of RH changes even with relatively minor alterations to interior materials: • By addition of MgO boards (less than 1/5 of the total wall area) the level of average RH by eliminated exterior RH drops at 7.56 percentage points in the TH and 1.37 points in the CH. • By the application of an interior finish (acrylic primer and earth plaster) on the MgO boards RH in the TH still drops at 4.30 percentage points, but in the CH the level of average RH compared to scenario 2 increases at 3.74 percentage points. • In the TH (ACH50 = 1.93) the finish seems not to have such a remarkable influence. • In the CH (ACH50 = 8.20) the vapour resistance (Sd value) of interior finish influences significantly the accessibility of the in-wall materials for water vapour. The deeper layers are therefore not freely available for sorption and the overall RH increases. 1285

13. Conclusions • This study shows that even small changes in materials can engender a significant influence on the level of reached RH in houses. • RH levels at the end of each testing period in the TH have been higher than in the CH although by each testing initial RH levels have been lower. • The lowest RH level was reached by adding MgO boards. The highest RH was reached in the TH with the original construction (plasterboard) and in the CH by addition of MgO boards with earth plaster. • The span between maximal reached RH each day by different scenarios was higher in the TH than in the CH. • Possible factors influencing the results differences: • Building envelope structure • Infiltration rate • Vapour diffusion resistance (Sd value) of used materials 1285

14. Conclusions • This study emphasises the need for a careful hygrothermal assessment of the construction, especially of the first layer from inside. • Air open structures need more energy for heating and cooling the interior due to the infiltration. This is not viable because of the general need for saving energy. • The solution would be a holistic approach to the combined heat, air, and moisture flow which include but are not limited to materials assessment, suitable ventilation, the orientation of the house, use of passive solar energy, shading, airtightness, and thermal insulation. 1285

15. Marcela BRAUNER marcela.brauner@aut.ac.nz 1285