Moisture convection performance on the joint of external wall and attic floor - laboratory tests and two-dimensional simulation model validation

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1 Moisture convection performance on the joint of external wall and attic floor - laboratory tests and two-dimensional simulation model validation Targo Kalamees, Jarek Kurnitski HVAC-Laboratory, Helsinki University of Technology Corresponding targo.kalamees@ttu.ee SUMMARY Full-scale laboratory measurements were conducted to determine moisture convection performance on the joint of external wall and attic floor. According to field measurements in previous studies this joint is one of the most typical air leakage paths. On this joint also the highest air pressure difference forms in winter. Two commonly used external walls: timberframe and autoclaved aerated concrete walls were measured. The attic floor was in both cases a timber-frame structure. Results from the first laboratory measurement series showed that in leaky joint the moisture convection due to positive air pressure remarkable raised moisture accumulation rate on the inner surface of sheathing. A two-dimensional heat, air, and moisture transport computer model was validated for future analysis. The simulation results showed that CHAMPS-BES program is useful tool in assessing the moisture behaviour of building components including moisture convection. INTRODUCTION Accurate assessment of hygrothermal performance of building envelope is important to prevent moisture damages and to guarantee longer service life as well better indoor air quality for buildings. Usually, in consulting companies, the hygrothermal dimensioning of building envelope is limited to the moisture accumulation in structures based on the Glaser-method that is a simple method for determining the risk of moisture condensation. Unfortunately, this method has many limitations: the method is the steady state, it does not consider hygroscopic materials, vapour diffusion is the only transport mechanism, condensation is only performance criterion, there are no coupling between the heat and moisture transport, and material properties are constant. Ventilated facade claddings are commonly used in timber-frame walls that should protected it from driving rain. If timber-frame wall haves a vapour barrier that controls the vapour diffusion, the major humidity load is usually the outgoing airflows due to air leakage. The role of air exfiltration for the hygrothermal performance of the building envelope is analysed in many studies [1, 2, 3, 4]. The uncontrolled air movement through a building envelope is not only hygrothermal problem, but may lead also to problems related to the health, energy consumption, performance of the ventilation systems, thermal comfort, noise, and fire resistance. Air leakage through a building envelope depends on the result of air-pressure differences over the envelope and air tightness of the building envelope. The positive air pressure difference over the building envelope can be utilised in the control of radon, particulate matter, fungal spore or other contaminant transport to the indoor air. At the same time high indoor humidity

2 load and positive air pressure conditions may cause intensive moisture accumulation in building envelope. As air pressure difference over the building envelope cannot be avoided even in typical residences [5] and buildings with timber-frame envelope are not totally airtight [6, 7], convection should be take into account in the process of hygrothermal design. As air leakages are concentrated to the joints of different envelope parts (wall, roof, floor, window), the air leakage is multidimensional and two-dimensional measurements and simulation should be done. In this study typical air leakages and their locations were determined from the field measurement data reported in [8]. A full-scale laboratory measurements were carried out to determine moisture convection performance on the joint of external wall and attic floor. A two-dimensional heat, air, and moisture transport computer model is validated for future simulations. METHODS Laboratory measurements Two commonly used external wall (timber-frame wall (Figure 2) and autoclaved aerated concrete (AAC) wall (Figure 3)) joints with timber-frame attic floor is measured in laboratory conditions between climatic chambers. The warm climatic chamber is equipped with heating and humidification units and it simulates the indoor climate. The cold climatic chamber is equipped with refrigeration, heating and humidification units simulating the outdoor climate. The climatic chambers are automatically controlled and continually maintain equilibrium of various climatic parameters, such as air temperature, relative humidity (RH), air pressure difference over the examined structure. Studied two-dimensional joint details consist of 1.2 by 0.7 m (width by height) external wall part and 1.2 by 0.9 m attic floor part. Studied setup was built as much as possible airtight and special well-defined air leakages were constructed for the joint. In timber-frame wall/attic floor joint the 10 cm wide air leakage was let between two air/vapour barrier foils. In AAC wall/attic floor joint a ~1 2 mm thick and 10 cm wide air gap was let between wall plate and AAC block. The real air inflow through the air leakage part and overall air tightness of the joint with closed air leakage part were measured with an electronic soap film calibrator (bubble flow meter), Figure Air flow, l/s y = x R 2 = Air flow, m 3 /h 0.2 y = x R 2 = Pressure difference, Pa Air leakage through the leakage path Air leakage through the untightnesses Figure 1 Measured air leakage thought the airgap between two air/vapour barrier foils (timber-frame wall/attic floor).

3 The laboratory measurements were done at -6 C and -11 C outdoor temperature conditions. The moisture excess (Δν, the difference between indoor and outdoor air s humidity by volume) was 4 g/m 3 in both tests. According to measurements in detached houses, this is the design value of moisture excess for dwellings low occupancy [9, 10]. The air pressure difference (ΔP) was Pa. According to measurements and simulations in detached houses the design value of air pressure difference over the building envelope should be at least ±10Pa [5]. Figure 2 Studied timber-frame wall joints with timber-frame attic floor (left) and the studied joint in laboratory conditions between climatic chambers (right). Temperature and RH measurement points are shown by red dots. Figure 3 Studied autoclaved aerated concrete wall connection with attic floor (left) and the structure in laboratory conditions between climatic chambers (right).temperature and RH measurement points are shown by red dots. Simulation mode Based on laboratory measurements the simulation model built with CHAMPS-BES program (Coupled Heat, Air, Moisture and Pollutant Simulation in Building Envelope Systems) (version 1.4.1) was validated for future analysis. This program is an outcome of a joint effort between Building Energy and Environmental Systems Laboratory at Syracuse University, U.S.A. and Institute for Building Climatology at University of Technology Dresden, Germany.

4 CHAMPS-BES modelling comprises the description of fluxes in the calculation domain or in the field (between volume elements including material interfaces) and at the boundary (between volume elements and exterior or interior rooms) by physical models. Also included are models for storage processes like adsorption, desorption and release. The numerical solution is done by semi-discretization in space (using a finite/control volume method) and subsequent integration in time [11]. The governing equation for the moisture balance is: mw mv mv [ j + j + j ] m w + v ρ = conv conv diff (1) t x where: m w v ρ + Moisture (liquid+vapour) density in reference volume, kg/m 3 j Convective liquid (capillary) water flux, kg/m 2 s m w conv m j v conv Convective water vapour flux, kg/m 2 s m j v diff Diffusive water vapour flux, kg/m 2 s The governing equation for the air mass balance is: m ρ a m [ a = jconv ] (2) t x where: m ρ a Air mass density in reference volume, kg/m 3 j Convective air mass flux, kg/m 2 s m a conv The governing equation for the energy balance is: U Q m m l g mv = [ jdiff + ul jconv + ug jdiff + hv jdiff ] t x (3) where: U ρ Internal energy density in reference volume, J/m 3 Q j diff Heat conduction, W/m 2 u l Specific internal energy of liquid phase, J/kg u g Specific internal energy of gas phase, J/kg h j m v v diff Specific enthalpy of water vapour, J/kg In the simulation for the material properties such as density, thermal conductivity, moisture permeability, and moisture storage function the measured material properties from Tampere University of Technology [12] were used. The example of the construction view of the simulated timber-frame walls connection with attic floor is shown in Figure 4. Figure 4 The grid of simulated timber-frame walls connection with attic floor. Temperature and RH measurement points are shown by shaded points. (green, yellow and red colours mark sheathing, mineral wool, and wood materials respectively).

5 RESULTS Laboratory measurements Full-scale laboratory measurements were performed to analyse moisture convection performance of the joint of external wall and attic floor and to get reference data to compare simulation results. Boundary conditions for the first two measurement series are shown in Table 1. Laboratory tests were made in two cycles: first, to stabilise the joint of external wall and attic floor, the test was done with closed (taped) air leakage path. This cycle continued 1 2 weeks. After that, the 10 cm long air leakage was let between air/vapour barrier foils (timber-frame wall/attic floor joint) and between wall plate and AAC block (AAC wall/attic floor joint). The measurements are continued with other indoor humidity loads, air pressure differences, air leakage conditions, and other sheathing materials. Table 1 Boundary conditions for the first two measurement series Test T in, C RH in, % T out, C RH out, % ΔP, Pa Δν, g/m There was condensation and frost formation on the inner surface of the sheathing of the attic floor in the end of the tests 1 and 2, Figure 5. During 30-day measurement period in Test 1 the increase in the mass-related moisture content of the wood fibreboard sheathing of the attic floor on the test 1 was 10 % and the final moisture content was 22 %. Figure 5 Condensation and frost with mineral wool pieces on the inner surface of the sheathing of the attic floor on the timber-frame wall (left) and the AAC wall (right) cases. Comparison of laboratory tests and results of simulations To develop the model for the future analysis, the simulation program CHAMPS-BES was validated based on full-scale laboratory measurements of Test 2.

6 Comparison of measured (Test 2, Table 1) and simulated temperature, RH, and the humidity by volume on the outer surface of insulation (behind the sheathing of the attic floor: T&RH sensors 4, 5, 7 in Figure 2) are shown in Figures Temperature, C o Positive air pressure Figure Time, dd.mm 4 Measured 5 Measured 7 Measured 4 Simulated 5 Simulated 7 Simulated Simulated and measured temperatures behind the sheathing of the attic floor in the case of timber-frame walls connection with attic floor Figure 7 Relative humidity, % Positive air pressure Time, dd.mm 4 Measured 5 Measured 7 Measured 4 Simulated 5 Simulated 7 Simulated Simulated and measured RH behind the sheathing of the attic floor in the case of timber-frame walls connection with attic floor. 6 Humidity by volume, g/m Positive air pressure Time, dd.mm 4 Measured 5 Measured 7 Measured 4 Simulated 5 Simulated 7 Simulated Figure 8 Simulated and measured humidity by volume behind the sheathing of the attic floor in the case of timber-frame walls connection with attic floor.

7 DISCUSSION There was condensate and frost formation on the inner surface of the sheathing of the attic floor in case of positive air pressure difference Pa and moisture excess +4 g/m 3. Moisture convection remarkable raised moisture accumulation rate: during the Test 1 (T out - 11 C) the increase in the moisture content of the wood fibreboard sheathing of the attic floor after 30 day period was 10 % and the final moisture content was 22 %. During the Test 2 (T out -6 C) the frost formation took just longer time. The high moisture content can offer suitable conditions for mould, wood decay fungi, reduce the thermal resistance of building materials and change building physical properties of materials and deform materials. Field measurements in Finnish detached houses [8] showed that one typical air leakage place was in the joint of attic floor with the external wall. Measurements in Estonian detached houses [7] showed, that more typical air leakage place was in the joint of interstitial floor with the external wall. The RH at the connection of intermediate floor and external wall can be rather high causing the risk of the growth of mould and rot fungi when there is positive air pressure inside the building [13]. As the air pressure difference over the building envelope on the joint of attic floor with the external wall is higher than in the joint of intermediate floor with the external wall, this joint was analysed in current study. It is not possible to take into account the geometrical change of materials in simulation programs. Nevertheless during measurements the fiberboard sheathing swelled due to moisture accumulation. The joints of timber-frame and AAC walls with the timber-frame attic floor were simulated as two-dimensional joint. In reality, the convection in joint was surely three-dimensional (look condensation areas in Figure 5). These two facts as well as the fact that studied joints consist small imperfections that cannot be modelled may be the main reasons for the difference in comparison of simulation and measurement results. The main difference between measurements and simulations was that the moisture accumulation continued in measurements, while in the simulations the moisture content remained to the fixed level, Figure 8. CONCLUSIONS The moisture convection performance on the joint of external wall and attic floor was studied by full-scale laboratory measurements. A two-dimensional heat, air and moisture transport computer model was validated for future analysis. Results from the first laboratory measurement series showed that in leaky joint the moisture convection due to positive air pressure remarkable raised moisture accumulation rate on the inner surface of sheathing. Despite of some discrepancies between the results of laboratory measurements and computer simulations, the simulation program CHAMPS-BES showed good performance and is useful tool in assessing the moisture behaviour of building components including moisture convection. The results will be utilized in further analyses aiming to specify air leakage properties allowing to expose studied structures to defined positive pressure and indoor humidity load. If

8 necessary, the studied structures will be further developed so that the moisture convection problem can be controlled in the construction. ACKNOWLEDGEMENT This study was supported with a grant from the Finnish Academy (grant ). The authors are grateful to prof. John Grünewald from Dresden University of Technology, one author of the CHAMPS-BES program that was used in this study. REFERENCES 1. Hagentoft, C.-E., Harderup, E Moisture Conditions in a North Facing Wall with Cellulose Loose Fill Insulation: Constructions with and without Vapor Retarder and Air Leakage. Journal of Thermal Envelope and Building Science;19(3): Janssens, A., Hens, H Interstitial condensation due to air leakage: a sensitivity analysis. Journal of Thermal Envelope and Building Science;27(1): Burch, D.M and TenWolde, A Computer analysis of moisture accumulation in the walls of Manufactured housing. ASHRAE Transactions;99(2): Ojanen,T. and Kumaran, K Effect of Exfiltration on the Hygrothermal Behaviour of a Residential Wall Assembly. Journal of Thermal Insulation and Building Environments; 19(3): Kalamees, T., Kurnitski, J., Jokisalo, J., Eskola, L., Jokiranta, K., Vinha, J Air pressure conditions in Finnish residences. Submitted to CLIMA 2007 conference, June, Helsinki, Finland, 8P. 6. Korpi, M., Vinha, J., Kurnitski, J Air tightness of timber-frame houses with different structural solutions. Proceedings of IX international conference on performance of exterior envelopes of whole buildings, 5 10 December, Session XIB, (CD), Florida, USA, 6p. 7. Kalamees, T Air tightness and air leakages of new lightweight single-family detached houses in Estonia. Building and Environment;42(6): Kalamees, T., Kurnitski, J., Korpi, M., Vinha, J The distribution of the air leakage places and thermal bridges of different types of detached houses and apartment buildings. 2 nd European BlowerDoor-Symposium, March, Kassel, Germany, 11p. 9. Kalamees, T., Vinha, J., Kurnitski, J Indoor Humidity Loads and Moisture Production in Lightweight Timber-frame Detached Houses. Journal of Building Physics, 29(3): Kalamees, T Indoor hyrgothermal loads in Estonain dwellings. The 4th. European Conference on Energy Performance & Indoor Climate in Buildings November 2006, Lyon, France. 11. Grunewald, J., Nicolai, A CHAMPS-BES Program for Coupled Heat, Air Moisture and Pollutant Simulation in Building Envelope Systems, version 1, User manual. 107p. 12. Vinha, J., Valovirta, I., Korpi, M., Mikkilä, A., Käkelä, P Rakennusmateriaalien rakennusfysikaaliset ominaisuudet lämpötilan ja suhteellisen kosteuden funktiona (Building physic material properties as a function of temperature and relative humidity). Tampere University of Technology, Department of Civil Engineering, Structural Engineering Laboratory. Research report 129. Tampere 2005 (in Finnish). 13. Kilpelainen, M., Luukkonen, I., Vinha, J., Käkelä, P Heat and moisture distribution at the connection of floor and external wall in multi-storey timber-frame houses. World Conference on Timber Engineering Whistler Resort, British Columbia, Canada July 31 - August 3, 2000.