Moisture management strategy in wood-frame stucco wall - observations from hygrothermal simulation

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Moisture management strategy in wood-frame stucco wall -

observations from hygrothermal simulation

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Moisture management strategy in wood-frame stucco wall – observations from hygrothermal simulation

Mukhopadhyaya, P.; Djebbar, R.; Kumaran, K.; van Reenen, D.

NRCC-46402

A version of this document is published in / Une version de ce document se trouve dans: XXX IAHS 31, World Congress on Housing, Housing Process & Product, Montreal,

Quebec, June 23-27, 2003, pp. 1-8

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XXX IAHS 31

World Congress on Housing Housing Process & Product June 23-27, 2003, Montreal, Canada

Moisture Management Strategy in Wood-frame Stucco Wall - Observations from Hygrothermal Simulation

Phalguni Mukhopadhyaya, Reda Djebbar, Kumar Kumaran, and David van Reenen

National Research Council Canada Institute for Research in Construction

1200 Montreal Road, Building M-24 Ottawa, Ontario K1A 0R6, Canada e-mail: phalguni.mukhopadhyaya@nrc-cnrc.gc.ca

Key words: moisture management, stucco wall, RHT index, hygrothermal modeling, hygIRC.

Abstract

This paper demonstrates the application of an advanced hygrothermal modelling tool for the moisture management design of the exterior building envelope. The exterior wall system considered in this study is a wood-frame stucco wall. The hygrothermal modelling tool used for this purpose is called hygIRC. The moisture management performance of the wall assembly is assessed at two Canadian geographic locations (Ottawa and Vancouver). The moisture response of the wood-frame stucco wall is analysed using a novel moisture response indicator called RHT index. The results from this study indicate that optimum vapour diffusion strategy in a wood-frame stucco wall can be determined if well defined input parameters such as construction details, material properties and boundary conditions are available.

1 Introduction

In a composite wood-frame stucco wall, each component of the wall assembly plays a specific role to ensure effective moisture management. The basic hygrothermal properties of the component materials and the various features of the construction details influence the moisture movement to and from the wall assembly, subjected to climatic loads (effects) such as relative humidity (RH), temperature, rainfall, wind velocity, wind direction, and solar radiation.

Studies conducted at the Institute for Research in Construction (IRC) of the National Research Council (NRC) Canada have shown that the hygrothermal properties of materials vary over a wide range and the variation in material properties have different degrees of influence on the overall simulated moisture response of the wall (Kumaran and Wang 1999; Mukhopadhyaya et al. 2002).

Results from the studies conducted by Kumaran and Wang (1999) showed that moisture diffusivity, water vapour permeability and sorption characteristics influence the drying characteristics of the material.

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22/08/03 Phalguni Mukhopadhyaya, Reda Djebbar, Kumar Kumaran, David van Reenen

Further detailed analysis of the IRC's material property database by Mukhopadhyaya et al. (2002) indicated that hygrothermal properties of the building materials used in North America do vary and can be represented by the upper limit, mean and lower limit values. However, not all materials or every material property affects the overall moisture response of the wall to the same extent. Among the four materials (stucco, sheathing membrane, sheathing board and vapour barrier) considered for the parametric analysis by Mukhopadhyaya et al. (2002), stucco had the most significant variation in the material properties that influence the overall moisture response of the ideal wood-frame stucco wall when exposed to a coastal Western Canadian climate in the simulation. More specifically, water vapour permeability (maximum value) and liquid diffusivity (minimum value) are the two most important properties of stucco that permit obtaining optimum moisture management in the wall assembly.

This paper further investigates the effects of various moisture management strategies, involving variation in construction details, on the overall drying and wetting characteristics of an ideal wood-frame stucco wall (i.e., without any deficiency that permits water entry to stud cavity) when it is exposed to weather conditions typical of Eastern Canada and coastal Western Canada. The moisture response of the each wood-frame stucco walls was simulated using an advanced hygrothermal modelling tool known as hygIRC. (Kumaran et al. 2002, 2003; Mukhopadhyaya and Kumaran 2001) and developed by researchers at the IRC/NRC (Karagiozis 1997; Djebbar et al. 2002).

2 Hygrothermal modeling and hygIRC

The modelling tool for hygrothermal simulation, hygIRC, is continuously evolving as a research tool. Interested readers can refer to the publications by Karagiozis, 1997; and Djebbar et al., 2002 for further details. The reliability of hygIRC outputs has been established through laboratory measurements and benchmarking exercises (Maref et al. 2002).

The effective use of these types of advanced numerical tools to analyse and obtain meaningful results, however, demands a proper physical understanding of the problem, an appropriate definition of input parameters and the ability to judiciously interpret the results (Mukhopadhyaya and Kumaran, 2001; Mukhopadhyaya et al. 2001; and 2002).

There are a number of major input parameters required for hygIRC simulation and following paragraphs provide details for these parameters.

2.1 Wall construction details

The basic construction details of the wood-frame stucco wall are shown in Figure 1. The wall remains the same for all the simulations done in this study, and only the specific parameters under investigation are varied as provided in Table 1.

2.2 Material properties for hygIRC

Eight sets of material properties are required for hygIRC simulation: air permeability, thermal conductivity, dry density, heat capacity, sorption characteristics, suction pressure, liquid diffusivity and water vapour permeability. These materials properties were obtained from the IRC/NRC's database and were determined in the IRC's Thermal and Moisture Performance Laboratory.

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XXXI IAHS, June 23-27, 2003, Montreal, Canada

2.3 Boundary Conditions

The outdoor climatic condition required for hygIRC simulations has seven major weather components (i.e., temperature, relative humidity, wind velocity, wind direction, rain fall, solar radiation and cloud index) recorded on an hourly basis. The hourly indoor climatic condition (i.e., temperature, relative humidity shown in Figure 2) is derived from outdoor climatic data using the weather analysis tool

Weathersmart (Djebbar et al. 2001) developed at the IRC/NRC.

2.4 Location and Exposure Period

In this study, two geographic locations are considered: Ottawa (year 1984), representing weather conditions of Eastern Canada (i.e., temperate continental with cool summer) and Vancouver (year 1963), representative of conditions in coastal Western Canada (i.e., temperate oceanic cool). The total exposure or simulation duration of two years (same weather year repeated) is used for both locations.

20 21 22 23 24 0 2000 4000 6000 8000 10000 Time (hours) T e m p eratu re (d eg . C ) 0 20 40 60 80 R e lative H u mid ity (%)

Temperature (deg. C) Relative Humidity (%)

20 21 22 23 24 25 0 2000 4000 6000 8000 10000 Time (hours) T e mp eratu re (d eg . C) 0 20 40 60 80 100 Relative Humidity (%)

Temperature (deg. C) Relative Humidity (%)

(b) Vancouver (a) Ottawa

25 100

Area [225x1.5 mm2_{] of the sheathing} board selected for RHT index calculation

Bott. Plate ht. [76 mm] Stucco Cladding [19 mm] Sheath Memb [0 23mm] Sheathing Board [11 mm] Insul Space [89 mm] Vapour Barrier [0.152mm] Gypsum [12mm] Top Plate ht. [76 mm] 240 0

Figure 1 Basic wall construction details Figure 2 Indoor climate

2.5 Initial Moisture Content

In this study, the initial moisture content of each wall component is assumed to be equivalent to the corresponding relative humidity of 50 percent, derived from the sorption isotherm of the respective materials.

3 Output from hygIRC and RHT index

An enormous amount of data are generated from each hygIRC simulation and subsequently post-processed for overall evaluation of the simulated hygrothermal response of the wall.

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3.1 Typical hygIRC output

Change of moisture content with time (i.e. drying curve) (Figure 3a) and contour plots for relative humidity (RH) and temperature (T) distribution patterns (Figures 3b and 3c), at any chosen time interval, are the major outputs from hygIRC simulation. In this study a novel hygrothermal performance indicator called RHT index, derived from the RH and T distribution patterns over a period time as defined in the following paragraphs, has been used.

Figure 3 Typical outputs from hygIRC

3.2 RHT Index

The RHT Index (Kumaran et al. 2002; Mukhopadhyaya et al. 2002; Mukhopadhyaya 2003) can be used to quantify and compare the localized hygrothermal response of any part of the wall component (i.e., ‘region of focus’). The ‘region of focus’ is that part of the wall cross-section where hygrothermal response is the most severe and critical for the long-term moisture performance of the wall assembly. Relative humidity (RH) and temperature (T) at different time steps are the values required to obtain the RHT index. The generic definition of RHT index at the 'region of focus' is:

RHT Index

∑

(

) (

………[1] =

−

×

−

=

n 1 t X X

T

RH

)

Time (days) Mo is tu re C o n te n t (k g /m ) 0 250 500 750 0 2 4 6 8 10 12 14 16 18 20 Total Stucco Sheathing Board Interior Gypsum Board Top Plate (Stud) Bottom Plate (Stud) Insulation (GF)

Wall width - Expanded (m)

W a ll h e ig h t (m ) 0.05 0.1 0.5 1 1.5 2 0.8750 0.8125 0.7500 0.6875 0.6250 0.5625 0.5000 0.4375 0.3750 0.3125 0.2500 0.1875 0.1250 0.0625 Region of Focus [225x1.5 mm2_]

OSB layer facing stud cavity

Wa ll h e ig h t (m ) 0.05 0.1 0.5 1 1.5 2 19.1367 18.3220 17.5073 16.6926 15.8780 15.0633 14.2486 13.4339 12.6193 11.8046 10.9899 10.1752 9.3606 8.5459 (c) T contour plot (b) RH contour plot

(a) Drying curve

T 19.9513 RH

0.9375

Where,

RH = relative humidity (%); RHx = threshold RH; T = temperature; Tx = threshold temperature (°C); t = time step or interval when RH and T values are recorded.

During any time step when either or both RH ≤ RHX% and T ≤ TX°C, the RHT value for that time step

is zero.

The RHT index brings out the long-term localized combined moisture and temperature response of the selected area of the material (i.e. ‘region of focus’). The part of the wall component selected in this study for the RHT index calculation, by visual inspection of the relative humidity contour plot (Figure 3b), is a thin layer (height 225mm; width 1.5mm) of sheathing board facing the exterior stucco cladding above the bottom plate (Figure 1). User-defined threshold values for RHX = 95%, TX = 5°C

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4 Parametric Studies

The main parameter under investigation in this study is the presence of overhang, for the prevention of wind-driven rainwater penetration, on the exterior cladding at two locations (i.e. Ottawa and Vancouver). The details regarding the simulated overhangs are given in the following paragraphs and in Table 1.

Table 1: Parametric variations considered Simulati

on ID

Geographic Location

Construction Feature

OT1 Ottawa Reference wall construction

VA1 Vancouver Reference wall construction

OT7 Ottawa Same as OT1 but 1/3rd height from top is protected from rain hit by overhang VA7 Vancouver Same as VA1 but 1/3rd height from top is protected from rain hit by overhang OT9 Ottawa Same as OT1 but entire exterior face is protected from rain hit by overhang VA9 Vancouver Same as VA1 but entire exterior face is protected from rain hit by overhang OT11 Ottawa Same as OT1 but 1/2 height of the exterior face is protected from rain hit by

overhang

VA11 Vancouver Same as VA1 but 1/2 height of the exterior face is protected from rain hit by overhang

4.1 Overhang on the exterior cladding

In a rain event the overhangs may significantly reduce the amount of water deposition on the wall but are not likely to completely eliminate it. However in this study it has been assumed that the net effect of the overhang is to bring about the complete exclusion of water deposition on the area protected by it. This is simulated by providing zero liquid diffusivity value for a very thin (≈2.5mm) exterior layer of the stucco cladding. This modification implies that though liquid water diffusion through the selected part of the exterior face of the wall was prevented but the water vapour diffusion process continued. Three different cases were considered to study the effect of overhang on the wood frame stucco wall (Figure 4). As shown in Figure 4, in three different cases the overhang protects one third, one half and full height of the wall respectively from liquid water diffusion.

h/3

Overhang

Wall height protected by overhang

h

Exterior Wall

Overhang

h

h h/2

Exterior Wall

Overhang

h

Exterior Wall

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5 Results and Discussion

The values of RHT indices calculated as described in the section 3.2 are presented in Table 2. The influence of various parametric variations on the overall moisture response of the wall is to be discussed in the following paragraphs with the help of these tabulated results.

Table 2: Result from parametric studies

Simulation ID Î OT1 VA1 OT7 VA7 OT9 VA9 OT11 VA11

RHT Index Î 2463 4552 2467 4555 0 5 2445 4558

5.1 Overhang on the Exterior Cladding

As shown in Figure 4, three different lengths of overhangs were considered. It is clear from the simulation output (Table 2) that if the 'region of focus' is not directly protected from the rainwater deposition then an overhang would not reduce the value of RHT index. In other words the simulated long-term moisture performances of these wood-frame stucco walls indicate that partial overhangs do not prevent liquid moisture entry over the entire height of the exterior face of stucco cladding would not be able to enhance the long-term moisture performance of the wall assembly. However, RH contour plots in Figure 5 indicate that overhangs are very effective in reducing the overall moisture load in the protected area.

W a ll h e ig h t (m ) 0.05 0.1 0.5 1 1.5 2 0.8120 0.7496 0.6871 0.6246 0.5622 0.4997 0.4372 0.3748 0.3123 0.2499 0.1874 0.1249 0.0625

W a ll he ight (m ) 0.05 0.1 0.5 1 1.5 2 RH 0.9369 0.8745 0.8120 0.7496 0.6871 0.6246 0.5622 0.4997 0.4372 0.3748 0.3123 0.2499 0.1874 0.1249 0.0625 VA11

W a ll he ight (m ) 0.05 0.1 0.5 1 1.5 2 RH 0.9369 0.8745 0.8120 0.7496 0.6871 0.6246 0.5622 0.4997 0.4372 0.3748 0.3123 0.2499 0.1874 0.1249 0.0625 VA9

W a ll he ight (m ) 0.05 0.1 0.5 1 1.5 2 0.8120 0.7496 0.6871 0.6246 0.5622 0.4997 0.4372 0.3748 0.3123 0.2499 0.1874 0.1249 0.0625

Figure 5 RH contour plots - effects of overhangs

RH 0.9369 0.8745 VA1 E xterio r face No overhang 1/2 overhang Full overhang 1/3rd_overhang VA7 RH 0.9369 0.8745

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6 Summary of Observations

The observations made in this study are useful, practical and applicable only for the input conditions stated in the paper. Simulated moisture performance of the wood-frame stucco walls with and without overhangs indicate that if the overhangs can deflect or prevent the liquid water entry into the stucco cladding then it is very effective in reducing the moisture load concentration in the area protected by it. However, the long-term moisture performance of the wood-frame stucco wall, based on RHT index, indicates that only the overhang capable of protecting the entire exterior face of the wall assembly would be effective to enhance the overall moisture performance of the wall.

Acknowledgements

The authors would like to acknowledge the financial support (Project # B1143) of the Natural

Resources Canada (NRCAN) to carry out the work presented in this paper.

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