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Retrofit strategies for a high-rise wall system and analyses of their hygrothermal effects
Djebbar, R.; Mukhopadhyaya, P.; Kumaran, M. K.
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Retrofit strategies for a high-rise wall system and analysis of their hygrothermal effects
Djebbar, D.; Mukhopadhyaya, P.; Kumaran, M.K.
A version of this document is published in / Une version de ce document se trouve dans : 11th Symposium for Building Physics, Dresden, Germany, Sept. 26-30, 2002, pp. 738-746
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Retrofit Strategies for a High-Rise Wall System and
Analyses of their Hygrothermal Effects
Reda Djebbar, Phalguni Mukhopadhyaya, M. Kumar Kumaran
Institute for Research in Construction - National Research Council of Canada Reda.Djebbar@nrc.ca
1 Abstract
An analytical approach to quantify and assess the effect of building envelope retrofit strategies on the long-term hygrothermal performance of masonry wall assemblies in high-rise buildings is presented in this paper. This approach has resulted from an on-going research project that has the main objective to predict effects of adding supplementary insulation and air-sealing retrofit options on several types of wall assemblies, used in both commercial and residential high-rise buildings throughout different geographical locations in the country (Canada). A set of three hygrothermal indicators are defined and used to compare between the relative moisture performance of different retrofit strategies. These hygrothermal indicators predict the degree and potential for moisture related damage due to frost, chemical or biochemical attacks as well as efflorescence-subflorescence and swelling of the masonry envelope components. The impact of indoor environment and type of ventilation, the outdoor weather are also addressed by using these hygrothermal indicators.
2 Introduction
As the stock of buildings in Canada ages, it is expected that there will be an increase in building envelope rehabilitation work. Such activities represent an ideal opportunity to add insulation and reduce air leakage to improve energy efficiency, occupant comfort and durability of the building envelope. However, the impact of such retrofit measures on the long-term performance of building envelope assemblies has not been fully assessed. Currently, the Institute for Research in Construction (IRC)/National Research Council (NRC) of Canada, in association with a number of Canadian government agencies, has embarked on a research program to address this concern. The main purpose of this research project is to develop a sound knowledge on how to assess the long-term moisture performance of retrofitted high-rise wall assemblies using detailed hygrothermal analysis tools. It is intended that, based on the assessment of hygrothermal analysis results, potentially problematic retrofit strategies can be identified while more promising measures can be advanced for additional assessment through full-scale laboratory testing or field demonstrations. Therefore, one of the main project objectives was to develop a simplified methodology to characterise the moisture
performance of the wall assemblies considered. This methodology will then be tailored to allow for assessing the impact of selected retrofit strategies used in different environmental conditions on the long-term performance of building envelope assemblies.
3 Research Background
The National Building Code of Canada [1] and CSA Standard S478-95 [2] provide guidance on the basic durability requirements and design service life of buildings and building envelope components. In Appendix A of the CSA standard there is some guidance also for the design service life for different components of buildings, including walls. The predicted service life of any building component, including repaired as well as new, noted in the first requirement of the CSA standard , is only approximate. This is primarily due to the fact that the prediction is based on the assumed environmental conditions. According to the same standard, one or more of the following methods may be used to assess predicted service life: (i) Demonstrated effectiveness, (ii) Modelling of the deterioration process and (iii) Testing. A detailed description of the three methods is included in the standard (i.e. CSA Standard S478-95).
Prediction of the service life of the building envelope components by modelling the deterioration process for given indoor and outdoor climates has many practical advantages. It is fast and less expensive, involving only computer calculations and using physical moisture-damage properties of the component being considered. The material’s damage properties constants are included in the so-called “moisture damage functions” used to predict the service life of a component subjected to particular indoor and outdoor environmental design loads. Several authors recently reviewed the literatures on modelling different deterioration processes [3], [4], [5], [6]. However, it is important to mention that most data available in the literatures are related to biological damage of wood and wood-based construction materials, and low-rise buildings with wood and wood-based framed envelopes were the type of wall and roof assemblies targeted in these studies. Very few data related to high-rise wall assemblies exist.
In this paper, the methodology developed to predict moisture performance used to analyze the Heat, Air and Moisture calculations results obtained from detailed hygrothermal analysis results, using IRC's hygrothermal simulation model hygIRC [7] is presented. Results using these hygrothermal indicators are given for the example of wall considered that is a brick veneer with a steel stud back-up wall assembly (BV/SS). BV/SS represents one type of wall assemblies out of the five addressed in the corresponding research project.
4 Objective
The primary aim of this paper is to demonstrate that a methodology can be developed to predict the long-term moisture performance of a retrofitted wall assembly used in high-rise buildings. Heat, air and moisture transport calculation results, obtained from IRC's hygrothermal simulation model hygIRC [7], have been used to develop the methodology that can predict moisture performance of a wall assembly with brick veneer and steel stud back-up (BV/SS).
5 Typical Moisture Failures in Wall Assemblies with Brick Veneer
and Steel Stud Back-up (BV/SS)
Information was gathered by the project team regarding the typical moisture-related damages that occur in Canadian buildings that were built with BV/SS wall assemblies. The objective was to identify the deterioration mechanisms that could be expected in the BV/SS wall assemblies. Analysis of the information gathered in this project can be summarised as follows:
• The masonry cladding elements (brick veneer), exterior sheathing boards and interior drywall as well as the joints that tie the cladding elements together (e.g., mortar) suffers from cracking or spalling due to freeze-thaw cycles.
• The masonry cladding elements (brick veneer), exterior sheathing boards and interior drywall as well as the joints that tie them together (mortar) showed cracking or spalling because of efflorescence or subflorescence due to salt crystallization.
• Steel elements embedded in the cladding and the exterior layer of the back-up walls (e.g., brick,) typically showed corrosion. Steel studs are also subject to corrosion.
• Interior and exterior gypsum boards showed deterioration in the form of biological growth and decay.
Analysis of these typical moisture failures in walls shows that there are five main damage mechanisms of concern: freeze-thaw, corrosion, efflorescence/subflorescence, biological deterioration, and thermal expansion and swelling.
Three processes — freeze-thaw, efflorescence/subflorescence, thermal expansion and swelling — result in either mechanical or physical changes of the susceptible component in the five walls. They are linked to temperature- or moisture-induced shrinkage or expansion or to internally generated stresses which change the state of compounds, leading to deformation, cracking and deterioration. One common condition in these three processes is sustained high-moisture content. Frost damage of components usually occurs after a number of freeze-thaw cycles when
materials are nearly at the saturation level. Warm temperatures are also necessary for both efflorescence/subflorescence, and thermal expansion and swelling. In addition to the critical hygrothermal microenvironment, salts present in brick, stone, mortar or adjacent materials will cause efflorescence or subflorescence. Corrosion of the metal components results from chemical changes that might be linked to a variety of reasons. Corrosion of metals exposed to environments outside and within buildings, and within building assemblies and materials, is the most costly and unforeseen problem in buildings. It is also the problem least likely to be addressed in the design process. Supplementary reading is available in CSA Standard [2] in which the whole of Appendix E is related to corrosion of metals in the building environment. Appendix C of this same standard states that corrosion usually occurs when the surface of the metal is covered by a film of water, something which is common when the relative humidity is above 80% or 90% and the temperature is above the freezing point.
High moisture levels in interior gypsum boards and exterior sheathing boards provide a favourable environment for biochemical deterioration. Sustained high-moisture levels and oxygen with warm temperatures ranging from 5°C to 40°C, as well as the existence of the food the various organisms or insects require, are prime conditions for biochemical deterioration of construction materials. Some organic and inorganic materials are susceptible to biological attack. Interior drywalls, wood and wood-based construction materials are certainly the most popular materials subject to biological attack.
6
Moisture Durability Analysis Using Hygrothermal Indicators
To predict long-term moisture performance of walls, several hygrothermal indicators are used in the present study. These indicators predict any potential moisture damage in the wall assembly of the nature described in the previous section. The hygrothermal indicators are not material-specific. However, it needs the history of the local hygrothermal environment within the component under consideration, which could be obtained from HAM simulations, and the critical threshold hygrothermal levels for moisture damage.
Use of hygrothermal indicators to predict moisture durability performance of building envelopes and the susceptible regions in the envelopes is increasingly becoming a common phenomenon [5], [8], [9]. It is also being considered for use in ASHRAE Standard 160P on Design Criteria for Moisture Control in Buildings.
There are different types of hygrothermal indicators available in the literature to assess the moisture durability performance of building envelopes. Three
hygrothermal indicators have been adopted for the present study to predict moisture-damage potential of the nature discussed in the previous section. The first and most popular index used in most hygrothermal analyses, is the so-called moisture mass index or MMi. The two other indices newly adapted for the present study are referred to as the freeze-thaw (FT) index and RHT index.
Moisture Mass Index
The first hygrothermal indicator is the test of continuous increase of moisture mass in the wall assembly components from year to year. This is, of course, not acceptable for any wall assembly. Walls should be able to dry out whenever the moisture load ends. This indicator is used today by all the hygrothermal models including the simplified ones based on the Glaser method.
The moisture mass index, MMi, as defined in hygIRC is defined in Equation 1: start u start u end u start u u MMi= ? = − (1) where end
u daily average of the total moisture in the envelope component considered (kg/m) at the end of one year period of calculation start
u daily average of the total moisture in the envelope component considered (kg/m) at the end of one year period of calculation Freeze-Thaw Index
This hygrothermal indicator, newly added to hygIRC, is used to predict the potential for moisture damage due to repeated freeze-thaw cycles in every part of the walls. The freeze-thaw index, FT, is defined in hygIRC as the number of cycles when temperatures oscillate between the freezing and thawing point for those envelope components that are almost at the moisture saturation level,
critical
φ . The summation is performed for the last year of the calculation. The higher the number of cycles, the more potential for freeze-thaw damage. The freeze-thaw index as defined in Equation 2 allows identification of the critical location for freeze-thaw damage in each envelope component of the wall assemblies. ∑ = = 8760hours 2 k Cycle(i,j,k) j) FT(i, (2) with k) j,
Cycle(i, =1 if T(i,j,k)*T(i,j,k-1) < 0 and φ(i,j,k) > φcritical
k) j,
Cycle(i, =0 if T(i,j,k)*T(i,j,k-1) > 0 or φ(i,j,k) <
critical
φ where
T(i,j,k) calculated temperature (K) within the considered part of the envelope component at a particular time step
φ(i,j,k) calculated relative humidity (%) within the considered part of
the envelope component at a particular time step critical
φ critical moisture saturation level (%) in the envelope component
i,j spatial indices for the considered part of the envelope component
k considered time step index
The critical moisture saturation level,
critical
φ , depends on the nature of the construction material being considered. A typical value of 95% relative humidity is assumed in the present study for frost damage to occur.
Figure 1 shows the results of a freeze-thaw index obtained from the 2-D HAM simulations of brick veneer with steel-stud wall assembly (BV/SS) located in the top corner of a 10-storey building. The analysis is for a residential building located in Halifax. Maximum freeze-thaw cycles are obtained on the exterior surface of the brick veneer cladding. This is the only location where frost damage occurred when this wall assembly was subjected to the environmental design load used for hygrothermal calculation. Results show the number of freeze-thaw cycles may occur in the exterior surface layer of the brick veneer. This number of freeze-thaw cycles will probably damage brick masonry if no appropriate measures are taken. The number of freeze-thaw cycles decreased toward the interior of the brick veneer.
RHT index
The RHT index, as used in hygIRC, was developed as an outcome of consortium project undertaken at IRC on Moisture Management in Exterior Wall Systems (MEWS) [8]. This hygrothermal indicator, newly added to hygIRC, is used to predict the potential for any moisture damage when sustained high moisture levels and warm temperatures occur for an extended time. This is the case for the examples of corrosion, swelling and expansion, efflorescence/subflorescence, and biochemical damages concerned in this study.
The RHT index is calculated by multiplying the two terms, temperature potential and moisture, for moisture damage (see Equation 3). Warm temperature and humidity levels are known triggers for particular damage. The summation is performed for the last year of the calculation.
x hours 8760 1 k k) j, (i, potential k) j, (i, potential T j) RHT(i, ∑ φ = = (3)
with critical T k) j, T(i, k) j, (i, potential T = − if T(i,j,k) > critical T 0 = k) j, (i, potential T if T(i,j,k) < critical T critical -k) j, (i, k) j, (i, potential φ φ φ = if φ(i,j,k) > critical φ 0 = k) j, (i, potential φ if φ(i,j,k) < critical φ where k) j, (i, potential
T temperature potential for moisture damage (K) within the considered part of the envelope component at a particular time step
k) j, (i, potential
φ moisture potential for moisture damage (%) within the considered part of the envelope component at a
particular time step critical
T critical temperature level (K) above which moisture damage is more likely to occur
critical
φ critical relative humidity level (%) above which moisture damage is more likely to occur
The critical temperature and relative humidity vary depending on the nature of the construction material being considered and moisture damage involved. For example, biochemical deterioration of interior drywall due to mould growth may require higher temperature and lower relative humidity, depending on the fungal species involved. Efflorescence/subflorescence and swelling/expansion may also need higher hygrothermal levels. On other hand, active corrosion of metal components may occur when temperatures are just above the freezing point, depending on the material and the surrounding chemical agents.
For this study, these two values of critical relative humidity and temperature are selected through a trade-off for the four damage mechanisms considered. They are selected so as to ensure that the four types of damages are captured by the RHT index. As a first degree of approximation and based on IEA Annexe 14 conclusions [10], typical values of 80% relative humidity for
critical
φ and 5ºC
for
critical
T are assumed for biochemical attack types of damage. For corrosion, swelling-expansion and efflorescence-subfloresence 80% relative humidity and 0ºC are assumed for
critical
φ and
critical
T respectively.
One example of RHT index calculation is shown in Figure 2. The RHT index distribution shown was obtained from HAM simulation of the BV/SS wall when subjected to Halifax outdoor weather and residential indoor environment. 80%
relative humidity for
critical
φ and 5ºC for
critical
T are the values. Results show that higher RHT values are located in the bottom mid-section of the brick veneer cladding. Hygrothermal levels in this area of the assembly are 15 times higher than the RHT values obtained in the other envelope components of the wall being considered. In this location, potential hygrothermal damage may occur if no appropriate measures are taken.
7 Summary
A theoretical approach to assess the moisture durability performance of high-rise walls analytically is documented in this paper. The approach uses three hygrothermal indicators to characterise the moisture durability performance of walls when subjected to different environmental design loads: the moisture mass index (MMi), freeze-thaw index (FT) and the RHT index (RHT). The MMi predicts whether continuous moisture mass increase will occur in any of the wall envelope components, the FT index identifies potential for frost damage, and the RHT index predicts the potential for corrosion, efflorescence-subflorescence, biological deterioration, and thermal expansion and swelling in the wall assemblies. Each envelope component of the different walls is characterised by three types of values: moisture mass index (MMi), maximum value of freeze-thaw index found for the component considered (
max
FT ) and the maximum
value of the RHT index found for the component being considered (RHTmax). Different outdoor climate and indoor environment corresponding to the expected building use will result in different values for the three hygrothermal indicators. The relative impact of using any of the selected retrofit strategies for the five base-case walls, i.e., indoor environment and ventilation system, can be assessed by comparing the three values.
8 Acknowledgements
The authors would like to thank Mr. David van Reenen from IRC/NRC and Mr. Khaled Abdulghani from Ottawa University for their technical support in implementing the hygrothermal indicators in hygIRC’s solver and postprocessor. The authors would also like to extend their gratitude for Mr. Bill Semple and Duncan Hill from CMHC and Mr. Allan Wiseman from PWGSC for gathering field information on the typical moisture-related damage that occurred in existing buildings constructed with the walls addressed in the project from which this paper is derived.
X Y 0.05 0.1 0.15 0.2 0.5 1 1.5 2 Cycles146 136 127 117 107 97 88 78 68 58 49 39 29 19 10 B ri c k v e n ee r A ir s p a ce E x te ri o r d ry w a ll G las s fi b e r in s u la ti on In te ri o r d ry w a ll
Figure 1: Freeze-thaw index distribution for BV/SS
X Y 0.05 0.1 0.15 0.2 0.5 1 1.5 2 RHT1 25416 23721 22027 20332 18638 16944 15249 13555 11861 10166 8472 6777 5083 3389 1694 B ri ck ve n ee r A ir s p ace E x te ri o r g ra d e d ry w a ll G lass fi b e r in su la ti on in te ri o r d ry w a ll
9 References
[1] National Building Code of Canada (NBC). 1995. Canadian Commission on Building and Fire Codes. National Research Council of Canada, Ottawa, Ontario, Canada.
[2] CSA Standard S478-95. 1995. “Guideline on Durability in Buildings: Structures (Design)”. Canadian Standards Association, Dec 1995, Etobicoke, Ontario, Canada.
[3] Nofal, M. 1998. “Hygrothermal Damage of Building Materials and Components: State-of-the-Art Report on Studies of Hygrothermal Damage and Proposed Approach for Damage Assessments”, Internal Report, Institute for Research in Construction, National Research Council Canada, 757, pp. 139, March 01, (IRC-IR-757).
[4] Nofal, M. and Kumaran, M.K. 1999. “Durability assessments of wood-frame construction using the concept of damage-functions,” 8th International Conference on Durability of Building Materials and Components (Vancouver, BC 5/30/99), pp. 1-14.
[5] Hagentof, C.E. 1998. IEA Annex 24 Final Report on Task 5 – Performances and Practice: The Impact of Heat, Air, and Moisture Transport on Energy Demand and Durability. Energy Conservation in Buildings and Community Systems Programme. K.U. Leuven, Belgium. [6] Viitanen H. and Salonvaara, M. 2001. “Failure criteria,” ASTM Manual on
Moisture Analysis and Condensation Control in Building Envelopes, pp 66-80, 2001.
[7] Djebbar, R., Kumaran, M.K., Van Reenen, D. and Tariku, F. 2002. Hygrothermal Modeling of Building Envelope Retrofit Measures in Multi-Unit Residential and Commercial Office Buildings, Client Final Report, pp. 187, (B-1110.3)
[8] Kumaran, M.K. Mukhopadhyaya, P. Cornick, S. M., Lacasse, M. A., Maref, w. Rousseau, M., Nofal, M. Quirt, J.D., Dalgliesh W.A. “A Methodology to develop moisture management strategies for wood-frame walls in North America: Application to stucco-clad walls”, Proceedings of the 6th Symposium, Trondheim, Norway, June 17-19, 2002, pp 651-658, 2002.
[9] Kurkinen, K. and Hagentoft C-E, Durability control by means of hygrothermal history in building components”, Proceedings of the 6th Symposium, Trondheim, Norway, June 17-19, 2002, pp 697-704, 2002 [10] IEA Annex 14 Final Report. 1991. Condensation and Energy, Sourcebook
Volume 1. International Energy Agency, Energy Conservation in Buildings and Community Systems Programme, K.U. Leuven, Belgium.
