Balancing the Control of Heat, Air, Moisture, and Competing Interests

Transcription

1 Balancing the Control of Heat, Air, Moisture, and Competing Interests Patrick J. Roppel, M.A.Sc. Mark D. Lawton, P.Eng. Brian Hubbs, P.Eng. Building Science Consultant Sr. Building Scientist Specialist Sr. Building Science Specialist Morrison Hershfield Ltd. Morrison Hershfield Ltd. RDH Building Engineering Ltd. Vancouver, B.C. Vancouver, B.C. Vancovuer, B.C. ABSTRACT The design of a building for either new construction or rehabilitation must consider the control of heat, air, and moisture (HAM) flows. A large number of buildings in the Lower Mainland of B.C. have been rehabilitated because of extensive deterioration of the building envelope as a result of rainwater penetration. Rehabilitation designs have focused on eliminating rainwater penetration as a damage mechanism, which has typically included an increased control of air flow through the envelope. Because these designs have resulted in buildings that are more air tight than before rehabilitation, there is an increased need for mechanical ventilation to maintain good building performance. Without adequate ventilation the balance humidity levels in the suites increase and, depending on the wall design, this can result in condensation on the windows or interior drywall, or within the wall assembly itself. This paper draws on one rehabilitated wall assembly with high humidity levels to explore the assumptions and decisions faced by the design team when completing a rehabilitation project with respect to controlling heat, air, moisture and the resulting affect on building performance. Decisions and assumptions discussed include: Indoor environment: acceptable and assumed ranges of conditions for temperature, ventilation, moisture generation, and the resulting balance relative humidity. Cost and existing construction: constraints of the existing construction including wall dimensions and the cost of rehabilitation. Current norms for building design and construction: current accepted norms for the control of heat, air and moisture transport and the importance of balancing these controls. This paper presents a combination of calculated values for the indoor environment using a moisture balance between the exterior and indoor environment, HAM computer modeling, and field measurements of a rehabilitated project to support this discussion. Conclusions and recommendations are drawn from the lessons learned from completing this work. INTRODUCTION The premature deterioration of multi-unit residential buildings in the Lower Mainland of BC due to rainwater penetration has been well documented in the past 10 years (Morrison Hershfield 1996, RDH 2001, Lawton 2004). Consequently, rehabilitation designs for such buildings focus on eliminating rainwater penetration as a damage mechanism. Typically, these buildings are more air tight after rehabilitation than they were before. This has the benefit of not only eliminating potential paths for water ingress but also reduces the amount of heat loss through and potential for condensation within the building envelope. Mechanical ventilation is important to the performance of an airtight building, as adequate ventilation is necessary to maintain a healthy and comfortable environment for occupants. Without adequate

2 ventilation, the occupancy may cause the relative humidity to rise to a level where, depending on the wall design and climate, condensation will form on the windows or interior drywall, or within the wall assembly itself. Moisture is the single leading cause of deterioration of building envelope materials. Moisture from the exterior, e.g., rainwater, if not restricted, can lead to premature deterioration of building materials. The path for rainwater penetration is usually a consequence of inadequate design or construction details. Conversely, wetting by moisture transported from the interior environment is typically a slower damage mechanism. The source of wetting from moisture transport from the interior is sometimes more difficult to determine in the field since damaged areas are a consequence of the combined heat, air, and moisture flow in four dimensions, time being an important factor in both the duration of accumulation and the onset of deterioration. If a wall has suffered severe deterioration due to rainwater penetration, then the challenge for rehabilitation projects is to determine whether there is damage by wetting from vapour diffusion from the interior as such damage may be masked by the severity of deterioration by rainwater penetration. After rehabilitation, with a more airtight envelope, the balance interior humidity levels may be difficult to ascertain. This paper draws on one rehabilitated wall assembly in Vancouver, B.C. as an example to illustrate the importance of balancing the control of heat, air, and moisture flows from both the interior and exterior. The primary cause of premature envelope failure was determined to be rainwater penetration, and the interior humidity was identified as a problem that required corrective action. The rehabilitated exterior walls consist of a rain-screen stucco wall assembly with 50 mm of semi-rigid mineral wool insulation, self-adhering SBS-modified bitumen membrane on fiberglass faced gypsum sheathing, 90 mm steel studs with fiberglass batt cavity insulation, and interior drywall with latex paint. Eight exterior wall cavities and two suites were instrumented with sensors on the east elevation during the rehabilitation of the building as part of a monitoring project (Finch et al 2006). This monitoring project revealed that moisture was collecting in the gypsum sheathing in winter. In addition, mold growth was present on the interior surface of the drywall on the exterior walls and excessive condensation was occurring at the aluminum windows. These symptoms were attributed to condensation due to the high interior relative humidity in the building. A follow-up investigation was undertaken to address the concerns raised by the monitoring program and to determine the following: The level of ventilation provided by the existing mechanical systems. The effectiveness of the provided level of ventilation with respect to control of interior humidity, other contaminates, and moisture collection in materials in the wall assembly. What changes to the ventilation strategy will provide appropriate control of interior humidity, other contaminates, and moisture collection in materials in the wall assembly. Alternative methods of increasing tolerance of the wall assembly for high humidity levels. The intent of this paper is to discuss how the rehabilitated wall assembly compares to current design practice to identify how the building operation and performance can be improved for future rehabilitation projects.

3 THE INDOOR ENVIRONMENT AND BUILDING OPERATING CONDITIONS Both exterior and interior environmental design loads must be considered when designing a wall assembly to control heat, air and moisture flows. Exterior environmental loads such as temperature, precipitation, solar radiation and humidity can be determined from climatic data. Interior moisture loads are comparatively more complex to establish for buildings with uncontrolled humidity since the indoor humidity is a function of the outdoor vapour pressure, ventilation rate, moisture generation rate, and absorption/desorption of interior hygroscopic materials (Roppel et al 2007). Ventilation controls the interior vapour pressure by exchanging moisture-laden air with exterior air, which in cool climates is typically at a lower vapour pressure. The rate of moisture generation and the control of ventilation and temperature of each suite in a multi-unit apartment building all have an impact on the interior humidity. Air that is removed from a suite by an exhaust fan will be replaced with air that originally came from the exterior. The path of the air flow will be: directly from outside through the envelope or an exterior make-up air supply, via the corridor pressurization system through the corridor and through intentional or unintentional openings (i.e., door undercuts) in the suite demising walls, from adjacent suites, carrying moisture and contaminated air generated in those suites. The following sections identify the key parameters governing the indoor operating conditions. Typical design assumptions are compared to actual operating conditions as determined from field measurements. PRINCIPAL EXHAUST VENTILATION RATE The suites of this building have individually ducted bathroom fans that have been provided (following rehabilitation) with time-activated switches according to the concept of the principle exhaust fan required under Part 9 of the current Vancouver Building Bylaw (VBBL) and the British Columbia Building Code (BCBC). The timers are located in lock boxes which minimizes tampering but complicates adjustments for conditions or to corrections for time changes or power outages. The airflow of the fans was measured with a flow hood under four different conditions: 1. With all windows and the suite entry door closed and the bathroom door open; 2. With all windows, the suite entry door, and the bathroom door closed; 3. With all windows closed, the suite entry door cracked open and the bathroom door open; 4. With a window cracked open, the suite entry door closed and the bathroom door open. Table 1 shows the measured flow of the bathroom flow rates in the first condition and compares that with the requirements of the current VBBL. There was relatively little difference in the four measurements (less than 5 CFM). Air tightness of the suites did not seem to affect the exhaust rate of the bathroom fans (it would affect where the replacement air was drawn from). This indicates that the flow rate is limited primarily by the resistance of the in-slab duct that services the bathroom fans. The current VBBL and BCBC have similar minimum requirements as ASHRAE Standard 62 which recommends 15 cfm/ person. Table A requires a minimum ventilation rate for the principal exhaust, which may be the bathroom exhaust fan, based on the number of bedrooms.

4 The VBBL requires that the principal exhaust fan be controlled by an adjustable time control device to provide a minimum of two 4-hour operating periods per day, or be designed to run continuously. A separate requirement states that the bathroom fan should have a capacity of 50 CFM if run intermittently or 20 CFM if run continuously. Suite Number of occupants Table 1: Bathroom Fan Operation (Condition 1) Number of Bedrooms Estimated fan operation time (hours) Flow Rate (CFM) VBBL Capacity Requirement (CFM) 15 CFM / Person (CFM) N/A N/A N/A < The range of measured values is 20 to 61 CFM, with an average rate of 43 CFM. From questioning the occupants of the suites, it appears that the fan timers were not set to the same time or to meet the expected peak moisture production or the code defined duration (two 4 hour periods). As can be seen in Table 1, four of 14 fans (29%) meet the minimum capacity as outlined in the current VBBL and two of 11 (18%) appear to be operating the minimum duration. Many of the bathroom fans were noisy (subjective opinion), and the noise level was identified by the occupants in Suite 611 as a contributing factor to lack of use of the bathroom fan. The noise levels of the fans were not measured, but the VBBL states that the principal exhaust fan should have a 1.5 Sone sound rating when controlled by an adjustable timer or 1.0 Sone when operating continuously. MAKE-UP AIR AND FRESH AIR The building had no provision to deliver fresh make-up air directly to the suites. Therefore, make-up air will likely be drawn from a combination of the corridors, adjacent suites and the exterior walls. The path of air exchange is dependent on the relative size of openings and pressure differentials between the suite and adjacent zones (corridor, adjacent suites, exterior environment). The corridor fan flow was measured, the unobstructed opening under the suite entry doors, and the pressure difference from the suite to the corridor, to establish how much fresh air could be expected to be delivered to each suite from the corridor. Tables 2 and 3 summarize the measured data.

5 Table 2: Measured Corridor Fan Flow Rates (CFM) Floor Level Corridor 1: Corridor 2: Suites 1 to 6 Suites 9 to Suite Table 3: Suite Entry Door Measurements Height of Space under Door (mm) Pressure Difference measured between Corridor and Suite A minimum capacity of 330 CFM for the corridor 1 supply (6 suites in total) and 210 CFM for corridor 2 supply (4 suites in total) is sufficient to supply the volume of air that would be exhausted if a principal exhaust fan, meeting the code defined minimum flow rates, was on for all of the connected suites. Generally, it appears that sufficient fresh air is being supplied to the corridors (but is not necessarily getting to the suites). A positive pressure difference measured from the suite to the corridor indicates that the pressure in the corridor is greater than the pressure in the suite. When the bathroom fan is running, makeup air will travel under or around the suite door at a rate dependent on the open area. Fresh air from the corridors is restricted by door sweeps installed in many suites. For the measured pressure differences and opening areas, the expected flow rate from the corridor is approximately 5 to 25 CFM, which represents 15% to 60% of the bathroom fan exhaust rate. A large amount of the make-up air for individual suites may be from adjacent suites. This route is likely since the bathroom exhaust fans in each suite appear to be operating at different times and durations. Carbon dioxide (CO 2 ) levels were measured as an alternative method of assessing whether adequate ventilation is provided with the current building operation. The ASHRAE Handbook of Fundamentals (2005) states that CO 2 is not normally considered to be a toxic air contaminant but high levels are associated with symptoms such as increased headaches and reports of people feeling tired. Measuring CO 2 can be used as a method of evaluating the level of ventilation.

6 Outdoor CO 2 is typically in the range of 330 to 370 ppm or slightly higher in urban environments due to automobile emissions. A level of 1000 ppm has been suggested as being representative of a CO 2 concentration when fresh air is being delivered at the ASHRAE s recommended 15 cfm per person. A summary of single point CO 2 measurements taken during the day when site visits were carried out is found in Table 4. Many suites had their windows open before arrival to take measurements; therefore, the lower readings are indications of the exterior ambient CO 2 levels. The lowest reading was 382 ppm, which is in line with the typical value stated by ASHRAE Handbook of Fundamentals (2005). Four of the 13 suites (31%) measured had readings above 1000 ppm. Table 4: Suite CO 2 Measurements Suite CO 2 (ppm) Long duration CO 2 monitoring to evaluate overnight levels and to determine the effect of continuously running the bathroom fans was also measured. CO 2 levels, RH, and temperature in two suites was measured every hour using a data logger. The occupant was instructed to record a period of time when the bathroom fan was operating continuously. Figures 1 and 2 represent these measurements for Suite 205. As can be seen in Figure 1 and Figure 2, the CO 2 peaked at 1800 ppm and was consistently above 1000 ppm, while the fan was controlled by the timer. When the fan was set to run continuously, CO 2 concentration peaked at 1200 ppm at midnight and for the most part was below 1000 ppm. The RH in the suite was in a range of 36% to 47%RH while the fan was continuously operating compared to a range of 42% to 56%RH while the fan was controlled by the timer. The effect of the fan running continuously on humidity can be seen more clearly by comparing the indoor vapour pressure to the outdoor vapour pressure as illustrated in Figure 3. This effect is most clearly identified by looking at the change in interior and exterior vapour pressure when the fan was shut off. Subsequent to shutting off the bathroom fan the interior vapour pressure increased 240 Pa while at the same time the exterior vapour pressure dropped 120 Pa. Calculations confirm that a change in the ventilation rate from 0.15 ACH to 0.35 ACH has the same effect on the vapour pressure as per our discussion below.

7 Figure 1: CO 2 and Temperature Readings in Suite 205 Figure 2: CO 2 and RH Readings in Suite 205 Figure 3: Comparison of Exterior Vapour Pressure to Interior Vapour Pressure (Suite 205)

8 The average difference between the exterior vapour pressure and the interior vapour pressure was approximately 370 Pa when the fan was operating continuously compared to a difference of 470 Pa when the fan was controlled by the timer. This difference in average vapour pressure represents a 4% difference in RH at a temperature of 21 o C. The BRE Model was utilized in the calculation of the indoor humidity (Roppel et al 2007) using a constant moisture generation rate, absorption/desorption of materials, and constant ventilation rates (fan running continuously or not at all), with the following assumptions ACH for the period when fan was not operating, 0.35 ACH for the period when fan was running continuously (this represents 37 CFM for a 800 ft 2 area or 6400 ft 3 volume), 4 kg/day moisture generation, admittance factors of alpha and beta of 0.6 and 0.4 respectively. It is important to note that the values are an order of magnitude approximations and a range of combinations of moisture generation, absorption/desorption, and ventilation is possible to match any single measured hourly data point. What is important is that the overall trend is consistent between the measured and calculated values and the assumed values are consistent with our experience and other studies. Differences between measured and calculated values can be explained reasonably well by peak moisture loads (showers, cooking, etc.) or peak ventilation (natural ventilation, exhaust fans, pressure differentials, etc.). The CO 2, RH, and temperature was measured in Suite 311 and the same trends were recorded. The affect on humidity is not as convincing due to the limited data, but the drop in CO 2 after turning on the fan is significant. The CO 2 dropped from approximately 4000 ppm during the course of one night to under 1000 ppm in a couple of hours after turning on the fan and peaked between 2000 and 2500 ppm during subsequent nights. These measurements are shown in Figure 4. Figure 4: CO 2 and Temperature Readings in Suite 311 The CO 2 levels still remains high during continuous operation of the bathroom fan. This suggests that the current operation of the bathroom fan as an exhaust, with the supply of fresh air from the corridor does not meet the ventilation requirements for this suite. This suggests that increasing fan operation by fan controls alone will not suffice for this suite.

9 MODELING AND ANALYSIS To establish recommendations to improve the conditions at the building, analysis was completed to determine how effective control of the building operation will impact the building envelope using a combination of hourly computer simulation and hand calculations. This analysis was completed to address the following impact of: 1. ventilation on indoor humidity, 2. wall construction on performance, 3. vapour resistance at the interior surface, 4. winter indoor operating temperature, 5. existing operating conditions on adjacent assemblies. THE IMPACT OF VENTILATION ON INDOOR HUMIDITY Ventilation controls the interior vapour pressure by exchanging moisture-laden air with exterior air at a lower vapour pressure. The moisture production and control of ventilation and temperature of each suite will have some impact on the relative humidity. ASHRAE standard , Thermal Environment Conditions for Human Occupancy, states limits for interior temperature and RH to satisfy 80% of sedentary or slightly active persons. The standard states an upper limit of RH at 60% and a lower limit of temperature at 20 o C. The suites do not appear to be operating within these conditions during the winter months. Some of the occupants of this building indicated that they often keep their windows open to provide fresh air. However, occupants are less likely to open their windows during cold weather at night since temperature comfort is likely to have priority. To analyze the benefit of providing more ventilation to the wall performance, the interior vapour pressures were calculated utilizing a moisture balance between the exterior and interior air and compared to measured vapour pressures. For Suite 611 measured data (Finch et al 2006, Roppel et al 2007), a moisture generation rate of 7 kg/day and 0.15 ACH ventilation was initially assumed for the calculation of the interior vapour pressure. This seemed a reasonable estimate since the bathroom fan was not running frequently, the occupants are home regularly throughout the day, and the suite is expected to have moisture transport from suites below. Figure 5 compares the calculated indoor vapour pressure and the measured indoor vapour pressure for the winter of The measured and calculated indoor RH during this same period is presented in Figure 6. The 7.5 kg/day water, 0.15 ACH assumption in the BRE model provide a good match for measured mid-winter condition but overstates vapour pressure and RH outside that period. One can speculate that this pattern is attributable to more frequent use of windows in warmer months. Note that three years of measured data was analyzed using the same procedure and parameters. A similar pattern and match was found for all the analyzed years for this suite.

10 Figure 5: Comparison of the Calculated to the Measured Indoor Vapour Pressure Figure 6: Comparison of the Calculated to the Measured Indoor RH Note that increasing the level of ventilation by 0.3 ACH has a major impact on winter indoor humidity levels. Using the matched calculated interior vapour pressures as a baseline, the wall assembly was analyzed for different ventilation rates using the 1-D hygrothermal software WUFI. Simulations were completed using both measured and calculated interior vapour pressures. The simulations were run from July 1, 2002 to July 1, 2003 and repeated for two years to demonstrate that equilibrium conditions are reached. A comparison of the WUFI simulated equilibrium RH of the exterior sheathing, at the interior surface, for different ventilation rates is presented in Figure 7. The measured and calculated RH values correlate well for the critical winter months from the end of November (day 150) to mid

11 March (day 250). These results show how the different ventilation rates during the winter will effect the performance of the wall assembly. The critical moisture content for gypsum may be considered where liquid capillary suction begins, which is approximately 90% RH or 22 kg/m 3, as illustrated in Figure 8 for the moisture storage function of gypsum. Figure 7: WUFI Simulation of equilibrium RH at Exterior Sheathing for Different Ventilation Rates Capillary Hygroscopic Figure 8: Moisture Storage Function of Gypsum (default WUFI material property) The minimum exhaust rate recommend in the VBBL is 0.45 ACH. This is equivalent to providing a 50 and 60 exhaust fan for a 800 ft 2 and 1000ft 2 apartment respectively (with 8 foot ceilings). If all the suites were timed to exhaust simultaneously at the exhaust rates as per VBBL, the risk of condensation at the sheathing would be minimal for the overall wall assembly since the calculated RH at the sheathing will be maintained below 90%RH.

12 THE IMPACT OF WALL CONSTRUCTION ON PERFORMANCE Not all issues that influence the design of a wall for rehabilitation are technical. Issues such as cost, property lines, and minimizing disruption to occupants can constrain the available design options. Often to accommodate these issues some risk is introduced into the design. It is the duty of the design professional to keep the risks to an acceptable level and make the owners of the building aware of these risks. One compromise that is common for Canadian buildings with steel stud back-up walls is to split up the thermal resistance of the wall by the inclusion of insulation in the stud cavity and exterior insulation outboard of the studs. Splitting up the insulation this way has the following benefits: Provides a thermal break for the steel studs, thus increasing the effectiveness of the cavity insulation, Less wall thickness than compared to an assembly with all the insulation outboard of the sheathing, Smaller structural attachments for cladding than compared to an assembly with all the insulation outboard of the sheathing. The drawback for this approach is that if the sheathing or sheathing membrane are low permeance materials then there is a risk of moisture accumulation on the interior surface of sheathing. A sufficient amount of insulation must be outboard of the low vapour permeance to keep the sheathing warm enough to limit the time that condensation may occur. During the rehabilitation the building, the decision was made to have low vapour resistance on the interior surface to avoid trapping moisture in the stud cavity and allow drying to the interior. Two questions arise for this type of wall construction, as follows: 1. Is the ratio of the insulation outboard of the low vapour permeance membrane sufficient? 2. Is the level of vapour resistance at the interior surface sufficient for the interior moisture load? The National and Provincial Building Codes and Vancouver s Building Bylaw define a minimum ratio (insulation outside / insulation inside of a low vapour permeance material) dependant on heating degree days in the climate. For Vancouver, Table of the 1995 and 2005 NBC requires a minimum ratio of 0.2. The rehabilitated wall assembly has a ratio is approximately 0.5 for the wall assembly with R12 batt and approximately 0.7 for R8 batt. This is 2.5 to 3.5 times more conservative than allowed by code. The NBC minimum ratio was originally derived by heat-air-moisture (HAM) modeling assuming an interior RH of 35% during the heating season. For the 2005 NBC it was confirmed using up to 60% RH for Vancouver s mild climate. The modeling also assumed a 60 ng/m 2 s Pa vapour barrier on the warm side of the insulation at the interior surface. Note that these assumptions are not stated in the 1995 NBC. In a paper published in the ASHRAE Journal, August 2004 (Lstiburek 2004), the author identifies a wall assembly similar to the wall assembly of the building where there is exterior insulation outboard of the exterior sheathing, insulated steel stud cavity, and low vapour resistance at the interior surface (for vapour permeance between 57 ng/m 2 s Pa and 575 ng/m 2 s Pa). In the discussion of the wall assembly, the author points out that the wall is applicable for marine climates and cold regions provided that the thickness of the exterior insulation is determined by hygrothermal analysis. According to the author s definition of a marine climate, Vancouver falls into this category, but is also very close to the definition of a cold climate. In the paper, the

13 author proposes to calculate the insulation ratio by assuming steady-state heat transfer, interior air at 40% RH at 21 o C (for marine climate), and exterior air at a temperature that is equal to the average outdoor temperature during the coldest three months of the year. The author indicates that the design guideline was evaluated using WUFI with the design conditions for the exterior and interior air as specified by the ASHRAE proposed standard 160P, Design Criteria for Moisture Control of Buildings and ventilation to meet ASHRAE standards 62.1 or Completing the steady-state heat transfer calculation for the conditions proposed in the paper would allow a minimum ratio of approximately 0.3, without a vapour barrier at the interior. The main differences between the example building from common assumptions commonly made about Vancouver wall performance/design and previous modeling done by others is: 1. The average monthly humidity for this building is higher than 60% RH; 2. The ventilation does not meet the ASHRAE standards of 62.1 or 62.2 and the NBC minimum principal ventilation rates. THE IMPACT OF THE INTERIOR DIFFUSION RESISTANCE The rehabilitated wall assembly has low vapour resistance at the interior surface (vapour permeance is approximately 180 ng/m 2 s Pa) and high vapour resistance (3 ng/m 2 s Pa) at the plane of the exterior sheathing. The impact on the wall performance of adding additional vapour resistance to the interior surface was analyzed using WUFI for the measured interior conditions in Suite 611 for July 1, 2002 to July 1, 2003, similar to the modeling previously discussed in the humidity control section. The wall was modeled assuming three levels of vapour resistance: 3, 35 and 180 ng/(m² s Pa) to simulate polyethylene, vapour retarding paint and the existing paint finish respectively. The results from the modeling are presented in Figure 9. Figure 9: Comparison of Interior Surface Vapour Resistance on Sheathing Moisture The initial moisture content of the sheathing was set at 80% RH. The modeling indicates that both the polyethylene and vapour retarding paint control the risk of capillary moisture in the exterior sheathing. The data was repeated for two consecutive years to show that a downward equilibrium will be reached (a function of the initial moisture content).

14 Some may argue that installing a sheet of polyethylene behind the interior drywall appears to work in theory, but in practice will trap moisture in the wall cavity from construction or if water were to enter the cavity from a leak since the sheathing membrane also has a low vapour permeance. An intermediate level of resistance such as vapour retarding paint appears to be the best compromise to sufficiently reduce the wetting from the interior air and provides some drying to the interior if the exterior sheathing were to become saturated from another source such as a water leak. Questions that arise from this discussion are: 1. Is the installed vapour resistance of the vapour retarding paint constantly to 35 ng/m 2 s Pa? 2. How much and how long will it take for a leak or construction moisture to dry-out if the sheathing was to become saturated? 3. If a more vapour permeable exterior sheathing membrane where selected, would this problem be avoided (note that the available alternatives during the rehabilitation of this were limited)? The application of a vapour retarding paint is difficult to review during construction to ensure proper application. Some assurance may be provided by the manufacturer to verify proper installation, but the applied vapour permeance of vapour retarding paints in the field will likely have a large variance. Reducing the vapour resistance at the exterior sheathing will maintain the moisture content of exterior sheathing for the rehabilitated wall assembly at an acceptable level (90% RH or 22 kg/m 3 ) as shown in Figure 10. Figure 10: Impact of Vapour Resistance of Exterior Sheathing Membrane THE IMPACT OF WINTER INTERIOR OPERATING TEMPERATURE For the rehabilitated wall assembly, condensation will occur on the sheathing whenever the sheathing temperature is below the dewpoint temperature of the interior air. This is due primarily to the low vapour diffusion resistance of the interior surface.

15 The impact of the low interior temperatures was analyzed by comparing the interior dewpoint temperature to the measured sheathing temperatures for the winter of (October 1 to March 31). Then the number of hours that condensation is expected to occur was calculated if the interior air temperature did not drop below 19 o C. For measured interior temperatures below 19 o C: the sheathing temperature was adjusted proportionally by the measured temperature difference between the outdoor and indoor conditions, the interior vapour pressure was assumed to be the same as the existing conditions, the calculated sheathing temperature was compared to dewpoint for air at 19 o C and the existing vapour pressure. Table 5 summarizes the number of hours the exterior sheathing is below the dewpoint for the existing building operation, and if the air temperature in the suite is maintained at minimum of 19 o C. A reduction of approximately 35% to 40% in the number of hours the sheathing temperature will be below the interior air dewpoint temperature was calculated for a minimum interior air temperature of 19 o C and approximately 60% to 70% reduction for a minimum temperature of 20 o C. Table 5: Number of Hours that Exterior Sheathing is below Interior Air Dewpoint Existing Operation Minimum Temperature of 19 o C Sensor Location Suite Number of Number of Percentage hours hours Percentage of of total below below total hours hours dewpoint dewpoint 2nd floor, below 3rd floor 211, balcony-wall interface % 4 0.1% 3rd floor, below window % % 6th floor, southeast corner % % 5th floor, below 6th floor 611, balcony-wall interface % % 6th floor, below window % % As seen in Table 5, the number of hours that the exterior sheathing is below the interior air dewpoint is highest at the sensor locations near Suite 611. It is worth mentioning that the occupant of Suite 611 indicated that the heat for the second bedroom is turned down and the door is often closed. This condition will result in the humidity in the room remaining high (same as the entire suite), cooler surface temperatures are increasing the potential for condensation on the windows and sheathing. Suite 311 also has lower than expected interior air temperatures during the winter months. IMPACT OF THE EXISTING OPERATING CONDITIONS ON ADJACENT ASSEMBLIES The low interior air temperatures with combined high humidity levels in the suites appears to be causing excessive condensation on the aluminum windows. For a minimum I-value of 0.4, the windows are expected to be below the dewpoint by approximately 15% to 20% for the measured interior conditions in winter The number of hours

16 that the window temperature is below the dewpoint drops to approximately 5% to 6% in winter for windows that have an I-value of 0.5 (optimistic). By comparison of the number of hours below the interior air dewpoint, condensation is more likely to form on the windows, if the window I-value is close to 0.4, than on the exterior sheathing. CONCLUSIONS The primary cause for poor performance of the wall assembly and elevated interior humidity appears to be from inadequate ventilation. Regardless of wall condensation issues, improved ventilation is required. This was concluded from the investigation of the building operation and monitoring program, given that: 1. The level of ventilation is insufficient since the occupant lifestyles can not be changed and ventilation is the only way to improve levels of the interior humidity, CO 2, and possibly other contaminates. 2. The existing ventilation system does not conform to the capacity or operating time requirements identified in Part 9 of the current VBBL. 3. High levels of humidity is leading to: mold on the interior drywall, excessive window condensation, and condensation in the wall cavity. 4. CO2 concentrations regularly exceed ASHRAE recommendations. Ventilation solutions must keep the occupants comfort in mind or risk them overriding that provision. Therefore, for this building we recommended that every suite be provided with a principal exhaust fan to meet the current VBBL for noise and have a capacity of at least that required by the VBBL. To address the potential of make-up air coming from adjacent suites, we recommended that all the fans run the minimum duration simultaneously to ensure that the exhaust air is directly replaced with fresh air from the corridor and/or exterior. Another provision may be to supply direct make-up air from either outside or the corridor in such a manner that limits cold drafts and reduces the chance of occupants covering the vents. Condensation forming on the exterior sheathing highlighted the risk of splitting the thermal resistance of a wall assembly between low vapour permeance materials. This type of wall assembly does have some advantages over fully exterior insulated wall assemblies. However, depending of the ratio of the vapour resistance of the interior surface to the exterior sheathing membrane, the performance of this wall assembly may be adversely affected if the interior humidity is not controlled within acceptable limits. The lessons drawn by the monitoring of the building assembly and the investigation of the building operation are: Increased vapour control at the interior surface of the rehabilitated wall assembly will likely keep the moisture content of the gypsum at acceptable levels for the existing operating conditions. A sheathing membrane with a higher vapour permeance than SBS self-adhesive membrane will lower the risk of condensation forming on the exterior sheathing for a wall assembly similar to the building and subjected to similar interior operating conditions.

17 REFERENCES Finch, G., Straube, J., Hubbs, B., Building Envelope Performance Monitoring and Modeling of West Coast Rainscreen Enclosures, Proceedings of the 3 rd Annual International Building Physics Conference, 2006 Lawton, M., Lessons to be Learned from Performance Failures of Framed Walls in High-Rise Buildings, Proceedings of the Performance of Exterior Envelopes of Whole Buildings IX International Conference, ASHRAE, Lstiburek, J., Understanding Vapour Barriers, ASHRAE Journal, August, 2004 Morrison Hershfield Ltd., Survey of Building Envelope Failures in the Coastal Climate of British Columbia, Morrison Hershfield Limitied for Canada Mortgage and Housing Corporation, 1996 Roppel, P., Brown, W., Lawton, M., Modelling of Uncontrolled Interior Humidity for Simulations of Residential Buildings, this paper has been submitted as part of the 11 th Canadian Building Science and Technology Conference and is found in these proceedings, 2007 RDH, Study of High Rise Envelope Performance in the Coastal Climate of British Columbia, RDH Building Engineering for Canada Mortagag and Housing Corporation, 2001