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

By: Patrick J. Roppel, Mark D. Lawton, & Brian Hubbs
           

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 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.

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 9.32.3.3.A requires a minimum ventilation rate for the principal exhaust, which may be the bathroom exhaust fan, based on the number of bedrooms.  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.

Table 1:  Bathroom Fan Operation (Condition 1)

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.

Table 2: Measured Corridor Fan Flow Rates (CFM)

Table 3: Suite Entry Door Measurements

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, make-up 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 (CO₂) 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 CO₂ 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 CO₂ can be used as a method of evaluating the level of ventilation.  Outdoor CO₂ 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 CO₂ concentration when fresh air is being delivered at the ASHRAE’s recommended 15 cfm per person.

A summary of single point CO₂ 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 CO₂ 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 CO₂ Measurements