The National Building Code of Canada [1], which is the basis of all building codes in Canada, requires that a vapour barrier ^[1] be included in most building envelopes. It defines a vapour barrier as “the elements installed to control the diffusion of water vapour”. Part 9 of the Building Code, which applies to housing and small buildings, requires that a vapour barrier be constructed “to provide a barrier to diffusion of water vapour from the interior to wall spaces…” Part 5 of the Building Code, which applies to all other buildings, requires that a vapour barrier be included “where a building component or assembly will be subjected to a temperature differential and a differential in water vapour pressure.” Taken together, these requirements effectively mean that a vapour barrier must be installed in the building envelope assemblies of all occupied buildings in Canada.

The uniformity of this requirement is a function of the fact that all regions of Canada are in primarily heating climates.  Air conditioning is increasingly common in houses of south central Canada, but even there, the direction of vapour pressure drive is generally outward over the majority of the year.
 

For the last 25 years, Canadian builders have found that the most convenient method of meeting the requirement for a vapour barrier in framed envelope assemblies has been to install polyethylene sheet as the vapour barrier.  It is typically installed between the insulated stud frame and the interior sheathing.  Polyethylene sheet, in the 6 mil thickness now typically used, has a very low vapour permeance; The ASHRAE Handbook – Fundamentals, states a value of 3.4 ng/(Pa·s·m²).  This value is lower than the code requirement of 45 ng/(Pa s·m²) in most cases and 15 ng/(Pa s·m²) is some cases. 

The code requirements were developed to be conservative so it is reasonable to say that, in most Canadian environments, the diffusion resistance of polyethylene sheet is more than necessary to control condensation within building envelope assemblies due to the diffusion transport of interior moisture.  However, it has been further suggested [2, 3] that the presence of a low permeance vapour barrier on the “warm-in-winter” side of the insulation is not only unnecessary, but that it can cause or exacerbate moisture problems.  The basis of this contention is that the presence of the low permeance layer restricts drying to the interior, conditioned space of moisture that may enter the envelope assembly as precipitation, or as vapour driven inward by solar heating of saturated cladding materials.  There are certainly situations where the ability to dry to the inside may be important.  These would include buildings in climates where the general direction of vapour drive is inward over most of the year, as would be the case in climates with significant outdoor humidity levels and a high number of air conditioning hours per year.  However, the concept that “poly is bad” has also been extended to temperate or mixed climates.

The promulgation of this point of view coincided with the realization of the extent of premature envelope failure attributed to rainwater intrusion into wall assemblies in the Pacific coastal climates of British Columbia and the northwestern United States.  Some parties have come to believe, or find it convenient to suggest, that the use of polyethylene vapour barriers caused the problem.  It has been suggested that if polyethylene had not been used the walls suffering from water intrusion could have dried to the inside and the epidemic of premature envelope failures would not have occurred. 

This paper takes a critical look at the contention that the presence of low permeance vapour barriers (i.e., polyethylene sheet vapour barriers) were, or are, a significant factor in the envelope failures of buildings in the temperate coastal climates of British Columbia and the northwestern United States.  It also assesses some of the risks engendered by using, or not using, low permeance vapour barriers in temperate climates. 

The Evolution of Vapour Barriers

The primary purpose of a vapour barrier is to control the transport of water vapour by diffusion.  Rose [4] provided a history on both the conceptual development and physical application of vapour barriers in the United States, tracing some of the concepts to ASHRAE publications of the 1920s and 1930s.  Rose attributed the application of diffusion theory to insulated buildings and the development of the concept of a vapour barrier to Rowley in the period between the 1930s and World War II.  In Canada, research into vapour diffusion control by the National Research Council extends back to the 1940s [5, 6].  The well accepted principles of diffusion theory show that the diffusion resistance required to control condensation in cold climate building assemblies depends upon: the outdoor temperature, indoor temperature and humidity; and the location, vapour permeance and thermal resistance of other materials in the envelope assembly.  There is a great deal of history, empirical evidence and analysis that show that the diffusion resistance required by Canadian codes (in most cases 45 ng/(Pa s·m²) and 15 ng/(Pa s·m²) in some special cases), provides more than sufficient control of diffusion in typical building envelope assemblies exposed to typical Canadian environments.  In all but a few situations, the higher level of diffusion resistance provided by a polyethylene sheet (about 3 ng/(Pa s·m²is) not necessary to adequately control vapour transport by diffusion.

However, even in its early research into condensation control, the National Research Council (NRC) recognized that air movement was an important moisture transport mechanism and that had to be controlled in the building envelope [6].  It is important to consider air leakage control because convective transport of water vapour, with even small amounts of air leakage, is much larger than diffusion through painted gypsum wallboard.  In fact, the benefit of the diffusion control provided by a vapour barrier can be bypassed if there is significant air leakage though the assembly.  The requirement for “sealed vapour barriers” in Canadian codes from the 1950s to the 1990s reflected that desire to control air movement in envelope assemblies [7].  The requirements for “sealed vapour barriers” also resulted in decades of confusion for “design professional” regarding the function and requirements of air and vapour barriers.  While researchers and more sophisticated professionals recognized that control of airflow and control of vapour diffusion were separate functions that could be performed by separate materials, the concept of a single “air/vapour barrier” of a low permeance material became a part of the conceptual base of most Canadian building professionals.  It remains so even after revisions to the 1985 (and subsequently the 1990) National Building Code clarified the separate and different requirements for air barriers and vapour barriers in Canadian construction. 

The intentional sealing of the polyethylene as the primary component of the air barrier system in very tight wood frame buildings evolved through research projects such as Saskatchewan House in the 1970s through the R-2000 Program in the 1980s, and is now part of common construction practice in Canada.  Even where there is no specific intent to seal the polyethylene, polyethylene can make a significant difference to air movement through a building assembly simply because it is installed as large sheets of air impermeable material clamped between elements that provide support. 

When advocating the deletion of the polyethylene vapour barriers in a wall system, it is prudent to recognize that not only will there be greater moisture transport by vapour diffusion but there will likely be a coincident, and probably greater, increase in moisture transport by convection. 

Limits to The Potential for Drying to the Inside

The merit of using a low permeance layer inside the insulation depends on the balance between the potential for accumulating moisture transported from the interior environment and the potential for drying (to the inside) of any moisture collected in the envelope from any source.  The hypothesis supporting deletion of low permeance vapour barriers in temperate climates is that the increased potential to dry accumulated moisture to the inside in warmer months overwhelms any benefit of reduced accumulation from interior sources in colder months.  It would then follow that this would provide some forgiveness of moisture entry from exterior sources.  This hypothesis needs to be tested.

There are several principles that define the need for, and requirements of, a vapour barrier:

·  A vapour barrier is only really required where the envelope assembly separates two environments with different vapour pressures or dew point temperatures, and where the dew point temperature of the high vapour pressure environment is, at times, higher than the temperatures of surfaces within the envelope assembly, thus giving the potential for condensation;

·  The vapour barrier needs to be placed where it will usually be warmer than the dew point temperature of the high vapour pressure environment, i.e., toward the high vapour pressure side of the insulation (the element with the largest temperature difference);

·   Air movement across the vapour barrier is controlled (or else any environmental separation is effectively bypassed), and

·   Surfaces on the outside of the low vapour pressure side of the vapour barrier must have a higher permeance than the vapour barrier. 

It is important to note that, technically speaking, the location of the vapour barrier should be defined by the vapour pressure or the mathematically related dew point temperature, rather than the dry bulb temperature. Furthermore, the diffusion resistance required for the vapour barrier is relative to the diffusion resistance of other materials in the assembly unless the resistance is so high that the quantity of moisture that can pass through it is insignificant.  This latter point is one reason that low permeance vapour barriers have been considered attractive. In order to evaluate the potential for drying to the inside, it is necessary to understand some of the relationships between indoor environments, outdoor environments, and environments inside an envelope assembly.  This is generally easier to visualize if one considers the dew point temperature of the air masses involved rather than vapour pressure.  Figure 1 identifies some of the key parameters.

_________________Int. Sheathing Temp (T_IS)

Ext. Sheathing Temp (T_ES)______

Outdoor                                                                                   Indoor

 Temp (T_O)                                                                                 Temp (T_I)

 Dew Point Temp. (DP_O)                                                 Dew Point Temp. (DP_I)

Figure 1   Insulated Cavity Wall

In occupied buildings there are many processes that add moisture to the indoor environment.  Without air conditioning or mechanical dehumidification, the average dew point temperature of the indoor environment, DP_I, will almost always be higher than outdoors, DP_O, regardless of season. 

Note that when an air conditioning system is in operation, it can be designed to condense out enough moisture to reduce the indoor vapour pressure to below outdoor conditions, but this should not be assumed.  In order for an air conditioning system to effectively dehumidify over a range of operating conditions, it requires variable cooling capacity or a reheat system.  These are not the norm in residential applications.  Mechanical systems that are designed to dehumidify are available and effective, at least for humidity levels down to about 50% RH.  They are, however, not common enough that one would want to design a residential building assuming their use.

Even in the hottest and most humid climates in Canada, DP_I will be higher than DP_O for the vast majority of the year.  This is especially true for residential buildings in the coastal climates of British Columbia, where air conditioning is rarely used. 

With highly air and vapour permeable insulation such as glass fiber batts, the dew point temperature of the air in the cavity, DP_CAV will be the same across the cavity.  On the warm side of the cavity the relative humidity will be lower, tending to dry materials on that side of the insulation. On the cold side the relative humidity is higher, tending to add moisture to materials on the cold side the insulation. 

The direction of moisture drive across the interior and exterior sheathing layers will be from the region of higher dew point temperature to the area of lower dew point.  The rate of moisture transport between environments depends on the magnitude of driving forces, which are a function of dew point differential (or vapour pressure difference), and the permeance and diffusivity of materials separating the assembly from the interior and exterior environment. 

Given that DP_I is most often higher than DP_O in Canadian climates, it follows that the direction of vapour pressure drive across the interior sheathing is most often from the interior space into the envelope, unless there is some accumulated moisture in the envelope materials from past condensation or moisture ingress from exterior sources.   

Even if there is accumulated moisture in the assembly, the potential for drying to the inside is limited. Consider Figure 1.  Even if there are saturated materials in the wall, the dew point temperature of air in the cavity will be governed by the temperature of the coldest surface of the cavity.  In cold weather, the surface temperature of the exterior sheathing, T_ES, governs.  Table 1 provides the calculated conditions for three temperature conditions assuming; an indoor moisture source rate of 6 kg/day, a ventilation rate of 0.3 Air Changes per Hour in an 80 m² apartment suite (1694 kg of air exchanged per day).  We have assumed that the exterior sheathing temperature is above the outdoor temperature by 10% of the indoor/outdoor temperature difference and the outdoor RH is 85% (which is the mean monthly relative humidity in Vancouver).

The Humidity Ratio (HR) of the indoor air can be calculated as:

6 (kg_w/day) / 1694 (kg_a/day) + HR of outdoor air (kg_w/kg_a). 

Table 1 - Psychrometric Relationships of Environments with a Saturated Wall

With these relationships, DP_I is above DP_CAV.  Moisture transport will be from the interior space into the wall.  In warmer weather, the cooler of the interior or the exterior sheathing temperature governs (DP_CAV).  DP_CAV may be higher than the DP_I, but not by much.  With such low vapour pressure drive, even a layer of paint provides enough diffusion resistance to greatly limit the mass of moisture that could be “dried to the inside”.  Furthermore the drying potential is decreased over time as layers of paint are added. 

We submit that relying on drying to the inside implies a reliance on mechanical dehumidification and control over what paints and finishes are used throughout the life of the building.

Protecting the Interior Sheathing

A good reason to be cautious about deleting a polyethylene vapour barrier and relying on drying to the inside of accumulated cavity water is that the most commonly used interior sheathing material, paper-faced gypsum board, is very sensitive to moisture exposure.  Exposure to high humidity or relatively small quantities of liquid water creates conditions that often lead to mold growth on the paper facing.  We suggest that exposure of the interior gypsum to the same high cavity humidity levels, necessary to drive vapour transport through painted gypsum board, is sufficient to support mold growth on the paper of the gypsum board.

It would be somewhat naïve to assume that if moisture is to migrate inward through the gypsum board, that it will not increase the moisture content to a level that will increase the potential for mold growth or physical degradation.  This is particularly true because painted gypsum board is not homogenous across its cross-section—it has a vapour resistant paint layer on the inside.  The moisture content of materials on the high vapour pressure side of this layer will be elevated.

The presence of a polyethylene sheet outside of the gypsum board can play a major role in the protection of this interior paper layer from incidental rain penetration, until it can dry to the outside.  Photos 1 to 9 provide a graphic display of this benefit.  Photos 1 to 3 are of wood framed walls and Photos 4 and 5 are of light steel framed walls in apartment buildings in the Vancouver region.  These buildings have all suffered from premature envelope failure due to rainwater intrusion.  In these buildings, water has accumulated in sufficient quantity and for sufficient time that the exterior sheathing and frame materials have suffered major degradation.  However, the gypsum board finish, protected by the polyethylene vapour barrier, is in good condition.  Photos 5 to 9 are of an apartment building in Seattle that has also suffered from rainwater penetration.  In this case, the framing materials are less degraded but there is widespread mold growth on the outside face of the interior gypsum sheathing—sheathing which was not protected by a polyethylene vapour barrier. 

One practical benefit of this protective function is that in the envelope rehabilitation projects in Vancouver, it is usually possible to repair the envelope from the outside without breaching the “barrier” created by the interior sheathing. 

HygIRC Simulation

In order to explore the impact of deleting polyethylene in a wall system in Vancouver’s climate, NRC’s advanced hygrothermal model, hygIRC, was used to simulate the performance of a wall assembly. 

The wall assembly modeled was a simple, undrained, stucco clad wall, one floor high (2.4m).  Figure 2 shows the base case that consisted of:

·         19 mm acrylic stucco

·         sheathing paper

·         11 mm OSB sheathing

·         38 x 89 mm wood framed wall (spruce) with

·         89 mm glass fibre insulation (11 kg/m³ density)

·         0.15 mm (6 mil) polyethylene sheet (base case only)

·         12 mm gypsum wall board

·         interior paint

Figure 2   Modeled Wall System

The basic input material properties were taken from the Institute for Research Construction’s material database, developed over the years.

The tested variable was the water vapour permeance of the interior assembly.  Six variations, as identified in Table 2, were modeled which were based on properties of four different types of paint, no paint or vapour barrier, and a polyethylene vapour barrier without paint.

Weather data for HygIRC consisted of hourly temperature, relative humidity, wind velocity and direction, global and diffuse radiation, rain and cloud index.  HygIRC uses rain and wind data to calculate the amount of rain that will hit a vertical surface and uses radiation and cloud index data to calculate radiant heat transfer between the wall and the surrounding sky.

The indoor boundary conditions assumed no mechanical dehumidification and recognized that the indoor humidity would vary with outdoor conditions.  Indoor temperature was kept constant at 21^oC.  Indoor dewpoint temperature was set at 3^oC above the outdoor temperature, but bounded by upper and lower limits of 70% and 30% RH (dew point temperature of 15^oC and 3^oC respectively).  Figure 3 shows the simulated indoor RH condition. 

Table 2 - Modeled Finishes

* Vapour Diffusion Thickness

Boundary Conditions

In all cases, an east-facing orientation was modeled. 

The same characteristics affecting air leakage were modeled for all simulation runs.  The outward air driving force was 10 Pa, plus the stack force over the sample height.  In these cases, air leakage was near zero because only air permeance was simulated (no hole in sheet materials was assumed). 

The initial temperature and moisture distributions across the wall were set by performing a simulation using weather data for an average year (1969) for Vancouver.  The boundary conditions used for the actual simulations were weather parameters for a wet year in Vancouver (1980, which had 1,213 mm of rain).

Figure 3   Simulated Indoor Humidity Profile

Figure 4   Simulated Rain Source to Wall, for Case 3

Cases

All six wall systems were modeled for three cases:

1.       No direct moisture source into the wall.  This is the reference case assuming no defects allowing rain penetration into the wall.  Moisture can be transported into the wall by diffusion, capillary suction, or air movement through materials.

2.       Assuming that a 240 mm high section of the OSB sheathing at the height 1.8 m (Area A on Figure 1) above the base of the wall is initially wetted to a fully saturated condition (34% Moisture Content). 

3.       Assuming that a defect allows water to enter and be absorbed into Area A each time the wall is subjected to wind driven rain.  The magnitude of the rain penetration depends on the amount of rain and wind pressure.  The pattern and quantity of water entry is shown in Figure 4.  The rate of water entry into the wall was based on a correlation developed by Mike Lacasse using the DWT (Dynamic Wall Testing) facility at NRC/IRC.  The entry rate ranged up to 58 gram/hr and the total annual moisture entry was 28.4 kg.  This represents 2.3% of the rain falling on a horizontal surface of the same size as the simulated wall. 

Output and Observations

Figures 5, 6 and 7 show output curves of the moisture absorbed into the sheets (2.4m high per m width) of OSB exterior sheathing and gypsum board interior sheathing for cases 1, 2 and 3 respectively.  The curves are based on calendar years. 

For all the OSB curves, we have provided a reference line at 3.1 kg/m width, which translates to 20% Moisture Content.  When OSB is maintained at moisture content in excess of 20% for prolonged periods in warm weather, decay and mold growth are likely to occur. 

For gypsum board, very small increases in moisture content can lead to mold growth on the paper facings.  NRC’s database of material properties shows that the equilibrium moisture content of gypsum board varies from 8.5% to 12.8% when exposed to air at 30% and 95% RH environments respectively.  We have provided a reference line on Figures 5, 6, and 7 at 2.6 kg/m width, which represents the equilibrium moisture content at 80% RH. 

Figure 5 shows that when there is no direct water penetration to the exterior sheathing (Case 1), the moisture content of the OSB varies over the year.  The moisture content in winter is higher than summer but, in all cases, the lower the permeance of the inside layer (paint or polyethylene), the lower the moisture content of the OSB.  This shows that diffusion of indoor humidity to cold exterior surfaces has an effect.  However, note that there are upward steps in absorbed moisture.  These are associated with wind driven rain events, which wet the stucco cladding.  Moisture is transported to the OSB sheathing by

Figure 5   Output for Case 1

Figure 6   Output for Case 2

Figure 7   Output for Case 3

capillary and vapour diffusion.  Note that this source of moisture is also strongest in winter in Vancouver.  The simulated moisture content of the high permeance finishes exceeded 20% in winter but all modeled cases dried in summer.

In this reference case (Case 1), the moisture content of the gypsum board varies only slightly over the year and is higher in the summer.  Permeance of the interior elements has little effect.

The one time addition of moisture into the sheathing simulated in Case 2 did not produce a major difference from the response of Case 1.  Comparing Figure 6 to Figure 5, one can see that the moisture content of the OSB sheathing was initially higher but it dried so that conditions at the end of the simulated year were not significantly different from Case 1.  Note that again, the lower the permeance of the inside surface, the lower the moisture content of the OSB.  Again permeance of the inside surface had little effect on moisture content of the interior gypsum board.

Figure 7, the output of Case 3 with a periodic moisture entry, provides results that are more interesting.  As could be expected with the high moisture entry rate, the simulation output indicates that the moisture content of the OSB reaches and maintains much higher levels.  Furthermore, the annual trend was upward.  The assumption of high permeance materials on the inside had a positive effect on the summer moisture content of the OSB.  However, when the assumed permeance of the paint film was below 500 ng/(m²sPa), the difference was minor and, even assuming unfinished gypsum board, did not control moisture content in the winter period. 

The output curves of moisture content of the gypsum board in Figure 7 are also of note.  The simulation indicated that moisture content of the gypsum board was controlled in two dramatically different cases; either when a polyethylene vapour barrier protected the gypsum board, or when there was no polyethylene and the gypsum board was unfinished (the “No Poly” case).  If a painted finish was assumed, then the amount of moisture accumulating in the gypsum board was inversely proportional to the permeance of the paint finish.

The results of the simulation exercise do not, in our opinion, endorse the strategy of deleting polyethylene and assuming drying to the inside.  Without the polyethylene, the moisture content of the exterior sheeting is generally higher except in the where there is periodic rain penetration (Case 3).  Any benefits of more rapid drying when there is a major source of rain entry will be compromised as paint layers are added over time.  The potential benefit also comes with an increased risk of moisture accumulation and mold growth on the interior gypsum sheathing. 

Protection from Solar Driven Vapour

Straube suggests that another reason to exclude a polyethylene vapour barrier from wall systems is to avoid moisture collection in the wall under the influence of solar driven inward vapour diffusion [3].  Straube used the Glazier method to show that, under certain conditions, solar heating of wet cladding materials can create a high inward vapour pressure drive that can lead to condensation on the outside face of the interior polyethylene, and that the rate of moisture deposition on the polyethylene could, at least temporarily, be much higher than cold weather condensation of vapour from interior sources on the exterior sheathing.  Straube suggests that deleting the polyethylene would avoid accumulation of this moisture by allowing the moisture to pass through the interior finish to the indoor space. 

This hypothesis also needs to be tested.  We note that if the polyethylene were eliminated, nearly the same quantity of moisture would be deposited and absorbed into the gypsum board instead of being deposited at the polyethylene.  Since there is a moisture resistant paint layer on the inside face of the gypsum board, the moisture content of the gypsum board would be elevated, and this could lead to mold growth on the paper face.  We suggest that in an environment where the predominant drying potential is to the outdoors, but with periodic solar driven, inward vapour drive, the presence of a polyethylene vapour barrier inside the insulation actually provides the benefit of protecting the moisture sensitive gypsum board against temporary exposure to a moisture source.  Deleting the polyethylene would increase the potential for mold growth. 

Conclusions

Before evaluating alternative vapour control strategies for insulated frame wall systems, it is necessary to acknowledge that acceptable moisture performance depends upon keeping precipitation out of the walls.  This is accomplished by providing a cladding system and interface details that ensure that any water that penetrates the cladding drains or dries to the outside without penetrating into the sheathing layer.  When assuming this first line of moisture management, optimizing the vapour control strategy then involves creating a favorable balance between the wetting and drying mechanisms to disperse seasonal accumulations and, perhaps, some small fraction of the water that may impact the wall.

Our analysis focussed on walls operating in the coastal climate of British Columbia and the northwestern United States, where the vapour pressure drive is from predominately inside to outside for the vast majority of year.  In this climate, residential buildings require a significant level of vapour diffusion resistance on the interior side of the insulation to control winter moisture accumulation.  However, the diffusion control element does not need to have the vapour resistance of polyethylene.  Nevertheless, this does not necessarily mean that eliminating the low permeance polyethylene vapour barrier reduces the risk of moisture problems. 

We are not convinced that eliminating the low permeance vapour barrier to maximize drying to the inside is a rational design principle for temperate climates.  In the absence of mechanical dehumidification, increasing the permeance of the vapour barrier results in increased winter moisture content of materials in the wall assembly.  There is limited drying potential to the inside if one assumes a painted interior finish.  Consequently, removing the low permeance vapour barrier may not significantly improve the wall’s moisture performance.  In high indoor humidity conditions, it will worsen it.

Furthermore, eliminating the low permeance vapour barrier may have some associated negative effects.  Increased airflow, and the moisture it carries, into the building envelope could multiply winter moisture accumulation.  Without the impermeable layer there is also an increased risk of periodic moisture accumulation and mold growth on paper-faced gypsum board.

Finally, we suggest that relying on drying to the inside to accommodate incidental rain ingress is not prudent.  To do so requires a juxtaposition of three things:

·         mechanical dehumidification

·         control of interior wall finishes used – both initially and over the life of the building, and

·         interior sheathings that are much less susceptible to mold growth than paper-faced gypsum board

References

[1]        National Building Code of Canada, 1995.

[2]        “Historic Summit Meeting Looks at Vapor Barrier” Energy Design Update Vol. 22, No. 8, Pgs 1-3.

[3]        Straube J.F., “The Influence of Low-Permeance Vapor Barriers on Roof and Wall Performance”, Conference Proceedings Performance of Exterior Envelopes of Whole Buildings VIII, December 2001.

[4]        Rose, W. 1997. “Control of Moisture in the Modern Building Envelope: The History of the Vapor Barrier in the United States” 1923-1952. APTBulletin, Vol. XVIII. No. 4, October 1997 (Association for Preservation Technology).

[5]        Handegord, G.O., CBD-9. “Vapour Barriers in Home Construction” National Research Council of Canada, 1960.

[6]        Wilson, A. G., CBD-23. “Air Leakage in Buildings” National Research Council of Canada, 1961.

[7]        Personal Communication with NRC officials, including A.G.Wilson.

About the Authors

Mark Lawton, P.Eng. is a Principal and Building Science Specialist with Morrison Hershfield Limited, Vancouver Office.

William Brown, P.Eng. is acting Technical  Director for Building Science with Morrison Hershfield Limited, Ottawa Office.

“This article was published as part of the Proceedings produced for the Ninth Canadian Conference on Building Science and Technology “Design and Construction of Durable Building Envelopes” held February 27 & 28, 2003 in Vancouver, British Columbia, Canada. 

The Conference was presented by the National Building Envelope Council (NBEC) and hosted by the British Columbia Building Envelope Council (BCBEC).

The NBEC will be presenting the 11th Canadian Conference on Building Science and Technology.  The Conference will be hosted by the Alberta Building Envelope Council (ABEC) March 22 and 23 in Banff, Alberta, Canada.”