by M.T. Bomberg and W.C. Brown

M.T.Bomberg and and Brown are senior research officers in the Building Performance Laboratory of the Institute for Research in Construction.

Originally published in "Construction Canada" 35(1), 1993, p. 15-18

Heat, air and moisture transport across a building envelope are inseparable phenomena. Each influences the others and is influenced by all the materials contained within the building envelope.

Often we simplify the process of architectural design by relating control of each phenomenon to a particular material. The thermal insulation, for example, is perceived to control heat transfer and the air barrier to control air leakage. Likewise, the rain screen and the vapour barrier eliminate ingress of moisture to materials. However, these materials perform many different and interrelated functions, and frequently participate as one of several factors in overall system performance (see Table 1). For instance, while controlling air leakage, the air- barrier system may also provide effective moisture control. Similarly, by increasing temperature in the wall cavity, thermal insulating sheathing may also reduce the degree of condensation in the cavity.

Thus the process of environmental control depends on strong interactions between heat, air and moisture transport. And to ensure that all aspects of the building envelope perform effectively, we must deal with heat, air and moisture transport collectively. In some ways, this approach is similar to the thinking of 60 years ago, long before detailed analyses were routine. The difference today centres on improved standards and requirements concerning performance of the individual elements that make up the building envelope. So, while we preserve the basic approach of the past, we are now better able to apply the fundamental concepts first introduced in the 1930s.

Table 1.Environmental barriers and driving forces

This article is the first of three that will address environmental control through design of the building envelope. Part 2 will review issues relating to materials. Part 3 will discuss systems issues.

Primarily, the building envelope provides shelter from the outdoor environment and encloses a comfortable indoor space. In doing so, the envelope must withstand many mechanical and environmental forces and this durability must extend over its service life. In response to climatic extremes, for instance the Canadian cold, the envelope must also be well in sulated provide the required level of thermal comfort at a reasonable cost.

Air Transport - a Lesson from History

Air transport represents a critical factor in environmental control. It underscores virtually all facets of environmental control as it also moves both heat and moisture through the building envelope.

Our understanding of the performance of walls comes mainly from the Prairie regions of North America where the climatic extremes magnify any faults in the ability of building envelopes to maintain environmental control. The pioneering work, carried out by University of Minnesota researchers between 1929 and 1932 on air leakage through frame walls, led to acceptance of the building paper weather barrier. Building paper impeded the movement of air and rain while permitting moisture to breath to the outdoors. In addition, building paper reduced heat losses by limiting air leakage, improved indoor comfort by reducing drafts, and reduced moisture damage to the walls by preventing the windwashing which decreases the inner surface temperature.

Later, in the quest for indoor thermal comfort, wall cavities were filled with insulation - first wood chips stabilized with lime, then shredded newsprint, and eventually mineral-fibre batts. Although water vapour passed through the thermal insulation as easily as through the air layer, the insulation lowered the temperature of the exterior sheathing and condensation appeared.

This situation led to the introduction of vapour barriers to control the flow of vapour from warmer indoor environments. Consequently, the walls of homes built as early as the 1940s already included the outside weather barrier and the inside vapour barrier.

In Canada, vapour barrier has become synonymous with polyethylene sheets, although building codes permit many solutions. One of them is a double-painted drywall in which the paint fulfils the vapour barrier requirement. And the 0.15 mm polyethylene sheathing that controls air leakage in houses functions as a vapour barrier as well.

Moisture Effects - Material Durability

In order for the building envelope to perform its role of separating the interior and exterior environments, it must have structural integrity and durability. Of particular importance is the prevention of moisture damage; of all environmental conditions, moisture poses the biggest threat to integrity and durability, accounting for up to 80% of damage in building envelopes.

Certainly, many construction materials contain moisture, most notably, masonry or concrete. These materials demonstrate excellent performance characteristics as long as the moisture does not compromise the structural or physical integrity. However, excessive moisture jeopardizes both the material and its ability to function.

Consider, for example, the ability of a material to withstand, without deterioration, natural periods of freezing and thawing. This so-called frost durability is not a material characteristic but a complex property that varies with both the material and the environment. For instance, in one school building, only the outer surface of the exterior clay-brick protrusions showed freeze-thaw spelling. These protrusions were more exposed to driving rains and the surface temperature of the bricks was slightly lower, compared to the plain facade where no spelling occurred. Both of these conditions typically result in freeze-thaw damage.

Corrosion of metals exposed to air similarly varies with surface temperature and humidity. Likewise, mould growth requires certain temperatures and humidities (temperatures above 5° C and relative humidity above 80%).

Heat Energy - Dynamic Performance

Assessing the thermal performance of the building envelope involves three considerations:

• the quantity of heat transferred through the walls, windows and other elements of the building envelope the conductive heat transfer;
• the quantity of heat needed to bring the temperature of the outdoor air to that of the indoor air-- the air-leakage characteristics or air exchange rate;
• the differences in temperatures on the inner surface of the building envelope - the mould and mildew control points.

Conductive heat transfer may be represented in four different manners, each with increasing precision. The first approximation considers only the plain, insulated areas of the envelope, ignoring the multi-directional heat flows caused by thermal anomalies. So a frame wall insulated with RSI 3.5 glass fibre batts is called an RSI 3.5 wall.

The second level of accuracy additionally considers how the actual thermal resistance of the wall differs from the one-dimensional flow model. Thus, the RSI 3.5 wall now becomes an RSI 3.1 wall.

The third level of accuracy adds two- or three dimensional calculations of heat flows, while assuming that the steady-state representation sufficiently describes the thermal performance of the building. The fourth level incorporates transient weather conditions into the calculations of thermal performance.

These last two levels of accuracy underlie the European trend to differentiate between the declared and design values when assessing the thermal characteristics of building materials. The declared value represents the expected thermal performance as measured at a reference temperature and thickness and stated with a certain confidence. The design value describes the performance under certain climate and use conditions.

The second component of energy performance -- air leakage relates to the rate of air flowing through the building envelope. This component is directly proportional to air-pressure differences across the envelope and inversely proportional to the airflow resistance of the building envelope.

The final component of thermal/energy performance valuation relates to the water vapour condensation on the surface of thermal bridges in the building envelope. At these locations, lower thermal resistance reduces the surface temperature. As a rule, surface condensation does not plague wood-frame walls, provided the wall cavity is completely insulated and air leakage is not significant.

Conversely, floor junctions in masonry construction and concrete decks connected to balconies do experience problems with condensation because of significant reductions in surface temperature. If a low rate of convective air movement accompanies these surface temperature reductions, then mould and mildew in bathrooms and closets, and deterioration of drywall in staircases can be expected.

Design for Environmental Control

Accommodating environmental control in building design requires iterative analysis and a willingness to change not only minor details, but to alter the basic concept itself if information indicates that this is desirable. Thus, the design must remain as flexible as possible until all the consequences are fully examined.

The design of an air-barrier system offers an example of how the process of iterative design might work. The information flow may start with a search for suitable materials. Typical questions are asked about possible materials and their air permeability, their ability to be extended; their pliability and adhesion; and their means of attachment, connection and support. The review would also address the long term performance, material aging, stress and deformations during service, as well as projected costs of repairs and maintenance.

After making an initial selection, the designer then specifies the architectural details such as intersections and joints between building elements (for example, foundations, walls, floors, windows and doors). Then, to achieve satisfactory performance in these locations, the designer must ask further questions concerning the performance of the whole system questions about the rate of air leakage, location of leakage, risk of drafts and impact on condensation. Throughout the design process, the designer consults with structural, electrical and mechanical experts to obtain answers to all these questions and to ensure that the selected materials will perform satisfactorily.

In addition, the designer reviews the buildability aspects such as material installation under different weather conditions, level of labour skill required for installation, and construction tolerance. Buildability, as the word suggests, reflects whether the design on paper can be constructed.

Finally, the complexity of heat, air and moisture interactions demands redundancy in the design. For instance, the air-barrier plane may be punctured, or not connected to some elements of the envelope, or a rain leak may develop. The designer must evaluate how the moisture could be drained, or if not drained, whether it could be dried out. How long would the drying take and what effect would it have on other materials? Could the prolonged presence of moisture cause corrosion, mould growth or rot?

The entire process of environmental-control design must occur off-site, and never at the building site. Addressing only a specific design problem on the job site, without reviewing all the performance effects, courts disaster since integration of other requirements may not be achieved.

Addressing the Duality

In designing for environmental control, professionals integrate two very different conceptual processes. One involves specific testing and analysis; the other encompasses broad qualitative assessments based on experience, judgment and knowledge of what makes a building envelope function.

On the analytical side is a complex array of tools, models and data which describe the material, structural and environmental factors relating to the building envelope. On the qualitative side is a sense of how a particular building envelope would function.

For example, a vapour barrier is typically classified at one perm, a unit that represents sufficient retardation of water vapour flow for wood-frame housing. However, if calculations were made using a complex model of heat, air and moisture transport for various climatic conditions in Canada, barriers ranging from 0.1 to 10 perms would be found suitable for various combinations of materials and climates. Similarly, some materials that qualify as vapour barriers may be ineffective when wet.

So, despite the move to establish a precise measurement to define vapour barriers, the selection of the most appropriate barrier involves both conceptual logic and mathematical analysis. Designers must still conduct an overall qualitative assessment to determine whether the barrier, chosen for its quantitative properties, would actually function in the specific application.

This is a strategy reminiscent of 60 years ago when builders took an holistic approach to performance. But it differs in one important area. In the past, this approach reflected a time of limited knowledge, less-demanding performance requirements and few analytical methodologies. Designers today understand and consolidate complex analysis and the lessons of experience. The net result is a building envelope designed for environmental control -- a building envelope that works.