Air & Vapour barrier design
Moisture problems in walls have been attributed in large measure to two mechanisms: vapour diffusion, and now more importantly, air leakage, specifically the deposition of moisture by moist air exfiltrating through the building envelope.
It is common architectural practice to specify a vapour barrier. Sometimes an air-vapour barrier will be
specified, and on rare occasions the term "air barrier" may be used. This shows a lack of consistency in the use of these terms but, more important, shown a confusion in the industry about the design, materials and methods of construction of the fundamental components of the building envelope. However, the basic confusion is with the functions of these different barriers.
A 4mm polyethylene sheet will make a good quality vapour barrier and twelve inches of cast-in-place concrete
will make a good quality air barrier. This would be too restrictive a definition, but the functions of these two
barriers are as different as polyethylene and concrete.
In practice many materials can be used as effective vapour barriers and many may be effective air barriers. The choice of materials or combination thereof and the method of assembly, are dependent on the function.
A clear understmding of the functions of the two types of barriers is imperative to good design, and to effective and predictable building envelope performance.
VAPOUR DIFFUSION CONTROL (THE VAPOUR BARRIER)
The principal function of a vapour barrier is to stop or, more accurately, to prevent the passage of moisture as it
diffuses through the assembly of materials in a wall.
Diffusion is the process by which water vapour migrates through a material. The rate at which water vapour migrates or diffuses depends on two factors: the difference between the water vapour pressure in the air inside the
building and that in the outside air, and the resistence that materials present to the migration of moisture by diffusion.
All materials have a resistance to moisture diffusion, some more than others. Water vapour migrates through air,
polyethylene film, fibre board, concrete and many other materials, but at varying rates. A vapour barrier is a material that offers a higher resistance to the diffusion of water vapour than most other materials. It is usually placed on the warm side of the insulation. Polyethylene film of sufficient thickness is the material most commonly used for this purpose; however, other materials such as aluminium foil, some paint types, metal, glass or even concrete of a suitable thickness may also be suitable.
For the vapour barrier to control condensation resulting from vapour diffusion it must be placed on or near the warm side of the insulation, which is normally the high vapour pressure side. Contrary to popular belief, a
vapour barrier need not be perfectly continuous. Unsealed laps and pin holes, minor cuts, etc., do not increase the overall moisture diffusion rate into a wall or roof cavity appreciably. It is worthwhile, however, to avoid these
imperfections if possible.
AIR LEAKAGE CONTROL (THE AIR BARRIER)
Water vapour diffusion is only one of the mechanisms by which water can be transported into a wall or roof cavity.
The provision of a vapour barrier within the wall or roof assembly satisfies only part of the requiement of controlling moisture entry into building enclosures, the other mechanism, which is now considered to be far more significant, is air leakage.
Air leakage occurs when openings (holes, cracks, etc.) in the building envelope form a continuous path from inside to outside and an air pressure difference occurs across it. Both mechanisms may, of course, operate at the same time. The principal function of the air barrier is to stop outside air from entering the building through the walls, windows or roof, and inside air from exfiltrating through the building envelope to the outside. This applies whether the air is humid or dry, since air leakage can result in problems other than the deposition of moisture in cavities. Exfiltrating air carries away heating and cooling energy, while incoming air
may bring in pollution as well as disable a rain screen wall system.
Moisture-laden air passing through an insulated cavity with a vapour barrier may deposit much more moisture
than would diffuse through the vapour barrier in the same period of time.
Air Leakage Paths
Holes or openings through the envelope can take maay forms, for example: cracks or joints between infill components and structural elements, or poor connections between the wall and the roof and at openings for building services. They may occur even in porous materials, for example concrete block, high density glass fibre, open cell polystyrene insulation, and fibreboard. Some openings follow direct channels from inside to outside, as in an uncapped steel deck at the perimeter of a floor or ceiling. Others may appear on an inside surface finish (behind a radiator cabinet or above a suspended ceiling), leading into a wall cavity and on through holes at some other location in the outside finish, or as weep or vent holes in brickwork. Same openings may develop after construction because of differential brick or block movement, caused by shrinkage of mortar, thermal expansion
and contraction of building elements, or deflection of beams. Other openings may result from an inappropriate choice of joint materials.
AIR BARRIER DESIGN REQUIREMENTS
Materials and the method of assembly chosen to build an air barrier must meet several requirements if they are to perform the air leakage control function successfully.
1. There must be continuity throughout the building envelope. The air barrier material of the wall must be continuous with the air barrier material of the roof (e-g., the roofing membrane). The air barrier material of the wall must be connected to the air barrier material of the window, etc
2. The air barrier system must be fastened to a supporting structure to resist a peak wind load, a sustained stack effect or pressurization from ventilation equipment; it must be sufficiently rigid to resist displacement.
a) The materials and configuration of the air barrier assembly must resist the highest expected air pressure load, inward or outward, without rupturing or detaching from the support.
b) The assembly must not creep away from a substrate or part at a joint under a sustained air pressure diflerence (such as stack effect or fan pressurization).
c) The deflection of the air barrier materials between supports must be minimized to prevent the displacement of other materials (such as insulation in cavities).
3. The air barrier system must be virtually air-impermeable. A value for the maximum allowable air permeability should be determined. However, materials such as polyethylene, many single ply roofing membranes, gypsum board, cast-in-place concrete, metal or glass qualify as low air impermeable materials, whereas concrete block, acoustic insulation, open cell polystyrene insulation or fibreboard would not.
4. The air barrier assembly must be durable in the same sense that the building is durable, and be made of materials that are known to have a long service life or be positioned so that it may be serviced from time to time.
AIR BARRIER DESIGN APPLICATIONS
Combined Air and Vapour Barriers
A wall or roof assembly will require an air barrier and possibly also a vapour barrier. They may or may not be the same material. But a combined system must meet the design requirements for both functions
Position of the Air Barrier
A vapour barrier is usually placed on the warm side of the insulation. It may also be positioned part way into the insulation but, for satisfactory performance, it should be no further in than the point at which the temperature of the inside air drops to its dew point.
While it is preferable that the air barrier system be placed on the warm side of an insulated assembly, it is not an essential requirement as it is with the vapour barrier. The position of the air barrier in a wall or roof is more a matter of suitable construction practice and the type of materials to be used. However, if this barrier is positioned on the outside of the insulation, consideration must be given to its water vapour permeability in case it should also act as a barrier to vapour which is on its way out from inside the wall assembly. This situation may be prevented by choosing an air barrier material that is ten to twenty times more permeable to water vapour diffusion than the vapour barrier material.
There are many situations in practice in which the air barrier of a wall or roof is on the outside of the insulation and performs quite satisfactorily. In the case of a typical medium rise apartment building, which has a decorative exposed cast-in-place concrete shear wall, the exterior facade is insulated on the inside. Further, it usually has a 4mm poIyethylene vapour barrier and a gypsum board interior finish. In practice, the castin-place concrete exterior walI is a continuous and structurally adequate air impermeable element of the wall and therefore acts as the air barrier. The vapour barrier is the polyethylene, and so long as it, the insulation and the interior surface of the concrete shear wall are in intimate contact, the wall will perform quite satisfactorily,
as it has in many existing buildings.
If the insulation is not in intimate contact with the concrete, convection within the cavity may seriously alter the thermal performance of the insulation. A prerequisite of any wall design is that the insulation be in intimate contact with the air barrier. But the general opinion among most researchers and practitioners is that the air barrier should be placed the warm side of the insulation, where thermal stresses will be at a minimum. (Inside of the insulation does not necessarily mean on the inside surface of the wall.)
CONCLUSIONS
The function of the vapour barrier is to prevent water vapour diffusion into insulated building envelope assemblies. Vapour diffusion control is simple to achieve and is primarily a function of the water vapour diffusion resistance of the chosen materials and their position within the building envelope assembly.
The barrier should be clearly identified by the designer and clearly identifiable by the builder. Air leakage control is a more complex objective, which must be considered as a separate and distinct function for any wall, roof, window and especially joint detail, and even for the below grade portion of the building envelope.
The function of the air barrier is to eliminate the through flow of air from inside to outside and vice versa. An air barrier must be continuous, be structurally fastened or supported to withstand a peak air pressure load, and be virtually impermeable to the passage of air.
It must be durable or easily serviced. Again, it is important that the designer clearly identify the air barrier and that the builder recognizes it as such.
It is desirable, but not essential, that it be installed on the warm side of the insulation, so that thermal stability will be improved and access for future maintenance be assisted. The vapour and air barriers may or may not be the same material; if they are the same material, then it must meet all the requirements of airtightness control and of vapour diffusion control.
If the barriers are not the same material, then the vapour barrier material need only meet the requirements for vapour diffusion control and the materials designated for the air barrier need only meet the requirements for air leakage control.