Template:All Systems - VAPOUR RETARDERS

1 Intent

Air and vapour barriers, along with thermal barriers, water resistive barriers and water-shedding surfaces, serve to separate the outside environment from the interior environments of a structure. Continuous air barriers are perhaps the most critical. Building Codes in force in each jurisdiction, and the National Energy Code (2011), require the selection and proper installation of “a continuous air barrier system comprised of air-barrier assemblies to control air leakage into and out of the conditioned space” (NEC 2011).

Continuity of the air and/or vapour barrier from the wall systems and roof systems is essential to the satisfactory performance of either or both. Therefore, proper connection between air and/or vapour barrier systems is essential, and the responsibility of both the design authority and trades constructing walls and roofs.

Air barriers control “flow of air through the building enclosure, either inward or outward” (Guide for Designing Energy Efficient Building Enclosures, Homeowner Protection Office). Controlling air flow into and out of conditioned spaces affects the performance of “thermally efficient enclosure assemblies” (ibid), impacts the potential for condensation in between materials, and directly influences rain water penetration of the building envelope. Some air barriers are considered permeable, others air-impermeable or ‘airtight’. The suitability of one over the other, in the application of a roofing system, is left to the discernment of the design authority and/or the roofing contractor. Consequently, the RoofStar Guarantee Program strongly recommends that designers and builders of roof systems intended to qualify for a RoofStar Guarantee carefully consider the regulatory design and installation requirements for effective, continuous air barrier systems.

Vapour barriers regulate or prohibit the movement of water vapour from one space to another by means of diffusion. Consequently, these barriers are referred to as either vapour-permeable or impermeable. Diffusion is a slow process, in contrast to air movement, and its regulation is not always mandatory or even desirable. Consequently, because continuous vapour barriers “are not needed within all climate zones and assemblies”, they are considered non-critical and may be left to the discretion of the design authority. Nevertheless, where continuous vapour barriers are required and specified by provincial or municipal building codes (current and in force), the RoofStar Guarantee Program requires that a suitable vapour barrier system be selected by the design authority and properly installed by the roofing contractor in conformity with the vapour barrier manufacturer’s published instructions, and with the design authority’s specified details.

Any references in this Manual to installation methodologies, and any construction details that show air and/or vapour barriers, are merely illustrative and not prescriptive. Installers of continuous air and/or vapour barrier systems are urged to understand and comply with best practices for their application.

2 Limitations and Exclusions

Air and vapour barrier performance is not part of the RoofStar Guarantee, and air/vapour barrier materials are not listed in the Accepted Materials section of this Manual. Therefore, the decision to specify an air and/or vapour barrier, the placement of a continuous air and/or vapour barrier in relation to a roof assembly and system, and the selection of suitable materials for that application, is the sole responsibility of the design authority.The design authority is urged to review and consider the performance characteristics of materials available for such applications.
Neither the RoofStar Guarantee Program nor the roofing contractor will accept any responsibility for damage to, or failure of, the roof system caused by the use or absence of air and/or vapour barriers.
Proper installation and continuity of air and/or vapour barriers within the roof assembly is the responsibility of the roofing contractor. The air/vapour barrier must:

extend past the roof membrane flashing by 100mm (4”) on new construction providing a positive (water-shedding) lap seal union between courses of material for the wall air/vapour barrier membrane applicator.
be sealed to the wall air/vapour barrier on roof replacement projects.

Installation must conform to the manufacturer’s published requirements and the design authority’s design details.

3 Material Selection

Fully supported air and/or vapour barriers must possess a minimum published static puncture resistance rating of 150 N (34 lbf) (ref. CGSB-37.56-M for both test method and standard limits) and be either self-adhering or torch-applied. For unsupported air and/or vapour barriers, see 4.3.3 below. Therefore, while responsibility for the selection of a suitable air/vapour barrier rests with the design authority, a roof designed and built to qualify for a RoofStar Guarantee shall not include either polyethylene sheet plastic or bitumen-impregnated kraft paper.
Where air and/or vapour barriers are specified by the design authority, they must be selected from the materials listed in the wind-tested assemblies reports for MARS, PARS or AARS roof systems (excluding polyethylene sheet plastic and kraft paper, as noted above). For a complete listing of current wind test reports, click on the linked system acronyms above.
Notwithstanding any of the foregoing, the RoofStar Guarantee Program strongly recommends that any air and/or vapour barrier system be installed over a smooth, continuous plane (for example, concrete or plywood). Consequently, a deck overlay board installed on corrugated steel roof decks in highly recommended. Where no deck overlay board is installed and the air and/or vapour barrier is partially unsupported (for example, on a steel deck), the air and/or vapour barrier must have a published static puncture resistance of at least 400 N (90 lbf). Furthermore, both the side laps and end laps must be fully supported.
Should the air/vapour barrier membrane be used as a temporary roof during project construction by either the roofing contractor or by other trades, a minimum 2mm thick bituminous membrane is recommended.
Because curing concrete releases considerable moisture that can compromise the performance of a roof system, a vapour barrier installed on new concrete decks (28 days or older) must be selected to prevent condensation inside the roof system. A membrane with a permeability of 0.01 perms (Class I) is recommended. Nevertheless, the selection of the vapour barrier product is the responsibility of the design authority.

4 Vapour Retarder Design for High-Humidity Building Interiors

Careful consideration should be given to the performance characteristics of air and/or vapour barriers when specifying such a membrane for roof assemblies constructed over high-humidity building interiors. These types of building interiors include (but are not limited to)

Swimming pools
Commercial laundry facilities
Large aquariums
Paper mills

Roof systems for facilities such as these, with high-humidity environments, may be susceptible to the accumulation of moisture within the roof assembly unless an effective vapour barrier is specified and installed.

Vapour retarders for high humidity facilities should have a perm rating as close to zero as possible, be durable, multi-layer membranes that are effective in preventing vapour drive into the roof systems from the interior of high humidity environments.

Thin, lightweight, single-layer vapour retarders are prone to damage during installation of roof assemblies and are therefore not recommended for use over high humidity building interiors.