The building envelope is the skin that protects occupants from the constantly changing climate outdoors. To achieve a healthy indoor environment, it is essential this skin be designed for thermal and moisture performance, have a continuous air-barrier system and be mold resistant. Within the Ottawa, Ontario-based Canada Green Building Council’s LEED program, Materials and Resources credit 8 for Durable Buildings encourages these performance objectives. MRc8 contributes 1 point to the building’s LEED rating but more importantly provides a six-step process to prevent building-envelope problems.


Present codes generally do not provide specific requirements for the durability of above-grade building envelopes. Durability failures—mostly hidden inside wall or roof spaces—can be prevented for typically less than $10,000 at the design stage. MRc8 is a preventive approach to achieve higher-performance, durable, healthy building envelopes to benefit our environment.

The origin of MRc8 comes from the $1.5 billion lawsuit, Mary Kimpton v. AG of Canada, Canada Mortgage and Housing Corp., and Province of British Columbia. The Ontario Building Code Commission ruled in July 2004 that a less-than-three-year-old exterior insulation and finish system be completely replaced. To avoid similar suits, MRc8 was established in December 2004, and CaGBC’s Durable Buildings Task Force was created to improve the credit in May 2005.

The first Canadian building to earn the durability credit was Radiance @ Minto Gardens, a Toronto high-rise condominium. During the past two years, CaGBC’s Durable Buildings Task Force has simplified MRc8. As a result, more than 25 percent of buildings seeking LEED Canada certification are striving to achieve the Durable Building Credit.


To earn MRc8, a design team must develop and implement a durability plan based on the Mississauga, Ontario-based Canadian Standards Association S478-95, “Guidelines on Durability in Buildings (R2001),” at the concept, design and construction phases.

The following are procedures that have been created to streamline achieving the LEED durability credit:

1. Determine Durability Targets CaGBC’s LEED sets a target lifespan of 50 years for a structure and building envelope. However a 25- to 50-year design service life can be targeted if the Durability Plan identifies components in a capital or reserve fund to cover replacement.

For example, most owners do not realize that based on code minimums the expected service life for a face-sealed, caulked system on a building is about 10 to 15 years. It is extremely important to determine durability targets at the concept stage to manage expectations.

2. Review Design Details Most building-envelope problems can be prevented on paper with appropriate design details. The review of details by a third party as required by MRc8 will help identify changes that should be made at the design stage and can reduce costs by thousands of dollars.

LEED Canada requires that a qualified building scientist who is employed with an engineering or architecture firm and has experience examining at least two buildings review the documents. The building scientist also must achieve one of the following: 35 hours of instruction, certification from a warranty program or be independent of the architectural firm of record.

The benefits from this type of review process are many. For example, on a recent building project, the cost and problems associated with concrete masonry suspended on steel, which is subject to corrosion, at grade were removed by bearing the masonry on an exterior recess in the foundation wall. This eliminated the steel and provided a natural stop against water entry into the building.

There are numerous details that achieve durable buildings in guides published by third parties, such as the Ontario New Home Warranty Program and Canada Mortgage and Housing Corp., Ottawa. Referenced in the 2005 National Building Code, the ONHWP High-rise Residential Construction Guide’s subsection dealing with durability states: “Designers and builders must be aware of the destructive effect of trapped moisture inside of the wall assembly and design for effective prevention of problems.”

The following are some tips to developing details that work:

• Deflection is the most-effective moisture- control principle because it keeps water off the outside face of the wall.

• Drainage is the second-most-effective moisture-control method because it redirects any water that enters the cladding. It is important to ensure drained water is directed away from the building.

• Wall systems should allow for drying but should not be relied on to the same extent as deflection and drainage.

• Wall and roof assemblies should be constructed of durable materials tolerant of moisture.

3. Durable Materials and Systems Durable materials, including concrete and concrete masonry products and curtainwall systems, provide moisture and mold resistance and contribute to a durable, healthy building envelope. Concrete, masonry and their associated products have the highest rating for mold resistance based on testing by an International Organization for Standardization-accredited independent laboratory in accordance with ASTM International C 1338, “Standard Test Method for Determining Fungi Resistance of Insulation Materials and Facings.”

The bulletin “Fungal Mould Resistance Testing of Common Building Materials According to MIL-STD 810E” and the High-rise Residential Construction Guide can be obtained by e-mailing

4. Declaration

Field review is required to confirm that the construction conforms to the design details. The declaration must be signed by a qualified building-science professional and general contractor. This step is important because substrates sometimes change after initial field reviews. For example, my firm currently is working on a sustainable renewal project outside of Boston where the substrate was not replaced and the entire cladding assembly requires replacement at a cost of more than $1 million.

5. Documentation

New tools have been created to streamline MRc8 documentation, including LEED templates for documenting the design service life of a project. A table covers building design data and another table documents the building assembly, material, design life and capital. By taking a few days to document the durability of the structural and building-envelope components, years of extended service life can be designed into a building and millions of dollars of premature failures can be prevented.

6. Training

Durable Healthy Building Envelope Training workshops have been delivered throughout North America in partnership with green-building chapters and warranty and insurance companies. The workshops contribute three hours toward the professional qualification requirements for MRc8. For information about a workshop near you, contact Diana Swift with The Construction Consortium, Kansas City, Mo., at swiftdia2@ or (816) 931-2278.


Durable envelope design allows owners and tenants to reap the benefits of lower operation costs and a healthier building envelope. By applying the six key principles and using the streamlined process established by the Durable Buildings Task Force, leaks, lawsuits and the environmental impact of premature building failure can be prevented.

>> BOB MARSHALL is the chair of the Ottawa-based Canada Green Building Council’s Durable Building Task Force and a member of CaGBC’s education committee. He is a senior consultant and cladding practice director at the Toronto office of Jacques Whitford Ltd., an international engineering and environmental consulting company. Marshall can be contacted at