Tags: assembly

17 Apr 2009, Comments Off

CFC Reviews CMHC Design Report: GUIDELINES FOR DELIVERING EFFECTIVE AIR BARRIER SYSTEMS – WHAT ARE WE TESTING FOR?

Author: admin

WHAT ARE WE TESTING FOR?

The National Building Code of Canada (NBCC), Part 5, Section 5.4, Subsection 5.4.1.2., stipulates four key requirements for successful air barrier systems: airtightness, continuity, structural integrity and durability.

Air tightness – Subsection 5.4.1.2. Sentence 1 states that “. . . sheet and panel type materials intended to provide the principal resistance to air leakage shall have an air leakage characteristic not greater than 0.02 L/(s·m2) measured at an air pressure difference of 75 Pa.” While there are many commercial air barrier materials that satisfy this requirement, these materials must be joined into a system so that the system is airtight under different indoor environmental conditions. Recommended maximum leakage rates for air barrier systems in exterior envelopes are provided in Appendix A of the NBCC.

Continuity – Subsection 5.4.1.2. Sentence 7 states that “The air barrier system shall be continuous (a) across construction, control and expansion joints, (b) across junctions between different building assemblies, and (c) around penetrations through the building assembly.” That is to say that not only is it important that no gaps exist in the individual components that comprise the system, but the components must be joined such that there are no gaps in the system as a whole. It is air leakage at the connections between air barrier components, and at penetrations through it, that usually determine the overall effectiveness of the system.

Structural Integrity - Subsection 5.4.1.2. Sentences 8 and 9 state that “An air barrier system installed in an assembly subject to wind load, and other elements of the separator that will be subject to wind load, shall transfer that load to the structure.” Specifically, it shall be “. . . designed and constructed to resist 100% of the specified wind load as determined in subsection 4.1.8.” The air barrier system must be able to resist peak wind loads, stack pressure effects or sustained pressurization loads without exhibiting signs of detachment, rupturing or creep load failure.

Durability - Subsections 5.1.4.1 and 5.1.4.2. detail the requirements for resistance to environmental loads and resistance to deterioration. The air barrier system must be durable, meaning it must be able to perform its intended function, be compatible with adjoining materials and resistant to the mechanisms of deterioration that can be reasonably expected given the nature, function and exposure of the materials, over the life of the building envelope.

These four requirements represent the minimum performance requirements of an air barrier system. In some instances, for certain buildings, the specifications on the particular project will demand that the performance standards of the system exceed those contained in the NBCC. Note also that the air barrier system must not only meet the requirements of the national code, but any provincial/state or municipal codes as well. Reviewed by Guiseppe Strazzeri.

3 Apr 2009, Comments Off

CFC Reviews CMHC Design Report: Window Installations – Shiplapping to shed water

Author: admin

Sheathing membranes, whether polymer or paper based, should be applied as shiplaps to direct water down and away from inner wall assembly. Sheathing membranes at the jamb should overlap the upstands of the impermeable membrane at the sill location.
At the head of the window, sheathing membranes should overlap window flanges and head flashings. Reviewed by Martin Lapedus.

1 Apr 2009, Comments Off

CFC Reviews CMHC Design Report on: Watertightness

Author: admin

The B rating of CSA A440 also defines a window’s ability to resist wind-driven rain, but it is not a measure of how the installed window with perform in the wall assembly.
Like the A rating for air leakage, the B rating is determined according to the results of laboratory testing. Table 2 shows the standards for B ratings.
Water is sprayed against the outside of the window at a known rate for at least 15 minutes, and an air-pressure difference is applied across the window, in an attempt to approximate wind and rain conditions.
The test method is described in the ASTM E547 Standard. The window is rated according to the highest pressure at which water ingress does NOT occur.
Note that “water ingress” can have several meanings. Under the pass–fail criterion of the ASTM E1105 procedure, water ingress is defined as any water that passes beyond the innermost projection of the test specimen, not including interior trim and hardware. Water ingress in CSA A440 means that:

 water has penetrated the window assembly and wetted room surfaces OR
 water has passed through the window into the wall below the sill OR
 water remains trapped in the window after the test pressure is released.

Clearly, the A440 criteria are more stringent than the E1105, and the designer should be clear on which criteria are more important in the performance of the window assembly.
As with the A rating, a window must pass the B1 level to meet the minimum requirements of A440 (and therefore, the Building Code requirements). The A440 Standard includes a User’s Guide — called CSA A440.1 — that gives some design assistance in selecting an appropriate B rating according to the location and building height. The specifics of the design process are best obtained by reviewing the A440.1 Standard. For tips on what to do with the design value once it is obtained, see the section entitled “Specifying ABC Ratings.”
Reviewed by Jan Luistermans

Tags

AIR Alexander Building Canada Canadian Funding Corp. Canadian Funding Corporation CFC cmhc construction Design IDP integrated design Moishe Alexander performance report Reviews Window Window Installation
  •