Introduction

This report summarizes the results of pre-and-post retrofit airtightness tests performed on 363 Broadway, Winnipeg, Manitoba. This is a 16-storey office building constructed in 1976 which underwent a major building envelope retrofit during the summer of 2011 to replace the curtain-wall glazing system. The pre-retrofit test was performed on April 14, 2011 while the post-retrofit test was carried out on June 12, 2012. In addition, supplemental tests were conducted on the building’s underground parking facility on June 2, 2012. The tests were carried out for two reasons: to quantify the impact of the retrofit on the airtightness characteristics of the building and to provide data on the airtightness of commercial buildings.

The impact of air leakage on building performance

Air leakage which occurs through a structure’s building envelope is a concern for a number of reasons. First, air exfiltration (defined as the unintentional movement of air from the interior of the building to the outdoors) can transport significant amounts of moisture into the building envelope where it may condense and form interstitial condensation. This can lead to a wide range of damaging and ultimately expensive conditions such as structural distress, corrosion, mould development, etc. Second, air infiltration (the opposite of air exfiltration) can create cold drafts and an uncomfortable indoor environment. Third, air infiltration has the potential to move outdoor pollutants into the building. Fourth, increased air infiltration can lead to unnecessarily high energy costs. Fifth and finally, a leaky structure is usually a noisy structure since the transmission of outdoor noise into the building occurs primarily through the same cracks and openings by which air leakage occurs. Since no structure is perfectly airtight, air leakage, and its undesirable effects, can not be eliminated but can only be controlled within manageable limits.

Test procedure

The test procedure used for 363 Broadway was a modified version of CGSB 149.10-M86 “Determination of the Airtightness of Building Envelopes by the Fan Depressurization Method” (1986). With this procedure, the airtightness is measured by depressurizing the entire structure to a series of indoor-to-outdoor pressure differentials and then measuring the corresponding air leakage rate at each condition. Intentional holes and openings in the envelope, such as ventilation system supply and exhaust grilles, are temporarily sealed so that only leakage through the envelope is measured. Using the air leakage versus pressure differential data, a linear regression curve is then computed to characterize the behaviour of the building. From this data, along with information on the building’s size, the airtightness results can be expressed using various area-or-volume-based metrics.

The major equipment used for the tests consisted of three Retrotec blower door assemblies complete with DM-2 micromanometers/controllers.

With large buildings some judgment is required to define the location of the air barrier and whether specific zones are considered inside, or outside, the test envelope. For 363 Broadway, one such zone had to be addressed: the underground parking garage. This is a three-level, heated garage which has its own HVAC system that supplies treated air to the garage through dedicated air inlets and outlets, although energy for the HVAC system comes from the building. Originally, the parking garage had been included as part of the overall structure being tested and was shown as such in the pre-retrofit test report. However, after consideration, the parking garage (for purposes of airtightness testing) was judged as outside the test envelope since its construction (cast concrete) was completely different from the curtain-wall construction of the rest of the building and because it is intended to be isolated (from an air leakage perspective) from the rest of the structure to prevent carbon monoxide and odour migration from the garage to the building. Therefore, the airtightness results shown in this report for the pre-retrofit condition are slightly different from those shown in the original pre-retrofit report.

There are several methods by which quantitative airtightness results can be expressed. In this report, results are reported using the Normalized Leakage Rate at an indoor-to-outdoor pressure differential of 75 Pa, NLR75. This metric was chosen for consistency with the method used in the National Building Code of Canada. It is also the most common metric used in Canada to express large building airtightness results. The NLR75‘s units are “litres per second per square metre of building envelope (l/s·m2)”, which essentially represents the average air leakage over the building envelope at a standardized indoor-to-outdoor pressure differential. This differential, 75 Pa, is equivalent to that which would be produced by a wind of approximately 40 km/hr blowing simultaneously and uniformly over the entire building envelope area. The building area used for the NLR75 calculation was defined as the total envelope area, including all above-grade components; the parkade was excluded from the tests.

The National Building Code of Canada (NBC) does not (and never has) contained quantitative requirements for airtightness in any type of building.  However, beginning with the 2005 edition of the NBC, it provided non-mandatory “recommendations” for maximum permissible air leakage rates of opaque, insulated portions of the building envelope (2005). This is not the same as the leakage rate for the entire building envelope, such as shown in Table 1, but rather for discrete portions only and does not include glazing or most of the joints and penetrations typically found in a building (and which produce most of the air leakage).  For an occupancy such as 363 Broadway, the recommended maximum leakage rate for assemblies in the NBC is 0.10 l/s·m2.

Blower door testing

Blower door testing

Results

The pre-retrofit airtightness of 363 Broadway, measured in 2011, was 1.23 l/s·m2, as shown in Table 1.  The 2012 post-retrofit measured airtightness was 1.03 l/s·m2.  This gave an absolute reduction in the Normalized Leakage Area at 75 Pascals of 0.20 l/s·m2, which represents 16.3 percent of the pre-retrofit airtightness. All results exclude the underground parking garage.

Comparative data

To provide additional perspective for the results, Table 2 contains some comparative airtightness data for various large buildings constructed in Canada and abroad over the last several decades (Proskiw and Phillips, 2001).  Most of these were existing structures tested some number of years after construction and would not have been designed or built using modern low-leakage construction techniques. Equivalent, comparative data for commercial buildings in Manitoba is extremely sparse.

Comparing the 363 Broadway results with those of other buildings shown in Table 2, the results from the pre-and-post retrofit tests indicate that the building is surprisingly airtight—displaying a measured air leakage rate which is lower than the mean leakage rates reported in the literature.

Table 3 provides some comparative data on the effectiveness of airtightness sealing in large buildings. Although only limited data is available, it shows that the reduction in air leakage achieved on 363 Broadway is comparable to what has been achieved on other major, building envelope retrofits. Also, of note, most of the buildings shown in Table 3 were initially much leakier than 363 Broadway. This is significant because it’s generally easier to achieve major reductions when the building is initially loose.

References

CAN/CGSB.  149.10, Determination of the Airtightness of Building Envelopes by the Fan Depressurization Method.  Canadian General Standards Board, Ottawa.

National Research Council of Canada.  2005.  National Building Code of Canada.  Canadian Commission on Building and Fire Codes.

Proskiw, G.  and Phillips, B.  2001.  Air Leakage Characteristics, Test Methods and Specifications for Large Buildings.  Report prepared for Canada Mortgage and Housing Corporation.

Table 1

Airtightness Test Results, 363 Broadway Ave., Winnipeg

Address

Pre-Retrofit

Post-Retrofit

Building height

57.6 m

Building volume

66,305 m3

Building surface area

10,436 m2

Date of test

April 14, 2011

June 12, 2012

Normalized Leakage Rate

1.23 l/s·m2

1.03 l/s·m2

Absolute reduction in NLR75

0.20 l/s·m2

Reduction in NLR75

16.3%

Table 2

Comparative Airtightness Test Data For Other Buildings

Type Of Building

Number Of Buildings

Normalized Leakage Rate (l/s·m2)

Range

Mean

Office Buildings

8

1.44 to 4.01

2.48

Multi-Unit Residential Buildings

12

1.18 to 6.37

3.19

Commercial Buildings

– Canada

– United States

8

68

0.23 to 2.14

0.73 to 24.56

1.35

6.18

363 Broadway, Pre-Retrofit

1.23

363 Broadway, Post-Retrofit

1.03

Table 3

Impact Of Air Leakage Sealing On Other Buildings

Type Of Building

Number Of Buildings

Percentage Reduction In NLR75

Multi-Unit Residential Buildings

3

15%

Office Buildings

5

24%

Schools

4

11%

Industrial

3

16%

363 Broadway

16.3%


By Gary Proskiw, P. Eng. of Proskiw Engineering Ltd. A report prepared for the Sustainable Infrastructure Technology Research Group (SITRG) at Red River College.