PERFORMANCE EVALUATION
5.5 SUPPLEMENTARY TEST PROCEDURES
curtain walls. There are a number of other procedures that are also applicable in testing the basic air leakage, water penetration resistance and structural performance of curtain wall systems. Advice should be sought from a specialist consultant in the specifics of these procedures.
ASTM E547- Water Penetration of Exterior Windows, Curtain Walls and
Doors by Cyclic Static Air Pressure Differential.
ASTM E1233 - Structural Performance of Exterior Windows, Curtain Walls
and Doors by Cyclic Static Air Pressure Differential.
ASTM E1424 - Determining the Rate of Air Leakage Through Exterior
Windows, Curtain Walls, and Doors under Specified Pressure and Temperature Difference Across the Specimen.
5.5 SUPPLEMENTARY TEST
PROCEDURES
A
side from the basic test procedures outlined in Section 5.3 further testing is often conducted when a formal mockup is prepared. Unlike the tests referenced in Section 5.3, these supplementary tests are not all subject to consistent test standards or procedures.1. Condensation resistance 2. Overall thermal transmittance 3. Thermal cycling
4. Seismic racking
5. Window washing anchor
5.5.1 Condensation Resistance
The resistance of highly conductive curtain wall framing to condensation under winter conditions is of significant interest in a cold climate. Testing or analysis to assess the condensation potential of a curtain wall system is carried out by one of three different means, each with its own limitations. These means include a simple, large chamber test, a formal thermal chamber test and computer simulation.
A simple, large chamber test is the most frequently used test method and arguably the least accurate. The method is often incorporated into an overall test programme and utilizes the same chamber and wall sample used for basic testing. This is its principal advantage.
In this test the mockup chamber is maintained as the warm chamber. An insulated chamber (referred to as a “cold” box) is placed on the exterior of the wall and the temperature is lowered to the design condition. Thermocouples placed on the interior surfaces of the wall record surface temperatures for given cold chamber conditions. The recorded surface temperatures are reviewed with reference to a psychrometric chart to assess condensation potential.
The test is useful in comparing, to a point, the performance of competing products. It has limitations, however, that must be recognized.
Figure 5.4: Simple large chamber test
Simple Test Procedure Limitations
• No forced air circulation occurs in the cold chamber. Therefore the effect of wind in cooling the frames is minimized.
• The interior surface of the wall is fully exposed to interior heat. This will not be the case on-site and therefore interior surface temperatures are effectively increased.
• The complete lack of interior structure and finishes promotes better air circulation and heat distribution over what would be present on a building site.
• These three limitations all result in an overstatement of the
condensation resistance that will actually be achieved in the building installation.
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The formal test chamber approach is very similar to the simple, large chamber test. The principal difference in this test method over the simple, large chamber test is the use of a wind generator in the “cold” box. A fan and flow straightening tubes provide an air flow perpendicular to the wall face. Thermocouples strategically placed measure surface temperature for review with psychrometric tables.
Formal Chamber Test
The formal chamber approach reduces errors by providing a controlled air flow but introduces errors by forcing a reduction in frame arrangement to fit the chamber.
Computer Simulations
Computer simulation is rapidly becoming the most popular means of assessing condensation potential. Using finite difference programs, such as
Frame (by Enermodal Engineering), curtain wall cross sections can be
modeled and thermal performances simulated. The modeling procedure is relatively simple involving the drawing of the frame section (complete with gaskets, thermal breaks and glass) within the program and then applying a temperature difference. The program considers the conductivities of the model elements and estimates the surface temperatures for later comparison to psychrometric charts.
While the creation of the models is a simple drawing exercise, the proper use and interpretation of the output from a computer simulation requires some experience. Comparison of simulated results with physical test data
sometimes reveals considerable error. However, proper use of the program is very effective in evaluating the relative impact of changes on the thermal performance and the ranking of different designs. Simulation data should be used with caution in predicting condensation potentials.
Even with accurate modeling, experience indicates that simulated surface temperature results should be reduced by 2˚C (±10˚F) for evaluation of condensation potentials using psychrometric charts. For critical installations an even greater correction should be considered.
Again these simulations do not normally consider interior finishes, or significant three-dimensional heat flow, that can have a significant impact on surface temperatures of the curtain wall.
Formal Chamber Procedure Limitations
• The principal limitation of the formal chamber is the size of the sample. Originally developed to test windows most chambers cannot
accommodate samples larger than 2.5 m square (8 foot by 8 foot). • The limitations of the simple procedure with respect to the lack of
interior structure and finishes still apply to the formal procedure, and will result in an overstatement of actual condensation resistance that will be achieved in the building installation.
5.5.2 Overall Thermal Transmittance
The overall coefficient of heat transfer (U-value) is an important property of a curtain wall system. When compared with the solar heat gain coefficient (SHGC) of the glazing an overall energy performance level can be
determined. A test for thermal transmittance means heat flow due to conduction, radiation and convection. A number of standard test methods are commonly referenced.
AAMA – 1503.1 - Voluntary Test Method for Thermal Transmittance and
Condensation Resistance of Windows, Doors and Glazed Wall Sections
ASTM C177 - Standard Test Method for Steady State Heat Flex
Measurements and Thermal Transmission Properties by Means of the Guarded Hot Plate Apparatus
ASTM C1199 - Standard Test Method for Measuring the Steady State and
Thermal Transmittance of Fenestration System Using Hot Box Methods
ASTM E1423 - Standard Practice for Determining the Steady State Thermal
Transmittance of Fenestration Systems
The test procedure set up is similar to the schematic shown for the
assessment of condensation resistance. Through the recording of temperature differences at specific locations and the power flow to a roomside
calorimeter overall thermal resistance and transmittance values can be obtained.
As with previously discussed thermal testing the principal limitation of this method is the size of the sample. Full curtain wall modules can rarely be accommodated in testing laboratory chambers.
As a result, computer simulation combined with standard test results from glass products is being used more frequently to determine U-values. Analytical procedures include consideration of an overall U-value based on an area weighted U-values of the framing, the glass edge and the centre of glass region.
5.5.3 Thermal Cycling
A sequence of thermal cycling is frequently included in a test programme which incorporates a simple large thermal chamber. The intent of this test is to cycle the wall through a range of exterior temperatures representative of the temperature extremes the wall is likely to be exposed to.
The temperature cycles induce movements in the wall and follow up air leakage and water penetration tests can then provide some indication on the potential effect of these movements. To be effective at least five cycles should be completed. The length of each cycle will depend on the features of the wall as more massive walls require longer to stabilize temperatures. Thermal cycling is not normally conducted on smaller formal test chamber samples as the components are generally too small to move sufficiently.
5.5.4 Seismic Racking
Basic structural testing to ASTM E330 does not impose any in-plane loading or differential out-of-plane loading as would be generated in a seismic event. Nor does it impact any rapid cyclic loading.
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While curtain wall design for high risk seismic areas on monumental buildings requires specialist input, most contemporary walls are evaluated based on their ability to accommodate movements from moderate events and their breakage behaviour in extreme events.
Anticipated differential floor slab movements, as can be supplied by the structural designer of the building frame, can be applied to the wall using hydraulic jacks. Initial movements should be representative of a more likely moderate event. Air and water leakage testing would be conducted after this level of loading. After all other testing is complete a final extreme movement can be applied to assess modes of failure.
5.5.5 Window Washing Anchors
Various designs of window washing tie backs are often incorporated into the curtain wall design. While qualified initially by calculation, tests of the anchor in a representative sample of framing may be warranted. Loading must be applied in the direction causing the most severe stress condition in the anchor.