Curtainwall Air, Water, Sound, and Thermal Performance
by Jimmy Huang, P.E.
Translated from the Chinese version which was previously published in the Taiwan Architecture magazine issue No.108
Majority of the curtainwall designer sub-out the structural calculation work to a licensed structural engineer due to the fact that it is a different professional field. A good structural engineer not only ensures a safe design system, but also pay attention to the cost saving in material for the contractor. However, when the design and material become overly trimed down, it has negative impact on how well parts fit together and the total sound and thermal performance.
A "top-notch" design work should be an "economical" design that does not adversely effect the curtainwall normal performance and its durability, and at the same time fulfill the architect conceptual requirement. Due to budge constrains and tight schedules, typical structural engineer only concentrate on fulfill safety requirements, but rarely advice the designer on how to optimize the design.
Therefore, the curtainwall designers needs to arm themselves with the ability of planning the entire process, as well as knowledgeable in construction practices and budget control.
Large curtainwall projects usually require passing a mockup test. The test evaluates the specimen installation procedures, penetration of air and water, and structural safety. Unless specification otherwise stated, typical mockup test procedures does not include sound insulation or thermal conductance tests. The air infiltration performance not only reflects the cooling or heating energy required, but also impacts the sound transmission level.
Low air infiltration also means reduction of noise transmitted by air. AAMA and CNS specification requires at least 75Pa pressure when measuring the air leakage, and many architects nowadays requested 300Pa or more. In fact, raising the pressure from 75Pa to 300Pa for the air infiltration test does not necessarily require a special design. This is easily obtainable with tight control on the construction quality.
Good air performance meant a noticeable saving in energy in a long run. The sound insulation not only has to do with air tightness, but also heavily link with surface panel thickness and composition. For example, laminated glass has better sound reduction than insulating glass, and dual pane glass is better than single pane glass. The larger the air gaps the higher noise reduction. Currently there is no fully-documented sound transmission
test data to evaluate the open joint stone panel system with backpan and mineral wool insulation, or the open joint aluminum panel system with backpan and mineral wool insulation system. In the past, we only consider the energy saved for the system thermal performance. Recently due to building code fire resistance requirements, insulation not only saves the energy, but also needs to be fire resistance. Therefore, non-mineral wool insulation such as glass wool
or PU foam insulation can not be used in cladding that has 1/2 hour or greater fire rating. Typical 5cm thick mineral wool will fulfill this requirement. On the other hand, large ratio of vision glass without proper Low E coating is one of the biggest energy hole. As the issue become more prominent, we believe the future curtainwall will become more diversify and be part of environmental friendly elements.
The air and water penetration test requirement for the current code is listed below. The sound and thermal insulation value will be calculated instead of testing. Fire resistance rating will not be in the scope of this article.
Air Infiltration Performance
1. Air infiltration performance means the volume of airflow through the wall specimen per square unit area and time at designated pressure difference. The metric unit is m3/m2 hr (fix window), m3/m hr (operable window)
2. The performance value is determined by CNS 13971 (similar to ASTM E283) test method for air infiltration test.
Air infiltration performance plays a strong influence in climate control equipment power loading. The test method is CNS 13971. The test pressure is normally at 75 Pa, but for high-rise building or building that requires higher standard of air and sound control, 300 Pa is used. This is equivalent to static pressure generated by 22 meters per second wind speed. Normal fixed window and panel allowable leakage is 1.08m3/m2 hr and for operable window is 1.5m3/m hr.
The air flow value for fix window is calculated per unit area, and operable window is calculated per unit perimeter length of the opening.
Water Penetration Performance
1. Water penetration performance means under the specified pressure and the required water spray rate at the exterior of the wall, there should be no water leakage found at the interior portion of the wall. The pressure unit is kgf/m2.
2. The performance is determined by CNS 13973 (similar to AAMA 501.1) dynamic water test and CNS 13974 (similar to ASTM E331) static water test. The operable window shall be treated the same as other fix window or panel wall.
Water penetration is one of most important performance test as it reduces the possible water leakage during heavy rain and storm. The test method is described in details in CNS 13973 and CNS 13974. The water performance level can be different for each type of door including fire-rated door. The requirement shall be clearly specified. When the door can not meet the water performance requirement, it is usually then installed at recess area to avoid direct contact with the rain.
AAMA recommends the testing pressure to be 20% of the maximum positive pressure. However, due to frequent typhoon activity in Taiwan, 720 Pa or above pressure is commonly used. The water spray rate is 3.4L/m2.min.
Sound Insulation Performance
Method of determining required sound insulation value:
Indoors Allowable Sound Pressure Level
L1 (db)
Outdoor Sound Pressure Level
L2(db)
TL = ( L2 - L1 ) +10 log S/A
S: Exterior Wall Area
A: Receiving Room Sound Absorption Area
L2-L1 : Required sound insulation pressure level
1. Sound insulation level is determined by CNS 8465 specified sound transmission lost curve.
2. The performance is determined by CNS 8466 sound transmission lost test method.
Sound insulation performance here is not represented by traditional average dB. Instead, the actual sound level curve (CNS 8465) is used. The test method for insulation design shall follow CNS 8466 requirements.
Thermal Performance
1. Thermal performance is determined by the R-value or resistance to heat flow. The unit is m2 (K/W).
2. The R-value can be obtained from a standard test or from the following equation:
R = Ro + Ri + {Ra + Σdi /λi }
Symbol Descriptions:
Ro: Outdoor side wall surface heat resistance(m2 (K/W))
Ri: Indoor side wall surface heat resistance(m2 (K/W))
Ra: Air space heat resistance (m2 (K/W))
d: Wall thickness(m)
λ: Wall thermal conductance(W/m K)
Thermal performance has a strong impact to air conditioner or heater load.
Heat resistance R represents thermal performance. Because other performance value is determined by test, the thermal performance in principle shall be determined by a standardlized test. However, because of the various factors involved in setting up the test, it is hard to obtain an uniform result. Therefore the theoretical calculated value is also referenced. Of course when the test is available, the result shall be used.
This article Ro and Ri are influenced by factors such as relative position of the wall and air, wall surface condition, wind direction, wind velocity, and temperature. Ra is influenced by factors such as wall thickness, temperature at two sides of the wall, wall surface condition, heat flow direction, air space thickness and position.
Further Reading:
Richard A. Behr, Ph.D., PE, and Scott A.Warner, "Earthquake-Resistant Architectural Glass: New design provisions and test methods", EarthquakeAdvisor.Com: 2003
|
