9. Slab-edge water condensation problem
Step 1: The water condensation on the slab-edge is caused by the surface temperature (Ts) being less than the dewpoint (D) of the surrounding. The dewpoint (D) is determined by the air temperature near the slab edge (Ta) and the relative humidity in the air (RH). The surrounding air temperature is always equal to dewpoint at 100 percent RH or higher than the dewpoint as represented by the following equation:
Ta ≥ D
Due to a warm indoor slab mass, for a slab edge in the ‘exterior air zone,’ the slab-edge temperature is always higher than the surrounding air temperature as represented by the following equation:
Ts > Ta
Combining these two equations results in:
Ts > Ta ≥ D
Since Ts is always higher than D, water condensation on the slab edge will never happen if it is in the exterior air zone.
Due to the slab edge being closer to the cold, for a slab edge in the ‘interior air zone,’ the surrounding air temperature is always higher than the slab-edge surface temperature as represented by the following equation:
Ta > Ts
Combining this with the first equation, there is no definite relationship between D and Ts—therefore, water condensation will occur for the condition of Ts < D. In a conventional window wall design, providing slab-edge insulation is the only method for increasing Ts to reduce the probability of condensation (i.e. ‘design for the effect’), however, it failed to eliminate the problem. Concluding from the above analysis, the cause of this problem is the slab edge being in the interior air zone.
Step 2: The air-loop window wall design places the slab edge in the exterior air zone due to the fact all joint cavities including the air space in front of the slab edge are pressure-equalized to the exterior air (i.e. the cause of the problem is eliminated). Therefore, the problem is forever solved without any slab-edge insulation.
For those accustomed to the second-generation design (i.e. concealed slab-edge), it is easy to continue to think the colder the region, the more slab-edge insulation is required. If one agrees Ts > Ta is always true without any slab-edge insulation, then this problem can be solved forever without needing any slab-edge insulation.
There might be another concern of interior water condensation on the floor slab inside the air-seal envelope in the extreme cold region. However, this is not likely to happen. The mass of the entire floor slab is a huge thermal sink—therefore, any thermal loss by thermal conductivity to the cold slab edge is unlikely to cause the interior slab temperature to go down to below the interior dewpoint.
This was demonstrated in an air-loop curtain wall renovation job in Minnesota where the base floor slab edges were completely exposed to the exterior cold weather without any slab-edge insulation—there has never been any reported interior water condensation on the interior floor slab surface.
In fact, adding the unnecessary slab-edge insulation to this design would have been counter-productive. This is because the effectiveness of the air-loop is to allow the pressurized exterior air due to positive wind to enter all cavities in the system as quickly as possible, and to enable residual water in the system cavities to dry out as quickly as possible after a rainstorm to prevent the possibility of mold growth inside the system cavities.
Securing the slab-edge insulation to the slab edge would reduce the air space in front of it, resulting in a slower rate of air-loop pressurization, as well as the drying of the system cavities. As mentioned earlier, eliminating the sun (i.e. cause of condensation) means no need for a parasol (i.e. slab-edge insulation).
The potential of slab-edge water condensation for hot and humid exterior air conditions is analyzed below, based on third-generation (i.e. air-loop) design only—that is, the slab edge is in the exterior air zone:
Ta > Ts
Combining this with Ta > D, there is no definite relationship between D and Ts. Therefore, water condensation will occur for the condition of Ts < D. However, for the air-loop system, if this is the condition of exterior water condensation, the condensed water will be harmlessly drained out in the instantaneous drainage mechanism, and all system cavities will be dried out quickly.
Again, adding the unnecessary slab-edge insulation would slow down the drying rate of the system cavities and increase the potential of mold growth in this hot exterior air condition.
Conclusion
A window wall has a distinct cost and time advantage over a curtain wall assembly due to easy erection from inside without hoisting equipment from outside. However, technical problems in a window wall system—such as water leakage, water condensation, and aesthetic and structural issues—have prevailed for decades without finding permanent solutions to the problems.
By using the concept of ‘eliminating cause rather than design for the effect’ as described in these articles, permanent solutions to nine major problems can be found. To implement a good air-loop design, the designer must have the courage to abandon some deep-rooted conventional thinking, such as requiring slab-edge insulation, for preventing problems caused by slab-edge water condensation.
Raymond Ting, PhD, PE, is the president of Ting Wall Inc., an exterior wall system engineering and design company. His expertise is in the exterior wall technology and composite foam panel technology. Ting has been involved in building product/system research and development since 1969, and is the inventor of more than 30 patents worldwide. He can be reached at ting@tingwall.com.
