In the August 2013 issue of The Construction Specifier, we included a Horizons column—“Introducing ‘Breathability’ to Curtain Walls”—by Raymond Ting, PhD, PE.
As a complement to our more straight-ahead technical features, Horizons examines still-emerging technologies and new ways of assembling buildings. In this particular case, Ting dealt with the issue of glazed assemblies and their impact on both energy efficiency and indoor air quality (IAQ). He acknowledged building codes advocated for airtightness and air barriers, but called for new thinking about how ventilation—a good dose of fresh air, in other words—might be more critical for the occupants living or working inside.
Due to space constraints, a short sidebar article was kept out of the magazine. For a further look into the rationale behind Ting’s call for breathable curtain walls, it is included here.
Calculating Energy Consumption for the HVAC
The ‘breathable wall’ concept discussed throughout the main article means it is advantageous to utilize the air leakage rate of the curtain wall for the purpose of executing the required air exchange rate as illustrated below.
For any curtain wall, the thermal insulation value is a given, invariable number affecting the calculated required energy consumption for the building’s HVAC. However, the air leakage rate through the curtain wall determines the required air exchange rate and, thus, the required energy consumption. Therefore, the energy consumption calculation provided below is limited to the part related to air leakage rate through the curtain wall and air exchange rate for the building.
The following definitions are used for the energy calculations:
- Ve: volume of air exchange rate required for maintaining IAQ (cubic meter [foot] per minute);
- Vg: volume of air leakage rate entered the windward wall and exhausted from the leeward (cubic meter per minute);
- Vf: volume of forced air exchange rate required to be created by the HVAC system;
- Ra: air leakage rate of the curtain wall (in cubic meter per minute square meter [square foot] of wall area);
- A: surface area of curtain wall (in square meter [square foot]);
- Ee: energy rate required for heating or cooling the exchanged air, Ve (in Btu per minute);
- Ef: energy rate required for heating or cooling the exchanged air, Vf, by HVAC system (in Btu per minute);
- Eg: energy rate required for heating or cooling the exchanged air, Vg, due to air leakage in curtain wall (in Btu per minute);
- Em: mechanical energy rate required for creating the forced air exchange rate, Vf (in Btu per minute); and
- Et: total energy rate for accomplishing air exchange (in Btu per minute).
From the above definitions, the energy calculations are:
- Vg = Ra x A
- If Ve < Vg, then, Vf = 0 and Ef = 0
- If Ve > Vg, then, Vf = Ve – Vg and Ef > 0
- Ee = Ef + Eg
- Et = Ee + Em
In Point 4 of the above energy equation, Ef is linearly proportional to Vf due to the HVAC, and Eg is linearly proportional to Vg due to air leakage. Ee becomes a constant since it is only a function of the predetermined air exchange volume. In Point 5, Em is also linearly proportional to Vf. It becomes apparent the most ideal condition for energy consumption, Et, is the condition represented by Point 2, meaning it is desirable to maximize the curtain wall air leakage rate, Ra, in Point 1. However, other curtain wall performance parameters must be considered in maximizing the curtain wall air leakage rate as presented in the following sections.
The above conclusion is directly in conflict with the conventional curtain wall design thinking of less air leakage rate for less energy consumption. The reason for this is due to the lack of concerted effort between the curtain wall designer and the HVAC engineer, and Point 5 has never been considered in curtain wall design. For the curtain wall designer, only the energy required for heating/cooling the infiltrated air (‘Eg’), is considered as ‘Ee.’ Therefore, the lesser the air leakage, the lesser the energy consumption—this leads to the conventional, but erroneous, thinking.