The typical large project can use 20 or more types of glass for optimal performance. Therefore, at this point, it would be better to leave the further refinement of the glass specification to the delegated design teams. Unfortunately, some architects pick the glass based on how it looks, leaving the design entirely up to the contractors, and often ending up unpleasantly surprised with the results.
The use of building information modeling (BIM) and various engineering simulations are sometimes criticized by those who note the discrepancies between the virtual and real world. Suffice it to say, software is only as good as the data fed to it. Such information includes physical characteristics of materials and spaces, including visible transmittance and reflectance of surfaces, which are often misunderstood by architects.
This author was once asked to calculate the daylight factor separately for mirror-like east and west quarters of a building, but the project’s architect did not respond to questions relating to the building surroundings. She also admitted ignorance with respect to material properties, such as surface reflectivity, which is important for such analysis. Therefore, predictably, east and west DF studies yielded identical results (at a double price), and neither result of the analysis was expected to emulate the real world.
Site conditions are not always fully taken into account. Walls and roofs of adjacent buildings and water surfaces sometimes send a strong reflection in the direction of a building, which should be addressed in the glazing design.
Other items to consider
Several factors besides glass performance affect a window’s VLT and SHGC.
Shading
Many aspects, such as external and internal shading, louvers, framing-to-glazing ratio, and opening projections all have an impact on physical glazing characteristics. In some cases, printed advertising media placed on a building can lower interior daylight factors significantly; in other instances, poorly designed sunshades can not only add to cooling energy expenditures, but also increase the need for electrical lighting when occupants keep shades down and lights on.
HVAC and interior design coordination
This author often finds a room’s windows are frequently not addressed by the HVAC system. In northern climates, this leads to damage of moisture-sensitive materials by water condensing on windows. In the South, lack of air-conditioning in part of the room adjacent to hot windows renders this area unable to meet ASHRAE 55, Thermal Environmental Conditions for Human Occupancy, and, therefore, the criteria of a conditioned space.
For example, this author was recently in a large media conference room in a new building in the Bahamas. The half of the room adjacent to the south-west elevation featured large strip windows and was not air-conditioned, with diffusers located close to the entrance doors and the return grilles.
A speaker was placed in a literal ‘hot spot’ because the podium was located in front of the windows.
The excessive window-to-floor ratio is sometimes caused by lack of coordination between building shell and core designs. Tenants often design and build the spaces long after the shell is constructed. This includes air-conditioning and internal shading.
Internal shades
Most sunshades are ‘roller’ furnishings coming down from a window head. Therefore, any mid-position leaves a horizontal strip of light above the sill (where glare hurts eyes the most and lights the room the least). Were the shades operated upside-down (or translucent strips introduced in a double configuration), a bright strip could be left at the top, providing some useful daylighting, saving artificial lighting, and preventing glare.
Cheap fixes
When a problem arises, there can be temptation to employ a ‘Band-Aid’ approach. However, in some cases, this may hurt more than help. This author is wary of interior-applied items that increase the glass temperature, such as the common aftermarket tint films or translucent insulation panels.
When incorrectly specified, they may induce glass breakage because the edge of the glass stays cool, chilled by its framing. It may be akin to pouring hot water on the center of a glass dish freshly removed from a fridge. A good example is glass cracking induced by users placing an aluminum kitchen foil behind a window glass to limit the sun heat transmission, frequently observed by this author in South Florida. For this reason, such an approach often voids the glass warranty; therefore, the glass fabricator should be consulted first.
Karol Kazmierczak, ASHRAE, NCARB, LEED AP, is the senior building science architect and president at Building Enclosure Consulting LLC. He has 17 years of experience in envelope design, engineering, consulting, and inspection. Kazmierczak can be contacted via e-mail at info@b-e-c.info.
