Last year the Building Science Group launched the Glazing and Winter Comfort Tool. This tool enables architects and engineers to study glazing properties (U-value, the use of room-side low-e coatings) and configurations and how these parameters affect occupant comfort in terms of radiant and downdraft discomfort (see graphic below). The issue is quantified by Predicted People Dissatisfied (PPD) and is a metric used by the ASHRAE 55 Standard.
Poorly performing glass facades typically require perimeter radiant heating to ensure occupant comfort. If architects begin early in the design process by planning the window configuration and glazing properties around occupant comfort, these remedial measures, which are an added upfront and maintenance costs, may be avoided.
A full in-depth article on both the issues of downdraft and radiant discomfort can be found here.
We shared our work on the research and the tool at a number of conferences across the U.S. last year. As we kick-off 2017, we wanted to share some of the frequently asked questions that came up when we present this research.
Glazing and Winter Comfort FAQs
The tool can help us quantify what is the highest U-value we can have in a given scenario to eliminate the need for perimeter heating. Is the U-value used in the tool the Center of glass (COG) U-value or assembly U-value?
We typically recommend using COG U-value. Radiant discomfort is based on the view factor, or how much of the surface the occupant is ‘seeing.’ If studying a single punched window, the occupant sees relatively little of the frame versus the surface of the glass. Therefore, the frame does not have a large effect on radiant discomfort even if it is at a much colder temperature. If the frame is a very prominent feature within the window composition where the mullions occur in relatively close spacing, then using an assembly U-value can make sense. Also, if designers or engineers want a more conservative approach in using the thermal comfort tool, then an assembly U-value is one approach to getting more conservative results.
Tall windows are more likely to cause downdraft currents that make occupants feel cold. Instead of reducing the height of the window, can a project use horizontal mullions to breakup downdraft and improve occupant comfort in the winter time?
Yes and no. In other words, it depends. This issue has been studied by researchers, and there are a couple of issues in play. The depth of the mullion or obstruction affects the breakup of the downdraft. If the obstruction is too small, the downdraft will just flow around the horizontal obstruction and affectively remain uninterrupted. This was shown for obstructions that were only 2” to 4” in depth. If the obstruction is deeper (approximately 7” to 10”), the downdraft will detach from the wall and mix with the room air, but there still remains a zone of downdraft near the face of the obstruction.
Another issue is the temperature of the mullion. Mullions made out of aluminum are typically colder than the glass and therefore the coldness of the horizontal is effectively an extension of the cold surface of the inner pane of glass. If one was considering wood mullions, the surface temperature is much warmer and will be a more appropriate solution in breaking up the downdraft.
One solution recently seen at the Kauffman Center for the Performing Arts is using a horizontal glass surface. This is visually unobtrusive, and since it is not part of the curtainwall framing system, it has a warmer surface temperature to aid in breaking up the downdraft.
How does this tool study the effect of the sun and solar radiation on occupant thermal comfort?
This study and tool is intended to be used during the coldest winter conditions. Typically, this occurs in the middle of the night when no solar radiation is present. While discomfort from solar radiation is known, the complexities of this phenomena added to the issues of downdraft and radiant discomfort is beyond the scope of this initial research project.
We know that room-side low-e coatings increase the potential of downdraft in the winter time. Can we use radiant heating to mitigate this downdraft?
Room-side low-e coatings function by effectively blocking the radiant heat transfer process. An uncoated piece of glass allows the radiant heat transfer process to occur, and the radiant heat warms up the interior surface of the glass. A room-side low-e coating is like placing a piece of aluminum foil on the interior face of glass so putting a radiant heat source against the surface will do nothing to heat up that interior glass surface. Therefore, it is not advised to use a room-side low-e coating with radiant heating to solve for the issue of downdraft. A forced air solution would be needed to address the increased potential of discomfort due to downdraft.