(JBED) - Summer 2006 - The Whole Building Design Guide

Feature
Window Comfort & Energy Codes
By Jim Larsen, Cardinal Glass
HEAT TRANSFER RATES THROUGH MOST windows are significantly
greater than the adjacent insulated wall. This includes both
winter heat loss and obviously summer heat gain. Figure 1
demonstrates how quickly the roomside surface temperature of
glass can drop in response to cold weather.
It’s important to analyze window comfort implications before
settling on an energy strategy that may compromise the livability
of a space. As an example, take the situation of trading out “efficient”
windows for an efficient furnace. On paper, the total energy
consumption may look to be the same, but Figure 1 tells us
that the occupant will be exposed to cold windows during the
extreme weather.
COMFORT BASICS
Comfort can be evaluated with a statistical index called predicted
percent dissatisfied (PPD) 1 . The calculation of PPD requires
a knowledge of room conditions (air temperature, air velocity,
humidity, and mean radiant temperature), and the occupant conditions
(clothing level and metabolic rate). When comparing two
conditions, a lower PPD is desirable as this reduces the risk of
occupant discomfort.
Some common examples where cold weather PPD will be
improved (lower):
• Increase thermostat setting;
• Adding layers of clothing; and
• Increase level of physical activity.
During hot weather the converse of these will improve comfort
as well as increasing air movement and/or reducing humidity.
WINDOW SPECIFIC INPUTS
Radiant conditions will be the primary driver on window comfort
issues. Mean Radiant Temperature (MRT) expresses the occupant
interaction with a window during cold weather. The value
of MRT varies with the occupant location relative to the window,
the size of the window and the room side surface temperatures
(typically taken as glass temperature). Figure 2 compares three
components of the MRT impact (size, proximity, and glass temp)
at 70°F inside/0°F outside to the change in PPD near the thermostat
(no MRT shift).
Solar gains represent a high temperature radiant source that is
handled independently of the room/ambient MRT. From research
work performed by the Windows and Daylighting Group at
Lawrence Berkeley National Laboratory 2 , a correlation has been
developed that shifts the occupant comfort based on total solar
gain. Figure 3 shows this offset for two levels of solar radiation
and two levels of window solar gain.
The National Fenestration Rating Council (NFRC) has completed
a research project 3 that carries these comfort concepts
forward in much greater detail than presented here. Interested
Figure 1: Roomside Surface Temperature vs. Outdoor conditions
Figure 2: PPD vs. Window Conditions and Thermostat Settings
Figure 3: Solar Offset to Window Comfort
Summer 2006 37