[GBlist] low E glass

[Richard Kadulski <kadulski@direct.ca>]

The fundamental issue to remember is that we are trying to create
comfort conditions in the house.  I've taken a piece out of my book on
heating systems as it may help clarify some of the discussion about when
and where low-e glass should be used.

We are most familiar with air temperature, which is a measure of heat
intensity.  The temperature within a room is usually not even. Often
there is a significant variation between the floor and ceiling. The air
temperature required for comfort depends on air movement, kind and level
of physical activity, type of clothing worn, age and the amount of
thermal radiation to or from the room surfaces.  People seated in a room
may be comfortable at an air temperature of 70°F (21.1 C), but if there
are large glass areas the air temperature may have to be 74°F (23.2 C).
Where people are physically active (as in a gym) then 50°F (10 C) may be
enough. Residential heating systems are normally designed based on
providing indoor temperatures of 72°F (22.2 C).

What is more important, and more difficult to define simply, is the Mean
Radiant Temperature.  Heat transfer from a body to its surroundings
happens when the surface of the body is at a higher temperature than its
surroundings. That is why surface temperatures of a room have a big
impact on comfort. A room has several surfaces, each of which can be at
different temperatures, depending on the room's finish and thermal
insulation. Windows are usually the coolest surfaces.

Radiant heat transfer is always from hot to cold. It can mean moving
through the vacuum of space - as the sun heats the earth through air
(without heating the air). The warm human body "sees" cold surfaces, so
it will radiate heat to them, thus creating a cooling sensation. Skin
responds very quickly to radiant heat. For example, at 50 F (10 C) it
may feel warm in the sunshine, as the body responds to the hot sun.
However, when a cloud moves in front of the sun, you instantly feel
cooler, although the air temperature has not changed.

How much body heat is lost by radiation depends on the surface
temperatures in the room. Engineers have developed the concept of mean
radiant temperature to define surface temperatures. This is one of the
most important elements affecting comfort. Mean radiant temperature is
the temperature of surfaces surrounding a person. It is also a measure
of the radiant heat exchange between a person and their surroundings.
The mean radiant temperature is dependent on the relationship of the
surface to the person and can vary from point to point.

The qualities of the surrounding surfaces are also important, as they
determine how good a reflector the surfaces are. With a perfect
reflector, the radiant energy loss from a person is reflected back, at
the same body temperature as the person, and there will be little
discomfort. Of course, there is no such thing as a perfect reflector.
When there are several surfaces at different temperatures, as is the
usual case, the effect of the mean radiant temperature becomes more
significant and can contribute to significant discomfort.

A simple formula has been developed that allows normal comfort level of
a room to be calculated by adding the two primary heating components:
air temperature and mean radiant temperature (average room surface
temperature). The formula is: air temperature ( F) + mean radiant
temperature ( F) = 140 ( F).

For example, with a conventional forced warm air heating system we can
maintain comfortable conditions at an air temperature of about 72 F (22
C) with a mean radiant temperature of 68 F (72 + 68 = 140). With a
radiant heating system, comfort is achieved at 68 F air temperature if
the mean radiant temperature is 72 F (68 + 72 = 140). The fact that the
air temperature in a room is 68 or 72 F is no guarantee of comfort. A
room with a temperature of 67 F (19 C) may feel warmer than a room in
which the air temperature is 72 F if the radiant losses are balanced. 

Drapes in front of large picture windows at night can improve comfort by
reducing the radiant heat loss, as the surface of the fabric now becomes
the radiant surface and the temperature of the fabric is warmer than the
cold window surface so we do not loose as much radiant heat. 

Heating registers and radiators are typically placed against the outside
wall, under windows to heat the coolest part of the envelope (the
windows), thus reducing the radiant heat loss, and also reducing cold
convective air currents washing down the wall.

The surface temperature of Low-e glass will be warmer, thus enhancing
comfort for the occupants of the room. Because of the cold night
temperatures, that is when it is most critical.  

(Now if we can only put meters on "comfort", it will be much easier to
deal with the issue, and there will be less talk about paybacks of low-e
glass vs clear glass).

-- 
Richard Kadulski Architect
204 - 1037 West Broadway
Vancouver, BC V6H 1E3

Tel/fax 604-689-1841

Climate adapted, energy sensitive, sustainable and healthy design
Editor Solplan Review: the independent journal of energy conservation,
building science & 
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