Solar Heat Gain Through a Skylight in a Light Well Page: 2 of 20
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radiative and convective interior and exterior surface heat transfer coefficients are assumed
to be small, and are neglected.
For projecting products, where the departure from planarity is much more significant,
application of equation 1 is more problematic. These product have what amounts to a
secondary interior space between the actual glazing elements and the aperture in the
envelope defined as the "fenestration" in equation 1. It has been shown (Klems 1998) that
for projecting "greenhouse" (or "garden") windows the interior surface heat transfer
coefficient is significantly modified by this space. Once this has been accounted for,
equation 1 can still be used to calculate nighttime thermal energy flows. For daytime solar
heat gain, one would expect that it would be significantly more difficult to calculate the
performance in a manner that would allow the use of equation 1; however, empirical studies
of this issue have not yet been done.
While a "roof window" type of skylight mounted in a cathedral ceiling can be from an
energy point of view very similar to an ordinary fenestration in a tilted surface, in the more
common installation the skylight sets at the top of a light well that passes through an attic or
plenum space. The "rough opening" of equation 1 then becomes the opening at the bottom
of the well, where the former joins the architectural space. This may be considered as a
projecting product, where much of the projection is in a different space, rather than out of
doors. The skylight may, of course, also project above the roof line, as may part of the well,
for example, to allow a tilted skylight to be installed on a flat roof.
Equation 1 is usually assumed to be applicable to such a situation. In the thermal
calculation the effect of the well is ignored, and the skylight is treated as though it were
installed directly in the ceiling of the space. In the case of solar heat gain, it is assumed that
all energy admitted by the skylight itself participates in the energy balance of the space.
However, it is well known that daylight is attenuated in passing through a long, narrow
space such as a light well. Accordingly, a "well daylight efficiency" is assigned to the
skylight to account for attenuation in the well. This constitutes the standard modeling of
skylights in the DOE-2 building energy simulation program. (Winkelmann, Birdsall et al.
1993)
This paper presents measurements of the heat flow through a skylight with a light well of
dimensions that would be reasonable in a residential or office situation. As will be seen, the
thermal energy flows are not well described by the above model.
MEASUREMENT PROCEDURE
An accurate, well-characterized and well-known outdoor test facility (Klems, Selkowitz et al.
1982; Klems 1992) was utilized for the measurements. Although normally used to study
vertical fenestrations, this facility was designed with ports in its nearly flat roof for the
installation of skylights. A commercial skylight adapter for mounting tilted skylights on a
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Klems, J.H. Solar Heat Gain Through a Skylight in a Light Well, article, August 1, 2001; Berkeley, California. (https://digital.library.unt.edu/ark:/67531/metadc785216/m1/2/: accessed April 24, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.