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re: condensation on thermal pane windows
21 aug 1999
david r. brown wrote:
>...scenario one [summer]
>air conditioning on in the house.
>inside temp is 23 c
>outside temp at 29 c and 100% relative humidity
>water condenses on outside of window, to centre of pane.
>
>reasoning:
>water condenses to centre of window because there is less insulating value
>in that portion of the window. the window is coldest there. therefore more
>condensation.
good windows are said to have more insulation value in the center than near
the edges. it seems reasonable to assume that's true in the summer as well
as in the winter...
>scenario two [winter]
>cold winter day
>outside temp -15 c
>inside temp 23 c
>inside humidity 50%
>
>water condenses on the inside of the window, at the edges of the pane.
>
>question:
> i can understand why the water vapor condenses out of the air with a
>decrease in temperature, dew point or saturation conditions.
especially in the first case with 100% outdoor humidity.
> but why is it that in one instance the condensation is evident in the
>centre of the pane and in the other instance at the perimeter?
that's a mystery to me, too. we might suppose the window has a us r-value of,
say 4 near the center and 1 near the edge, and the indoor air film has a
us r-value of 0.68, and the faster moving outdoor air film has an r-value of,
say 0.2... then we have something like this:
70 f --------------
| |
w w
^ w 0.68 ^ w 0.68
| w | w
ic | ie |
|-tci |- tei
| |
inside pane of|glass
| |
w w
center w 4 w 1 edge
w w
| |
outside pane of glass
| |
|-tco |- teo
| |
w w
w 0.2 w 0.2
w w
| |
to = |90 f summer -------------
|30 f winter
in the winter, ic = -(70f-30f)/(0.68+4+0.2) = -8.2 btu/h-ft^2 flows out
through the center of the window, so the inside pane of glass has a center
temperature tci = 70-8.2(0.68) = 64.4 f. near the edge, tce = 55.5, so
we'd expect condensation first near the edge.
in the summer, ic = (90f-70f)/(0.68+4+0.2) = 4.1 btu/h-ft^2 flows out
through the center of the window, so the outside pane of glass has a center
temperature tco = 70+4.1(4.68) = 89.2 f. near the edge, teo = 87.9. if the
temperature at the edge were lower than the temperature at the center,
we'd also expect condensation first near the edge...
nick
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