8 skylights and a glass canopy in the center of the roof
jackofalltrades777
| Posted in Green Building Techniques on
How bad is a home that has 8 skylights and a glass canopy in the center of the roof? To me, this is an energy disaster and a constant maintenance issue. Not to mention it’s in a Zone 3 climate area with a desert sun beating on these windows during summer.
The home supposedly has a R-50 attic area but these skylights (Velux) & glass canopy have to reduce that number drastically. Are there any calculations to show the reduction in R-Value? Velux skylights have a R-2 or R-3 value at best but the solar gain in summer is bad as is the loss during winter.
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Replies
Most skylights are designed for a low SGHC in the 0.2s, sometimes lower, and a high visible light transmittance, but are usually about U0.3- U0.5-ish. eg:
https://www.energystar.gov/sites/default/files/VELUX-2_spec_windows_doors_skylights_v6.pdf
Wasco aerogel insulated skylights can have U-factors in the low 0.2s, but often higher SHGC, taking a hit in visible transmittance and clarity/crispness of the view.
https://www.wascoskylights.com/product/lumira-aerogel/#tab_3
http://mattrisinger.com/wasco-aerogel-filled-high-performance-skylight-review/
To get a low SHGC it needs low-E coated glass on one side, which means less aerogel, and a higher U-factor, eg:
https://www.wascoskylights.com/uploads/lumira-residential.pdf
In my area (New England) the low-U/high-SHGC version isn't necessarily a bad thing unless there's just too much glazing overall. In your region with a lot of skylight area it may be worth going with a higher U-factor to be able to get the SHGC under 0.2.
As a retrofit there are no really cheap or great solutions, but 8 skylights plus a dome is a lot. Building a standard roofed cupola over the skylight dome so that it only gets direct sun when the sun angles are low may (or may not) be something worth pursuing.
Dana,
Are not skylights in a Zone 3 climate a constant energy loss? With eight R-2 holes in a roof and a summer sun that will beat down for months on end, heating the interior and putting greater loads on the AC unit.
Peter,
The only time you get an energy benefit from a skylight is during peak sun hours on very cold days, when the solar gain through the skylight helps heat the house. At all other times -- at night in all seasons, during cloudy weather in all seasons, and during sunny weather during the summer -- a skylight performs worse (from a thermal perspective) than an insulated roof.
In other words, for most hours of the year, these skylights are either holes in your home's thermal envelope, or they are helping your house overheat during hot weather.
Your guess -- that these 8 skylights and the glass canopy are "an energy disaster and a constant maintenance issue" -- sounds about right to me.