Optimum air gap for skylight
Context: a 14x7x6′ tinyhouse with 4″-thick polyiso insulation on all surfaces except the windows, which, for reasons that aren’t relevant here, must be on the roof.
Several hours of research shows that the optimum air gap for a typical double-glazed window ranges between 13-30mm depending on whom you ask. Charts show that U-value tends to bottom out at around 13mm and slowly climbs up from there. From this research I’ve gleaned hints that the glazing material and the angle the window is mounted at are factors, but I’ve been unable to find out how they matter, so I turn to the Community.
I’m using acrylic (Plexiglas), and the windows (approximately 22″x22″) are mounted on the roof, horizontally of course.They will be at least triple-pane, filled with air, sealed airtight, and with dessicant in the spaces. How can I figure out the optimal spacing?
Or to approach the solution from another angle – for thermal bridging reasons, and with regard to available (mostly reclaimed) materials, it’s best to have the windows 5″ thick. I would rather not have 10 half-inch spaces with 11 panes; assembling that correctly would be a, um, pain. How big a gap can I have before convection substantially decreases the R-value of the assembly?
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I don't have any answers for your question, so hopefully someone else can chime in, but maybe you can sidestep it.
Why do you need the glazing portion of your acrylic skylights to be 5" thick? Can you sketch up a detail explaining? In "normal" windows, the frame is much thicker than the the glazing. Why can't the skylights you are making be the same?
On a related note, my gut feeling is that your 14'x7'x6' tinyhouse with 4" polyiso is going to heat up super easily, so if you can't eek every single bit of performance out of your skylights (i.e., you don't feel like making those undeca-pane windows (had to look that prefix up haha)), it's probably going to be okay.
Geva,
First of all, a homemade skylight is very likely to leak -- especially if it is horizontal rather than sloped. I advise you to buy a skylight.
Second, here are links to two web pages with information on optimal gaps for IGUs:
https://windows.lbl.gov/adv_sys/hi_r_insert/GapWidths.html
https://www.greenbuildingadvisor.com/community/forum/energy-efficiency-and-durability/25330/what-ideal-thickness-dead-air-space-insulated
In your case -- building a homemade skylight with air rather than argon between the glazing layers -- I don't think the gap size is crucial. Other factors -- thermal bridging at the perimeter, workmanship problems, air leaks, and water leaks -- are likely to be the dominant issues here. The minor thermal performance differences between a 1/2 inch gap and a 3/4 gap will be overwhelmed by other factors.
Not worried about leaks; I already built a single-pane skylight some months ago that's withstood off some serious rain, wind, and snow, so I trust my construction techniques. Long-term remains to be seen of course.
Here are two screenshots of the SketchUp model I've got - cross-sections showing the hatch closed and open. The static and moving frames are 1.25"x1/8" angle aluminum because that's what I have. Yellow is polyiso, 4" thick. White is alu, including 1x1.5" beams supporting the roof; they're the main thermal bridge that would result unless my window assembly extends below them. 4" thick ceiling + 1.25" window frame + miscellaneous spacing = about 5" optimal window thickness. Brown is plywood, blue (shows white in places) is Plexiglas; this prototype is four panes 30mm apart. Details such as weatherstripping and fastening strips are left out, can be inferred.
Geva,
I'm glad you've developed details that won't leak (although over time, thermal expansion and contraction can change the leakiness of a homemade skylight glazed with Plexiglas).
For the rest, I stand by my answer. Thermal bridging through the aluminum frame will overwhelm any thermal performance differences related to gap thickness between glazing layers.
Still wondering what equations govern convection cells... short of actually poring over a thermodynamics textbook, the best I've been able to find are graphs that seem to indicate...
1. In triple-pane windows, there's a higher temperature gradient at the outer air space than the inner, and
2. With a higher temperature gradient, optimal air gap is lower than with a lower gradient. Peak R-value with US testing methods is at about 14mm with a 39C gradient; >18mm with European methods at a 20C gradient.
Putting those results together, I came up with this 4-space arrangement with 15mm in the top space and growing exponentially to 43mm in the bottom. It avoids a thermal bridge with the alu frame, mostly by the skylight being deep enough to sit below it. There are two layers of weatherstripping (grey) - one at the very top, one at very bottom.
Paying $190 for a 2-pane skylight of optimistically R3 when I can build something 3 times as good (or better) for less than half the cost, even with new materials... there are reasons I DIY and so far they've been validated by experience.