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Re: Shooting windows

COW Forums : Lighting Design Pros

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david granthamRe: Shooting windows
by on Sep 5, 2011 at 6:55:27 pm

Just noticed this excellent string after rigging a system for DFN for a regular location with relatively large S-facing wood-framed residential windows. (My -sort of budget - method's described at the bottom of this post)

I've got training in architecture, and - for what it's worth - a thought below about the (theoretical, at least) risk of heavily gelling multiple (and possibly single) pane windows on sunny days when weather is extremely cold.

The risk is that the light-absorbing gelled assembly gets so incredibly hot against the window that it causes the superheated inner pane to expand much more than the frigid outer pane. Like a two-strip thermocoil, the window then is subject to curving forces. This could cause the window to break. Or it could bend and thus crack the seal at the bottom, damaging the window's performance by a) compromising the insulating vacuum between the panes which also b)causes fogging and droplets to form on the colder outside pane. This wasn't a risk in a summer assembly for me, but I'm planning to use it in winter for a few days at a time when the outdoor temp could be -40 celsius. I"m quite sure the temperature differential on a winter day across the window pane could be at least as high as 80 degrees celsius, and I'm not sure that wouldn't stress even a single pane of glass to the breaking point.

Maybe someone can confirm or refute this possible risk (I hope I'm wrong) but I know there are severe limitations to how differently manufacturers will treat inner and outer panes for this reason. I'm going to ask window manufacturers, but I think they will almost certainly say there's a risk so as to indemnify themselves against liability.

If this is a genuine risk in very cold cimates (I'm sure it's not a problem in mild ones) the cumbersome solution would be to sandwich the layers of soft gel material between 2 layers of plexiglass (or stretch them all tightly enough to sell them as looking like glass) mount it on spacers in front of the windows, and perforate the assembly at the top and bottom well enough to allow for circulation of air to moderate the temperature at the inside face of the glass. (An unvented space probably wouldn't work as convection currents conduct the heat to the oustide very efficiently.) Would have to figure out how to let air - but not light - through the assembly into that gap. Fussy, time-consuming and expensive, but I'm thinking about it to ensure the windows are safe in a Canadian winter.

For my recent (summer) DFN assembly I used 3 layers of ND9 - wish I'd known about that cheap vinyl - under a layer of ND9 plexiglass. (Not $$$ hard gels but std - much cheaper - 1/8" plexiglass which is avail in the same transmissivity ranges.) Held this assmbly wrinkle-free against the windows with 3/16"x3/16" window stops stuck to the window sash with outdoor carpet tape. Looks perfect no matter how close the camera was to the window. (Stops need to be pushed against window sash every couple of hours as carpet tape softens with the accumulated heat.)


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