What kind of temporary cover for vertical “skylights” in CZ-2?
michaelbluejay
| Posted in Energy Efficiency and Durability on
SHORT QUESTION: What kind of temporary cover for these windows (for hottest and maybe coldest days) will afford decent energy savings in CZ-2?
DETAILS: I have a rental house in CZ-2 that has four 44″ x 30″ clear plastic windows at the top of the ceiling in the finished attic room. They’re kind of like skylights but they’re installed vertically like regular windows. (see pic) They’re partially shaded but are still letting in staggering amounts of heat in the summer. Using my infrared camera this summer, the windows are clearly the energy offender, walls and ceiling are sufficient. Replacing the windows with low-E glass windows is out because of the cost and because the roof is too steep and dangerous for my taste (and expensive if I hire out the work).
My main difficulties in devising a solution are these unknowns (to me):
…..(A) Is stopping radiant heat enough (e.g., radiant barrier) or should I also be looking at stopping conduction (with insulation)?
…..(B) In the winter, does the radiant heat gain in winter from the sun outweigh the conductive heat loss from the room to the outside?
Here are my various ideas to remedy, all done from the inside.
…(1) A hinged cover (to uncover the window when it’s not super hot or cold), like a 1×4 frame with a radiant barrier facing out. (Weighs ~9 lbs.) If I also need insulation then it gets heavy for someone to raise/lower: 27lbs for the wood plus 9 lbs. for fiberglass or mineral wool batts (or next to nothing if I use bubble wrap and sacrifice some R-value). Polyiso at 22 lbs. doesn’t seem worth the weight for the extra R-value.
…(2) A slidable reflective curtain made out of an emergency blanket or radiant bubble wrap. Not sure how well it would work since it wouldn’t be tightly pressed against the window, and if I need insulation and not just reflection then it’s no good. Similarly, there are cellular shades and solar roller shades, but I doubt they’d be more effective and they’re considerably more expensive.
…(3) Buy new acrylic sheets, put low-E film on them, and then layer them against the existing plastic. Would dramatically reduce solar gain, but again, if I need insulation too then it’s not enough. If I need to let winter sun in for heat then I could hinge them to open them in winter. The cost is about $80 each for the sheet + the film. I couldn’t install the film on the existing windows, b/c there’s no way I’d get a good install on large windows wile working on a ladder (plus I couldn’t hinge them if I should be letting in sun heat in the winter).
What does the community think?
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Replies
WHy not just ~1 inch iso foam cut to fit in the space. plain side facing the window. glue cloth to the inside leaving a flap at the top to act as a hinge, tacked to the frame above the window. Rig strings and pulleys to open when you want light.
Foamcore, the white stuff they make signs out of, should be cheap, might be translucent.
corrugated plastic, cut to size leave in place.
plastic greenhouse material, kinda expensive
metalized mylar film[old school what they did in offices for this purpose] double sticky taped and leave it there
I would just cut a piece of polyiso to fit in/over the windows. Using foil-faced polyiso, you get a radiant barrier "for free" from the foil facing, and the extra R value from the foam. If you want to make it nice, trim it out with thin materials such as luan on the face and some basic casing around the edges. No need to frame it out as a full wall section or anything like that.
The suggestion in #1 to use "greenhouse material" is a good one too that may work for you. This material is usually known as "triplewall", and is a sort of multicellular polycarbonate (similar to acrylic, but much more durable) sheet material. The air spaces formed by the material offer some R value while still allowing light to pass through. This stuff is "clear", but more diffused, so not the same as regular window glazing.
Bill
michaelbluejay,
This is complicated by it being a rental unit. So the solution you may be comfortable with in your own house might not fly with tenants if it is too labour intensive, or Heath-Robinson.
You should also be careful about adding materials (like foam) with represent a fire risk if exposed.
Thank you for the replies. About that:
(1) Foam plastic must be covered by an approved thermal barrier as per code (and is a good idea even if where it's not; it's flammable and the smoke is toxic).
(2) As I mentioned, it's hard to plan without knowing the answers to the supplemental questions (A) and (B).
Code normally applies to permanent assemblies, so temporary things aren't usually an issue -- but that can depend on the nature of the "things". If you went with polyiso, you could use something like Thermax that is rated for exposure, but it's only rated on one side. You could use 1/4" hardboard on the other side, which is, I believe, the thinnest material (aside from metal sheet) that is allowable as a thermal barrier for this type of material.
Regarding your two questions:
A: I expect most of the heating is due to radiant heat, as that's what the low-E coatings primarily deal with. You could block that with aluminum foil in the window if you wanted -- you don't need anything fance. I think something rigid would be easier to handle though, which could be some 1/4" hardboard again, painted white, or 1/2" polyiso, with the foil facer. Anything that blocks that light AT the window should help prevent the sunlight from heating other things inside the structure. That's probably where most of your solar gain is coming from.
B: this is difficult to answer without knowing a little more about the structure, but if you're planning on removeable barriers (which seems to be the case), this is irrelevant anyway -- you could just remove the barriers in the winter when you want solar gain again. If you go with Dow Thermax polyiso, you could probably cut the panels to fit tightly in the window frame against the glass, which would leave the "rated to be left exposed" side facing the interior, so no extra thermal or ignition barrier would be needed, and the other side would be against the glass, so no barrier would be needed there, either. You could pop the panels in and out of the windows as needed.
Another option I can think of, if your windows are setup to allow screens or removeable storm windows, is to get some tinted or "one way mirror" polycarbonate panels cut to fit in place of the screen or storm window. I used 1/4" polycarbonate like this once to build some dog barriers, since I had a dog years ago that liked to jump up against the windows and broke two of them. The polycarbonate barriers acted as dog-proofing in that case, but in your case, the tint or mirror coating would act as your radiant "barrier", and the extra air space between the polycarbonate panel and the existing window would provide some thermal insulation.
BTW, polycarbonate is a much better choice for removeable window panels than acrylic. Polycarbonate is MUCH more durable, and not prone to fracture the way that acrylic is. Cost is similar, as are optical properties, between the two materials.
Bill
I guess I made the assumption that the OP is the tenant, thus the unwillingness to fix the fundamental problem
I don't think code applies to window treatments, not that covering plastics up is a bad idea...
THey also make high shade screen available at big box stores that I use outside over 2 of my skylights that pretty effectively cuts down glare and to some extent heat gain
I decided that while I don't know whether solar heat gain in the winter would be greater than conduction losses (and therefore whether I should be using insulation as well as a radiant barrier), I'm certainly losing heat at night, so I might as well make the covers with insulation. For during the day, I can test how much energy my heat pump heater uses on a day with the covers open vs. a similar-weather day with the covers closed.
Thermax is hard to find in my area, especially in small quantities (2 sheets), but I'll give it a shot. Does it not require a thermal barrier b/c of the nature of the insulation itself, or because of its thick foil faces? Because it's faced on both sides but the edges would be exposed. I plan to cover the edges with foil tape, would that be enough to avoid needing a thermal barrier on the edges of the foam?
2" Thermax plus a 1x4 on one side and 1/4" lattice moulding on the other (to attach the hinges, and the hooks), plus the hinges, is about 9 lbs. per cover.
Thermax has a thicker foil faced on the side that is rated to be exposed. I’ve never had an issue with exposed edges, but taping them with foil tape certainly wouldn’t hurt.
I’d probably try to use 3/4” material if you can find it (John’s manville’s CI Max is another polyiso rated to be left exposed), then use 1x boards to frame it out, and conceal the edges.
Bill
I can't find any documentation that says only one side can be exposed (as an exception to the thermal barrier requirement), certainly not in the installation instructions: https://www.buildsite.com/pdf/duponttyvek/Thermax-Sheathing-Installation-Instructions-1931868.pdf
Do you have a source for the idea that one side doesn't qualify for the thermal barrier exception?
Sorry, I was thinking the exposed side on the "white" Thermax, which has one side finished white for use in basements, for better appearance. That product has one white side and one "regular" side, but both sides are OK for exposure in terms of fire resistance.
I did just reverify this reading some of their datasheets. I'm more familiar using this stuff in basement type applications, so I'd had that "exposed side" stuck in my head for the fire part too, but for fire resistance, either side, or both, can be the "exposed" side.
Bill
Thank you very much for double-checking. In the meantime, I did see on the product description that it's 1.0 mill of aluminum on both sides. If only one side could be exposed, my solution was gonna be that instead of a 1" sheet, I'd get two 1/2" sheets and glue the non-exposed sides together, I'm so clever.
I wouldn't bother with wood
I wouldn't bother with more than an inch
Unless you can really seal it airtight, worrying about that level of heat loss is not very productive
I cannot easily calculate gain, but look at loss:
36 square feet
At 40 degrees outside and 70 degrees inside:
Current[R1] 1080 BTU hrs
1" [R1+R6] 154 BTU
2" [R1+R12] 83 BTU
That 71 BTU's is in the noise.
By limiting the weight, you can allow yourself schemes to open them for light if you want it. You won't need hinges so much as just flexible mounting. Like a couple spots of velcro to hold them up and a long stick that sits in the corner to flip them up or down.
A sheet of clear plastic with low E film in it would at least cut heat loss in half and drop gain significantly
IF gain is the real problem, tint guys that do cars could throw a dark tint on pretty quick
Thank you for your advice and helpful calculations, gusfhb. I see now that there's marginal difference between 1" and 2" polyiso.
I think I can get the covers pretty tight. I plan to "weatherstrip" the perimeter: the panel will press against the foam weatherstripping.
Hinges are simpler than removable panels (hinged = locked in the uncovered position with hook+eye), because then you don't have to carry the panels up and down a ladder, and don't have to store the panels anywhere. Each panel is only 9 lbs. total, easy to lift and latch.
Conductive heat loss is non-trivial and window film won't help with that. Window film would also prevent beneficial solar heating in the winter.
Using the calculator below, I got 5125 BTU/hr for solar radiation (with full sun on each window), so that's huge, and radiation is the bigger factor vs. conduction, even though conduction is non-trivial.
https://www.borstengineeringconstruction.com/Passive_Solar_Fenestration_Exposure_Calculator.html
INPUTS: Latitude 30.3, Month 7, S. wall deviation 24°, Glass height 25’, Height of overhang from top of glass 0, depth of roof overhang, feet: 0), SHGC 0.95, sf 36.8, Ground Reflectance 0.2, Climactic sunshine 100%, hrs/d 1
Answering my own questions (A) and (B), and showing the calculations which I just learned:
(A) Conductive heat losses through the windows are non-trivial. U-Factor 0.94 x 9.2 sf/window x 4 windows x 30°F temp. difference between inside + outside = 1038 BTU/hr.
(B) Conductive loss in winter is similar to summer. Summer 104° vs. 74°F, winter 70°F vs. 40°F, so 1038 BTU/hr either way. Solar heat gain when sun hits windows in January is 2500 BTU/hr, as per the calculator I referenced above.
Conclusions:
(1) Super hot: Close the covers
(2) Cold and no direct sun: Close the covers
(3) Cold but direct sun on windows: Open the covers
(4) Not super hot or cold: Either way is fine
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