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SteveGerber

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Posts: 21
Reply with quote  #1 
I now have a solar hot air panel supplying warm air to two rooms in my house.  I've also been experimenting with covering the windows in those rooms at night to minimize heat loss.  I uncover them during the day unless it's going to be totally overcast.  My question is, at what low outside temperature point, on a sunny day would it still make sense to keep the windows covered with insulating panels?  Just as a rough example assume the daytime high is only 10F or maybe 0F and the windows are just average low-e double pane, NOT a super high performance window.  In my climate (Shenandoah valley of Virginia)  it's not very common to have extreme low temperatures on a sunny day in winter but I'm still curious as to approximately where this cross-over point might be.  The next couple days will be about as low as it typically gets here on a sunny day, around 15F then it will go to near 0F overnight.

Gordy

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Posts: 122
Reply with quote  #2 
Steve,

I can't answer your question, but thought I'd share what I did with my windows.

Single layer of bubble wrap on a double pain sliding patio door.  A side by side test at -10f outside, the bare glass inside reads 34f with the IRT, the bubble wrap reads 55f. Bare foot and bare legged, standing next to the door I could feel the cold air cascading down and across the floor before, not as noticeable now. Obscures the view but lets plenty of light in.
window insulation 001 small.jpg

 
1"x2" frame with shrink window film on both sides and closed cell foam seal around the edges. This gives 2 dead air spaces, bare glass reading about the same as the door. The film is tough to get a consistent reading on, but pointing the IRT through the film on to the wood between the two film layers reads 58f on a north facing window. Note the wood was pre-drilled before screwing it together to keep the end grain from splitting.
window insulation 002 small.jpg


Handles on each side for easy removal. You can see I bent the handles before mounting, to give more knuckle clearance to the window sill so I can grab them. Also before mounting the handles I put some clear packing tape on the film so the handle won't cut the film.
window insulation 003 small.jpg

Most of these window inserts stay in year round, because they also work in the summer to reduce the need for the AC to run.  These and my small door solar collector have reduced my LP usage by almost 1/2.
 


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Gordy,
Minnesota
stmbtwle

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Posts: 2,544
Reply with quote  #3 
I did a similar setup, but the 1x2 is set like a picture frame and I set acrylic window "glass" in a saw groove. Like Gordy I leave 'em year 'round. Weatherstripping holds them in place. IMG_0673a.jpg 
IMG_0674a.jpg 
The acrylic is probably tougher than shrink film but it's a lot more expensive.



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Willie, Tampa Bay
KevinH

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Posts: 547
Reply with quote  #4 
I found this old study about net heat gain that was done in Canada.  It takes into account both the solar gain and losses due to the temp differential.  I haven't spent a lot of time looking at it, but it appears that for a south window even in the coldest months there was a good net heat gain when it is sunny.  See the figure 9 graph for January for a double pane window in Winnepeg showing the net heat gain peaking at 600 W/m2 mid day.

http://web.mit.edu/parmstr/Public/NRCan/nrcc18674.pdf

The rest of the article is focusing on the total net gain/loss (day and night for a month).  In that case there are periods in the winter where the loss exceeded the gain for the south window.

I think that no matter how cold it is, the window should be left uncovered if there is sun coming through the window.  Of course, if you want to go to the trouble of doing night insulation that will help.  Myself, I just insulate some windows that don't get any sun in the winter where I don't care that they are blocked.  For example, a sliding glass patio door in the basement.  I also do a few windows with the shrink film, although I do something similar to what Gordy showed so the film gets multiple seasons of use.  I don't think there is much benefit versus the cost when the shrink film is only used one season.  Years ago I used the calculator at Builditsolar.com to estimate the benefit of one season for the shrink film and in my case (if I remember correctly) it was close to breakeven or very little benefit (based on natural gas heating).

I also have a couple reflectors outside to bounce more light into two windows to increase the gain (only practical for rooms you don't use much since the light is bright).

Kevin H
MN




Gordy

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Posts: 122
Reply with quote  #5 
Hi guy's,

I had used the window film single layer and thought if that worked why not try two layers (two dead air spaces). I did side by side testing and found that the double dead air space was better. As to durability I have no youngsters or critters to wreck the film. And most of these inserts have been in place for 9 or 10 years. A lot of the film was bought a garage sales so cheap. The duck stuff is not bad but the 3M has less distortions and the fold seams seem to disappear better.

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Gordy,
Minnesota
stmbtwle

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Reply with quote  #6 
Nice to know. The double air space SHOULD be better, and if you're putting it on a wood frame it's not much more work to do both sides of the frame. Great idea!

Did you seal the frames with anything or does that tape stick well to bare wood?


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Willie, Tampa Bay
Gordy

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Posts: 122
Reply with quote  #7 
Willie,

I wiped the wood down to make sure it was clean and use two double sided tapes side by side on the edge of the 1x2 against the sill. Then applied film to one side, then repeated the double row of tape and applied the film to the second side. Then sticky backed foam weather stripping. Not in the drawing is closed cell foam rope wedged in between the 1x2 and sill against the other foam. There is enough pressure from the foams to keep the tape in place.

widow insets.jpg 


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Gordy,
Minnesota
Gordy

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Posts: 122
Reply with quote  #8 
Willie,

Forgot to add. With single layer film on the inside of the house window. The air in the dead air space will warm up against the film and rise, while the air against the cold glass will cool and drop. Creating an air current, similar to what we see in your collectors if allowed to back draft at night but in a constant loop. The double dead air space slows down the heat transfer by making it do it twice and lowering the temp differential between each space.

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Gordy,
Minnesota
stmbtwle

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Reply with quote  #9 
Thanks Gordy I'd heard that.  I understand there's an ideal gap width which will not allow convection currents, I think it's around 1/2 inch but I'm not sure.  3/4 would probably be close enough (kinda like the glazing/screen gap in a ZP). At least that's what I understand.  It's kind of hard to control the width of the gap between the film and the window.  I've got a room which is not heated except by solar, and it has big windows... might be worth a try if I can get my old ass in gear before our short winter is over.


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Willie, Tampa Bay
Gordy

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Posts: 122
Reply with quote  #10 
Thank's Willie,

As I was typing the last comment I was wondering about convection currents in tight gaps and how small the gap needed to be to stop it.

If I were to mount single layer film in the standard way, to the trim around the window sill. I would end up with a separation between film and glass of 5.25" on the bottom glass and 6.5 on the top glass. That would get a good flow going, I think.

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Gordy,
Minnesota
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