I went store to buy double layer of heavy duty black garbage bags to black out a bright bedroom at night, and spoke with salesperson and his supervisor. They both recommended go outside to balcony and tape these bags on outside window. Don't stay inside and don't tape bags between blinds and inside window!

My English is bad, and I can't remember their explanation. Can anyone think why? Because weather? Condensation? My condo's in Toronto, Canada. Toronto snows and gets cold in winter.

  • 4
    What are the bags for? That sounds crazy the wind will blow them off.
    – Ed Beal
    Sep 2 '20 at 18:50
  • 1
    @EdBeal That's what I thought! I edited my post.
    – NNOX Apps
    Sep 2 '20 at 18:50
  • 11
    I use 1 inch Styrofoam insulation board, painted white on the side that faces out and black on the side that faces in, on my bedroom window. The sun goes down around midnight and comes up at 3:30 in june/july in anchorage. There are tin-foiled windows all over town.
    – Alaska Man
    Sep 2 '20 at 19:41
  • 6
    Also, a plastic bag baking in the sun may emit harmful chemicals, you don't want that inside the house Sep 3 '20 at 6:15
  • 1
    There is a danger that a window - bag - curtains order could cause heat buildup and resultant glass cracking.
    – Hot Licks
    Sep 3 '20 at 22:16

Because black surfaces absorb sunlight and convert it to heat. You don't want to accumulate that heat inside the house, usually. Of course, this assumes that you don't install and remove the covering daily.

You could also use aluminum foil and avoid most of that issue. However, check where the light shining on your window will reflect to before using aluminum foil (you might annoy your neighbors or fry your plants).

Caveat (thanks, Max, and others who confirmed): With double or triple glazed windows, it can be destructive to mount bags or reflective foil from the inside. It can cause uneven expansion of one glass panel which can result in fractures or breakage of the window.

  • 1
    @isherwood I've not experienced this by myself yet but one of my relatives had that problem and companies that manufacture reflective films for windows also warn about that
    – Max
    Sep 3 '20 at 13:06
  • How would mounting the plastic on the outside make the temperature differential less, anyway? On the outside, still one pane of glass is immediately adjacent to a hot sheet of plastic, and the other is not. I could even see it being worse: with low-E glass a significant amount of solar energy is reflected from the outer pane, so the plastic receives less energy and gets less hot. With plastic on the outside it is exposed to 100% of the solar energy, getting hotter.
    – Phil Frost
    Sep 4 '20 at 14:25
  • 1
    Very little of the heat accumulated in plastic outside the window will make its way inside the house. Air conduction, reflection, and the window's insulation will prevent transfer. On the other hand, any heat generated inside is already inside.
    – isherwood
    Sep 4 '20 at 14:40
  • 1
    "you might annoy your neighbors or fry your plants" Or blind traffic, if the window is even remotely road-facing.
    – Mast
    Sep 5 '20 at 17:20

It does work better to have them outside. The filtering in the bags + window after is better than window + bags. I have actually tested this.

Outside though like Ed mentions, really really hard to secure these.

Inside they can kind of smell and it looks really bad - sunlight heat sits between bag and window.

Those are your main pros/cons.

If this is long term I would think about tinting solutions or shutters that close.

  • Contractor bags or a roll of 6 mil poly would resolve that in a hurry.
    – isherwood
    Sep 2 '20 at 18:55
  • 1
    the order of the layers has no bearing on the total amount of light filtered. Layer interaction could slightly change refraction (glare between layers), but also keep in mind that glass has a much higher reflection, which means a "glass outside" arrangement would actually be darker inside and cooler on the surface (further compounded by the relatively high thermal emisivity of black anything)
    – dandavis
    Sep 3 '20 at 4:04
  • 1
    Smelling and looking bad (from the inside) seem like the most compelling reasons.
    – Phil Frost
    Sep 4 '20 at 14:28
  • Shutters are wonderful. In Germany, we have something called Rolladen. They can be put down to completely block out the light at any time of the day. It keeps our house really cool on hot days.
    – Knossos
    Sep 4 '20 at 17:36

If you are doing this for blocking light I would possibly do something different. I used aluminum foil. I applied it with a water sprayer on the glass and wiped it down. It only had to be taped at the edges and the window could still open. If you want to open the window or door, often 1/2 the glass being sealed on the inside and the other half on the outside with whatever you use is the best option. Being careful I have foiled the insides of my windows for 100% light blockage but sliding the window scratches the foil and it will slowly let light in over time.

  • 1
    What sort of windows slide in such a way as to scratch something flush to the glass?
    – Chris H
    Sep 3 '20 at 7:59
  • 1
    Almost all windows have a seal strip that seals on the space between the 2 windows when they close this makes thick materials like plastic almost impossible to hang on that side and in my case after a few years it wor through the foil.
    – Ed Beal
    Sep 3 '20 at 13:07
  • 1
    Here the seal is always on the (usually plastic) frame, so covering only the glass would mean the foil was over 1" from the seal. You could cover the glass then tape to the sides with no risk of catching on any moving parts. You could even do this is you wanted a space between glass and foil for insulation
    – Chris H
    Sep 3 '20 at 14:36
  • Since the original post linked to a question about what to use, it seems like this answer would be better on that question instead of this one. Sep 3 '20 at 15:03
  • No need to post meta-comments in your answer, Ed. Just use the rollback feature in the edits list. That said, the edits looked good to me with one small exception. Such misunderstandings are the risk you take when you write so many run-on and incomplete sentences.
    – isherwood
    Sep 4 '20 at 13:39

As others have answered, the main consideration is indeed where you are trapping the heat. Even in watery, winter sunlight, you'll be amazed how much heat ends up being absorbed into the black plastic.

Depending on the glass though, it may not make all that much difference heat-wise - especially once the plastic is saturated with heat, heat will be radiated through the glass. Although, with the plastic on the outside, you should expect less of it as it will be cooled by any breeze, and some of the heat will be radiated away from the glass too.

It's really a matter of compromise - it's likely to be trickier to affix the plastic outside, it's also going to get wet, wind is going to bother it, and you may even have insect issues (here in Australia, we definitely would!).

If this was me, I'd be installing blackout blinds - you can open them easily to let light in when you want and there won't be maintenance issues. However, there is of course a cost issue (they sure aren't cheap) and it's not an option for everyone (renting, etc). So if I couldn't install blackout blinds, I also wouldn't be putting foil on the outside - as effective as it is, it'll be bright and ugly. What I would consider, is using dull foil (like the insulation foil used behind plasterboard) as a layer on the inside of the glass to reflect a decent amount of energy, and then use the black plastic on the inside behind that to block remaining light if desired.

  • 1
    "you may even have insect issues (here in Australia, we definitely would!)." Oh my god! What you mean? Why insects like plastic garbage bags???
    – NNOX Apps
    Sep 5 '20 at 4:28
  • @ElaineNai I think the lesson here is to use empty garbage bags
    – Andreas
    Sep 5 '20 at 16:42
  • 1
    @ElaineNai they don't necessarily like plastic garbage bags, but if you provide them a cozy, dark place that they can crawl under or into, they will.
    – James D
    Sep 6 '20 at 22:31

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