Should I put black garbage bags on the inside or outside of my windows to keep my house warm and cold from getting in?

Question

I live in an older house and I can feel the cold air coming in through the windows. Should I put black trash bags on the outside or inside to keep my house warmer?

Answer 1

TL;DR Inside

For a bunch of reasons, inside is better than outside, including:

• Damage - Inside, the plastic will basically last forever, or at least through the winter. Outside you have birds, wind, rain and other natural, uncontrollable, things that can mess up the plastic.
• Aesthetics - Admittedly, black garbage bags inside won't look fantastic from the outside. But almost any plastic over the windows on the outside will look awful. In a worst-case scenario, someone may think black plastic covering the windows is due to broken windows, and broken windows (even if they turn out not to be broken in this case) encourage vandalism and other crime.
• Ease of Installation One - This varies from house to house, but at a minimum second-story window coverings are as easy to install as first-story window coverings when done from the inside. But from the outside that means ladders, possibly over dirt or bushes or other things besides flat pavement.
• Ease of Installation Two - To be effective, you need a good seal at the edges. Whether that is by using shrinkable plastic or duct tape, it is fairly easy to do inside but outside there are often different materials such as brick around the sides of windows that make it harder to seal the plastic.

That being said, consider a couple of alternatives:

• If you must use bags (i.e., cheapest readily available material), use white bags instead. That will hopefully let at least a little bit of light through. They aren't generally as large as the black bags, but it should be possible to tape them together (for the larger windows) and make it work.
• If you can spend just a little bit more, consider getting a window insulation kit like this one from Amazon:

The details vary. Some have thicker plastic than others. They have different types of tape for the edges. But the premise is generally:

• Clear plastic to cover the windows, so you can still get natural light
• Something to hold the plastic at the edges
• Use a hair dryer (a heat gun might be better, but the average homeowner has a hair dryer and these are designed for easy DIY installation with no special tools) to "heat shrink" to hold the plastic really tight

The idea of "dead air between plastic and windows" is exactly the same with taping up plastic bags. But these kits do a much better (and more attractive) job of it.

Answer 2

Should I put black trash bags on the outside or inside to keep my house warmer?

No.

Your question reflects a common misconception about radiant heat. Black surfaces do indeed build heat from sunlight, but so do most of the other surfaces in your home. Also, even if you were to use black plastic on the inside, much of the heat generated would be lost to leakage back out the window. Then, the poor environment* that blackened windows creates is not worth the minor improvement in radiant heating you might gain.

Let the light reach more of your room for more benefit and greater comfort. Use clear plastic on your windows to block air leaks (by far a more significant benefit) and provide some level of insulation value while still allowing light transmission. Window kits exist for a reason and aren't terribly expensive. They allow you to tighten the film so it's nearly transparent.

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* Mental and physical health depends on adequate exposure to light. The light and views that windows provide are more important than you might think.

Answer 3

Not.

From second hand experience I can tell you that if you get an unexpected moment of strong sunshine on them, they can melt stuck to your window. Use something like cloth instead. That I'd put on the inside to prevent it getting wet.

Answer 4

I would tend to go with isherwood's answer. Clear plastic would be the superior choice, mounted on the inside.

Whereas black surfaces do indeed collect heat readily, they also are superior radiators of heat and the laws of thermodynamics show that the rapidity of heat transfer from hot to cold is dependent on the difference in temperature between a hot area and a cold area (the difference is called the 'delta-t').

In your case, the black plastic is at the window (arguably colder than the room it adjoins), so a black covering would tend to radiate heat beck out in that direction, toward the window, simply due to the color of the plastic.

In essence, it would have a tendency to absorb heat from your room and radiate it back out the window, probably not what you had in mind.

Clear plastic, on the other hand, would let the sun in and act as an improved barrier to heat transfer, not the most efficient, but better than no second barrier at all.

Go to Home Depot or your favorite big-box home improvement store in your part of the country and get a roll of painters dropcloth, the thin lightweight stuff.

Mount it with blue painter's masking tape so it doesn't pull the paint off your walls when you take the plastic down. That would be your best bet.

Answer 5

I would lean towards putting them on the inside. There are usually gaps between the window frame and the rough opening in the wall to facilitate installing a square and plumb window in a not-square-and-plumb rough opening. Air can get through these gaps. In more modern homes these gaps are insulated and filled, but it is possible they may not be in your home.

If you attach the plastic to the wall around the window, it will cover the whole window and the gaps that may be leaking air behind the window frame and trim.

Note that connecting the plastic to the wall may involve tape, which opens the question of how to keep the wall finish intact when removing the tape when the weather gets warmer. This could be the subject of a new question.

Also, putting the plastic inside keeps the outside of the house looking like it usually does.

Note that black garbage bags will not be easy to see through. Sunlight is good for mental health. Maybe clear plastic may be better, with thick curtains when light needs blocking out.