curtains and airflow across windowpanes

Mike Jay wrote:Thekla, that sounds like a wonderful way to stop all the thermal losses with curtains.  Unfortunately the curtains I have to hang were made by my loving mother in law and they're bigger than the window so I have to hang them several inches inside of the wall surface.  I can't really block the top since they have a fair amount of gather/undulation on the curtain rod.  Our window sills are only an inch deep so I can't really do the draft dodger sock system you use.  

I wonder in the case of curtains that can't block air flow, are they a little better than no curtains, much better, the same or worse?

Hi Mike

I've always wondered about that too.  It seems like if I don't block the airflow or slow it down, then curtains may be increasing the flow of heat OUT, increasing the cooling effect of the window glass, because of that stream of warm air across them.

Seems like the channel with enough room for the air to flow, (between window pane and curtain) makes the air flow more concentrated, more heat transfer out through the windowpane. .

I've never thought of a way to test this.

Maybe someone else has.  I've just decided to proceed as if there is significant heat loss through air crossing the windowpane, and the draft it creates adds to feeling cold.

I think this was the origin of valances, little boxes that housed the tops of the drapes.  They held the hardware that the curtains were hung on.  And I think if  the drapes hang to the floor, that stops the current at the bottom.

I think if you can stop the airflow at either the bottom or the top, then the air doesn't get that loop going, down the window a warmer inner surface across the top and down the window again.

With your one inch depth the windows are set in to the wall, you might be able to get a tension rod and hang sheer curtains that are underneath the gift curtains.

As you so correctly observe, heat loss and efficiency and curtains are surprisingly complex.

All curtains can block/reduce the radiant heat loss from a warm body to a cold window, some better than others.

If the curtain is open at the top and the bottom, and there is an inch or more between the window and the glass, it will set up a convective loop with the chilled air "on" the glass falling, and the relatively warm air against the curtain rising.  This convective heat loss can be numerically worse than the naked window.  But...even if that is true, the curtain is still stopping the radiant heat loss from your body to/through the window.


Of course, the ideal situation is to stop both.  For that to happen, you have to close off the top and bottom both.  If that's not possible, then it's more effective to block the bottom than the top.

Having the bottom open, the chilled dense air against the window pane will still fall, and that will still draw warm room air up to replace it, even with the top blocked off.

Thanks for setting up the new thread!  

I think a way to test it would be to pick three nights with the same low temp and try three different approaches.  One night with blinds open, one with them closed but not sealed at the top/bottom, and a third with them sealed off.  Potentially a fourth with just the top or bottom sealed off.  Put a thermometer in the room and see what it says at 7 AM.  As long as your furnace thermostat is in another room, the heat provided to the bedroom should be consistent and the heat loss would be the main variable.  Especially if it was done in an unused bedroom so the occupants wouldn't affect the trial.  I also wonder if cloudy vs clear nights would affect the results?

Now that I've laid out this wonderful experiment, I hate to say that I can't run it.  I heat with wood so my heat input is quite variable

I like the valence idea.  That would be a sexy way to block the top while letting the curtains still open.  The few that I've seen have open tops but it would be simple to block them off.  And I also think that if you block just the top or bottom you still get most of the benefit of blocking both as long as the sides are relatively snug to the wall.

Regarding radiant heat loss, does the loss increase as the temperature differences of the two surfaces increase?  So at night you have a 65 degree interior that is "visible" to the wide expanses of space that are at absolute zero, so that gives a high radiant heat loss?  But if it's cloudy the loss is 65 degrees vs -100 degrees (or however cold clouds are)?  I think the surface finish/color/texture also affects radiant heat transfer.  If that's the case, the backing on the curtains could be important.

Ok, I thought of a way to block the top and reduce the gap at the bottom without a valance.  Take or make a metal curtain rod bracket and bend it up at a 20 degree angle.  Then have the rod in the normal divot during the day and when you close the curtains at night, just push the rod towards the wall and it will slide/roll down the bracket until the curtain is pressed against the wall.  Then in the morning when you open the curtains you pull the rod back out to the normal location.  Here's an amazingly crude sketch of the system.  Bracket is in brown, daytime curtains are orange and bunched up at the side of the window, green is the position of the curtain at night and tight to the wall.  The two black circles are the two positions of the curtain rod at each time of day.  This is a side view with the window glass in blue.  Sorry again for the crudeness of the sketch...

I understand the sketch.  It's enough to get the idea across.  It might help me too.  I have some very heavy drapes I have been thinking I would put across the whole 3 window section of the wall. NOT put it in the window recess.  And possibly retain the current situation with the two separate layers of curtains on each separate window.

I have often thought about that reflective coated bubble wrap stuff, and about making panels of it that just go right against the window at night, get removed during the day.  Out of sight and out of the way storage is the drawback on that one.

"Thanks for setting up the new thread!"


It was my pleasure...



"I think a way to test it would be to pick three nights with the same low temp and try three different approaches.  One night with blinds open, one with them closed but not sealed at the top/bottom, and a third with them sealed off.  Potentially a fourth with just the top or bottom sealed off.  Put a thermometer in the room and see what it says at 7 AM.  As long as your furnace thermostat is in another room, the heat provided to the bedroom should be consistent and the heat loss would be the main variable.  Especially if it was done in an unused bedroom so the occupants wouldn't affect the trial.  I also wonder if cloudy vs clear nights would affect the results? "


Your idea experiment is a good one, but with one flaw.  Thermometers mostly measure air temperature.  Humans perceive temperature through all three heat transfer methods, conduction, convection and radiant.  The perfect example of why this matters is two rooms with the exact same air temp, let's say 72F.  One room has a big picture window and it's 11F outside, and the other has no window at all, just a nice insulated wall.  The air temp will read exactly the same, but the human will find the room with the picture window noticeably colder due to the radiant heat loss through the window.

So even if one method produces a clear winner with a higher air temperature, it's not necessarily the room that the human would find most comfortable.

That's one of the biggest advantages of a rocket mass heater, you can sit on it.  Conduction is the most effective heat transfer method there is. Radiant is next and convection is the least efficient.

Wood stoves in general, and rocket mass heaters in particular also heat you with radiant heat.  So you could be in a room with pretty chilly air (54F) and think it's cozy because your sitting on the hot mass bench of the RMH, and right next to that big hot radiating barrel.  Positively toasty...

There are people doing interesting work on thermostats that pay attention to the room's mean radiant temperature, to better modulate the furnace, but they are uber rare.  Like so:

https://blog.adafruit.com/2014/02/10/radiant-temperature-thermostat-measures-air-convection-and-mean-radiant-temp-for-better-comfort-arduino-opensource/



"Regarding radiant heat loss, does the loss increase as the temperature differences of the two surfaces increase?  So at night you have a 65 degree interior that is "visible" to the wide expanses of space that are at absolute zero, so that gives a high radiant heat loss?  But if it's cloudy the loss is 65 degrees vs -100 degrees (or however cold clouds are)?  I think the surface finish/color/texture also affects radiant heat transfer.  If that's the case, the backing on the curtains could be important. "

Exactly.  Your intuition has served you well.  All three heat transfer methods are linearly driven by the temperature differential, and color/texture matters for radiant.


Einstein complained that thermodynamics gave him a lot of trouble, so don't feel bad that it seems a bit convoluted.

Good, if Einstein had trouble, I'm in good company.  I took a semester of Thermo in college and didn't like it much.

So on the proposed experiment, I can see that the human body can feel radiant cooling through an un-curtained window.  Can't the surfaces of the room also lose heat to the window?  And then those surfaces in turn get colder and the air temperature surrounding them would go down?  And then the thermometer would notice the air in the room was colder?

I guess if I was doing the experiment, I'd still test that just to have a baseline.  When that code/trial fails, we'd have the curtains closed anyway and the radiant cooling direct to the human would be resolved.  Then it would be just down to if closing off the curtain top, bottom or both is best.

Thekla, related to your bubble wrap idea, one thing I've also been wanting to try is to make interior storm windows from shrink plastic film.  It gives you two extra panes of material.  And it's pretty clear so you don't have to take it down each day.  Here's a guy who did it with Mylar instead of shrink film.  Maybe Mylar is better...?...  Interior Mylar Storm Windows

"So on the proposed experiment, I can see that the human body can feel radiant cooling through an un-curtained window.  Can't the surfaces of the room also lose heat to the window?  And then those surfaces in turn get colder and the air temperature surrounding them would go down?  And then the thermometer would notice the air in the room was colder? "

Yes, everything in the room that can "see" the window will lose heat to the window by radiant means if there is a temperature differential.  And yes, that will eventually cause the air temperature to also drop, which the thermometer could measure.

But the thermometer could not measure how much colder the room feels to a human due to the radiant heat loss.  In the end, the thermometer is still just measuring air temperature.

If the sides and bottom of the curtain are sealed, there can be no convective loop. All the air against the window wants to go down, and if it can't, then nothing will move at the top. Of course, a perfect seal is highly unlikely, so practically speaking it will always be beneficial to seal the top as well.

If top and bottom are sealed instead of just bottom, I think you would get distinctly less convection. Some would still go in at the top of the sides and out at the bottom, but it would be less free to move.

And, if the top is "sealed" with the traditional valance, and nothing else is that well fitted, just kind of close to the wall say within an inch, I think you are still miles ahead.

Thekla McDaniels wrote:I have often thought about that reflective coated bubble wrap stuff, and about making panels of it that just go right against the window at night, get removed during the day.  Out of sight and out of the way storage is the drawback on that one.

I actually read somewhere that you can just press bubble wrap up against the glass and it'll stick there, so you don't have to make panels or anything. I haven't tried it yet, but sounds like a nice addition to curtains for the very coldest nights of the winter.

"I actually read somewhere that you can just press bubble wrap up against the glass and it'll stick there, so you don't have to make panels or anything. I haven't tried it yet, but sounds like a nice addition to curtains for the very coldest nights of the winter."


We have actually done that.  We bought a huge roll of the large bubble wrap, cut to fit, spritz a little water on the window and press, and voila, it sticks.


Yeah, except in real life it falls down after a few days when the dry air evaporates the water film.  It lasts a little better if you put a few drops of soap in it, and a few tablespoons of glycerin in the spray bottle.

The reduced draft and improved comfort are real enough though.

Great, Rebecca and Troy.
Any suggestions about day time out of sight storage?