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Curtains block radiant heat loss

 
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I was thinking about the concept of how heat is lost through our windows at night.  I couldn't quite get it straight in my head why it's worse at night than during the day but now I've figured it out and I thought I'd share.  We loose heat through our widows by radiant heat loss, this is in addition to conduction and convection. Radiant energy travels through our windows in the form of invisible light.  

The temperature difference between the stuff in your house and the objects outside is what drives the radiant heat loss.  When the sun sets the outside temperature drops and this increases the rate of energy loss through your windows.  If you close your curtains to block visible light from shining through your windows you also block the invisible light (radiant energy).  So closing your curtains and blinds at night reduces the radiative heat loss and will keep your home warmer through the cold winter nights, reducing your heating costs.

A thrifty way to reduce heating costs, simply by hang up curtains on any bare windows and remembering to close them at night.
 
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Hi Ann,
Welcome to permies, if no one has already welcomed you.

I like your post.  And I think the curtains that slow the radiant heat loss also slow heat loss by insulating, and by preventing the convective currents inside the room from providing a continuous supply of warmer air to lose through the window.

Hooray for curtains, and other window coverings.
 
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Hmm, I guess I should get those curtains hung after all.   Honey-do item number 43...

But now here's a thought.  If the top and bottom of your curtains aren't particularly tight to the wall, does that create an area of cold air between the curtain and the glass that tries to sink?  Then it pulls warm air down from above the curtain, past the window to cool, etc.  In effect creating a cooling loop of air?  

I'm still wrapping my brain around radiant heat loss versus conductive or convective losses that I'm more used to so maybe I'm underthinking the whole thing.  

 
Thekla McDaniels
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The conductive and convective heat losses are real and important.  Just add the radiant heat to the list of losses.

I am with you on stopping the airflow the cold window surface creates (even with all the fancy window glass) the glass is still cold ( just not as cold).  It makes quite a down draft as the air next to the window cools and falls, and creates a reciprocal up draft some other part of the room, making it feel drafty and cool, kind of like wind chill factor.

My curtains are longer than the windows.  I tuck the extra length under buckwheat hull stuffed "snakes" which rest on the window sill.  The top are also quite tight.  I have inset windows, so the top of the curtains are within the width of the wall, the room side of the curtains is even with the walls.  They are on pressure rods and the little ruffle above the rod fits up against the upper side of the window recess.  I have two layers of curtains on each window, and they are hung on separate rods.  There is an air space between the glass and the first curtain, then air space, then the second curtain.  Both inner and outer curtains are gathered independent of one another, so they each are trapping air, and obstructing air flow.  

And, with both layers of curtains and all those air spaces, I hope I'm cutting down the radiant heat loss.  
 
Mike Jay
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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.  

Hopefully I'm not hijacking the thread, but 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?
 
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Any curtain is better than no curtain. If there is no way to block the top, bottom or sides, it's only a little better, but still a net gain.

Even a restriction (short of total blocking) on top or bottom would increase the usefulness of your curtains.
 
Thekla McDaniels
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Mike,
I created a new thread where we could discuss airflow and curtains' effect on it, so we won't take this thread off topic.

Glad to see Glenn's comment, it's my conclusion too.

https://permies.com/t/59728/curtains-airflow-windowpanes#507496
 
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Mike Jay wrote:But now here's a thought.  If the top and bottom of your curtains aren't particularly tight to the wall, does that create an area of cold air between the curtain and the glass that tries to sink?  Then it pulls warm air down from above the curtain, past the window to cool, etc.  In effect creating a cooling loop of air?  



Since I've lived in a solar heated house in a cold climate for the past 20 some years, I've got a lot of experience with trying various things to keep the heat in the house. Even the simplest loosest curtain at night helps make the room feel warmer. You can clearly feel the difference in comfort. And a huge difference by morning. I think that's probably the radiant heat, and a bit of conduction out that big cold plate of glass.

When it gets to the coldest months of winter, January, I add an extra curtain made of a blanket. I don't have a fancy curtain rod assembly, I just screwed some hooks into the wooden lintel, and sewed some little loops on a blanket. It so happens that some small summer blankets we have fit the window almost perfectly when folded in half. The window is in the middle of the thick adobe wall, so there's a 6-inch sill on the inside. Usually I push the bottom of the curtain back towards the window on the coldest nights, but it doesn't probably make a huge difference. When the weather warms up and the days get longer in February I either leave a corner uncovered so the room won't be so dark in the morning, or I get lazy and don't put the curtain on at all.
 
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