Window Convenction

Corrach-the-Blue's picture

Greeting fellow green wizards,

As a complete newbie to the word of green wizardry, I'm hoping to learn a lot here.

Currently we have no window coverings (except in our daughter's room) and a lot of window space on south-easterly facing walls. In the summer, its impossible to keep cool and we lose a ton of heat in the winter. The windows are weird, we're told, in that we have two panes of glass, but there's a six inch gap between them. We'd like to put up curtains of some kind to reduce heat loss in the winter and lower solar heating in the summer, but are wondering about the convection we've read about. We used incense in our daughter's room to see that there is a breeze coming from below the curtain while they're closed and there's about thirty degree differential between the outside temperature and the inside temperature.

Our question is how do we go about measuring how much heat loss there is from the convection behind the curtain. We're trying to decide if the cost of building some kind of curtain box to prevent convection is worth it or not.

Thank you all for any ideas you have!

Corrach the Blue

mountainmoma's picture

Welcome. First, I would suggest you buy the book, Green Wizardry, if you haven't already, and read the sections on this subject, as it is covered. It is a very good book, I have read it thru a few times and I have other expertise in this area already, I think it is a great baseline with ALOT of information.

The overall answer is that instead of spending time thinking you can measure every location, is instead to use the generalizations that we know about which items of energy saving give the most bang for the buck. In order this would be: air infiltration taken care of first, caulk and weather strip. Insulate, especially the relatively easy to get to attic and crawlspace. Then when those are done, do window coverings. Do the windows first in the areas you spend the most time in, the common areas like living room most likely, and when you do those, which you do not have right now, do them as correct as you can manage, so make shades that don't allow much convection currents. If you are like most of us, this list will take so long that it will be quite a while before you get back to the existing window covering in the one room, that is the last one to tackle.

The air gap between the window panes is like having a window and a storm window, it is not worth changing out windows, your windows are good. ( from the book when it talks about storm windows vs modern double one, part 3, lesson 22 window coverings, first paragraph)

You definitely have convection currents with the curtains, but it is not the first job to tackle. The first thing to tackle is air infiltration ( so caulking and weatherstripping, and that includes making sure your operable windows have weather stripping and all windows are caulked as appropriate) the second thing is to make sure the attic and crawlspace are insulated. Then move on to window coverings.

Not in that book, but from other sources, your feeling of comfort in a room will be enhanced by window coverings, even if they allow convection currents, because of radiant heat loss. your feeling of comfort in the home temperature wise is affected a lot by the sum of the surface temperatures surrounding you. The ceiling temperature (insulate) the floor temperature( insulate) the wall temperature and the window temperature, and glass loses heat the quickest so your window glass will be colder or hotter following the outside temperature. This is all about radiant heat loss, part 3 of the Green Wizardry book, Energy, lesson 20 on caulking and weatherstripping gives a simple and concise definition of the types of heat transfer, conduction, convection, radiant. Radiant heat transfer will make you feel hotter in front of a window with the sun hitting it than if the window has the shades drawn even if in both cases the room air temperature is the same. On a winter night, same thing the other way around, you will feel colder with no shades, even bad shades, on the windows in the room compared to windows with shades and the same room air temperature. In the winter night case, you are the hot surface radiating heat out and the night sky on a clear night especially will suck out that radiant heat to the cold depths of space.

(But of course when you make and add new window coverings, do them right and do them so they do not get the convection currents and so don't lose heat by conduction at the window pane, it is just that that particular window is at least covered so is not the first chore to tackle, go get out your caulk gun)

Corrach-the-Blue's picture

Thank you for the detailed response. Green Wizardry is where I heard about window convection originally. I had vague memories of it from high school science class, but that was a...couple...of years ago.

Using the incense method, we haven't noticed much air leakage around the windows. When we moved in, none of the windows had screens so we suspect that they had not been opened since the house was built twenty years ago. There is no attic and the crawlspace is well insulated already. The only place weatherstripping probably needs to be replaced is our back door, but I think we need an entirely door back there anyway.

My question was mainly prompted because my wife and I agree that we need window coverings, but she is not convinced that the heat loss from convection is enough to justify the added expense of preventing the convection. She is more likely to be convinced by hard numbers than abstract theories about heat transfer and air flow so I was hoping to provide the numbers by measuring the convection.

mountainmoma's picture

yeah, there is no easy way to measure it, as you say, you can show it is happening as you have done. This type of thing was supported by experience in the '70's during the energy crisis. you could feel the difference in heat, your neighbors bill went down more than others who did not stop convection, etc....

It used to be that places like Rocky Mountain Institute had hard numbers and recommendations for "common folk". but they sold out and their website has been scrubbed of useful information like that.

You maybe could estimate it. Just thinking about the physics involved makes me see that it is worth it, don't know how to convince someone else who thinks different. Ok, the larger the temperature difference the more energy you are going to lose as that temperature difference will make the convection current move faster. glass transfers heat readily. The air touching the glass loses heat, assume it loses all heat it is holding as it touches the glass ( close enough). Still air loses less heat then than moving air. You have shown that you can feel the convection current by your one window compared to the ones with no curtains. calculate the cubit feet of air that would pass by the windows, something like sq foot of window area times distance between curtain and window. calculate the cubic feet of air in the room. lets say 20ftx30ft living room with 8 ft ceilings is 4800cu ft. lets say window area is 2 sliding glass doors and one window, 5x3x6.5 ( my window is like one sliding glass door half, so 5 sliding glass door halves 97.5 sq ft times 3 inches (.25ft) is 24.4 cubic feet of air space between curtains and windows. the room has 200 times the airspace as the space between the windows and curtains, so air making one cycle across the glass 200 times means all room air has been in that space ( this is not in reality, we are doing a thought experiment, air currents may not mix entirely in the room, but I think this is good for seeing why it is an issue) how long does it take your incense to make it front eh bottom of the curtain to the top ? lets say 1/2minute, so 1/120th of an hour so 200 cycles across the glass takes 2 hours. Obviously not all the air molecules in the 3 inch airspace are losing all their heat energy on each pass, but you can get the idea on this.

So, for my example, every 2 hours all the room air has passed thru the gap between curtain and window. Likely it is much quicker than 30 seconds, so most likely more like every hour if it is really cold outside or their is wind from a storm both making the outer glass of the window colder.

So, what is the function of curtains for heat transfer ? They can help in 2 ways. The first, for immediate comfort is to stop radiant heat transfer, so you feel more comfortable in the room. Ok, fine, don't need to be sealed around edges for that. But, the second reason is to help convection/conduction losses from the window. They do not help this at all if they allow convection currents and often can make it worse if they set up faster convection loops. So, then they reduce radiant heat transfer and make convection/conduction heat transfer problems worse.

Good luck

mountainmoma's picture

in my house, even though I have good windows and doors and those are weather stripped well, I have ALOT of infiltration problems. So, the things you mentioned you have like weather stripping and insulation are not addressing infiltration. Try to find infiltration issues. GO under your house with spray foam and caulk and look for gaps and fill them. this will likely be in your rim joist area( under your walls) and places that plumbing and electrical go thru the floor.

I also do not have an attic but do have a crawlspace like you.

Your walls can also get a chimney effect inside of them, like you have between your window and the curtain. So, you want to seal the leaks in the walls, especially the top and bottom. You will need to pull back your batt insulation under the house in suspected areas to do this. SO, you pull it back from the edges of the house bit, you will push it back in place when you are done, and you seal the cracks between edges of wood under the area that is your outside wall ( the rim joist area) Then you have measured and got an idea where your bathroom drain sink areas are so you see if you can pull insulate back from those areas to use your spray foam, often the gaps are very large there ( my gaps there used to let tarantulas into the bathroom, so I found mine and sealed them !)

Since you do not have an attic, you have an area where your roof rafters connect to the top of your outside wall, and then what they do is just put blocking between each one, and maybe you can or can't see this area from the outside of your house, in my house I can. And, I can see I have spaces there that need to be caulked, you should caulk that upper seam or around your roof joists or what you can see. ( DO not caulk the bottom edges of siding though as it is for moisture to leave) My spaces there on the outside are so large, I had to spray foam)

Corrach-the-Blue's picture

Thank you for all of the great suggestions!