Temperature of Windows

Hello,
Is there a rule of thumb on calculating temperature on the surface of a window? I had "Pella" designer series triple pane glass with argon between the first two panes installed (includes double hung and bay window) and it appears that the third pane is not much better then the old windows when I had just two panes of glass.
The problem is the temperature differance between glass surface temperature and room air. This is causing a flow of cold air that mimics an air leak around the windows.
I don't believe that I need to seal off the windows with plastic wrap because that is why I went with triple pane. The third pane is removable.
The only ideas that I thought of in solving this is to place a 100 watt lamp or small heater in the bay window and to keep the shades up on my double hung windows to allow better warm air contact with the surface of the glass.
This may not stop the air flow but at least it will be warmer, I think.
Does anyone have ideas that I might try?
During the day, temperatures are around 40 degrees and drop down to about 32 to 34 degrees overnight on average. I keep the room thermostat set at 71 degrees during the day and 67 degrees after 10:00 pm.
My house is a row type dwelling and the exterior walls are not insulated.
But the exterior walls are much warmer then the surface temperature of the glass.

 

Not sure what you're looking for, but make sure your expectations are realistic. Bricks and drywall (or plaster) are much better insulators than glass, so the glass will always be the coldest spot along the wall. Low-E, argon, krypton, etc. aren't going to eliminate this. However, just having glass in contact with the air shouldn't be generating a noticeable air flow -- what happens if you hold a lighter or match in front of the window? It shouldn't exhibit signs of blowing inside your house.

If you're getting that much noticeable air flow, like enough to cause the flame to flicker, it's probably coming from the meeting rail or where the sashes sit in the frame, not just the relatively cold glass meeting the relatively warm air. Of course, properly installed windows shouldn't have air leaks coming in anyhow, assuming they were properly manufactured and the weatherstripping is functioning properly.

 

JPM121,
It may be a combination of both windows and lack of insulation in the exterior walls. The bay window was installed by "Pella" contractors and I installed the double hung windows. I have the same problem with the bay window as I do with the double hungs. I have a question posted on the Insulation forum about the possibility of installing solid foam sheathing and/or house wrap on the inside of the exterior walls. I only have about 3/4 inch space behind the drywall. If the walls are contributing to the air flow, then insulating them will help.

 

Bay windows are generally colder than any other window in the house. This is because they protrude out past the walls of the home and are surrounded by the cold exterior on 3 sides plus the bottom and top. If they project out at all, they are kind of isolated from the normal flow of air in the home. For instance- say you have a window in your kitchen that is a straight window- maybe 3 double hungs that are all common mulled together. You have a HVAC vent directly below them, and when the heater turns on, it blows warm air over the window. Doesn't feel as cold. Then you replace those windows with a bay window that projects out, has more sq inches of glass, and more area to heat. Plus the glass is too far away from that vent for it to warm the glass- the seat blocks the airflow from reaching the windows. Additionally, maybe the installers did not add any type of insulation to the head or seat of the window? That would make it cold as well. So there are really multiple reasons why a bow window could feel cold. Glass is a terrible insulator.

I suspect that the reason Oberon has not replied yet is because he's not done typing his long articulate reply. Unfortunately my short attention span prevents me from staying on one subject for very long. But he would be able to give you a better idea of what temperatures you might expect from certain types of glass. I believe that when the temperature outside is 0F and the temperature inside is 70F, you should expect your glass to be about 45F. With the most efficient types of glass, you might get 55F. (as I recall from my Libbey-Owens-Ford education) Not bad when you consider that a single piece of glass would be about 16F under the same conditions. The center of your glass will be the warmest, while the glass edge will be the coldest.

Moving air over the windows by pointing a fan at them would be your best bet, because it would circulate warm air from the home out into the bay and would prevent the temperature out there from getting so low. The colder the air in your bay area is, the colder the glass will become... it's like a downward spiral, so try to keep it warm by keeping those curtains up and circulating some air. The heater is probably unnecessary. It'll just go "right out the window" so to speak.

I will take exception to something jpm mentioned:

>>properly installed windows shouldn't have air leaks coming in anyhow

This is incorrect because every window is manufactured to meet certain specifications, and one of those specifications is maximum air infiltration at X cfm. What this means is, ALL windows leak air. But it also means that there are maximum limits to what the industry recognizes is normal and tolerable. It would be true that properly installed windows will "minimize" air leaks, because some air leaks may be attributable to poor installation.

I don't like it when people jump on something I've said, so I don't want to give that impression, but I thought I should clear that up. This may be one of my longest posts ever!

 

Xsleeper,
Thanks for your reply. Its more confirmation that the air flow is caused by the temperature differance of the glass and room air mostly. My heating ducts are on the opposite side of the living and bedrooms. So the windows do not get sufficient heat to overcome the temperature differance.
I think I would object to fan noise to circulate air to the windows. Keeping the curtains and shades open during the day has helped a lot. It appears I may have to seal the windows using clear plastic. I did this in the basement where there is a large glass block window which gets ice cold and it does a good job. I made a 1 by 3 wood frame, routed dado's in it and attached the plastic with spline material. When I remodeled my basement, I installed insulation on the exterior walls and later saw that I would have to do something with the glass block window. Thanks Again.

 

XSleeper, your point is well taken -- I should have worded my response more carefully. Good info in your post, too.

 

rjordan,

As a follow-up to what has already been posted (good stuff!), I believe that you have suggested your own answer to part of the question in your last post.

In most situations, HVAC folks like to place heating elements on the exterior walls, often right below windows.

I have read "how-to" manuals (especially those written in the 70's and early 80's) that preach the concept of placing heating elements not on the exterior walls, but on the interior. What the folks making these arguments suggest is that it is more energy efficient to place the ducts on the interior than the exterior because heating the exterior walls allows more heat to be drawn directly to the outside before it ever has an affect on the inside home temperature. I am not by any means an HVAC expert, but I can think of a number of counter arguments that very much disagree with that assessment.

One very simplified version suggests that having the heat ducts on the interior walls will result in a convection current that will draw the warm air from the interior walls to the exterior walls, but it will "heat up" the interior space in the process better than having exterior heat ducts (rather than "wasting" heat to the outside as is suggested by exterior wall ducts, is the thought)... while this may not be untrue, and this heating set up will probably result in a convection current, it fails to consider the result of the movement of cooler air as well...cooler air from the window and into the living space.

As XSleeper pointed out, bow and bay windows are generally colder than other styles of windows. This may become obvious when a person leans into the space created by the bay or bow...you can often feel a noticeable drop in temperature within that space.

In your situation, I suspect that you have an excellent flow of warm air from the interior heating ducts to the exterior walls and the warmer air in your home is zeroing in on the coldest spot it can find - which turns out to be that particular window unit. The cooler air coming off that window might be 50 degrees or better, but when you feel it moving across your body as part of the convection current, it feels cold and uncomfortable.

Obviously, as XSleeper already suggested, warming that space would be the best possible solution. Plastic sheeting, as an additional insulator, should help to raise the temperature there as well.

Good luck!

 

Oberon,
Thanks for confirming my suspicions also. I will put up some temporary sheeting for this season and this Spring I will make up permenent wood frames with plastic sheeting. I prefer my own manufacture. The plastic window sheetings that are available are too thin. Home Depot sells a clear sheeting material that is much thicker and better suited for a permenent fixture.

 

A lot of good info has already been presented on this topic. I'll just add a few items that haven't been mentioned.

You may wish to buy a digital infrared thermometer to measure glass temperature. They cost $49.99 at RadioShack last time I looked. Someone said they saw them at Home Depot too but I haven't seen them there. If any of you window pros don't have one, GO GET ONE NOW! It will show you the surface temperature of whatever you point it at, as well as the differences on center of glass, edge of glass, etc. Incidentally, my 60 watt light bulbs are 212° and my ice cubes are -6°

There is a rule of thumb I'm familiar with for calculating center of glass temperature on the surface of a window, but it's based on 0°F outside. Here goes:
Single glazed 14°-16°F
Clear IG 44°
Single glazed with storm 45°
Double glazed Low E/Argon gas 57°

The Pella glass system you described has a glass temp of 59°-63° according to Pella's literature.

Incidentally, on sealed insulating glass units, the edge of glass temperature is much colder than the center of glass. Here's the stats:
Clear IG with aluminum spacer 29°
Low E/Argon with aluminum spacer 32°
Low E/Argon with warm edge spacers mostly 37° (steel) to 39° (non-metalic)

The Pella system you have, since it has an interior glazing panel built into the sash, has an edge temperature of approx 52°.

One last variable to mention regarding glass temperature is the presence or absence of roomside window coverings (shades, blinds, shutters, etc.). At 0° outside, a Low E/argon unit (57° roomside temp) will drop to 36° when covered with a shade, that's a whopping 21° drop. You didn't mention if your Pella's had between-glass blinds or shades (which would actually avoid the temperature drop completely) but that would make a considerable difference in surface temperature.

 

Tru blue,
<The Pella glass system you described has a glass temp of 59°-63° according to Pella's literature.>

Thanks for replying Tru blue. I'll see if Radio Shack has an infrared temperature tool in my area.
My glass temperatures feels much lower then the 59 - 63 degrees. If I were to guess, they feel more like 35 - 40 degrees. But maybe thats because I have a sheer drapery covering the bay windows and shades covering the other double hungs. My heating ducts are on the oposite side of the room and may be a contributing factor also. I placed plastic sheeting on all windows and the differance it makes is about 50% better. So the remaining draft that I feel is coming from the uninsulated exterior walls. I only have a 3/4 inch air space between my sheetrock and masonary walls and wonder if you have any knowledge about my options in taking down the sheetrock and adding rigid foam between the 1 by 3 furring strips. I am thinking about Dows product called Super Tuff. It has a foil vapor barrier on both sides. Using Tyvak is also being considered.
I posted simular questions on the insulation forum but the link they gave me on how to do masonary walls does not cover my situation due to the limited wall space.

 

Sorry I can't help you with the insulation thing. If you do get an infrared thermometer, experiment with it a bit. Keep the shades open on some and closed on others; also try some with the sheeting and some without. I hate to hear of anyone needing to use sheeting. It's best to take measurements at night so the sun isn't skewing the measurements by adding solar heat gain.

 

Tru blue,
These are all brand new windows from "Pella" The third glass which is removable is a lot colder then I think it should be. They installed the Bay window and I installed the double hungs. The reason for the sheeting is that all windows leak air but these maybe leaking more then they should. I would feel a cold draft at my feet. I think most of the discomfort is from room air making contact with the cold windows and the uninsulated walls and then dropping to the floor, hence the cold draft. I eliminated about 50% of the draft. I agree with Oberon's reply. I got forced hot air heat and hate it. I wish I had hot water heat with radiators under the windows.

 

Your thinking that the glass is colder than it should be is precisely why it would be helpful to have that infrared thermometer to test that theory. I would venture a guess that the glass temperature is pretty darn good since it's triple glazed double Low E with argon gas. It's hard to screw that up. You could compare that glass with a neighbor's glass.

The draftiness is interesting. Unlike glass temperature, draftiness can very much be a result of less than perfect installation, or a lack of insulation around the windows. There are numerous other factors potentially as well, such as living in an unusually high windload area, the windows themselves, defective or missing weatherstrip, a serious negative air pressure in the house, or it could be all in your mind (I'm not saying it is but it's been known to happen). You might want to call their service department to check it out - labor is included in their warranty I believe for the first few years.

 

Tru blue
I know I get leakage from where the sash rides against the vinyl insert in the jamb area. The manufacturer may call this normal. I suppose if the sash was air tight, I would not be able to open the window. However, I am partially satisfied with the plastic sheeting holding back leaks from any part of the windows. I will make my decision about taking down the exterior wall sheetrock and adding rigid insulation between the furring strips and using foam insulation where needed in the Spring. The infrared thermometer will be purchased next winter as I do not want to take apart the plastic sheeting. The sheeting tears too easily.

 

Great thread,

Thought I might add a geek's 2 cents worth.

You said "My glass temperatures feels much lower then the 59 - 63 degrees. If I were to guess, they feel more like 35 - 40 degrees."

From that I assume that by "feel" means you are measuring by touch. Glass conducts heat very well, more like metal than wood. For example, if you left a piece of wood and a piece of metal outside at 0deg F they would both be at 0deg F. If you stick your tongue on the wood it won't stick but if you stuck it on the metal it would. Same temperature, just the metal "feels" colder because it conducts heat much faster than wood. Glass will "feel" colder than wood or drywall even if it is at the same/close-to-the-same temperature.

Long-story short, for the same reason you don't touch your tongue on a flagpole, you don't touch a window to read its temperature.

That said, the actual temperature of the window is very important in determining the warmth of the air in the room, as convection (heat transfer through air) is directly proportional to the temperature difference between the window/wall/wood and the temperature in the room. The air around a Bay window will always be colder than the rest of the room, blowing a fan on it will just push more air across the windows to get cooled. This will cool your house more than it will be making the Bay area feel warm.

All of these useful tidbits, don't change the fact I don't know how to replace my own windows, but are fun at parties (where other geeks are present)

 

By ZekeMowatt,
<Same temperature, just the metal "feels" colder because it conducts heat much faster than wood. Glass will "feel" colder than wood or drywall even if it is at the same/close-to-the-same temperature>

That's interesting. great tidbit. I'll remember it. Thanks.