06:01AM | 06/13/05
Member Since: 06/12/05
2 lifetime posts
We have a new home with low-e glass, double hung, vinyl windows. We have several walls of windows and noticed that some of the windows form condensation on the outside in the early morning.

Why does condensation form e.g. on 4 out of 10 window sections on the same wall in the same area? This wall has a southwest exposure. At the same time, the windows in the living room (opposite side of house)consists of 9 sections with 3 having condensation on the outside.

In total, on the first floor, we have 33 window sections with 16 having condensation on the outside. This happens on all sides of the house. The top window section might be dry and the bottom of the same window will be wet. On an adjcent window it will be the opposite or no condensation at all.

The inside air temperature is 77 degrees (AC is on and twin dehumidifiers running in the basement which is 2300 square feet). The outside temperature is 71 degrees.

If anyone has any answers, I would appreciate it. The window manufacturer suggested that the areas around the windows were not caulked or insulated very well. I can assure you that they are all caulked. I don't know about the insulation.


03:37AM | 06/14/05
Member Since: 05/13/05
40 lifetime posts
Good morning Jim,

Condensation forms on an object when that objects surface temperature goes below the dew point. The dew point is defined as the temperature where the air is 100% saturated with moisture - or where the air is at 100% relative humidity.

Your windows are have condensation because the surface of the window is below the dew point. Based on the temperatures you mentioned, I suspect that the air is carrying quite a bit of water.

This is NOT a bad thing. What is happening on those windows with the condensation is that the window is performing exactly as designed...It is blocking the heat from in your home (77 degrees) from reaching the outside of your home (71 degrees) thus allowing the outside windows to be cooler than the morning dew point.

Exterior condensation on energy efficient windows is quite common and it is perfectly normal.

I should probably say that I am surprised that the window manufacturer didn't tell you this; but I am not. While the company's engineers probably know the answer, I suspect that you dealt with marketing or customer service folks who haven't a clue. Anyway, nothing at all to worry about there.

But, the next obvious question is why don't ALL the windows have condensation?

Whether on not a window developes exterior condensation is actually a rather complex bit of environmental and performance issues. For example, you are much less likely to develop condensation on a cloudy night. Any trees or other obstructions close to the windows? Bushes under the windows? Even the legth and angle of the soffit or other overhangs can affect the formation of condensation.

It is also possible that those windows that have not condensated have some energy leakage going on that is warming the exterior of the glass to a temperature above the dew point. If that is the case, it could be a very minor "leakage" that might raise the temp just a tiny amount; just enough to avoid the formation of the condensation. A tiny change in either temperature or humidity might raise or lower the dew point just a little bit and you might see a whole different level of condensation.

Anyway, that was really long, but the ultimate result (again) is that condensation forming on the exterior of energy efficient windows is quite normal and nothing to worry about.


05:16AM | 06/28/05
Member Since: 06/12/05
2 lifetime posts
Thanks for the response. You have narrowed the cause down to energy leakage.

The wall of windows involved has no shrubs, trees or other obstructions. It faces wide open space. The wall is 19 feet high. The soffit is about 1 foot wide with the closest window starting about 1 1/2 feet below it. Three windows are in the same proximity to the soffit with two windows forming heavy condensation while the third one does not. The exposure on this wall is Southeast.

Condensation forms on some window panes on all four exterior walls of the home. We are in a new development which was farm land, no trees or other obstructions are present.

The fact that condensation forms on all exposures and only on some panes tells me that energy leakage has got to be the cause. What is more perplexing is that on a double hung window, the top or the bottom will be covered with heavy condensation while the other half is completely dry.

The question is, are the windows not made properly or is it a faulty installation by the builder? I paid a significant price for the low-e upgrade.

I called the window manufacturer and the customer service rep told me initially that they had a problem previously and some of the widows that were supposed to be low-e that were shipped to customers were not. She said this might be the problem.

Shortly after that, I was contact and advised that this was definitely not the problem with my windows. They did not have an explanation as to what the cause was.

Thanks again for your response.


03:21PM | 07/03/05
Member Since: 07/02/05
10 lifetime posts
Window company on some type of windows models coat some of the glass on one side to reflect heat back into the house and allow heat from the sun into the home kind a a passive heat gain. So if the company installed the glass the wrong side facing in you would have heat loss to the out side of the glass. I forget how you check it but not hard to do.



03:31AM | 07/07/05
Member Since: 05/13/05
40 lifetime posts
Actually Jimspring, I would say that there really doesn't have to be either faulty construction or installation to have the differences in condensation on exterior lites.

There are a number of minor things that could affect the formation of condensation that it can be very difficult to pinpoint a specific one way or the other.

I have seen the same question as yours your case it was cooler outside and warmer inside. Well, what if it is cooler inside and warmer outside?

Can you still get condensation formation on the exterior of the windows? Yep.

Ultimately, the window, specifically the LowE coating, is designed to stop heat flow thru the glass. If the outside is cooler, as in your case, then the space between the lites can warm up to temperatures close to inside the house but the LowE will block that warmth from penetrating to the exterior of the lite, thus allowing the outer surface to be below the dew point, and condensation will result.

On the other hand, if it is cooler inside, then the cooler indoor air will penetrate into the spacing between the lites as well. But, in this case the warmer outside air is prevented from passing into the space because of the LowE coating...which is applied on the number 2 surface of the IGU (insulating glass unit - the term for dual or triple pane glass). The number 2 surface is the inner side of the outer lite.

Anyway, the LowE coating is metallic, so it can cool down based on the temperature inside the glass airspace and that would cause the exterior lite to cool to a point below the exterior dew point and again, condensation would result.

Really simple concept, but many variables involved! That some windows condensate and some don't, even if right next to one another, is not necessarily a flaw. It may be an environmental issue as simple as the shape of your indoor ceiling related to airflow against the insides of the unit.

Jackblack, you were referring to the LowE coating I believe. I hope this helps clarify it a bit for you as well.

And the way to check for a LowE coating is to hold a match up to the window when it is dark and watch for the flame reflection.

You will see three reflections in a LowE unit. The middle one is the LowE reflection and depending on whether it is closer to the outdoor side or indoor side (other reflection) will tell you which lite has the LowE coating.

Jackblack, good point about installation of the IGU and which lite the coating is on. The coating being on the #3 surface, versus the #2 surface, could have an effect on condensation and performance.

There is a difference on which surface you want the coating on depending on which coating you use (there are really three different species - and several sub-species), and your specific environmental location (heating, cooling, both). But, about 85% (or more) of all LowE coatings in North America are on the #2 surface. About the only exception is if you live in parts of Canada or extreme mountain or northern areas of the US.


09:56AM | 07/14/05
Member Since: 07/12/05
1 lifetime posts
This discussion string has been a great help to me, but I'm not quite out of the woods and would appreciate your help.

I recently put high-end vinyl windows in a cute little house where I now live. I will be selling this house soon (job transfer), but I'm looking forward to building a new house in the new area.

I've had some exterior window condensation on extremely high-humidity mornings that sounds a lot like the problems encountered by jimspring.

A key difference, though, might be that, in my case, the outdoor morning temperature was NOT cooler than the interior. (This is the very hot, very humid area around Washington, DC.)

After reading Oberon's great explanations, I'm about 90%-ready to just accept that the darn super-duper insulated and coated-up-to-the-wazoo windows are gonna sweat on hot, humid mornings. (And hot, humid ready-to-storm afternoons, too.)

I don't completely understand all of the technical details, but I've heard enough serious people tell me that the exterior condensation happens that I'm OK with it and no longer wonder if I got a bunch of lemons instead of windows.


When I build my new house--small and plain, but very high quality--I'd like a few big windows. I'm really loving the "H" Windows that were developed in Norway. (

But if I'm going to have condensation on the large windows, I'm not sure I want them. Yes, I'll have some windows, but the design of the house will change if the insulated windows require exterior sweat during the high humidity times. (My transfer will not get me away from the hot-and-humid environment.)

I know the folks who show the H Windows will tell me that their windows are the best--and I believe them--but is it even possible to get big modern windows without the exterior condensation?

I'd even be willing to do a couple of the "soundproof" window arrangements with 2 windows in one frame just to get away from the condensation. (I just mention this as an example of how I might vary things just to get away from the exterior condensation.)

So, can anybody tell me about this? Is there anyway to be reasonably sure that the exterior condensation problem is under control?

Thanks for your interest and help, including the info already offered in this discussion string. There's NOTHING so fine as learning the facts!

Best wishes to you all in your work and with your windows,



11:39AM | 07/17/05
Member Since: 05/13/05
40 lifetime posts
Actually, you will have condensation anytime that the temperature of the surface of the glass drops below the dew point temperature.

Generally, unless it is really steamy outside and the inside air conditioner is cranking, you won't see exterior condensation on the window surface during the day...but, generally does not mean always.

Also, a slight breeze or the sun shining directly on the surface of the window will affect the formation of condensation...or lack of...but, given all the right conditions and you are going to have some condensation.


06:39AM | 10/01/08
Member Since: 09/30/08
1 lifetime posts
I have read your response to the widow condesation issue for condensation o n the outside of the windows,

How can the exterior panes get below dew point when they are insulated?

I believe that the only way for condensation to form is moist air traveling across a cold surface, which would mean that the windows are leaking around their seal in the frame letting the warm inside air to travel across the cool outside pane and then condense.


08:57AM | 08/22/13
""Actually, you will have condensation anytime that the temperature of the surface of the glass drops below the dew point temperature.""

Oberon seems to make a lot of sense here. You certainly get morning dew on grass, and metal surfaces like the hood of a car when the air cools overnight.


05:32PM | 08/23/13
This is a very educational post on windows. Thank you Oberon for the in-depth but reasonably simple explanation. I see so many posts where people are blaming the window manufacturer, when a lot of times, I think it's just aspects of nature -- dewpoints (because having insulated windows doesn't mean there is zero hot/cold transmittance), high indoor humidity (which really doesn't have to be all that high to make condensation on windows), or lack of circulation inside the house around the window (such as having your shades pulled most of the time).


08:04PM | 09/09/13
I am having a similar issue on my new house. Some windows have condensation issues on the outside and some do not. In my case it even happens when I leave the windows open at night. Wide open window so the the temperature is exactly the same on both sides of the glass but the outside pane gets condensation and the inside stays perfectly dry. Any ideas?


12:56PM | 09/27/13
Hi everyone,

I found this forum due to a similar issue with new low e windows that are doing the same thing at my house. Only 4 of the 5 new ones get condensation on moist, foggy mornings but in all honesty, I didn't just pay $$$$ to get the periodic "aquarium view" in my picture windows. Also, Kansas makes a really good point, shouldn't the windows stay above dew point if insulated? Furthermore, my condensation is rectangular and soesn't cover the entire window pane (pic attached).

While I appreciate some of the "weather" explanations, Now I'm also concerned that the soon to be built retirement house will suffer similar problems and that's something I'm not willing to accept.

Thanks for any more info!

20130927 074815


03:50PM | 09/30/13
The hardest thing for people to understand is the CAUSE of the condensation...

It comes back to 2 simple points:

1. TEMPERATURE of the surface of the glass.
2. AMOUNT of moisture in the air.

these two factors can explain condensation.

Air can hold a certain amount of water.
The warmer the air, the more water it can hold. (lots of science out there on this one)
When saturated air cools down, it has to release some of the moisture - Think DEW on the grass... Think about your ice cold drink when you are outside on a hot day - what forms on the glass? why?
The moisture collects on the cool surfaces.

Because the air temperature, moisture, and surface temperatures are always changing, it is hard to get an idea of the circumstances involing certain windows - BUT the science is always the same.

Condensation forms when the temperature of a surface drops below that of the dew point.

Was there some air movement that dried the glass off?
Air movement that brought in less moisture in the air?\
Is an area of your yard lower than another? (think fog)
Is there a bush that is giving off moisture under the window?
Is that bush giving off heat instead?
Is the window sheltered more than the others? Less?


07:16PM | 10/15/13
This is all very good information and we too are experiencing moisture on the outside of the windows, only the top half of the window only.

Another concern I have about this moisture is will this be an issue in the winter and become frost on the windows?


10:29AM | 10/28/13
Check out these two articles for some good tips on avoiding and removing window condensation. Like the others say, it's all about temperature on the surface of the glass and amount of moisture in the air.


08:15AM | 12/16/13
Andersen windows has a very good video that explains window condensation - both outside and inside your home:


10:21PM | 08/22/14
you may want to look at whether the sash that have moisture are covered by a half screen and the ones that are not do not have the screens. The screen mesh will alter the glass surface temperature which could be part of the reason you are seeing the issue on some of your sash but not all


04:08PM | 08/26/14
We have replaced four of the 31 windows in our home with High efficiency and these are the only windows with morning condensation outside and no views until it clears. We have a significant mountain view that is gone until this clears. This is unacceptable to just say this is normal. Why didnt the manufacture of high E have an answer to correct this and not just accept this as normal. There has to be someone that can answer how to resolve this issue without going back to lower efficiency. I am stuck with the cost of these windows and would rather go back to spending a few cents more on air conditioning to have a window I can see through.


10:01PM | 09/06/14
The point of having double or triple glazing is to insulate the interior from the exterior weather. The only explanation for exterior condensation that makes any sense is faulty seals on the windows that get exterior condensation.


01:15AM | 09/07/14
i had this same problem with early morning condensation on the exterior of my triple pane Shuttco windows - a perfect rectangular forms with a perfectly clear 2" margin on the outside of the condensation area. It only occurs on north and south facing windows that do not get direct sunlight and disappears after a couple hours - sure would like to know what causes this to happen - have a hard time buying into the weather causes and reasoning - but if this is indeed true , why wouldn't distributors be up front about this situation prior to you shelling out big $$$$ for these windows - whats funny is my east west windows replaced at same time are fine


08:41PM | 10/01/14
We too have recently had a problem with condensation on a newly installed double-pane window. My problem is that the condensation only forms on 1 window on the outside of the outer pane. I read through all of the other posts and I must conclude that there is probably no problem. But, my husband, being an engineer just can't stand the fact that the window does this and wants it replaced. I don't want to have to go through the hassle. Should I just tell him not to worry or would it be any different if we did in fact have that window replaced? Thanks.
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