Window U-Value

Oberon

As was pointed out in a couple different posts, krypton performance peaks in a narrower airspace than does argon. Krypton works well at 1/4" spacing and argon at 1/2" spacing - give or take a thirty-second or two.

There are a number of argon-filled triple-pane windows on the market that have performance numbers in U-.19, or better, range. These units are designed to support a wider IG unit than would be required for a triple with krypton fill. Even with slightly narrower airspaces, a triple with argon can reach the U-.20 range or so.

Triples with argon work quite well and it is a good bit easier to keep argon in an IG than it is to keep krypton in an IG unit; and since krypton simply isn't very common, it is a good bit more expensive than argon.

enforcer

Oberon,
when looking at a triple pane w/Argon,is it important to see if the spacing is wider? will Argon fail if spacing is too narrow?

Oberon

Enforcer,

The unit will be no more likely to fail than will any other IG, but argon is simply less effective at the narrower airspace width that is often common with triples.

At 3/32" width airspace, which is optimum for krypton, and not uncommon for some triples, argon isn't much more effective as an insulator than is air.

Argon's effectiveness as an insulator begins to be "noticeable" at about 3/8" and as the airspace widens, the benefit of argon increases while krypon's effectiveness as an insulator decreases. Argon's effectiveness peaks and stays fairly constant up to about an inch or so before performance begins to gradually decline.

However, even at 1/2" width airspace krypton will outperform argon but, krypton's slight advantage as an insulator at the wider airspace doesn't offset the additional cost of the krypton fill, so argon is clearly the better choice at wider airspace.

Ultimately, it comes down to both the economics and the reality of the availability of krypton. Simply, there isn't enough krypton to fill all the windows made in the US - forget the rest of the world.

easy sider

It seems that the newer energy saving coating negate the passive heat gain that the green builders specify. Most of these "green" projects want old style hard coat low emissivity glass to increase solar radiation to help heat the house. These homes are using shades to retain heat on cold dark days instead of triple glaze and the newer low e coatings.

So, I still feel that any energy savings in most of the USA that isn't eligible for a rebate is overkill or over-sell.

enforcer

to be honest,is there really an extra benefit to having a triple pane with a U-factor of 18 opposed to a U-factor of .22 ? a noticable difference?

MarkC

We use a unit with a .22 U-Value and .27 U value to be competitive with the market.

MarkC

Oberon

Much depends on the individual homeowner's desire to participate in their home's energy performance.

While some folks really enjoy helping their home to achieve maximum efficiency, other folks prefer to build maximum efficiency into the home so that they are not required to become intimately involved in their homes energy performance.

Insulating shades can be a positive and a negative. Positive because at their best they can help to keep warmer air in the home when the home isn't gathering solar energy, but negative because few people like the idea of living in a cave when the sun isn't shining directly on their window or of having to constantly adjust their shades when the sun is / isn't shining on the window.

And from a practical application, windows need warm air from in the home to flow over the surface of the glass in order to avoid condensation build up on the glass. One of the biggest contributors to condensation on windows is shades or blinds blocking indoor heat from the glass when it is cold outside.

Looking at it from another perspective, is there any extra benefit to increasing wall insulation by 18%?

Realistically, both .18 and .22 are really good numbers for windows and I would be more than happy with either. It probably comes down to doing a cost-to-benefit analysis to see if whatever extra price is worth it.

dayspring

I don't know if anyone is familiar with this software but it totals the u-value of a home, everything added together. pretty cool. It's called Res-Check
http://www.energycodes.gov/rescheck/
check it out if you haven't

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Ed the Roofer

Does anyone else think that this post should become a "Sticky"?

I think it is one of the most informative threads about widow values I have read.

Ed

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dayspring

Stick(Y) that sucker

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easy sider

[quote=Oberon;530338]Much depends on the individual homeowner's desire to participate in their home's energy performance.

While some folks really enjoy helping their home to achieve maximum efficiency, other folks prefer to build maximum efficiency into the home so that they are not required to become intimately involved in their homes energy performance.

Insulating shades can be a positive and a negative. Positive because at their best they can help to keep warmer air in the home when the home isn't gathering solar energy, but negative because few people like the idea of living in a cave when the sun isn't shining directly on their window or of having to constantly adjust their shades when the sun is / isn't shining on the window.

And from a practical application, windows need warm air from in the home to flow over the surface of the glass in order to avoid condensation build up on the glass. One of the biggest contributors to condensation on windows is shades or blinds blocking indoor heat from the glass when it is cold outside.

__________________________________________________ __________
However......
When the sun is out, the blinds or shades can be open to max out passive solar heat gain in colder weather. In warmer weather, a shade would be closed when the sun hits the glass or possibly an awning could be used. Most home owners close the blinds or shades at night for privacy and most home owners all ready own some sort of blind or shade.

So, wouldn't the insulating window shades,drapes and blinds be a better value than extreme window upgrades such as triple glaze ?

Condensation is attracted to the coldest areas of a room which is usually the window opening. With a vinyl framed window the cold spot becomes the glass where the attraction is the greatest. This is where water vapor turns into water. Even with window upgrades this will always be the coldest area of the home and susceptible to condensation.

This water that forms on the glass doesn't hurt anything and can wiped off with very little effort.

MarkC

U Value is very important, but the air infiltration rating on the windows can be just as important. The lower the air leakage, the tighter the seal and this will also signifigantly contribute to energy savings and reduce energy loss.

MarkC
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Oberon

[quote=easy sider;532015]Again, it is more often than not simply the homeowners desire to participate in their home's performance. Some people like the idea of actively controlling their enviroment, others want to install it and forget it. As you said, awnings are great and they actually work. But, in today's world few people really want awnins on their homes. I remember as a kid seeing awnings on many homes in the old neighborhood...not something we see very often today, at least not where I live.

I once saw a system that automatically raised and lowered insulated window shades based on glass temperature readings. These were not loose shades, each shade had a track that it travelled in for a tight seal in the opening and weather stripping at the bottom of the shade for a tight seal to the sill.

The folks showing the system had all these great numbers on potential energy savings - etc, etc, etc. Okay, so every window in the home has sensors on the glass and every window in the home has motorized shades - so what would a system like this cost? $500/window? $1000/window? $2000/window? I can't imagine why anyone would see that as a good idea...what would be the payback time?

In a retrofit, using (non motorized) insulating shades, curtains, blinds, etc is a very real option versus replacing or upgrading the windows. But in new construction wouldn't it make sense to upgrade the the best available (and affordable) option versus planning upgrades before the home is even completed? Higher quality insluating shades can easily reach $200 - $300 per window which is often more than the cost to upgrade the window in the first place.

In most parts of the country a good quality dual pane window with a LowE coating is going to be more than adequate. Triple pane can certainly be overkill in many situations. The triple with LowE on two surfaces will save money over the dual pane with one surface coated, but the payback may not be worth the additional cost.

Per high solar gain versus low solar gain coatings, a properly designed home or building certainly can benefit from using high solar gain windows in the winter months. However, few solar designers are going to recommend using high gain windows on the east, west, or north walls of the home. I would suggest that most will likely recommend maximum solar gain on the south wall and a minimum number of higher insulating windows on the other three sides.

Curiously, the latest addition of Energy Star limits SHGC to .55 even in the far north. The reason is likely due to limiting solar gain on east and west windows - which can potentially result in poorer performance because of excessive heat gain in summer and even possibly in winter given the right circumstances.

Condensation occurs on a window when the temperature of the window/glass goes below the dew point temperature of the air surrounding the window.

A window exposed to warmer room air during colder winter months is much less likely to have condensation than is a window that is covered. Given that most folks do close blinds or shades in winter and condensation is often a related issue on those windows.

In order to stop condensation from forming on the surface of a window, it is necessary to either lower the dew point temperature of the air to a level below the dew point temperature of the window surface, or to warm the window surface to a temperature above the dew point temperature or a combination of both.

Relative humidity is a comparison of actual vapor density versus saturation vapor density at a particular temperature.

Put a bit more simply, dew point is 100% relative humidity or the point where the air - at a certain temperature - is no longer able to hold any more moisture. If the air has reached vapor saturation (100% relative humidity), then the air will release moisture which will show up as condensation on any surface that is at a temperature below the dew point temperature of the air.

There are two ways to lower relative humidity – first, increase the air temperature or second, decrease the moisture content of the air. Lowering the relative humidity, which is the common “remedy” for wet windows, may have absolutely no effect on controlling window condensation or it may completely solve the problem – depending on how the relative humidity is lowered and what affect the “how” has on both the moisture level of the air and the temperature of the window.

Increasing air temperature will lower relative humidity but it will not change the dew point temperature which is based on the amount of water vapor in the air and is not based on the temperature of the air. So while RH is lower with higher air temperature, it may not effect condensation on window surfaces at all – unless the rise in air temperature also results in a corresponding rise in window glass temperature to a level above the dew point temperature.

But lowering the amount of water vapor or moisture in the air will also lower the dew point temperature as well. If the dew point temperature is lowered sufficiently to drop it below the temperature of the window glass there should have no more condensation issues.

The amount of moisture in the air is typically measured in grams per cubic meter, which is kind of nice for our metric folks but not so nice for our non-metric folks; but the metric version is much easier on the calculator than the English version. So in the interest of making this stuff a bit easier to understand for all of us non-metric types, I am going to covert back to Fahrenheit rather than use Celsius temperatures in the calculations (actually I am going to mix the two which is easiest for me in this case).

Okay – consider a home at 65 degrees F and with a relative humidity reading of 40%. In this scenario there are 6.25 grams of water per cubic meter of air which equates to a dew point temperature of 38 degrees F. So at 38 degrees the air will be at 100% relative humidity or at saturation vapor density.

If a home hygrometer measures the relative humidity at 60% while the temperature is 70 degrees, the dew point temperature is just about 51 degrees – meaning that if the temperature of the window surface is below 51 degrees there will likely be condensation on that window surface.

There will be a difference between edge temperature and center-of-glass temperature which can be significant when dealing with condensation issues. Also, consider that the interior glass temperatures are based on the fact that moving, warmer, indoor air is actually in contact with the glass at a given time.

A triple pane with dual LowE coatings and argon or krypton fill (depending on width of the airspace) will have an indoor surface temperature approaching 60 degrees in the zero outside, 70 degree inside scenario. There has to be a lot of moisture inside the home to have condensation on that window.

Curtains, shades, other obstructions can cause problems by blocking airflow across the glass – airflow that can have a huge effect on the condition of the window relating to condensation. Also, bay and bow windows can be more prone to condensation – again because of the possibility of decreased airflow over the glass.



Sorry so long, I tend to ramble...

Oberon

Okay by me