The Canadians came up with an "ER" window rating system. It's supposed to account for the U-Values and SHGC values and then give you an indication of total performance based on calculations done for a standard window size. It also takes into account the air infiltration of the window. Doing so will show you the relative net gain of a window.

This is supposedly a better rating system than what we have here in the USA. As a window with a lower SHGC can actually outperform one with a higher SHGC ( passive solar applications) if it has a higher U-Value that is enough to reduce heat loss by an amount large enough to more than offset the lower passive SHGC number. Throw in the windows air infiltration number. This will result in a higher net total gain and therefore a better "ER" rating.

Anyone have more insight into this ratings system and will the US DOE embrace this type of rating system?

The "ER" ratings used in Canada are only applicable to heating-only or very much heating dominated climates - OK for Canada, but not for much of the U.S. Therefore, it will never be adopted for the U.S. as a whole. The ER ratings can be useful in those heating-only climates in the U.S.

It would be better to use RESFEN or BEopt or similar to perform tradeoffs for windows for specific climate and house applications than to simply use the ER ratings, but they are a start.

Much of the northern US is the same as Canada as 90% of us live within 160km of the border. Of course I haven't seen any palm trees yet here...........

Why would the Er system be only good for heating dominated climates?

Posted By MikeSolar on 29 Sep 2012 05:12 PM

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Why would the Er system be only good for heating dominated climates?

The ER rating is made up of three factors: 1) solar heat gains; 2) heat loss through frames, spacers and glass; and 3) air leakage heat loss.  A rating of zero means that the window gains as much heat through solar heat gains as it loses through the other two factors.  A positive rating means that the window results in a net gain of heat energy.  A high solar gain is NOT desirable for those parts of the U.S. that have significant air conditioning loads compared to heating loads, since solar gains often dominate all others in determining cooling loads.  

Of course, in a mixed cooling and heating environment, it is often possible to mix high and low solar gain windows, using appropriate overhangs and shading to try to limit the solar gains to correspond to the times heating is required in the house.  That is where the use of a computer model with house details is useful in the window selection process, and provides better information than a simple ER rating.      

A Canadian study done by NRCC in 2007 concluded, "The study indicates that overall energy savings will be realized with high SHGC, Low-E residential glazing in Canada and in the Northern half of the blue, Northern Climate Zone in the U.S.A" So basically half of the USA.





Here in the USA where electric prices are around 11 cents per kWh, people don't really care about energy builds. If we had energy prices like they do in Germany (35 cents per kWh), then maybe people would think differently. Until then, we are stuck with what we have here in the States. Europe and Canada are leading the way in energy builds, we are lagging behind.

US and Canada are not that far away in pricing but we will be getting much higher costs over the next few years. We have some very old transformer stations and switchgear which is one reason why PV is not allowed is some circumstances here. When we do a PV system we have to call the utility and check on "capacity allocation", is there enough room on the grid. It is a BS argument but that is what we must do.

Europe has a much newer grid and it is partly because many of the decisions are taken centrally and the grid is a different entity from private producers. Because the grid management has a long term plan, the cost of upgrading the lines is built in to the cost.

Here, we have to argue over $5b for new lines rather than tack on 2-3cents/kwh to cover it.......all in the name of LOOKING competitive.........

That map shows that 40-50% of the US could benefit from the rating system. Maybe a computer modeling system could incorporate all the shading and cooling dominated issues. I'm not up on this stuff.

For heating-dominated climates, the ER ratings for windows are certainly superior to simply choosing a minimum U-factor for windows, which some people focus on.  The ER ratings include solar gain, which in some climates is very significant in the overall tradeoff for window selection.  As pointed out by the Canadians, a good window will have a POSITIVE ER rating, meaning that it is better than having a highly insulated wall in place of the window since the window results in a net energy gain, not loss.

In my particular climate with high solar insolation and heating-only requirements, the windows on the E, S, and W sides are all computed by RESFEN to show net energy gains.  These windows, except for two, are all high solar gain windows.

On a related note, much of the new home construction in the U.S. in the past 20 years or more has been in the southern half of the country.  For that reason, there has been an over-emphasis in the U.S. on windows suitable for those areas.  A good (positive) ER rating would be a bad selection for those areas, since the high solar gain would lead to excess cooling requirements.

I have computed the tradeoff between high and low solar gain windows for a number of cities across the U.S. at http://www.residentialenergylaborat...codes.html.  Not surprisingly, all the cities in the north and most in "middle" America show lower energy costs with high solar gain windows, while those in the south and not at high altitude show lower energy costs with low solar gain windows.  Building codes and ratings such as Energy Star tend to force builders toward low solar gain windows due to an over-emphasis on U-factor, a bad choice in many areas.

I guess I am confused. The Canadian rating says that the high SHGC is better than LowU, Even in the areas that have a lot of cloud cover such as WA, and OR west of the cascades?

Posted By whirnot on 10 Oct 2012 10:50 AM
I guess I am confused. The Canadian rating says that the high SHGC is better than LowU, Even in the areas that have a lot of cloud cover such as WA, and OR west of the cascades?

For south facing windows that would be true, even the foggy-dew western slopes of the Cascades or even the damp coastal mountain ranges.   Those "bright clouds" winter days still have some punch on the south face, and clear sunny winter days with higher solar gains have high correlation with lower outdoor temps.

For other orientations that would not be the case.

Posted By whirnot on 10 Oct 2012 10:50 AM
I guess I am confused. The Canadian rating says that the high SHGC is better than LowU, Even in the areas that have a lot of cloud cover such as WA, and OR west of the cascades?

You have hit upon the shortcoming of using a simple rating factor like the Canadian ER value rather than using a model that includes climate factors.  That is why I suggested earlier in this thread using RESFEN or BEopt or some other model that includes climate factors that can discriminate between areas with lots of cloud cover versus sunny areas.  Unfortunately, many folks are reluctant or fearful of using models, and for them, trying to come up with a rating factor seems to be limit of their "research" capabilities. 

However, as long as the user understands what is included in the ER rating, it can still be a useful guideline.  If the user is in a very cloudy area, then the high solar gain is obviously of less significance.   

Edited to add:
I notice in the calculations that I did with RESFEN that even Seattle WA, that I think of as "mostly cloudy," shows a 9.1% overall energy savings using high solar gain windows on all four sides compared to low solar gain windows, even though the assumed U-factor is higher for the high solar gain windows.  Likewise, San Francisco that can be cloudy some of the time, shows a 34.6% predicted reduction in energy use using high solar gain in place of low solar gain windows.  Details for these calculations are given at http://www.residentialenergylaborat...codes.html.

But Lee, that's at a very small difference in (not so great) U-factors and a large difference in SGHC: SGHC of only 0.30 for the U0.34 window vs 0.53 for the U0.37 window. It would be more instructive to run the comparative numbers on something that's actually code legal in those places.

In Seattle U0.34 is only code legal if the glazing/floor ratio is less than 13%, and even U0.32 is only legal up to 25%.

http://www.ci.seattle.wa.us/dpd/publications/cam/cam303a.pdf

And to meet Energy Star in in the northern region a U0.32 window has to have an SGHC of greater than 0.40 (see Table 7 on your referenced page), which is substantially more gain and slightly lower U than the windows you ran your RESFEN model on.

Run it on something code-legal and Energy Star before taking the lesson too deeply to heart.

Posted By Dana1 on 10 Oct 2012 06:49 PM
But Lee, that's at a very small difference in (not so great) U-factors and a large difference in SGHC: SGHC of only 0.30 for the U0.34 window vs 0.53 for the U0.37 window. It would be more instructive to run the comparative numbers on something that's actually code legal in those places.

In Seattle U0.34 is only code legal if the glazing/floor ratio is less than 13%, and even U0.32 is only legal up to 25%.

http://www.ci.seattle.wa.us/dpd/publications/cam/cam303a.pdf

And to meet Energy Star in in the northern region a U0.32 window has to have an SGHC of greater than 0.40 (see Table 7 on your referenced page), which is substantially more gain and slightly lower U than the windows you ran your RESFEN model on.

Run it on something code-legal and Energy Star before taking the lesson too deeply to heart.

The small differences in U-factor and large differences in SHGC are inherent in the physics of the coatings used in low-E windows, not something that I chose.  I did not make up those performance factors for the windows; they came straight from the window database in RESFEN 5.0 as index numbers 321 (high solar gain) and 341 (low solar gain). 

To make the lowest U-factor window, you must choose a coating that has the lowest emissivity across the electromagnetic spectrum while still allowing visible light through the window, so transmission between 0.4 and 0.7 micrometers, but reflective elsewhere.  This results in reflecting a lot of the solar energy spectrum which extends well beyond 0.7 micrometers, in fact, to about 2.5 micrometers.  The high solar gain windows allow transmission out to 2.0 micrometers or so, but necessarily have increased emissivity since emissivity = 1 - reflectivity at a given wavelength. 

Now RESFEN is not some secret proprietary code that only I have access to.  It is available for free download.  If you think choosing a lower U-factor design window will make a difference, then run the numbers and show the results.  In fact, I have run numbers for Seattle for what RESFEN 5.0 calls insulating windows, with the low solar gain having U=0.26 and SHGC=0.31, and the high solar gain having U=0.29 and SHGC=0.56.  Whoops, notice that for these windows also the change in U factor is only 0.03, just like for the other windows that I chose originally to do the calculations on, while the change in SHGC is 0.27.  The physics of coatings does not change even when I change the windows framing material.  This should be no surprise. 

So consider four cases: windows on all four sides are low solar gain (LLLL), windows on the south side only are changed to high solar gain (LLLH), windows on the east, south, and west are changed to high solar gain (LHHH), and all four windows are changed to high solar gain (HHHH).  Total heating and cooling energy for Seattle WA are predicted to be:

LLLL   LLLH   LHHH   HHHH
29.7   27.9    26.9    26.8

The total savings for using high solar gain windows in place of low solar gain windows is predicted to be 9.8%, essentially the same savings as I reported for the higher U-value windows (9.1%).  Also notice that in contrast to your suggestion of using high solar gain windows only on the south side, that RESFEN says to use high solar gain on three or four sides. 

You seemed to miss the point of the on-line article that choosing a window that does NOT meet Energy Star is a better choice than choosing one that does meet Energy Star in those cases of heating dominated climate and missing the Energy Star rating by changing coatings to get high solar gain at a small penalty in U-factor (typically 0.03). 

Another interesting point is that with European & Canadian Tilt & Turn operable windows, the screens go on the outside of the window. You can actually control some of the unwanted or wanted SHG by getting different colored screens, or removing them all together during winter to let in full maximum SHG. With casements the screens are on the inside so they don't work as well as T&T which have the screen on the outside before the suns rays can hit the glazing.

If one is not using widows to help heat their home as well as keep heat out of their home, they are only winning half the battle. This can only truly be accomplished with triple pane windows. They in essence allow one to have their cake and eat it to. With dual panes, in a heating dominated climate, you are stuck with buying and setting the coatings for winter and then having to do shading in the summer. With triple panes one can reflect the infrared rays of the sun out in the summer and reflect the infrared rays back in during the winter with triple panes by properly placed coatings on both the #2 and #5 surfaces. There is a reason why triple pane windows dominate European, Canadian and Asian markets. R-5 windows will become the Energy Star standard eventually here in the USA. This will only be accomplished by triple panes or double panes utilizing heat mirror technology which is more expensive than just going with triple panes.

Often overlooked is the glass thickness. Nothing thinner than 4mm is what most experts say. The most important factor is the air-tightness of the windows, makes no sense to run all these SHGC & U-Value calcs and then have a window leaking huge amounts of air. Sliders and single/double hung are notorious air leakers due to design.

In the proper design and cold climate, windows can outperform walls by capturing solar heat gain, that is a fact.

Posted By Lbear on 11 Oct 2012 06:14 PM
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With triple panes one can reflect the infrared rays of the sun out in the summer and reflect the infrared rays back in during the winter with triple panes by properly placed coatings on both the #2 and #5 surfaces.
...snip...

I believe somebody has been pulling your leg there.  I don't think those window coatings know the difference between summer and winter.  

Posted By Lee Dodge on 11 Oct 2012 07:11 PM

I believe somebody has been pulling your leg there.  I don't think those window coatings know the difference between summer and winter.  

I was oversimplifying it. What I meant is that window coatings only work half as well in one direction as they do the other. With triple panes one can block out heat by utilizing the proper coating on surface #2 and then to capture heat use the proper coating on surface #5. This type of setup cannot be done with typical dual or single pane windows, the added glazing is what makes it possible.

One can have a high SHGC = 0.62 / U-Value = 0.15 or a low SHGC = 0.25 / U-Value = 0.10. These types of numbers are not seen in US Manufacturers besides SW which uses heat mirror technology. These glazing numbers are found in European and Canadian manufacturers that manufacture triple pane glazing.



With the heat mirror technology you utilize dual pane glazing but the heat mirrors allow you to build upon the technology I mentioned above. They can create 3 Heat Mirror films suspended inside of an insulating glass unit, creating four air spaces. This would be the most energy-efficient glazing product in the world. You can essentially create a U-value of 0.05 or R-value of 20. Now with that being said, I don't think anyone produces such a window but in scientific theory, it is possible.