Who still makes high solar heat gain windows?

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#16 Post by Bill » Wed Jan 18, 2006 10:52 am


I have never seen the studies that Simonton used to base their decision on when they dropped the hard coat low-e and the regional glass packages I asked the question why and they stated the decision was based on more recent studies that found when you factor in cooling in a mostly heating zone the advantage was back to Low-e sc, but not by much. I would think this would be welcome news to a national manufacturer; stocking both types of low-e I’m sure created inventory and production problems.

Your in depth research seems to have developed a consensus among manufacturers that the soft coat is the way to go in all regions.

I doubt you will find hard coat low-e available in Simonton products from Noradex. I have three Simonton lines in my system and the hard coat option has been removed from all of them.

Uneeda Window of NJ
Last edited by Bill on Wed Jan 18, 2006 12:36 pm, edited 1 time in total.

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#17 Post by FenEx » Wed Jan 18, 2006 11:03 am

It's my understanding that "most" window manufacturers are using soft coats inside insulated glass units as it performs better. On non-sealed glass panels (i.e. Pella's hinged glass panel) the hard coats must be used as they are more durable and are not degraded by air and moisture like the soft coats.

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#18 Post by researcher » Wed Jan 18, 2006 11:28 am

I just got off the phone talking to Sunrise Windows, and she said that the blinds do have Low-E coating on one side of the blinds and they come in four different colors. In the summer to help keep the heat out, you would close the blinds so that the Low-E faces out and in the winter you would close the blinds so the Low-E faces is (at night) and open the blinds on those sunny winter days. So you effectively can eliminate the Low-E when you do not want it for a higher solar heat gain. If i were to do this, i would order the glass with without Low-E. I doubt if you could order just the blinds for external use but maybe you could. I do not see why Low-E would not work even if were outside of the glass unit if the Low-E is protected by some means.

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#19 Post by ColoJ » Thu Jan 19, 2006 2:24 am


I wonder if the Simonton studies were based on differential placement of hardcoat and softcoat windows. That is, hard coat on south-facing windows with a good overhang that shades windows in summer but exposes them to sun in winter, and softcoat on the remainder of windows in the house. I can understand why manufacturers and installers would prefer using a single product since it reduces inventories and simplifies installations. Organizing which window belongs on which face of the building must be a pain on the job site.

When I looked at the Norandex web site they only discussed softcoat windows and described the Cardinal 172 coating so I don’t think that they offer a hardcoat. Among the American companies, Milgard offers a product with a pyrolytic hard coat. In Canada, Gienow is the one company that I know that produces a hardcoat product. Other than the Pella designer series, which sounds from the descriptions here to be unusual, these manufactures may provide the last of the windows with a pyrolytic hard coat.

My understanding from the Canadian sites is that the hardcoat will give a positive ER rating, meaning that the window will provide a net gain of heat during the winter months. This would be particularly impressive if they get a net gain in Toronto where much of the winter is cloudy. A good description of the Canadian rating system can be found at http://www.thermotechwindows.com/NRC6.htm

I think that I remember reading someplace that the Europeans were also considering inclusion of solar heat gain in their window ratings systems, but I can’t recall the reference.

It is interesting that suggestions for windows in a solarium for passive solar heating ranged from clear glass to LowE triple pane. I think that it is fair to call that a lack of consensus.

I cannot see why putting a softcoat on blinds in an IGU would be helpful unless the coating helps direct heat radiation from the blind itself.

My understanding is that the softcoat is a thin, nearly atomic layer of silver that has been sputterd onto the glass in a vacuum. The soft, thin layer is subject scratching if exposed. The hard coat is a tin oxide that is applied to the glass when it is hot to become a hard surface annealed to the glass. Hardcoats can theoretically be exposed on the #1 or #4 surfaces because they are resistant to scratches.

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LowE stuff

#20 Post by Oberon » Thu Jan 19, 2006 8:24 am

Great topic!

FenEx is 100% correct that soft coats must be protected so they are only available when sealed in an IGU. And, as ColoJ pointed out, hardcoats are basically tin oxide that was applied to the upper surface of the glass during the float process so that they don't need the same level of protection as does a softcoat.

Float glass is called such because it literally floats on a bed of molten tin during the manufacturing process. This leaves a microscopically-thin layer of tin on the bottom layer of the glass - hence the terms "air-side" and "tin-side" which applies to all glass manufactured in the float process.

The consumer (or window company for that matter) can't really tell one side from the other, and from a window performance stand-point, it doesn't matter a bit.

But, it does matter to the folks who coat glass. Softcoats consist of multiple layers of metals and metallic oxides that are "sputtered" on to the air surface of the raw glass. Note air-side. As mentioned, hard-coats are tin oxide applied to the upper surface of the glass during the float process. Soft-coats simply work better at blocking UV and at blocking both longwave and shortwave IR than do hardcoats.

But, that certainly doesn't mean that hardcoats have no value...and, the hardcoat manufacturers are constantly working hard to come up with product that meets the standards of the soft-coat world.

The ironic thing is that while the hardcoat guys advertise that their lower SHGC numbers are an advantage in heating-dominated climates, they are also advertising how they are working to "improve" their SHGC numbers (not generally in the same publications at the same time, however). :wink:

Canada does use solar heat gain as a calculation in their energy performance numbers. Never mind that the numbers used in that particular calculation were actually developed by the hardcoat manufacturers...of course R-factor was originally introduced by fiberglass insulation manufacturers...which coincidently happens to be some of the same companies. :wink:

Obviously passive solar heating does have both real and imaginary advantages over the alternative. Heck, I really enjoy the feel of the sun, thru clear glass, warming me in the middle of the winter. There's a great feeling when you get into your car on a cold, sunny, winter day.

Unfortunately, as soon as a cloud passes between the sun and that glass then that feeling goes away really quickly - to be replaced by a sudden chill as that warmth is sucked back thru the window in the other direction. Thus the need for a LowE coating.

ColoJ, you have a pretty good idea of how much heat you gain via the solarium. You don't know how much you lose (measurable BTU's), but you do know that the 80-90 degree daytime temp in the solarium drops to freezing or so at night?

I think I would recommend a log measuring temperature changes and conditions...for example, "1/19/06 - noon, heavy clouds, temp in the solarium is 65 degrees"...or, " 1-29-06, 2:pm clear and sunny, temp 93 degrees". I think that this would allow you to chart the temperature changes versus conditions and with charted data you could calculate the potential gain and loss advantages of various LowE coatings and even clear glass - in the solarium - and that would allow a calculation of payback versus the cost of the various options.

An even better way (but more expensive) would be to invest in an electronic thermometer with charting capabilities...then you would really know what is happening in there.

The idea of a "LowE" blind is technically correct, but kind of misleading in my opinion. Emissivity is the inverse of reflectance. ALL blinds or shades are by definition “low emissivityâ€Â￾ in that they block something, but by adding a layer of reflective material to the surface of the blind you increase reflectance and subsequently decrease emissivity…of course if the window had a LowE coating already, then you could set up something of a “hot spaceâ€Â￾ between the blind and the window and that can lead to all sorts of interesting issues.

So while the customer service person is technically correct in saying that the blinds are “low emissivityâ€Â￾ it is at the cost of things like visible light and the view out the window. Heck, aluminum foil makes for great low emissivity numbers when used to cover a window!

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#21 Post by researcher » Thu Jan 19, 2006 9:50 am

"So while the customer service person is technically correct in saying that the blinds are “low emissivityâ€Â￾ it is at the cost of things like visible light and the view out the window. Heck, aluminum foil makes for great low emissivity numbers when used to cover a window"

Oberon, Thanks for the clarification, i understand what what your saying.

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#22 Post by FenEx » Fri Jan 20, 2006 7:27 pm

I was out of the office for a few days but can see that ya'll have been busy. Just wanted to touch on a few things are being overlooked here.

Radiant barriers or reflective coatings (including Low-E) are ONLY effective when combined with an airspace, as I had noted earlier. "EFFECTIVE" was the word I chose, not "available" as the counter clerk at Sunrise responded. Do not fall into the advertising trap of hearing "Low-E" in other building product applications and assume it is all the same and will save you energy or money. It's simply not true. It's use in blinds is not nearly the same as it's use in insulated glass units.

Understanding how energy moves is key here. Conduction, convection and radiation are all vehicles, but are not equals in any given scenario. Heat energy takes the path of least resistance. Low-E works in sealed glass units as it returns the heat through highly conductive glass it's applied to on one side while also returning heat from the other direction which reheats the air-gas space. As that air or gas is confined, it still convects but has no where to go- thus the heat chooses the next easiest path which is conduction through the next pane of glass. In blinds, a Low-E coating would bounce the daytime solar heat back at the IGU but convective currents would carry most of it up and away and into the room before it's conducted through a solid surface such as glass. Even it's a best case scenario where Pella conseals the blinds between an unsealed third pane of glass and an IGU the "speculated" U-factor improvement of the blinds is about 0.01. This is NOT accepted, tested or approved by the NFRC. Blinds and shades are very effective because they are blocking radiant heat from reaching the surfaces and items in a room that would directly absorb and store it during the day.

In this gentleman's case, he wishes to allow radiant heat throughout the day and seal it off at night. I recommended insulating blinds at night to help reduce the loss through the glass. Low-E blinds are NOT insulators. If they were installed and only dropped at night they would simply bounce radiant heat provided from the room which WILL find the weakest link, which would be the glass behind them or any points of air exfiltration to the outside. This effect will be pronounced even further as the poster closes of the space in the evening.

Researcher, it's very important to not confuse reflectance with insulation... two completely different things. As you are posting more frequently and making more recommendations, please consider the potential consequences. The blinds option you have suggested for this consumer would cost approximatley $150-$200 per window and would have yielded a very poor return in results. Keep researching.

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#23 Post by researcher » Fri Jan 20, 2006 9:56 pm

"Researcher, it's very important to not confuse reflectance with insulation... two completely different things. As you are posting more frequently and making more recommendations, please consider the potential consequences. The blinds option you have suggested for this consumer would cost approximately $150-$200 per window and would have yielded a very poor return in results. Keep researching."

I do not see anywhere in my post where you would think that i do not know the difference between reflecting radiant heat and insulation which slows conduction. Oberon understood what i was saying and where my misunderstanding lay at and cleared it and i am thankful for that.

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Between-glass blind performance

#24 Post by tru_blue » Sat Jan 21, 2006 12:28 am

The following performance values are what I pulled off of Pella's website regarding U Values and Solar Heat Gain for windows with between-glass blinds. I hope this makes sense or is helpful. Numbers are for total unit (29 x 59 casement), not center-of-glass.

Single glazed plus Low E hardcoat glazing panel:
U = .49
SHGC - .50
With white between-glass blind (closed):
U = .35
SHGC = .17
With RD cellular shade between-glass (closed):
U = .29
SHGC = .13

Insulating glass softcoat Low E 2/argon gas plus clear glazing panel:
U = .29
SHGC - .25
With white between-glass blind (closed):
U = .28
SHGC = .15
With RD cellular shade between-glass (closed):
U = .26
SHGC = .11

Numbers drop even lower if the clear glazing panel is changed to a Low E panel, but that's not germane to someone seeking a high SHGC.

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#25 Post by ColoJ » Sat Jan 21, 2006 1:33 am

Will replacing my old clear double pane windows with more efficient windows give me too much of a good thing?

Thank you all for your input. Oberon, you are absolutely right in that I have no idea how much heat I am losing through the clear double pane windows. I have a nagging concern that by reducing the SHGC a little with replacement hardcoat windows, while improving the U value by a greater extent, that I could turn my plant friendly or at least plant tolerant solarium into a solar cooker; a kind of roach hotel where visible light enters and infrared doesn’t get out.

I haven’t kept detailed logs of inside and outside temperatures, but I can provide some data. Temperatures in the solarium approach freezing only at night when outside temperatures are below zero and even then only when a cold night follows an overcast day. To avoid damage to the houseplants on these nights, which are far more infrequent here than in Wisconsin and Minnesota, I crack two windows to permit air to move from the living space into the solarium. This is the only instance when heat is deliberately moved from the living space into the solarium. In January along the front range of northern Colorado, the average daily highs are about 43 and the lows are about 16 degrees F. Under these typical circumstances, the temperature in the solarium reaches a maximum of about 85 to 90 F between about 2:00 and 3:00 PM. Nighttime temperatures in the living quarters typically reach a minimum of about 62 to 65 F by morning. (I often turn on the gas forced air at this point to warm the living area to about 72 and avoid the “when I was a lad I struggled…â€Â￾ discussions with my teenage kids.) Temperatures in the solarium by dawn range between the upper 30s on cold nights to upper 40s and low 50s on warmer nights. On sunny days from late November through mid February, the solarium temperature warms to about 70 by about 9:30 AM. By opening doors and windows that separate the solarium from the living space, heat moves from the solarium to heat the living space by thermal siphon. The forced air furnace system has cold air returns in the solarium so that I can use the furnace blower without turning on the gas burner to move air from the solarium to the living area. The blower system works, but is not very efficient and needs to be improved, perhaps with a more efficient fan system. On a good day, I can get the temperature in the living area to reach the upper 70’s to low 80’s by late afternoon. At this point, the solarium is beginning to cool, and when equilibrium between the living area and the solarium is reached, the solarium is closed off from the living quarters. This all takes some attention, and works fairly well since my children arrive home from school at about 4:00, and when they are conscious of the world around them can close the solarium from the living area to maximize the heat gain. Thermostatic controls that control high efficiency fans to move air from the solarium to the living area would be a big help. (Any suggestions thermostatic control and air movement with fans would be appreciated).

Yesterday was overcast and the high temperature was in the mid 30s the solarium temperature yesterday eveing was in the low 50s.

As part of the renovation, about 650 sq ft of living space will be added to the existing 1900 square feet. There will be about 80 sq ft of south-facing glass added to the living area in the new addition that will provide some solar gain. No more glass area will be added to the solarium. The south-facing solarium glass will remain the same at about 270 sq ft. There are about 70 square feet of east- and west- facing windows in the solarium that are more heat sinks than sources since they are clear glass. I worry that by putting lowE on the west-facing windows and probably on the east-facing ones as well, I could also increase the heat retention in the solarium to intolerable levels. Is this a valid concern?

The discussion of the effects of thermal shades is also of interest to me. I don’t plan to use them in the solarium, but I plan to use them on the windows in the living area.

Please bear with me as I try to puzzle this through and tell me when I leave the tracks.

Windows lose heat via three routes, conduction, convection and radiation. I think that I understand the conduction. It is similar to electricity moving through a wire. The difference in temperature between inside and outside is the driving force, which for electricity is voltage; the movement of heat through the conducting medium (the frame, spacers, glass and gas between the glass) is equivalent to current; the ability of the window to resist this heat conduction is the resistance expressed as its reciprocal, U. (For electricity, V=IR or Vg=I where g=1/R; for heat flow TU=H where T is the temperature differential, U is the heat conductance, and H is the flow of heat per unit time). Adding cellular shades on the inside of a window provides an insulating barrier and lowers the U value so that the rate of heat movement from the warmer to the cooler region is slowed.

The cellular shades also help with convection because the cold glass and frame that have lost heat by conduction cool the adjacent air that then sinks along the window to cool the room air and create drafts. Cellular shades separate the air from the cooler surfaces and reduce convection.

All of this seems fine for me when considering standard clear glass windows. Where it begins to break down is with the concept of emittance and its effects on radiant heat loss. To me, emittance seems very close to mysterious emanationsjavascript:emoticon(':shock:')
Shocked. The way that I think of it, the LowE coatings, especially the hardcoats, are a kind of optical filter in that they pass shorter wavelength visible light, but reflect (or maybe absorb and reemit) longer wavelength infrared, which is heat. Visible light comes in and longer wavelength infrared is more or less trapped. The effect of a translucent cellular shade on this process seems complex to me, and I don’t see how to evaluate the effect of cellular shades on the radiant heat loss aspects of a window’s U value. I suppose that a reflective backing on the shade would reduce radiant heat loss, but I envision a positive feedback loop between the reflective backing and the E coating that could lead to heating the glass to damaging temperatures. The best backing would seem to permit both visible and infrared radiation to pass through the shade when going from outside to inside and reflect infrared radiation going from inside to outside. Although this would be good in winter, it would be opposite the desired effect in summer.

So, are cellular shades of much value when used with LowE hardcoat or softcoat windows?

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good questions....

#26 Post by Oberon » Wed Jan 25, 2006 8:13 am

Heck of a question...or questions...not sure where to start.

I would say no, the addition of LowE units wouldn't be too much of a good thing. What they will do is moderate the solar energy gain and loss by blocking some solar gain (I know we are talking about High Solar Gain products, but even HSG will stop some solar energy - at least more solar gain than clear glass will allow to enter), and it will keep some of that heat from escaping when the environment wants to remove warmth from the room.

Nature wants a balance. Nature does not like things that are unbalanced. Nature wants to reach equilibrium. If the room is warmer than the outdoors, then nature wants that additional warmth in the room to equalize to the outside temperature. The trick of any sort of insulation is to keep that nature from doing so...keep the warmth where we want it despite nature's idea. Be it wall insulation, LowE on windows, additional window panes, whatever, we are trying to thwart natures attempt to equalize things.

Heat travels in three ways - conduction, radiation, convection.

Imagine an electric heating element. Touch the element with an aluminum rod, and soon the rod will be too hot to hold (and you might be electrocuted - but that is another discussion). A conductor is a conductor – be it conducting heat, electricity, light, water, doesn’t matter. So the electrical analogy is valid…although the specifics of various conductors would certainly differ.

If you were to hold your hand above that heating element, you would feel the heat radiating from the element. Some of that heat is actually conduction as well – heat is being conducted by the air molecules – but some of that heat would be radiant as well.

LowE coatings affect radiant heat. The airspace and gas infill in a multi-pane unit affects conduction.

Baseboard heat is a good example of a convection system. So is a whole house forced air system. In simple terms, convection is the movement or displacement of air mass. When warm air is attempting to equalize with cool air, the warm air mass will attempt to “moveâ€Â￾ to the cooler air. This sets up air currents. In the case of a baseboard heating system, warm air rises and cool air falls. When the heater warms the air it is in contact with (by conduction and radiation), that warm air rises and is replaced by cooler air…again, air mass movement or a convection current.

And, here is a quick (relatively!) explanation of R value and U value...

R-value measures the resistance to heat flow of a material.
U-value measures heat conduction thru a material.

The formula for computing U-value is: Btu / (hr x degrees F x sqft)
The formula for computing R-value is: (hr x degrees F x sqft) / Btu

To illustrate a simple example, imagine that we have one sqft of fiberglass insulation (thickness not important for this exercise).

We have a temp of 70º on one side of the material and 0 º on the other, we can now find the U-value if we know how much energy it took to keep the 70º side at 70º for one hour.

If it took 3.68 Btu’s to keep the 70º constant for that hour, then we have 3.68 / (1 hr x 70º x 1 sqft) = a U-value of .05257.

The same calculation for R-value would be (1 hr x 70º x 1 sqft) / 3.68 or an R-value of 19.02.

19.02 = 1 / .05257 or .05257 = 1 / 19.02 - Thus the inverse or reciprocal relationship between R-vale and U-value.

So why use U-value for windows and R-value for about everything else?

Walls are built to stop the outside from coming in while windows are added to allow the outside to come in. Walls stop heat and light…windows pass heat and light.

Really simplified - when a person builds a wall they want to know how much of the outside is going to stay outside…and when a person installs a window, they want to know how much of the outside the window is going to allow inside. Walls resist heat flow and R-value measures resistance to heat flow; U-value measures heat flow and windows pass heat…

Makes sense?

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#27 Post by wayside » Wed Jan 25, 2006 10:12 am

> Makes sense?

It might to someone in the industry, but to your average consumer who is much more familiar with R values, it would have been easier for us if everything was R values, for several reasons:

1) With R values, bigger is better. This is much more intuitive than smaller is better.

2) Most homeowners have some sense of what R values are "typical" (i.e., R-13 for a 2x4 wall, R-19 for a 2x6. Having the windows use R values allows you to directly associate with what you already know.

3) U values are so small it is hard to get a good feel for how meaningful the difference is. For me, comparing R values of 4.0 and 3.0 is simpler and more meaningful than comparing U values of .25 and .33 .

4) Given that one is the reciprocal of the other, they are exactly equivalent, and interchangeable. Why have another system?

My $0.02 .

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#28 Post by FenEx » Wed Jan 25, 2006 3:00 pm

Great question, and I think the answer will suprise you.

R-values rate the thermal resistance of building components and can be added together from inside to outside i.e. drywall (R.45) + insulation (R-11) + sheathing (R-1.32) + siding (.81), = Overall R-13.58 for this section of wall. Differences in R-values across a surface however cannot be added to determine heat loss.

U-values (called U-factor for windows) rate thermal transmittance, and cannot be added together through a wall section, however U-values, not R values, are used to calculate energy loss/gain to determine heating and cooling needs across surfaces.

Here's an example of why. Lets say you have a 10'x10' (100sq/ft) bedroom wall that has a 2'x5' (10sq/ft) window. Let's say that the 90 sq/ft of wall area has an R-value of 10, and the 10 sq/ft single pane window has an R-value of 1. If you wanted to rate the entire combined average by adding (90sq/ft x R-10)= 900, + (10 sq/ft x R-1)= 10, you would come up with 910/100sq= R-9.1 avg. This would be incorrect as heat follows the path of least resistance.

To properly calculate the combined R-values across a surface, you need to convert them to U-values first. As you will see, the correct numbers are drastically different. The R-10 converted is U-0.1 and the R-1 converted is U-1.0. Now lets combine (90sq x U-0.1)= 9, + (10sq x U-1.0)= 10, giving us 19, divide that by 100sq/ft and you get a U-.19. This converted back to R would equal R-5.26. That small 10% window reduces the entire wall's overall resistance by almost 50%.

Now for fun, lets increase the 90 sq/ft of studwall to an impossible R-100, but leave that little window alone at an R-1. Through the same heat loss calculations, the overall 100 sq/ft surface would only achieve an R-9.17. If you increase the thickness of a boat's hull without plugging the few holes in it.. it will still sink.

Next scenario. We leave the wall at it's original R-10, but change that little window to an R-5 (U-factor of .20). Now we have (90sq x U-.1)= 9, + (10sq x U-.20)=2 gives us 11, divided by 100sq = U-0.11per sq/ft which is an R-9.09 overall for the wall surface.

With windows, you have several different materials used across the surface which all have different R-values. To determine the heat loss overall, they need to be converted to U-factors anyways... thus U-factors are used. There is a need for both R and U values, depending on what you are trying to calculate. Hope this didn't confuse you further.

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#29 Post by wayside » Wed Jan 25, 2006 4:14 pm

> Hope this didn't confuse you further.

Nah, the math makes perfect sense.

But I doubt Joe Sixpack is doing heat loss calculations when trying to compare two windows and decide which to buy. For him I think it would make more sense to use R values, which he probably already has some hazy idea what it means, instead of some random number like 0.31 .

In fact, what would make the most sense is for some sort of industry-standard calculation that compares a given window to a single pane window of the same size. Sort of like SEER is used for air conditioners. Who knows what a SEER of 8.0 really means, anyway? But you know if the unit right next to it has a SEER of 8.1, they are practically identical energy-wise, and you can pay more attention to price and feature. If you find one with a SEER of 12.0, that one is much better and is probably worth paying more for, because it will save energy.

You could even have different ratings based on the region of the country you are in, which would help take the pain out of trying to figure out what the heck SHGC is and how it will affect my utility bill.

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#30 Post by ColoJ » Thu Jan 26, 2006 2:15 am


Thank you for the information about SHG and the insulating value of the windows. I agree with you, and I am betting that you are right. In fact, in the next few days I plan to buy the windows and test the hypothesis.

If I am thinking about this right, clear glass will reflect about 16% of the long wavelength infrared (IR) radiating from objects in the solarium and absorb the remaining 84%, which will be reradiated from the glass as long wavelength IR. More or less half of the IR absorbed by the inner pane of glass (half of the 84%) will be radiated to the outside per unit time and half will be radiated back into the room so that about half of the radiated heat absorbed by the window is lost. With clear glass, this provides a net gain of heat and is the source of much of the heat in a greenhouse on a sunny day.

The hardcoat glass has an E of about .215 so that of the 84% of the IR absorbed by the glass something like 78.5% of this long wavelength IR will be reflected back into the room instead of about 50% with clear glass which is reradiated from the glass. In this way, the hardcoat mid lowE coating makes the old fashioned greenhouse effect more efficient. I think that the solar mass, although far short of the suggested 150 pounds of masonry/sq ft of glazing, will help buffer the solar heat gain.

FenEx and Oberon,

Thank you for the discussion of U and R values. It is now much clearer to me why U values are used for describing the insulating properties of windows. I think that the electrical analogy that I discussed earlier still holds. As I remember from basic circuit theory, resistances (R values) add linearly when they are in series with one another. When resistances are arranged in parallel they add as the inverse (1/R), and current follows the path of least resistance in the circuit. As you say point out, heat also follows the path of least resistance. For the conduction of heat, 1/R=U. As an analogy with the electrical circuit, thermal resistance values of the insulated wall and the window are arranged in parallel so the U values add together. Again, as you point out, there are several ways that heat can leave through a window, and these pathways are mostly in parallel with one another so U values are a much better way to calculate heat loss from a window.

It seems to me that FenEx’s argument about the path of least resistance argues strongly for insulating covers over windows at night in the winter to reduce the U value for the opening. I have not looked much for window coverings, but I can’t recall seeing IR reflectance values for any of them, yet this would seem to be the greatest contributor to slowing heat loss through lowE windows.

How are the R published values for cellular shades calculated?

Do cellular shades provide a sufficient reflectance value to be of much value in affecting the combined U value of the window and the window covering?

Would the ideal window covering be translucent to visible light, but reflective to long wavelength IR?

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