There are three separate issues here, and let us be specific in isolating them. First, there is the reflectivity of the shade, as defined by the amount of energy reflected off the shade divided by the amount of energy that reaches the shade, where the reflected energy is measured before that radiation passes through any glazings. Second, there is the amount of energy reflected off the shade that makes it back through the glass and coatings to the outside world. Third, there is the effect of the shade on solar heat gain. Let us focus specifically on item 1, the reflectivity at this point, and leave the other two items as issues to be discussed separately. I have provided a reference (http://www.blindrage.co.za/files/green/TheEnvironmentalBenefitsOfUsingSolarShades.pdf) that states that the reflectivity for a "Blackout White" shade behind clear glass is 77%. Low-e coatings limit the UV and IR reaching the shade, wavelenths where the shade reflectivity would be less than in the visible. So the reflectivity, defined as the amount of radiant energy reflected divided by the amount of energy that reaches the shade, as measured at 77% for clear glass, would only be increased for low-e glass. Do you agree or disagree?

You have stated: "I may have understated the reflectivity of titanium paints- while rutile process TiO2 is fairly absorptive in the blue & UV, a super-bright-white shiny anatase TiO2 pigments it about 90% reflectivity in the visible & infrared, even though they're still absorbing half the UV, so it's at least remotely possible that a titanum paint would exceed 75% reflectance of whatever initally passed through the low-E coating."
This discussion only makes since if you are making the discussion more complicated and confusing by talking about reflectivity back through the glass, which is an interesting but separate topic. From the figure at the bottom of page 7 of http://www2.dupont.com/Titanium_Technologies/en_US/tech_info/literature/Coatings/CO_B_H_65969_Coatings_Brochure.pdf, the reflectivity of anatase TiO2 is well above 90%, and closer to 95% from 400 nm to 1000 nm, which is the highest wavelength given. Below 400 nm, the reflectivity drops off sharply. If the light reaching the TiO2 surface were filtered through low-e glass first, the amount of UV in the light would be reduced, increasing the reflectivity, as defined by the amount of energy reflected divided by the amount of light reaching the surface, and that reflectivity can be estimated to be about 95%. Do you agree or disagree? If you agree, then where does the 75% number come from in your statement, "... so it's at least remotely possible that a titanum paint would exceed 75% reflectance of whatever initally passed through the low-E coating." Looks like 95% to me.

Before we can compute the light reflected back through the glazings, and the effect on solar heat gain, we must agree on the reflectivity of the shade itself as measured by the amount of energy reflected off the shade divided by the amount of energy reaching the shade.

In terms of the total SHGC, what doesn't make it back out of the glass is by definition a gain- a cooling load located inside the thermal & pressure boundary of the house. It's just a concentrated cooling load that's cookin' the hell out of your sealed glass, unless you allow that gain to be distributed at a lower temperature via convection.

What's reflecting back to the glass and NOT going back through the glass that's the problem for the sealed glass- it's heating it up!

Low-E coatings on windows usually aren't designed to reject the blue and UV that some titanium paints absorb. Since most of the non-visible solar spectrum is infra-red, that's where the performance is best gained. It's true the indium tin-oxide reflects some in the UV spectrum, but it's still passing all of the visible blue to be absorbed by the less-reflective TiO2 paint (which is still WAY more reflective than any non-aluminized fabric.) I'm not sure why this detail has become so important in the discussion of fabric shades though- it was simply thrown out as a comparison to just how reflective that fabric would have to be to have a 50% reduction in gain.

Dana1-

You have jumped off topic to items 2 and 3 that I outlined above. We cannot determine the answer to items 2 and 3 until we agree on item 1, the reflectivity of the surface of the shade, where reflectivity refers ONLY to that surface. Please tell me if you agree or disagree with the fact that the shade reflectivity measured behind clear glass will be very similar to the shade reflectivity measured behind low-e glass.

Also please tell me where the 75% figure comes from for your comment about TiO2.

Here is a GBA article on this topic:

GBA - Do Window Shades Save Energy?

Thermal Performance Data


Window Attachments

Posted By Lbear on 15 Nov 2012 02:31 AM
Here is a GBA article on this topic:

GBA - Do Window Shades Save Energy?

Note particularly the comment by Gordon Clements who found cellular shades with side seals to be effective at reducing heat losses in the winter and heat gains in the summer.

He reports a value of R4 to R5 for cellular shades with side seals, while I found as high as R3 only at low differential temperatures between inside and outside, dropping down to about R2 at high differential temperatures, and that high only for light-blocking shades. My testing assumed a constant R-value for the windows independent of temperature, which may not be true.

Posted By Lee Dodge on 14 Nov 2012 05:46 PM
Dana1-

You have jumped off topic to items 2 and 3 that I outlined above. We cannot determine the answer to items 2 and 3 until we agree on item 1, the reflectivity of the surface of the shade, where reflectivity refers ONLY to that surface. Please tell me if you agree or disagree with the fact that the shade reflectivity measured behind clear glass will be very similar to the shade reflectivity measured behind low-e glass.

Also please tell me where the 75% figure comes from for your comment about TiO2.

The 75% number is a WAG at the average reflectivity of the raw rutile pigment across the solar spectrum as seen in figure 5 p,7 ,less some derating factor for the paint binders, etc. 

With derating for paint binders would probably be no more than 75% but even assUming the full paint product is 90% when averaged across the full the solar spectrum (which would be even better than the raw pigment), only about half the IR portion of the  solar spectrum incident on the exterior of the low-E glass is reaching the paint.   A larger than 50% fraction of the visible blue and UV that gets absorbed by the TiO2 pigment still reaching the paint, so it's spectrum skewed to the less reflective/more absorptive end of the TiO2 spectrum. That makes the total reflectivity across the low-E filtered spectrum is even lower.  Without data on the actual window coating used we can't speculate with any precision exactly how much of the spectrum incident on the paint is being reflected by the paint, but pretty safe to say it's nowhere near 90%, and probably doesn't exceed 75%.  The transmission spectrum of typical moderate-gain low-E coatings cuts  IR far more than the visible or UV, and TiO2 pigments absorb significantly more blue & UV than IR, so it's a reduced average reflectivity when the incident spectrum is considered.

But whatever it is with TiO2, only an aluminized fabric would approach anything like the 90% reflectivity that it would take to make a 75%+ reduction in SHGC for the selective coated single pane seen in the SHGC vs. shade reflectivity data plotted in Figure 5 of Mills & McCluney's document.   According to their plot a shade reflectivity of 75% yields only a ~50% reduction in SHGC for the selective-coated single pane, and only a ~60% reduction for the clear glass model.

Posted By Dana1 on 14 Nov 2012 05:01 PM
In terms of the total SHGC, what doesn't make it back out of the glass is by definition a gain- a cooling load located inside the thermal & pressure boundary of the house. It's just a concentrated cooling load that's cookin' the hell out of your sealed glass, unless you allow that gain to be distributed at a lower temperature via convection.

What's reflecting back to the glass and NOT going back through the glass that's the problem for the sealed glass- it's heating it up!
...snip...

Not so much. Consider the case for a high SHGC, low-e, double-pane window. The radiation that makes it to the shade has already had much of infrared radiation removed by reflection and absorption as the light goes through the glass and coating (and similarly for the small amount of UV). Therefore, the light passing back outward through the glazings will be mostly in the visible spectrum, or just outside it, where the tranmission of the glass and coating is higher than it is for the overall solar spectrum. The heat gain in the room will be similar to the case for a clear, double-pane window.

Let us be more specific and consider the case for an insulated glass unit (IGU) from Cardinal glass, that consists of a clear outer glazing and the inner glazing with a LoE-179 high SHGC coating. Properties are given at http://www.cardinalcorp.com/wp-content/uploads/pdf/residential-brochure.pdf, and specifically the tranmission spectrum in Fig. 19-1, and the optical properties in Fig. 12-1. We can define an idealized transmission curve for the low-e window by taking the 50% of maximum tranmission points (43% actual tranmission), and making a rectangular transmission curve that covers the range of 365 nm to 860 nm. As shown in Fig. 19.1, this tranmission range is mostly in the visible range, with slight transmission in the UV and IR. Thus, the radiation reflected off the shade will be mostly in the visible region, and that radiation is fairly transparent to this window.

I did a first-order calculation comparing Cardinal Glass's clear, double-pane window with the low-e window. Note that for the clear window, the light reflected off the shade has not been spectrally filtered very much, so the absorption characteristics are similar for the reflected light as for the initial light.

For the clear window, I compute the energy absorbed in the 1st lite and 2nd lite as 12.4% (including the incident and first shade reflection) and on the shade as 16.8% of the incident energy. For the low-e window, the 1st lite absorbs 13.9%, the 2nd (coated) lite 17.5%, and the shade 13.8%. Assuming the 1st lite exchanges heat with the outdoors, while the 2nd lite and shade heat the interior, then the clear window with shade down dumps 29% of the solar energy into the room, while the low-e window dumps 31%, not much different. With the shade up, the clear window leaves 73% of the energy into the room, and the low-e window 60%. These values are similar to the results shown in Fig. 5 in the Mills and McCluney paper for single-pane windows.

Therefore, I do not see the double-pane, low-e windows as trapping an abnormal amount of heat compared to clear, double-pane windows, or single-pane windows. In all cases, it may be necessary to dump some of the heat absorbed by the shade and the inner lite into the room to avoid damaging the window seals. In all cases, the shades are effective in dramatically reducing the solar gain in the room. (I consider a factor of 2 dramatic.)

I will provide a spreadsheet with the detailed (but only first-order) calculations by private message (PM) to anyone that cares to see them. Send me a PM and identify yourself by name, not screen name.

Lee, I plain give up- you appear to be arguing with yourself!?

I never made statements regarding the relative heat trapped by low-E double panes vs. clear double panes (that you seem so ardent to prove are similar.) OK, I'll buy that, so?

I DID in an earlier post suggest that a double pane of similar transmittance would more trap more than a single pane at a high shade reflectivity, and that's still my position.

Any reflected radiation that doesn't make it back out is still a gain, single pane, double pane, whatever.

Anything that is absorbed by the shade is still a gain (such as the ~ 10%+ of the transmitted spectrum that falls in UV & blue fraction absorbed by Ti02 paint.) That is, unless the shade is SO insulated and the window gets SO hot that it is convecting and radiating a good fraction of that gain to the exterior.

Yes, the factor of 2 is dramatic (if not as dramatic as the factor of 4 or 5 you were initially suggesting), and it takes a highly reflective shade to get there. Getting much more than a factor of 2 without stressing the window would still be a shock.

I'm done with this thread.

I think that we can now feel comfortable using interior shades as part of a convenient energy conservation strategy, both summer and winter, without feeling that it is a futile effort based on statements like: "If you let the thing draft to cool the windows to avoid blowing the seals, the shade effectively [becomes a] collector surface of a thermal air panel."

We have also been served notice that in using interior shades, we should be aware of potential increased condensation problems in the winter, and potential window overheating in the summer. User beware.

Posted By Lee Dodge on 16 Nov 2012 05:10 PM
I think that we can now feel comfortable using interior shades as part of a convenient energy conservation strategy, both summer and winter, without feeling that it is a futile effort based on statements like: "If you let the thing draft to cool the windows to avoid blowing the seals, the shade effectively [becomes a] collector surface of a thermal air panel."

We have also been served notice that in using interior shades, we should be aware of potential increased condensation problems in the winter, and potential window overheating in the summer. User beware.

Would a fair compromise be a thermal blind or a thermal curtain/drape vs. the sealed thermal shades? The latter requires some vigilance to make sure it is not overheating the glass while the first two would not pose an overheating or condensation problem because they allow some airflow.

Interesting discussion and there are many variables at play. Window positioning, time of year, SHGC, glazing coatings, double or triple pane, glazing widths, etc.

My position is that thermal shades do work and some work better then others. Currently I use thermal drapes and they do a pretty good job of keeping the SHG in check. Window coverings are sort of a "necessary evil", we need them for privacy and light control. They are not inexpensive, especially the custom made solar shades. The thermal drapes are the least expensive as they typically have standard sizes and install is DIY. Just get a curtain rod, mount it to the wall and hang the drape. Another pro to thermal drapes is they tend to help with sound control as they absorb certain soundwaves because of the fabric.

I know this is an old post but would someone be willing to shed some light (no pun intended) on how to calculate SHGC on any variety of shades such as blackout or solar? I found some cheap solar shades on a site called shademonster.com and was wondering if opacity has anything to do with it.