The problem with their theory is what UV wavelengths do each block?
Yes, plain glass blocks a certain % of UV but the added layer blocks different wavelengths... the more destructive ones. That is why the two combined block in the upper 90% of the most harmful wavelengths.
Actually, just the opposite of what you said. Plain glass (soda lime) blocks UVC and UVB rays 100% (wavelengths <315nm). These are the most harmful. Plain glass also blocks considerable UVA energy (400-315nm) because the cut-off (50% transmission inflection point) occurs at about 340 nm. Again, this means blocking of the most destructive UVA wavelengths as well. Standard acrylic has more UVA blocking efficiency and will reach the 50% cut-off of UVA at about 360 nm, allowing just enough transmission at 365-370 nm to enable optical brightener fluorescence. OBA activation generally peaks at 370 nm. In the 380-390 nm band (i.e., the weakest UVA component) plain glass transmits 80-90%. Museum glass or OP3 plexi is designed to cut off essentially all UVA energy, so its cut-off curve inflects right at or near 400 nm. Hence, full UV block glazing is designed to block all UVC, UVB, and UVA whereas other glazings all do UVC and UVB but only various percentages of UVA. However, there's no free lunch. When blocking all UVA the spectral transmission inflection point at 400 puts a slight yellow tinge into the viewing condition since the UVA-block glazing's spectral transmission starts to significantly decline in the visible blue wavelength region as well.
What many conservation professionals have failed to grasp over the years in this whole "UV is really damaging" argument is that ordinary window glass and even painted walls, carpeting, fabrics, etc (titanium dioxide used to make white paint is a strong UV absorber) already absorbs a very large percentage of UV energy entering homes and offices. Hence, art that is only indirectly illuminated by natural daylight scattered within an interior environment is already much reduced in the UV/ViS ratio compared to objects exposed outdoors to direct sunlight. Moreover, when indoor UV energy is at its highest ratio with respect to visible light, this condition also precisely correlates with sunlight coming through windows and directly striking the artwork!!!. Wherever that happens, overall light intensity is also incredibly high on the artwork, often up 1000x over other indirectly illuminated areas even in the same room. In these situations, full UV blocking glazing helps reduce fade rates at best 2-3x over plain glass, but it's like rearranging the deck chairs on the Titanic. The absolute intensity associated with the direct sunlight goes up 100-1000x, and it is this peak intensity (UV filtered or not) that causes the primary damage to the artwork.
To put this argument in perspective, just 6 minutes average per day of sunlight streaming directly through a window onto a work of art will typically produce a 2x increase in average daily illumination (because the sunlight is so intense compared to indirect daylight conditions) and thus a 2x increase in fade rate on average. Moral of the story: Use UV blocking glazing if you want the absolute maximum amount of light fade protection, but pay even closer attention to where sunlight reaches actual surfaces in your home, even for just a few minutes per day on average. These are the locations, UV protected or not, where you are going to see faded art, faded upholstery, faded drapes, etc.
cheers,
Mark
http://www.aardenburg-imaging.com