UV protection from windows Part 2

I did some more research on UV protection of windows since I got stuck on window films the last time I posted about this.

I found a great review article discussing the photoprotective properties of glass and what is being done to increase broad spectrum UV protection.

Tuchinda C, Srivannaboon S, Lim HW. Photoprotection by window glass, automobile glass, and sunglasses.J Am Acad Dermatol. 2006 May;54(5):845-54.

I wish that everyone could have a copy of this article, as it has some useful tables, but here's my attempt at summarizing it.

Clear glass allows almost full (90%) visible light transmission and blocks about 30% of solar heat and 20% of UV radiation. Depending on the use of the glass (commercial v. residential) and how much visible light transmission is desired, different methods of fortifying clear glass are available. Here are some properties of glass and how they affect light transmission.

1. Thickness - minimal effect on light transmission
2. Double-glazing - moderate reduction in UV and heat radiation without sacrificing much visible light transmission
3. Tinting - more effective than double-glazing in UV and heat radiation, but sacrifices visible light transmission
4. Reflective coating - significant reduction of visible light, UV, and solar heat transmission
5. Low emissivity glass (coating of microscopically thin, transparent layers of silver between layers of antireflective metal oxide coatings) - significant reduction of UV and heat radiation without sacrificing much visible light transmission
6. Laminated glass (bonding two pieces of glass together with polyvinyl butyral) - dramatic reduction of UV radiation but only moderate reduction of solar heat while retaining good visible light transmission
7. UV-blocking coating - dramatic UV reduction with minimal solar heat and visible light transmission reduction

According to this article, using double glazed glass, where one piece is low-e glass and the other has the UV-blocking coat offers the best UV and heat insulation without sacrificing visible light and can block up to 99.9% of UV transmission. The thing to remember is that UV blocking is not uniform over all wavelengths. It is easier to block shorter wavelengths than longer ones. So the 0.1% of UV transmitted will not be a cross section of UV wavelengths, but will be predominantly UVA1 (long wavelength) waves.

According to this report from the government, a ten-window house can spend $75 more per window ($750) to get a more efficient frame with low-e double glazed glass instead of plain clear glass. The energy savings per year for heating/cooling are estimated at $319 per year. In less than 2.5 years, the energy savings would make up for the increased cost, not to mention the sun protection benefits. With housing design including more windows, the sun protection factor cannot be discounted.