For an option that is easy to use daily and easy on the eyes, I like honeycomb cellular shades. The double-cell variety can offer a gain of almost R 4. Levelor has a sheer efficiency version of the Accordia that we are looking into: http://www.levolor.com/products/cellular-shades/energy-savings.php. Single cell, but more transparent with R 3.33 rating. Hunter Douglas has the Duette Architella which has an insulating cell within the cell. But I haven't been able to find the R value for this series yet, but their energy guide appear to indicate R =4.0. If so, that would be a 300% efficiency gain over an R 2 storm window according to their docs. Total R-value ÷ Window R-value = Energy Efficiency Increase (4.0 + 2.0) ÷ 2.0 = 6.0 ÷ 2.0 = 3 Energy efficiency is tripled (300%). The U-factor of an R-2.0 window is 0.5 (R = 1/U; U = 1/R). To calculate the heat flow through this window if its size is 48" x 60" (20 sq. ft.) and the temperature is 70° inside and 0° outside: Heat Flow = U-factor x Area (sq. ft.) x Temperature Difference Heat Flow = U x Asf x ΔT = 0.5 x 20 x 70 = 700 BTU/hour Over a 24-hour period, heat loss would total 16,800 BTU. To calculate heat loss with an R-4.0 shade over the R-2.0 window: R-value = 4.0 + 2.0 = 6.0; U-factor = 1 ÷ 6.0 = 0.167 Heat Flow = U x Asf x ΔT = 0.167 x 20 x 70 = 234 BTU/hour Over a 24-hour period, heat loss would total 5,616 BTU. The R-4.0 shade cuts heat loss by two-thirds (67%).