Windows give you natural light and solar heat gain, but they are also the most strategically vulnerable part of your home or retreat and largest source of heat loss in conventional construction. The R value (measuring insulation or resistance to heat flow, the higher the more insulting something is http://rvalue.net ) may be an incomplete, and “apples to oranges” number as pointed out by David South of Monolithic Domes http://www.monolithicdome.com/plan_design/rfairy/ but, for my purposes here it does give us a reasonably quantitative basis for comparison. Remember, the higher the R number, the higher the degree of insulation. Here in Wisconsin, …