I am looking at the DIY methods of reducing heat loss in the winter. Well, I have the answer. I'm afraid no one answer is not entirely correct and if applied could lead to a significant error if multiple sheets are involved. Assuming you are stacking multiple sheets close together, the U value is composed of the sum of the individual values of I plus two additional constants, the internal and external surface resistances known as Rsi and Rso an average figure of the two is 0. 168, as told by Rivergirl, first calculate the R value of a single sheet, taking 1 / (U value) deduce the result 0. 168. Multiply the result by the number of sheets of proposals, then add back 0. 168. Then get the total value of U dividing this figure by 1, ie 1 / (R-value combo, you just made). You can check the error that would have occurred to try the first method would give a value of more than generous U. If you are stacking multiple sheets, you can make a considerable improvement in the U-value by providing a sealed air space between each sheet. If you want to know how this works please contact me. I assume that the polycarbonate sheets which you refer are the specific roofing sheets used with the built in air space. But if they are solid single sheets you will be disappointed with the results.