I am considering insulating my basement with Polyethylene sheet boards. I am wondering if anyone knows about its R-value or its thermal properties.
What you are refering to is POLYURETHANE insulation, not polyethylene. Polyurethane is also commonly referred to as simply "urethane" insulation. It has a very high initial R value of approximately 7.5 per inch, comes in foil faced sheets that are light weight and easy to install. This is a very good choice for retrofitting and remodeling. Because it covers large areas, it also performs well as an infiltration barrier. The sheets commonly come in thicknesses ranging from 1/2" to 1". Be sure to tape all the joints with a quality metalic tape( not duct tape) such as a SMACNA approved metalic duct tape available from wholesale air conditioning parts houses and some building supply stores. It is an expensive tape but you will recognize the quality with the first strip that you apply. The urethane sheets are relatively strong but must be completely covered with gypsum board or panelling and cannot be exposed to abuse or wear. Also, the urethane core will degrade when exposed to ultraviolet light (sun). Lastly, the R value degrades with time. Aged R values drop to around 6.0 per inch when in place for one to two years and exposed to high temperatures. This is still a much higher R value than most other insulations.