You can easily buy 25mil insulation board and cut down to size.would also be cheaper as well. Choice of manufacturer all about the same. Just compère prices first.
Biuld your shed as normal and buy the 25mm insulation sheets normally comes in 8 x4 then cut to fit in between the beams. Although not as good as 50 mm better than nothing. Further to this check that 38 mm is not availbile. And always ware eye protection when cutting with a saw i know.
