John, I welcome your message and am only too glad that it arrived before I started any further work. My father was a Carpenter all his life and believe it or not, I am also a Carpenter who teaches in our local College. Now anyone reading the recent posts may think that it's time I hung up my boots and retired from teaching if thats the sort of questions I am firing out to a modelling forum. However, recently the country has gone mad on 'air-tightness', 'super-insulated', 'passive housing' and 'thermal bridging'. Whatever happened to good air circulation around a building? My fear is that introducing polythene barriers will promote sweating and more moisture and although Eoin is correct to direct me towards architects details on the matter, these details are fine in housing but not necessary in small garden shed. I'm in two minds about the vapour barrier but I will certainly insulated the roof.