DIY Doctor UK DIY Forums

Hello, I have a bug in my head about insulation which I cannot shift and I cannot find the answers to.
If I have a cup of tea out of a stryrofoam cup, I can barely feel any heat coming through the 3mm of styrofoam from the 98 degree tea, so I cannot really understand why we need these massive amounts of insulation in our lofts.
If a house has no loft insulation at all it will obvioulsy lose a lot of heat through the roof. If I lined the outside of the roof timbers of this house with say 10mm of 8 x 4 sheets of celotex (or Kingspan) yellow foam insulation boards, and taped all the joints with reflective tape, what percentage (roughly) of the heat that was previoulsy escaping would I be stopping? 95%? If so, if I made it 130mm of Celotex rather than my measly 10mm, would I then be stopping 98% of the heat loss through the roof rather than 95%? Does anyone know of the actual figures for lining a roof on the outside with rigid foam per 10mm of depth? It would be interesting to see the difference between no insulation, a little and a lot.
Thanks,

Stuart

Each insulation product has its own Uvalue this is the insulation data that the building industry uses to establish the heat loss through roofs walls and floors.
Part L is the building code for all insulation properties,so if you "google" each product it will tell you what the U value is.The lower the Uvalue the better.
some of the best products are Kinspan,ISO superquilt,Celotex