Roof level insulation
This is a summary of the calculations I made to assess the insulation values of the materials I was putting in the roof and some links to helpful information on doing these calculations.
K value, R value and U value
These are important measures of insulating values of materials and are useful for comparing them. Rather than go through what all these mean, I suggest looking at the following:
Basically for a material to have good insulating properties it should have a low k value, a high R value and a low U value. The U values that building regulations look for are calculated by looking at the combined insulation values of the materials being used.
What I used
I bought the Kingspan Kooltherm from Seconds and Co, cheaper than buying new. The boards had minor imperfections which did not detract from their performance. The material was actually not the K7 board I had expected but K9 which only has foil on one face. However since I was putting Kingspan between the rafters and under the rafters I could put the two non-foil faces together and ensure there was still a vapour barrier at each side of the insulation.
I bought 60mm Kingspan for between the rafters and 40 mm Kingspan for fixing to the rafters, had 20mm battens under this and then 9mm plasterboard.
Calculations
The building regulations now look for a U value of 0.2 for pitched roofs in attics and that was what I was aiming for.
I used Celotex's U value calculator (http://www.celotex.co.uk/Other-Resources/U-value-Calculator). This gave a U value of 0.23. However, I think the result is better than this because the 60mm Kingspan Kooltherm has a higher R value than the 60mm Celotex. Against this is the fact that the rafters are at 350mm centres rather than 400mm centres (the option in the Celotex calculator) which would lessen the insulation value of the construction. Kingspan have tables from which one can calculate approximate U values and although I am not using exactly the method shown in the Kingspan examples, they do suggest similar values for this construction.
Insulation calcs