Interstitial Condensation Confusion

15 Feb 2008
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United Kingdom

I'm renovating the bedroom in our late 19th century end of terrace and the original lime plaster is in poor shape. I may need to bring it all down and start from scratch and although this would be a pain it would allow me to internally insulate the pitched part of the ceilings as well as the end wall. However I'm concerned about introducing damp problems. Looking at Kingspan's literature for their roof boards in combination with their insulation backed plasterboard suggests a massive difference in the risk of interstitial condensation between their 62.5 mm and 82.5 mm products. I've plugged the figures into the ChangePlan U-value calculator, can't for the life of see the difference between the two and so have no idea if I would be creating problems for myself if I insulate my ceiling in a similar way.

I have 100 mm joists at 350 mm centres which need to be ventilated, so max depth of board will be 50 mm. The wall is only a half brick with 50 mm of render on the outside which then has slate to keep the rain off.

Can anyone give my any advice if I can safely insulate this roof and wall from the inside?

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