insulating exposed pitch roof

[Robbox]

One of our rooms has an exposed rafter, pitch roof. The exposed rafters don't go all the way to the apex, there is a small flat area that constitutes the ceiling.

The extension was built in 2001, but the room gets cold in the winter, compared to the rest of the house. I'd like to batten out the rafters and cut Kingspan to fit in between, then get a plasterer to come to board and skim.

Do I have to take care about condensation behind the the new Kingspan or not?

 

[freddiemercurystwin]

Is there a membrane beneath the tiles? Presumably so as it was built in 2001. If so what is it, traditional felt or a breathable membrane? Knowing this is key to understanding how the condensation issues can be dealt with.

Seems a bit weird that an extension built that recently is not insulated?

 

[Robbox]

It is insulated, 75mm Kingspan type sheets in between the roof rafters. So I would strongly assume that membranes etc have been included, since it was built in 2001.

I am trying to understand why it's the coldest room in the house. It has a radiator, but compared to bigger rooms it loses it's heat much quicker. The average temperature in the main house, in winter, is around 16c, but this room goes down to 12c.

It is cavity wall construction, two external walls, two windows of total 2 sqm area. Other rooms in the house are solid stone walls, larger windows.

So I am thinking that the roof insulation of 75mm is the big culprit, since cavity walls are better U factor than solid walls.

 

[noseall]

So I am thinking that the roof insulation of 75mm is the big culprit, since cavity walls are better U factor than solid walls.

Yeah, it's nearly double that now.

You would be required to add at least another 40mm underdraw to that spec' to meet current rerg's. Probably.

If you do go with a solid underdraw, use foil tape on all joints and use duplex plasterboards.