Insulating the roof of an old house

[Mike313]

Hi,
I'm looking at buying a small house the roof of which has been re-tiled some years ago with concrete tiles but there is no felt under the tiles. It's a storey-and-a-half house so the upstairs floor is in the roof space/rafters, and I want to insulate this roof space to use as bedroom space. What I envisage doing is:
(A) have someone strip the tiles and battens, put on some felt and then refit the battens and tiles (replacing any battens that are rotten).
Then (B) I will myself put some 100mm Kingspan (or similar) rigid boards between the rafters (leaving an air gap above of about 75mm), tape all the joints for air-proofedness and then fix some insulation-backed plasterboard under the rafters to eliminate any thermal bridging through the rafters.
So I have two questions:
(1) Does that all sound ok? and,
(2)How much do you think it would cost me to get (A) above done by a roofer? The house is small about 6metres square with the double-pitched roof at about 45degrees so I guess about 50sq.metres altogether of tiles. If I'm going ahead I will ask a roofer but just to get an idea before I get much further

Ta in advance.

 

[Mr Chibs]

Sounds good, are you sure the roof rafters are deep enough to take 100mm insulation and 75mm air gap?

I’ve found smaller, older properties quite often have smaller depth rafters than 175mm.

should you have deep enough woodwork, once you get the roof stripped you might find it easier to add the insulation whilst the battens and tiles are off.
Then membrane, batten and re tile.

Separate insulation and plasterboards are usually cheaper than an insulated plasterboard.

No idea on cost, but you could do all this yourself.

 

[Mike313]

Thanks Mr C. Good to know the overall idea is OK. Unfortunately I can't work safely at heights so I'll have to get someone to strip/felt/re-tile for me. That's why I was going to do the insulation from the inside myself.
Has anyone else got an idea of costs?