DIY Doctor UK DIY Forums

This is my first post on the forum so hello everyone. I moved into a 1960’s semi-detached dormer bungalow a couple of years back and the bedrooms particularly (rooms in the roof) are bloody freezing in the winter months. There is no insulation on the sloped sections of the roof or dwarf stud walls, so I’ll be stripping out all the old plasterboard and reinsulating one room at a time. Before anyone suggests getting a roofer in to do the work, I live on my own and simply don’t have the budget as the whole house needs updating so have no option but to do as much of the work myself as I can.

I’ve started with the main bedroom as per photos which has a flat roof above the dormer and have a few questions if anyone can help please?

I’ll be installing warm roof deck with rubber membrane to the flat roof above the dormer window using a kit from Rubber4Roofs who provide pretty comprehensive details on how to do the job but their guidance is a little light on details for where the warm roof deck and membrane ties into an existing sloped roof (as per mine). Does anyone know where I could get this type of specific detail / construction drawing / guidance from?

The photo of the underside of the flat roof shows what looks to be tongue and groove boards through which you can see the bitumen has seeped. So, I’m guessing the roof is simply the roof felt directly onto the T&G boards. When I put the warm deck on can I remove the existing felt and build up from existing T&G boards (i.e. vapour barrier, rigid insulation, overlay board, rubber membrane) or should I lay OSB3 board over existing T&G boards first?

On the dormer cheek I could install 50mm of insulated plasterboard on the existing battens shown in the photo but that would only leave approx. 10mm of the window frame showing where the insulated board would butt up against the window frame which I don’t think would look right. The existing battens are 25mm deep, is it possible to remove those battens and bond the insulated board to the existing timber cheek board to allow for a thicker board to be installed?

The rafters on the main roof are only 100mm deep so the plan is to friction fit 50mm of rigid board between the rafters (to maintain 50mm ventilation gap) and then overboard with insulated plasterboard. This is also the plan for the dwarf stud walls. Does anyone know if this is an acceptable solution and do I have to install any kind of vapour barrier as that isn’t a topic I’m very familiar with? Or would the insulation supplier advise on that?

Should I stop the rigid board between rafters at the level of the purlin shown in the photo or continue it all the way up to the apex/ ridge of the roof? I’m thinking that I should stop the rigid insulation at the level of the purlin and then just regular Rockwool roll insulation to the ‘standard’ loft areas.

Should I insulate the chimney shaft (to the right in the main roof photo where you can see some exposed brick) with insulated plasterboard as I would be worried about it being a cold bridge? The chimney isn’t used, the fireplace is bricked up on the ground floor.

I’ll also be replacing the window but don’t know if a dormer is just a standard window or needs to be strengthened in any way because it’s a dormer window. The flat roof joists don’t appear to be sitting on top of the window frame as they look to be fixed to a timber lintel that runs the width of the window (see photo). Anyone know if a dormer window is just a standard window frame or needs to be strengthened?

Final question. I’ll be installing soffit vents along both sides of the roof to maintain ventilation of the cold areas of the roof space, but do I also need to install ridge vents as well or is that overkill?

Apologies for the lengthy post. Any help, advice, tips from experienced people would be very greatly appreciated.

Hi Bertie bumwhistle
Suggest you do not mix ventilated and unventilated roofs in the same property. Stick with a cold roof on the dormers and insulate below the rafters.
Use a higher thermal resistance board for the dormer cheeks so you do not seal against the timber used.
Use rigid expanded polystyrene between the rafters and joists between the purlins, overboard with more polystyrene and then use standard plasterboards to cover. This will be much cheaper than using plasterboard backed with insulation. Don’t bother taking the insulation up to the ridge.
Yes insulate the chimney - here you could use backed plasterboard.
The window will be a standard window but if the is a central vertical mullion make sure this is reinforced in any replacement window.
Soffit vents on both sides of the roof will be fine for removing condensation but to reduce temperatures in the ventilated area, ridge vents are advisable. These will also reduce the room temperatures.
Regards S

That's a big help Stoneyboy. Thank you for advice, very much appreciated.