UKH

My partner has a 1930s semi-detached house with bay windows.

It needs a new roof, and she's got a very reasonable quote for that from a builder who comes with a personal recommendation.

The vertical tiled areas between the bays are still in reasonable condition, but they won't be insulated so she figured it might be a good idea to get them redone and insulated at the same time - was quoted almost half the price of the roof to get the two bays done.

I'm thinking there must be cheaper ways to go about this, perhaps DIY. 

I gather the normal way to insulate these is to take the tiles off, add celotex type insulation to the back of the void, leaving a suitable air gap under the laths to allow any condensation to air, and re-tile.

I've never done any roof tiling before, however I like to think I'm reasonably handy - anyone on here done this before? Was it a difficult job? I'm assuming the quote is because it's a more time consuming job than the large roof tiles, rather than expensive materials.

There are a few houses down the road with render over the bay, so I also wondered about doing that. 

Internet seems to throw up a few different ways people have gone, but one fairly authoritative sounding post suggested insulate at the back but leave an air gap, install membrane onto wooden frame, battens on top of membrane to create another air gap, a layer of builders paper (or another membrane) to prevent render getting into the gap, then staple on expanded metal lath and render over.

Likewise any experience of DIYing this job? 

I guess the biggest concern with rendering a previously tiled bay would be that there could be movement in the frame that the tiles are done with but would crack the render?

Bit of a ramble, but if you got this far any and all thoughts welcome, even if it's just "your gonna trash the house, leave it to an expert".

We (1955 property) needed to replaster anyway and you are right there was zero insulation under the window, so on the recommendation of our plasterer he used insulated plasterboard. He also inserted rock wool insulation above the plasterboard on the ceiling area under the flat roof of the bay. 

Im sure it's nowhere near as good as cellotex or a cavity wall, but better than we had before. In our case we wanted to do the work from the inside as was decorating anyway.

Would it be possible to put insulation on the inside?

We had an extension with walls that were a single skin of breeze block with minimal insulation(ie the polystyrene that is usually used to line walls before papering) and it was cold.

We had the walls lined on the inside with 100mm of Kingspan and the difference was amazing.

Dave 

Did my roof, guttering, fascia board off and stuffed the void with rockwool, couple of hours work, did make a difference as snow seems to stay on it now.

As a relatively green DIYer I did a cement tiled roof on a lean-to (4-5 m2). It was a pretty straight forward job and hasn't blown away or fallen down in the last 8 years. 

The cost of the pro job may be due to the tile type, trying to remove them without breaking them so they can be reused. Maybe the builder knows that they are friable and difficult to replace. Maybe it's difficult to get enough insulation in to meet building regs. If the framing of the bay is only 4 inch timber you could add on an inch of roofing batten to give yourself enough room for an air gap.

There's a whole load of possible issues. Your plan sounds ok but personally I'd avoid rendering (it's a bit of an art like plastering). Otherwise I'd probably give the tiling option a go myself.

Personally I much prefer the look of hanging tiles than render.

just a couple of things to "help" [help muddy the water?]:

when you say "membrane" - vapor barrier on the warm side of the insulation, and a breathable membrane under the tiling. the more breathable the better imo.

and: if you're doing the outside yourself, don't underestimate the value of a scaffold / platform. theoretically you can do a lot from a ladder or even two; but you'll be doing so much up and down it'll drive you insane.

Thanks folks, since there's only a couple of broken tiles probably just going to replace those myself and go from the inside for the insulation.

Looking at the insulated plasterboard, I didn't realize how thick that stuff goes - for a little loss of space in the bay we should be able to make a big difference to the heat loss.

Yes, this bit is important. If water vapour from the room makes it through the insulation it can condense inside the wall.

Some of the insulated plasterboards are themselves a vapour barrier, so that's worth looking at.