Insulating old extension roof?

andre_xs

We have a little extension in the front (~2-3sqm). The (flat) roof is basically pure concrete (approx 15cm thick) with roof felt on top. From bottom, it's just painted. There's no insulation anywhere. As a consequence, we have horrible condensation in the winter.

We're currently short of money, so I thought whether such a quick fix would work at least for the coming winter:

On top of the current roof (on top of the felt, which is still sealed, no leakage or so) I could put some wooden frame, maybe 10cm thick, and fill the frame with rockwool. The top of that would need waterproofing, of course, e.g. plywood plus a thick wheather resistent foil, or plywood plus felt, or just plywood, treat it for wheather resistance, and put it in a slight slope on the roof, so that there's no standing water.

Does that sound reasonable? (I would have most of the materials lying around, so not really expensive) At least for just one winter. Should I consider something in addition?

Edit: From the inside the ceiling is quite low and we just had a plasterer to skim it smoothly, freshly painted. So basically if possible we don't want to do anything from the inside, unless there's really good reason for this.

FreeBear

You could insulate from the outside - Would recommend Kinspan/Celotex type PUR foam sheets. Much better insulation than the same thickness of rockwool. Attention to detail is essential. You'd want to run the insulation down the sides so that the risk of cold spots are reduced. As long as you don't puncture the existing felt, you could even lay sheets of PUR foam on the roof followed by a DPM and top it off with some thin paving slabs.

Search for "Warm Roof" to get a better idea of what is involved.

Insulating from inside would mean the room heats up quicker, but would also cool down faster. And as you have already plastered the ceiling, you missed a chance there.