Ideally a flat roof should be a 'warm roof', i.e. using good insulation and a vapour barrier so that heat is kept in and moisture can not reach any cold spots to condense and cause problems. But what do you do if you can't or won't spend the money refitting an entire flat roof? This was our situation - the roof was only refitted a bit over five years ago, so has plenty of life in it yet. When it's time to renew it, we'll get the insulation done properly at that point. Until then, we needed to come up with another solution...
The sections of flat roof we have are above our dormer windows, and as such they link into the small loft that is higher than them. Above the windows they are not airtight at all, which is useful as it lets air flow through the roof space and loft, preventing any condensation forming. However, a few months ago I finally got round to investigating what was in the roof space, and I found this:
Yes, that's right - plasterboard, over 100mm gap, then the roof deck. So all that fresh air blowing through was chilling the plasterboard and therefore the bedrooms below. I knew it wouldn't be great, but I'd not realised there'd be no insulation in there at all! A further complication is that the access from the loft is obstructed by joists. You can just see the edge of one under the floorboard here:
The only thing to do was find something flexible that I could get in through the gap. It would never be perfect, but anything would be better than nothing! The other thing that was needed was to ensure that there was still a gap for air to flow after the insulation was installed.
A quick trip to B&Q gave me some materials to experiment with. I found something a bit like tough bubblewrap coated with aluminium. This actually has an insulation value equal to about 50mm of polystyrene, so is quite useful.
This could easily be cut into strips and fed into the gaps:
After a bit of work, it was lying down in the gap between the joists, though not lying completely flat:
The next step was to feed in a 25mm polystyrene board on top of this. Polystyrene isn't as good an insulator as some of the fancier insulation boards (eco-therm, cellotex), but the important thing for me was that it's flexible:
I used one of my chimney sweeping rods to put some weight on it while pushing it to the end of the roof:
Once I had enough board in, it was all lying pretty flat:
I came up with some improvements during the process, such as taping the end of the foil layer to make a ridge for the polystyrene board to push against:
For the sections that had easier access, I actually taped the first board onto the foil layer:
This also made it easier to pull it all back a tiny bit to ensure an air gap at the far end.
Before buying materials to do the whole roof, I did a couple of sections and then tested the results. I don't have access to a fancy thermal imaging camera, but a friend lent me this very handy Thermal Leak Detector, made by Black & Decker. Basically, you point it at a spot and it gives you a temperature readout. You can also set it to a reference temperature, for which it projects a green light at the spot it is measuring, and then the light changes to red or blue to show if the temperature has gone up or down. Anyway, here's the readouts with the room temperature at about 19C and the outdoors at about 2C, first with insulation and then without:
Quite a difference! So I quickly went on and insulated the rest of the flat roof, and the temperature was a couple of centigrade higher than normal the next morning, with similar weather and the same use of our woodburner.
If you want to see a bit more about the Thermal Leak Detector, here's a short video of it in action:
Sweeping the chimney
Installing the wood stove
Plumbing and testing the wood stove
Installing solar water heating
Insulating the cavity walls
Insulating the loft
Installing off-grid solar PV (part 1)
Installing off-grid solar PV (part 2)
Upgrading the off-grid solar PV to 750W