Tuesday, 7 February 2012

Retro-fitting flat roof insulation

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:

Retrofitting flat roof insulation IMG_0525

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:
Retrofitting flat roof insulation IMG_0523

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.
Retrofitting flat roof insulation IMG_0526

This could easily be cut into strips and fed into the gaps:
Retrofitting flat roof insulation IMG_0527

After a bit of work, it was lying down in the gap between the joists, though not lying completely flat:
Retrofitting flat roof insulation IMG_0532

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:
Retrofitting flat roof insulation IMG_0533

Retrofitting flat roof insulation IMG_0534

I used one of my chimney sweeping rods to put some weight on it while pushing it to the end of the roof:
Retrofitting flat roof insulation IMG_0535

Once I had enough board in, it was all lying pretty flat:
Retrofitting flat roof insulation IMG_0538

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:
Retrofitting flat roof insulation IMG_0545

For the sections that had easier access, I actually taped the first board onto the foil layer:
Retrofitting flat roof insulation IMG_0651

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:


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ChrisJS said...

Ingenious! Our solution to similar problem was to insulate on top or the flat roof - OK till the roof springs a leek!! Like the bit telling me about the thermal leak detector which I didn't know about.
Chris (ecodiy.org)

Mike Pepler said...

Insulating on top of the roof is an accepted way of doing this - but in our case, this would still have left an icy gale blowing between the roof deck and the plasterboard...