Flat roof under pitched roof - An insulation tale

Several years ago, we had an extension put onto the back of the house, over an old flat roof. However the extension didn’t cover the whole of the flat roof, so we had a pitched roof put over the remainder so that it gave a more reliable rain shield.

The builders insulated the new roof, but of course they didn’t remove the old flat roof first! The insulation was just piled up over the old felt and left. The room underneath this is the bathroom and I had been wondering why it was still quite cold. Lots of insulation in the roof, so should be warm etc. So I had a look. It was then that I discovered what had happened.

So effectively what I had was a vented flat roof with a load of insulation above it. This meant that the wind was venting the old flat roof still and so I was only really benefiting from the amount of insulation that was in the flat roof structure!

So one of my jobs has been to remove the insulation, then remove the flat roof, inspect the insulation in the old structure (I wasn’t expecting much!) and then do some remedial works. Fun, especially since the new roof only gave me around 1.2 metres of headroom at the apex. Being a 1.9m tall person, I was expecting a bit of a squeeze. I wasn’t disappointed!

So I set about cutting out the old roof with my reciprocating saw and drill. What a lovely job. I had to cut out large rectangles of felt and chipboard to expose what lay beneath. I did this as close to the edge as height would allow to ensure that the final solution would be accessible as possible.  Anyway, as expected I uncovered a right old mixed bag. Some spaces between the joists had 5cm of insulation, some a collapsed 10. Some, none at all. No wonder is wasn’t too warm in there.

The solution needed to be thought through, but was in the end, basic. The warm moist air from the bathroom will primarily vent through the extractor, but will also partially vent through the ceiling and insulation (just like in a standard loft). The insulation therefore had to allow this into the main pitched roof space and so I also needed to keep the eaves clear to give the required draught in the void (this avoids the risk of any condensation forming on the now cold underside of the pitched roof). So effectively I just piled up the insulation, that I had removed prior to starting, on top of the exposed ceiling boards to a depth of around 30-40cm and kept the required 5cm gap at the eaves for the ventilation.

Not so worried about autumn and winter’s imminent arrival now, though will keep a check on the moisture issue since the insulation that I re-instated is recycled plastic rather than my preferred warmcel insulation. I would have preferred the recycled paper since it is breathable and hence any excessive moisture would be more easily transmitted through it than the plastic. Still another job done.

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