Hi,

I have a few questions regarding insulation.

I live in a top floor apartment with a pitched roof. The building is 30 years old.

We have a cold loft. In the loft there is no insulation between the rafters.

In the bedrooms we have pitched walls. Behind the plasterboard there is about a 6cm air gap, followed by the rafters. The rafters have fiberglass between them. I'd say it's initially about 10cm deep and has probably expanded a little with time.

The rafters are 17cm deep, above which there is a roofing membrane, which looks like it is only made of plastic.

There is no noticeable air gap between the plastic sheeting and the insulation.

I've looked in the loft and I cannot see any ventilation at the top of the roof, the plastic just rolls over the top of the rafters from one side of the roof to the other.

I would like to change the insulation with one with higher performance. I am thinking if using wood fiber insulation for its higher decrement decay factor. I'm not sure what to do about water vapour and condensation however, given that there seems to be no ventilation on the inside of the plastic roof sheeting.

Is it worth adding a 25 or 50mm air gap if there is no ventilation at the bottom of my pitched wall? Underneath it's my neighbour that also has a pitched wall, and there is probably no air gap in her wall either.

Is it worth adding an air gap if there is no way for the water vapour to escape at the top of my roofing space?

If I were to leave an air gap, do I need to add a ventilation at the bottom and top of my pitched walls to let air flow behind the insulation?

I haven't seen any obvious signs of condensation in the loft. The plastic sheeting is dry and I cannot see any mold on the rafters.

I've attached a photo where the rafters meet at the top of the roof and another at the floor of my bedroom, you can see the fiberglass that is coming up from my neighbour below and the plastic sheeting.