Is fibreglass insulation in-between rafters good enough?

[Harveypuss]

We've recently moved into a house which has a useable loft room with lounge area, bedroom and en-suite. We have realised that there is only fibreglass insulation in-between the rafters rather than Celotex/Kingspan and no insulation at all separating the internal walls from the attic storage space (the building regs documentation signed off the side extension but not the loft. We suspect our solicitor may have been incompetent in his cross-checking but that's another thread entirely!).

Consequently in this hot weather the loft area it is uncomfortably hot up there and I suspect it may be freezing in the winter, though there are radiators in the rooms.

I am considering getting a builder to rip the ceilings down, take out the fibreglass and replace with Celotex/Kingspan everywhere plus put another two Velux windows in, which should help with ventilation.

I have had a quote so far for about £15k to do this work (it's a large area) but I wonder if it's worth it. I'd hate to spend that sort money and it make no difference to temperature. I could just get some air-con installed, but am worried about the gas bills in the winter if the fibreglass rafter insulation doesn't retain the heat in the winter.

Has anybody any experience of something like this? Advice and opinions are welcome. TIA.

the rigid foam has about twice the insulating power as the same thickness of fibreglass.

Usually a layer is put between the rafters, them another layer is fixed to the face of the rafters, then plasterboarded.

I think you will find diagrams and specifications on the Kingspan and Celotex websites, or Knauf.

This may also be a good time to re-roof the building and include a modern breathable membrane under the tiles (unless you happen to know for certain that it has already been done to a good standard).

If you are in Scotland you may also need sarking.

But in this weather you will still boil. Winter temperatures are a different thing though.

Yes insulation is good for keeping heat out as well as in. Glazing lets a lot of heat in. If you can insulate well it should stay stable whatever the weather is outside.

although hot air will rise up through the house, and tend to collect at the top.

If you're doing some building work, and spending £lots, I'd consider vents or windows in the gables (if any) as well as your velux windows. If you have an opening at opposite ends or sides of the house, any wind will tend to blow in one side and out the other. With veluxes, people do sometimes open then in hot weather, and come home to find a sudden summer storm has drenched the room. You can also get tile vents and mushrooms that are weatherproof. A ventilator that you can leave open fairly permanently may prevent the temperature rising too much, because the hotter it is, the faster hot air will flow up and out.

Roofers are usually much better at doing watertight work than general builders.

p.s.
good news! some clouds arrived here! It's only 30C!

Totally agree on the heat as our attic bedroom is a furnace. It's never cold in the winter however.
If it's not been signed off on building regs are you 100% sure that it's got its escape routes and fireproof doors etc so it can be used as a bedroom?