Insulating garage ceiling under room

[EuphoricExplorer]

Looking for some advice on the best way to insulate the ceiling of my garage.

It’s an attached garage and a heated room above built in the 80s. The garage ceiling isn’t currently insulated, and during winter, the floor above gets quite cold, even though it’s carpeted.

I'm planning to insulate the garage ceiling from below to improve the comfort of the room upstairs, but want to make sure it is being done correctly. My thinking is:

Use Knauf Loft Roll or Rockwool between the ceiling joists and just board it with moisture resistance board, not going to bother with skimming it either.

A few questions I have:

Is this the right approach, or is there a better alternative?

Do I need an air gap to avoid condensation and if so, where should it go?

Do I need a vapour barrier?

The garage isn't heated and has small draughts under the garage door. The ceiling is boarded up with some sort of beige/yellowish plasterboard at the moment which I haven't seen before. No sign of mould or condensation so far.

My aim is simply want the floor of the heated warmer during the winter without overcomplicating things and doing more harm than good. Appreciate any pointers.

 

[Ivor Windybottom]

Check the fire regs. As it was built 40 years ago, it probably doesn't comply with modern rules, as there's nothing but floorboards and plasterboard between the garage and bedroom. As you're now working on it, it really needs to comply with current standards. You'll probably need fire-rated plasterboard (prepare for backache) and non-combustible insulation. But I don't know, you'll have to check for yourself and ensure it's compliant.

Edit: The boarding you already have is probably the fire barrier. Possibly cement board, and theoretically may contain asbestos as I believe it was still in use in the 1980s. Get a sample tested before doing anything.

The vapour barrier should be on the warm side of the insulation, so directly under the floorboards, which is going to be impossible without lifting the floor from above. I'd be inclined to use PIR between the joists, banged up against the floor above, so its foil face will act as the vapour barrier. You could put polythene under the floor and wrapped under the joists, but I'd worry that this would encapsulate the joists in a container that potentially could become full of water.

Officially you probably should inform building control, who'll check and give you a very expensive piece of paper at the end. Personally I wouldn't bother, others would say you absolutely must or something awful will happen but your choice.

I don't think you need an air gap. But if you bang the insulation up against the floor you'll inevitably have a gap between it and the ceiling. I wouldn't worry about ventilating it as it's a sealed indoor cavity. But others' opinions may differ.

I'd probably nail some tiny battens to the sides of the joists to hold up the insulation, to ensure it can't drop down onto the plasterboard, leaving the floor colder.

 

[Ivor Windybottom]

You'll probably find this useful too...

Gapotape - PIR Insulation Gap Tape

Gapotape is a simple yet effective PIR insulation gap tape that helps eliminate air gaps and gives you a perfect finish

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