Insulation advice for bedroom above garage

[meza]

House built in 1991 (Bovis)

Regular block and brick, had cavity wall insulation 5 years ago.

The bedroom above the garage gets a cold floor in winter. The garage ceiling is double skin insulated plasterboard (about 50mm total thickness). But the ceiling / floor joists are empty and end with a vented soffit above the garage door. Cold air enters and travels down the joists cooling the bedroom floor.

I'm prepared to take the bedroom floor up (chipboard). Looking for advice on what steps I can take to add/improve the insulation to this room.

I wonder what kind of inslutation I can put under the bedroom floor? Can I block off the ends of the joists behind the soffit?

What about in the void, and the small wall that runs under the dormer (with radiator on)? I'm prepared to take the rad off to allow a large-ish access hole behind the rad to access this space if need be.

bedroom insulation.jpg (61.79 KiB) Viewed 958 times

[dewaltdisney]

I think the better option would be to take the garage ceiling down and fill it with Celotex from below. Depending on the joist layout you may only have to open one end to slide the insulation in. The chipboard floors will most likely be glued along the board's tongue and grooves (to stop squeaks) so you will muller them trying to pull them up. The plasterboard is easier to replace from below using a deadman. Either direction you will have a fair bit of remedial.

DWD

[meza]

Hi DWD, sorry I should have added that taking down the garage ceiling will be a royal PITA because of the junk in the garage! I'm alright at getting up the chipboard, and it's only nailed and yes it does squeak so I'll be replacing the nails with screws.

So assuming I go in through the top, just PIR will be fine? Fill the whole joist depth? Need the tape anything up? Vapour barrier needed? Cheers!

[dewaltdisney]

I would say you need a bit of clearance to allow air to circulate, it has to have a bit of ventilation. The insulation will stop heat loss and cold penetration. Minimal gaps are unlikely to cause a cold spot but you can suss out vulnerable heat loss spots as you go.

DWD

[wine~o]

So assuming I go in through the top, just PIR will be fine? Fill the whole joist depth? Need the tape anything up? Vapour barrier needed?

Don't go the full depth, 50mm or 75mm PIR boards will be enough. You want to keep the airflow beneath the PIR boards. Cut to size using a bread knife to give a snug fit between the joists, level with the top of the joist. no vapour barrier required as the foil face will act as a vapour barrier.

[big-all]

no expert on this

what are the fire requirements above a garage ??
will this effect what insulation' gaps and plasterboard you choose

[dewaltdisney]

It has got the double layer of plasterboard above the garage

DWD