Insulating garage ceiling

31 Oct 2013
Reaction score
United Kingdom

Our 1930’s house has an integrated garage (used for storage, not a car), with a bedroom above. We’ve had the asbestos ceiling removed so now have the joists and can see straight up to the floorboards of the room above. I want to insulate and plasterboard the ceiling. Would it be ok to feed through a layer of rock wool to lie on top of the joists and then fit celotex or something between the joists and then fit plasterboard to the underside of the joists?

if so, what thickness for each would you suggest please? I’m not bothered about skimming the board or anything.

thank you

Sponsored Links
I would rockwool between the joists and overboard with as thick PIR as you can (40mm?). Then pink plasterboard.
If you put mineral wool between the joists (I expect it will fit about 170mm) you will not need celotex. It will provide more than enough insulation for a floor, and can be stuffed in tightly to prevent draughts, with no precision cutting needed.

Mineral wool does not burn, or emit poisonous fumes in a fire (unlike Celotex)

Two layers of pink (fire resisting) plasterboard will give best resistance to noise and fire. You should stagger them to avoid the joints coinciding. Use pink fire foam round the edges and any penetration such as pipes or cables. Do not make downlighter holes.
Thanks for the responses. Can I ask how you would deal with the wiring please? Do I just leave those sections empty of insulation? I’ve attached photos. Please ignore the white wire, the grey wiring is what our electrician is currently sorting when fitting the new consumer unit.


  • 19FD9D89-AFE0-4FBB-8ADE-6B173269780C.jpeg
    261.4 KB · Views: 127
  • 933CF6E5-EAA0-4444-AF54-F245C177AE71.jpeg
    381.4 KB · Views: 106
  • 5F34253D-B86E-429E-AA20-948DF18DAF17.jpeg
    355.7 KB · Views: 137
Sponsored Links
I hope the electrician is sorting out that dog rough junction box.
You need to speak to your sparky.

Depending on the loads, they maybe able to cope with being buried in insulation. There is a calculation (derating factors) to ensure cables can withstand not being able to shed excess heat
cables should not be buried in insulation. They can lie on top of it, or they can lie on the ceiling with the insulation on top

either is OK.
There is a derating factor for being buried in insulation, so if the cables can take it, it is possible
I've had a bedroom built above garage and bco has stated 150mm celotex between joists and then 50mm across underside to stop cold bridging then plasterboard..... crazy.
I’m thinking of just putting as thick a layer of insulation under the joists then board over. As the consumer unit is in the garage, there is wiring going around and out to the house between many of the joists so would be really awkward to insulate around. Not ideal I know.

DIYnot Local

Staff member

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.

Are you a trade or supplier? You can create your listing free at DIYnot Local

Sponsored Links