Im currently insulating the house (large dormer bungalow) from the inside, using mainly glass fibre and rockwool. The rooms are room in roof design with a knee wall.

The upstairs master bedroom is above the small garage and I discovered today, whilst foam blocking the subfloor cavity under the bedroom floor, that the plasterboard garage ceiling is separated only by an air gap before the flooring of the bedroom above.

Blown in cellulose would fill in this void perfectly, however this would not be viable for such a small area.

This led me to thinking maybe I could insulate from the garage side.

EPS foam is light and could be mechanically fixed to the upstairs floor joists, through the existing plasterboard.

Using a good depth of EPS would serve as a great insulator to the room above, however Im aware of the associated fire risk.

Would using a fibre cement panel lower this fire risk, or should I look for another method to insulate this area?

I think some pics would help here.

 

A little difficult to get my head around this, but if there's any airflow then the insulation upgrade would be pointless / useless, if I am reading this right that is. 

 

How thick an insulation board can you fit? 50mm or more?

 

Have you considered PIR (Cellotex / Kingspan etc) as the results vs effort would then be maximised. EPS is not great by comparison, particularly with thinner layers of it. 

20 minutes ago, andeebee said:

Would using a fibre cement panel lower this fire risk, or should I look for another method to insulate this area?

Are you referring to a fire in the garage rising to the habitable spaces above? This should already have been addressed by building control and there should be plastered FR plasterboard (even 2 layers of it) already creating the fire-break?

Don't fix eps as an exposed soffit. It is highly flammable and melts as is it burns, spreading gobbets of fire.

Pir is not as awful but still not good.

 

Incredibly these were once standard ceilings.

But you could apply it then cover with plasterboard or cement board.

 

Preferable would be to remove the ceiling and fill between joists with rockwool. 

13 hours ago, Nickfromwales said:

I think some pics would help here.

 

A little difficult to get my head around this, but if there's any airflow then the insulation upgrade would be pointless / useless, if I am reading this right that is. 

 

How thick an insulation board can you fit? 50mm or more?

 

Have you considered PIR (Cellotex / Kingspan etc) as the results vs effort would then be maximised. EPS is not great by comparison, particularly with thinner layers of it. 

I was hoping to get a good layer of maybe 200mm as I have loads of head room in the garage, but you are right the air gap above will probably negate this.

1 hour ago, saveasteading said:

Don't fix eps as an exposed soffit. It is highly flammable and melts as is it burns, spreading gobbets of fire.

Pir is not as awful but still not good.

 

Incredibly these were once standard ceilings.

But you could apply it then cover with plasterboard or cement board.

 

Preferable would be to remove the ceiling and fill between joists with rockwool. 

Yes, you are right, my fears about using EPS are confirmed then.

 

I think a better option, albeit more invasive, would be to lift the floor in the bedroom and fill the void with fibreglass.

The joists are 150mm so I can fit a good layer in there. This was on my plans but as its so disruptive wouldnt be a job I could do over a weekend as it requires complete bedroom teardown

16 minutes ago, andeebee said:

Yes, you are right, my fears about using EPS are confirmed then.

 

I think a better option, albeit more invasive, would be to lift the floor in the bedroom and fill the void with fibreglass.

The joists are 150mm so I can fit a good layer in there. This was on my plans but as its so disruptive wouldnt be a job I could do over a weekend as it requires complete bedroom teardown

Why not take the garage ceiling down and do it from below, you can then update the fireproofing between the garage and bedroom. 

A garage ceiling built to modern regs should be 2 layers of 15mm pink fireblock plasterboard, with the joints on both layers staggered.  I very much doubt you have that much now.

 

So best bet is take down the plasterboard ceiling you have, insulate between the joists with rockwoll etc then re board with 2 layers as above to modern specs.

3 minutes ago, Russell griffiths said:

Why not take the garage ceiling down and do it from below

I think that's what I said.

 

25 minutes ago, andeebee said:

but you are right the air gap above will probably negate

I don't know what you mean.  An air gap is additional insulation unless draughty.

 

I think that's what I said.
 

see your learning. 
 

@saveasteading

Thanks both, sorry I missed the bit about removing the ceiling and fitting the insulation that way.

Of course makes sense and less disruption.

On 12/01/2025 at 10:52, saveasteading said:

An air gap is additional insulation unless draughty.

It's over a garage so will be a separate (cold) air envelope, so will likely be awash with convection currents through every single nook & cranny.

Do not use PIR or polystyrene in an intermediate floor above a garage. You will not get the fire resistance you need. Speak to your local Gypsum rep who will give you a specification for a floor build up that gives you the fire resistance and good thermal resistance.