Garage Insulation

[demogorgan]

Hi,

I want to convert my garage to use it as a study area, It is single brick with flat roof and as such I plan to create a frame out of wood to allow me to install 50mm foil backed cellotex insulation on all external walls whilst leaving a gap between the warm side and cold side & then plasterboard over the top - would I need a vapour barrier between the wall and the insulation here being as the insulation is foil backed ?

On the flat roof it is fairly new so do not want to take it off to create a warm roof so instead want to insulate internally, again using the 50mm cellotex - the roof joists run side to side on the garage and my understanding is I have to allow airflow between each joist to prevent moisture build up, I plan to fit a vented board if this is the right name so each joist has the airflow as I wanted to fit the insulation in-between the joists and then plasterboard over the top, does this sound right or am I barking up the wrong tree ?

The floor Is concrete but due to head height I cannot install a floating insulated floor so instead plan to use carpet & carpet insulation, it will be heated by a standalone diesel heater which will be installed outside and the outlet ran through a wall so will only be heated when it is being used.

Hope this makes sense

Thanks in advance
Demo

 

[23vc]

The vapour barriers (wall and ceiling) go on the plasterboard side of the insulation, not on the cold facing side. I’d try and go more than 50mm if you can (stating the obvious)
The airflow above the ceiling insulation needs to come via gaps in the soffit board at each end, which tends to be done using soffit vents

 

[demogorgan]

Thanks for the clarity, do I still need it if the board is foil backed ?

I have already got the board as it was left over from when I did my roof, can I double it up, or use a combination of rock wool and then the board ?
I currently don't have any vents as the roof joists to straight to brick which I didn't realise at first - what's the best way to tackle this do you think?

Forgot to mention 1 side of the garage is against a wall so venting on both sides is not possible, only one, does this pose a problem?

 

[23vc]

Yes still need vapour barrier with foil backed plasterboard
You can double up plasterboard but it won’t make much difference to thermal insulation
You could use rockwool but won’t be as efficient as PIR for the same thickness
Only venting along the rafters at one end isn’t a good idea, it should be cross ventilated for air flow. If you really can’t cross ventilate the roof plan may not be a go-er. You could do a warm roof which wouldn’t need ventilation but would end up maybe 100mm ish higher, which tends to blow the 2.5m max height rule if it applies

Edit: or you could possibly batten the rafters at right angles to the rafters and put the OSB on top, and vent it side to side, to avoid the issue of the existing wall

 

[demogorgan]

Sorry I meant foil backed cellotex board, but sounds like I still need the vapour barrier.

For doubling up I meant could I either use ,50mm of cellotex and stick another 50mm to it some how making 100mm then the vapour barrier then plasterboard over the top, or use rock wool in the wooden frame I'm making and then 50mm of cellotex over the top then the vapour barrier then plasterboard over that?

I did think about the cross batten for ventilation but didn't know if it would be enough due the ups and downs of the rrafters, so I would batten at right angles, then osb, then cellotex then vapour barrier then plasterboard?

 

[thomp1983]

Yes you can double up your 50mm celotex to make a 100mm piece.

 

[23vc]

The roof, from bottom to top, if you’re sticking with a cold roof, would be plasterboard, vapour barrier, celotex between rafters, air gap vented at both ends created by the cross battening, osb, felt/rubber
If you went for a warm roof it would be plasterboard, rafters, osb, vapour barrier, celotex, another osb, felt/rubber and no need to vent.

 

[demogorgan]

That's great - thanks for your help!

Hope you had a good one!