Concrete garage conversion insulation and vapour barrier

7 Aug 2020
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United Kingdom
Hoping for some advice. We have a large concrete panel garage that we are about to convert into a games room by adding 4by2 stud walls, insulation and plasterboarding. We heard there are often problems with cold concreate/supporting steels having condensation problems. Any advice on the best way to insulate and best prevent?

We currently thinking two options:

1) Attach foil vapour barrier (SuperFOIL SFTV Thermal Vapour Barrier) to the rear of a constructed 4 by 2 stud wall leaving a small gap between that and the concrete exterior to allow for some airflow. Fill the 4 by 2 with rockwool and on the warm interior side fix foil backed plasterboard.

2) As above but use 70mm celotex insulation instead of rockwool

Our fitter would rather use rockwool vs the mess of cutting celotex. We know that celotex offers better thermal insulation and we would need thicker rookwool top get the same level of insulation. Our other key concern is that rockwool may sag and fall back against the vapour barrier/exterior concrete wall. Any thoughts on rockwool vs celotex?

Someone has also suggested that it would be better to use moisture resistant plasterboard as this sort of room can be prone to damp (especially as it won't be heated at all times).

Thanks for your help, any advice appreciated!
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Fix 50mm foil sandwich sheets to the concrete with foam.

Foil tape all the joints.

Trap them back with with 3" x 2" studs.

Fill between the studs with 75mm celotex.

Foil tape the celotex.

Drape some 1200g polythene to the bottom of the studs. Fix it 300mm up and let it drape 300mm onto the floor.

Fix foil backed boards to the studs.

Lay polythene on the floor and float some more foil insulation boards and deck it with T&G chipboard.
Many thanks for the advice -really appreciate it. Sounds like attaching foil direct to the concrete may be the way to go rather than leaving an air gap between concrete and the stud wall. To do that what kind of foil product would you recommend to attach directly to the concrete? Would the following be ok or is there an alternative you've seen work well?
Wickes Thermal Foil Insulation Roll 600mm x 8m |
You mentioned 50mm foil but assume it could be thinner so long as it seals against the concrete? Assume its purpose is to press right against the concrete to try and seal it right up there is no air access to the cold concrete wall at all?
The builder has 100mm Rockwool slabs already so we will need to use that in the stud and as you outlined we will request a foil vapour barrier/foil backed boards on the warm side.
Is there anything else we're missing? Should we request any air vent be placed in the concrete wall?
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Is there anything else we're missing? Should we request any air vent be placed in the concrete wall?
Not especially no. It would make sense to have an extractor if supplementary ventilation is not achievable by an openable window or that you are expecting high volumes of moisture within the room. All depends what you use it for.

As with all PIR foil sandwich insulation, the more the merrier. Nothing wrong with the rockwool/PIR hybrid solution but I would robustly install the PIR against the concrete ensuring you foil tape everything and use the foam wisely, i.e. filling the panel cracks etc. It will be a bit of a faff having to wedge the PIR sheets until the foam has grabbed but it will be worth it.

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