26 Oct 2016
Reaction score
United Kingdom

I am looking for some advice on insulation.

Building an insulated wooden office (walls/floor and roof) in an uninsulated garage. I am looking to find the solution for the best insulation but I am tight on space.

I have two thoughts at the moment for construction of walls, floor and roof.

Option 1

100mm insulation (probably foil backed PIR) in 100mm stud work

Standard vapour control layer onto stud work

Standard plasterboard/plywood for floor

Option 2

80mm insulation (probably foil backed PIR) in 100mm stud work

Foil vapour control layer onto stud work. This leaves 20mm min air space and reduces cold bridging on stud work.

Standard plasterboard/plywood for floor

My question is what would give me the best insulation for the room? I like the idea of the 20mm space to help with electric cables on option 2.

Any other ideas are also appreciated.

Sponsored Links
Not sure how the air space reduces bridging, for that you would fill the studs and then use celotex backed plasterboard to cover the studs. If you want a service void then fill the studs, cover with 25mm celotex and seal the joints, then batten through and plasterboard the battens.
However this is inside a garage, so you may end up spending more on insulation than you'll save, but if it's a fun project that's OK. Good luck!
Hello John

Thanks for coming back.

Was thinking of using a foil vapour membrane so this would cover the studs under the plasterboard to reduce bridging and keep the depth low. The air gap was listed by one of the suppliers to increase the R value. The air gap had been asked for on either side. Details at

With all of this I think that you may be correct that the new room is already inside a garage so I am probably not going to save what would be spent so may just stick with the 100mm PIR and plain plasterboard/vapour layer.

Don't start the foil debate! Suffice to say the foil won't insulate at all unless it faces an air gap so it can reflect the heat back. If there's no gap then heat loss is by conduction, so you need celotex to prevent it. You could foil and then counter batten to leave your air gap but I'm not convinced you're going to need such care.
Sponsored Links
Ordinarily an air gap would be on the cold side - for venting. In this case an air gap is pointless. Fully fill the void with foil skinned Celotex including any gaps with ex-foam. The foil on the Celotex will help with the vapour control. Use foil back plasterboards for additional VCL.
Well, you don't really need foil faced if the gap is to be fully filled.. foil creates a low emissivity/low absorption/high reflectivity surface when it has an adjacent air gap. Radiant heat is reflected back by the surface of the foil, and it's a rule of physics that a shiny surface radiates less heat than e.g a black one, hence when a foil faced board is installed with air gaps, less heat escapes because some is reflected back, and less is radiated away on the cold side. Reasonable rule of thumb would be each foil surface improves the u value by around 0.01. No air gaps, none of this mechanism applies.. and hence you can buy cheaper non foiled insulation from the likes of secondsandco

I think you'd find that 38x63mm CLS stud work would be fine, you can then overboard it with 25 to 40mm of celotex and have a 100mm ish wall thickness, with better insulation cover and fewer cold bridges, for about the same money. The big difference will come from attention to detail with regard to gaps. Carefully align all mating edges of the PIR and either glue them together with expanding foam, or tape over them with aluminium foil tape.

If you're desperate for your wall to have a 100mm structural footprint you can stagger 63mm studs so half are internal, half are external. Might even make your life easier to turn them round so the 63mm dimension is visible in the room, then use 50mm celotex sheets cut to 600 widths and also staggered to give an effective continuous insulation. Fully glued with a urethane glue (egger d4, expanding foam, lumberjack thixotropic gunnable etc) it would have sufficient racking resistance for anything you could throw at it without needing to sheathe one side. As you're effectively building a small timber frame house part inside your bigger brick shell, if it's to be durable, then fit a vapour control layer to the inside and a breathable membrane to the outside. Products like tyvek house wrap, supro roofing felt or competitor alternatives would be fine for the outside, and anything from polythene sheeting to tyvek air guard for the inside
Do you actually need stud work? Could you not go mechanical or dot/dab of something like Celotex PL4060. This is 60mm insulation and 12.5mm plaster board. Depending on the construction of the garage at the moment - e.g. 4" block, you'd still be within Building control requirements.

Avoiding studs will improve the thermal efficiency a few %

Just as a note of caution - 100mm studs are usually 95-96mm dried - I learned that lesson on my man cave which resulted in me having to nail gun 5mm strips of ply to the studs to get the plaster board on.

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