Cilotex to insulate exterior wall

[plonker]

Have a new build bedroom extension that I am told has to be insulated on the interior of the external walls to neet Building Regulations.
The walls are brick exterior with Rockwool cavity fill and Thermolite interior.
What thickness Cilotex board is required to satisfy Building Reg's and what size batterns (do i need an air gap?).
Advice appreciated

 

[regsmyth]

Well your building control inspector should tell you.

However, I'd have thought an insulated cavity wall with a thermalite inner leaf would be satisfactory.

Did you not have to supply plans?
Did you not have to supply such details to b.c before starting?
Who has been specifying the build construction so far?

 

[^woody^]

Assuming this extension has only something like 50 or 75mm cavity insulation, then you would use 25mm Celotex/Kingspan and 12.5mm board and 3mm skim

No air gaps required

 

[plonker]

Thanks for replies.
Plans refer to 12.5 paster board finish, builder suggests should be 50mm Cilotex then plaster board.
Have e mailed Building Inspector to get clarification.

 

[^woody^]

Thats an expensive and poor design with an insulated cavity wall and then internal insulation too.

 

[plonker]

Nothing wrong with the design or materials used.
Outcome and cost will be determined by the Building Reg's and Local Inspectorate. Lets hope the Builder is incorrect, although I doubt it as he has completed the same type extension on my estate a number of times.

 

[^woody^]

A cavity wall with 100mm cavity and full fill insulation and thermalite standard/shield will achieve compliance and will be the cheapest and less time consuming method in terms of material cost and time to build

By building a cavity wall, and insulating the cavity and then insulating the internal face is a poor design and expensive in terms of the material cost, the time to build and the messing about in doing it, and actually fixing the insulation to the inner face of a thermalite wall

The other alternative would have been to just put the Celotex in the cavity in the first place

 

[noseall]

The only time we have to insulate the inner face of an insulated cavity wall is (as Woodstck has already pointed out) when we are tying into walls that pre-date 100mm cavities.

It is sometimes necessary to run the inner walls parallel, like when wall plates have to line up for instance, so it is more logical to build the new wall at say 75mm cav's as opposed to 100mm, to achieve this continuity.

In these instances insulating the inner face is inevitable.

Otherwise, if you have room to do so and have no structural restrictions then building 100mm fully filled cavity walls is simpler and cheaper. To actually opt for insulation boards on the inner face is ludicrous.

 

[Richard C]

if you have room to do so and have no structural restrictions then building 100mm fully filled cavity walls is simpler and cheaper.

By far & away the most sensible way to go even if it means some "disguising" is required.

To actually opt for insulation boards on the inner face is ludicrous.

Totally

 

[plonker]

Thanks all for constructive advice.
New extension was built on top of existing house wall and cavity is below 70mm therefore building Inspector confirms it has to be insulated.

Over the phone he suggests 25mm Celotex is OK but my Builder says 50mm is the norm, this I have to confirm when Inspector comes to site.
Once again many thanks to all that have commented.

 

[^woody^]

25mm will do. 50mm is not "the norm" because this wall construction is not normal.

Its too late now, but I would have some serious words with whoever designed this or decided to build it like it. It does get my back up

The blockwork could have been oversailed at floor level to form a wider cavity. Or, you could have used better insulation within the narrower cavity.

Because of this poor design you are paying more and losing more space off your internal room area. The designer obviously didn't give a toss, and the builder now makes more money

 

[plonker]

Woody - thanks for your comments, I agree.

Interesting to note that Authorised plans/Details agreed by Local Authority specify only Plaster Board & dab for inner walls.

Easy to be wise after event will know better next time!

 

[legs-akimbo]

Hang fire before you do anything rash, let me know what make and thickness of insulation you have used in the cavity and the cavity size and i will do you a U value calculation to see if it complies before going to the expense of insulating internally, I would gladly e mail you the calculation detail if it does comply.

 

[legs-akimbo]

Just done a quick calculation, if there is 50mm of insulation in the cavity then alongside brick, thermalite and plasterboard on dabs then unfortunatly the u value is 0.33 if you had used full fill polypearl platinum bead at 50mm in cavity you would have scraped in at 0.29 which is the UK compliance for walls in the UK (perhaps someone could confirm that) You will need to internally dryline but the good news is there is absolutely no need to batten out the walls, either dot and dab the walls with 38mm thermal lined plasterboards or mechanically fix with nailable mushroom plugs, on the plus side it will be a warmer room so the few quid extra spent will be well worth it in the end.
A 38mm thermal liner board will give you a u value of 0.25 well within acceptance of regs ...50mm is overkill.
If I get the time I will post up a photo of my back room which I am currently linining in therma board with mushroom plugs to give you some idea of what you should be looking for