Vaulted Ceiling Insulation

[Millwright]

We have had a single storey extension built with pitched roof. 100mm Celotex between joists and the plasterboard has 12.5mm polystyrene backing.

Is this enough? I was under the impression that the plasterboard had to be backed with celotex rather than polystyrene. Will the performance of polystyrene be enough?

 

[John D v2.0]

The material doesn't matter it's the effective u value that maters. You can have 270ish of mineral wool, 150ish of celotex, or 70ish of aerogel if that's what you want to pay for.
Having said that, your build up does sound slightly thin, you can find u value calculators online or work it out from the data sheets.

 

[chappers]

You won't be anywhere near with that, the target is a u-value of 0.18 100 celotex between with 40 celotex under is bang on.
Just to give you some idea 100 between and 15 under (the thinnest as a laminated board) comes in at 0.24 and polystyrene won't be as good as that let alone at 12.5mm

 

[motorbiking]

Looks like the builder is is aiming to hit building control minimums. But its relative if you've got a roof full of double glazed vlux or a wall full of bifolds a U-value of .3 isn't that bad.

Tiles can perform quite well also.

Even at 22mm the polystyrene backed plaster board is only performing at around 1/2 to a 3rd of the capability of a 15mm celotex equiv. But its about half the price.

 

[Millwright]

Thanks for the replies. I've just spoken to the building inspector and he's not so sure at the moment that it could be a 'it won't pass at all' problem. He's going to come and have a look and see what he thinks.

I've just looked at Knauff's website (not sure if that is the actual brand of board) and for this thickness of board they're giving for a 2.88m2 sheet a U value of 0.038W/mk.

 

[motorbiking]

R values are around 4.5 for 100mm celotex
0.3 for 22mm of polystyrene on plaster board
tiles about .1

U Value is probably closer to .20 + a guestimate for the joists approx. .22-.24

Depending on the thickness there may even be an air gap. I'd be very surprised if this wont pass.

 

[chappers]

Looks like the builder is is aiming to hit building control minimums. But its relative if you've got a roof full of double glazed vlux or a wall full of bifolds a U-value of .3 isn't that bad.

Tiles can perform quite well also.

Even at 22mm the polystyrene backed plaster board is only performing at around 1/2 to a 3rd of the capability of a 15mm celotex equiv. But its about half the price.

But BC minimums are based on individual elements and what the OP has doesn't pass. If you went for an area weighted average because you had an extension full of glass then the requirements would be for even more insulation in the other elements.
The requirement for an extension pitched roof is 0.18 and the OP currently has worse than 0.24 based upon the cleotex calculator

 

[Millwright]

I spoke to our building inspector today, he looked at the drawings to take into consideration the size of doors and windows and concluded that in summery it's marginal but perfectly ok. Although the celotex or thicker polystyrene would have been better, he said we're unlikely to know any different and the room will probably be warmer than any upstairs room we currently have anyway.

Though I'm satisfied with the building inspector's conclusion, it just adds to the frustration of builders doing what they want and not following drawings. Had I have spotted this before the plasterer had been and we've started painting and getting the interior ready, it'd all be coming down on the basis that although it's ok, it's not what we wanted - on the drawings is what we wanted, that's exactly what the drawings are for.

 

[noseall]

Let's hope that meagre layer of polystyrene is enough to prevent cold spot shadowing through the rafter.

 

[chappers]

That's interesting it's not down to whether you will notice the difference or not or how the new elements compare to the rest of the house.
It's down to the requirements of the building regulations. which state that reasonable provisions should be made for the conservation of fuel and power. It then goes on to in the guidance to clarify what reasonable provisions are when building an extension, where it states that newly constructed elements should reach the laid out standards which for a roof is a u-value of 0.18.
I agree with noseall, with polystyrene that thin you could potentially get pattern staining, I would tell your builder that your not happy and if there are any problems you will be expecting him to rectify them.

 

[Millwright]

Well, the building inspector doesn't seem to think it will be a problem at all. The rafters are 200mm, I'd be surprised to see pattern staining coming through, but I take your point.

In fact, on my porch, it has 4" rafters and plain regular plasterboard attached underneath. I've never seen spot staining on that. When the house was built, the porch roof was never insulated and it was very cold as a result, but I have replaced the roof covering and added 50mm celotex inside some years ago and haven't had any problems with it.