Cold (but new) extension - what should I do

tunatuna

Hi, we moved into our house earlier this year which has a single storey flat roof extension on it. The hope was it would be lovely and warm, unfortunately we're finding it's not. 

I took a downlight out yesterday to have a look into the ceiling void. 
The ceiling is polystyrene backed plasterboard, there's then a void of around 200mm with a wooden deck. I am guessing (hoping) there is insulation laid above this underneath the roof itself. 

First question: am I right in thinking this is a "warm roof" construction? The tiny amount of polystyrene insulation on the back of the plasterboard would not meet modern insulation standards and building control would not have signed it off (I am guessing). 

This void appears to be pretty drafty and cool, so guessing there's some leaks somewhere. It's very difficult to find out exactly though as there's no access of course.

One thing I am considering is overboarding the existing ceiling and lantern vertical sections with 50mm backed PIR insulation and then making it as airtight as possible to minimise moist air reaching the cavity. We have plenty of ceiling height to do this.

Any thoughts on this approach? 

WIAWSNB

When was this extension built? 
But pretty much regardless, I'd say it nowhere near meets recent building regs in terms of insulation.
That is a pitiful amount of ceiling insulation, and it isn't even particularly high value - it looks like Jablite or similar, effectively expanded polystyrene, and only an inch thick?! 
I don't know if that is a warm or cold roof design - it appears to have elements of both, whilst not being fully either.
You can check whether the upper decking has insulation by comparing the distance from the ceiling through that hole to the tops of the joists, which should be decked, and compare the overall thickness to that on the outside of that extension - there should be a very noticeable few inches difference. 
What seems pretty obvious, however, is that since the void is draughty, it doesn't provide any insulation, so even if there is a further layer up above, it'll serve no purpose.
Can you find this extension on your LA's Planning Portal? I'd like to see the Build Control plans, and a completion cert.
However, your proposal to line the underside with insulated p'board is ideal, and should be transformative. I'd also look to fit flush LED ceiling lights, so all you have there a teeny cable holes. These JCCs are fire-rated, but I doubt well insulated. 
Sorted.
That leaves just the walls and floor...

grumpy_codger

tunatuna said:

...
The ceiling is polystyrene backed plasterboard, there's then a void of around 200mm with a wooden deck. I am guessing (hoping) there is insulation laid above this underneath the roof itself. 

...

I wonder if this void can be filled with mineral wool insulating matts. 20cm is a lot.

tunatuna

WIAWSNB said:

When was this extension built? 
But pretty much regardless, I'd say it nowhere near meets recent building regs in terms of insulation.
That is a pitiful amount of ceiling insulation, and it isn't even particularly high value - it looks like Jablite or similar, effectively expanded polystyrene, and only an inch thick?! 
I don't know if that is a warm or cold roof design - it appears to have elements of both, whilst not being fully either.
You can check whether the upper decking has insulation by comparing the distance from the ceiling through that hole to the tops of the joists, which should be decked, and compare the overall thickness to that on the outside of that extension - there should be a very noticeable few inches difference. 
What seems pretty obvious, however, is that since the void is draughty, it doesn't provide any insulation, so even if there is a further layer up above, it'll serve no purpose.
Can you find this extension on your LA's Planning Portal? I'd like to see the Build Control plans, and a completion cert.
However, your proposal to line the underside with insulated p'board is ideal, and should be transformative. I'd also look to fit flush LED ceiling lights, so all you have there a teeny cable holes. These JCCs are fire-rated, but I doubt well insulated. 
Sorted.
That leaves just the walls and floor...

It was built in 2020. I checked the local planning portal and no documentation is available unfortunately. 

Good call to measure and compare the ceiling height relative to the height of the deck and roof level itself, I will look to do this and see if there is a difference between.

I just put a portable thermometer in the void, room temperature was reading 18 and void reading 15 so not a huge difference, suggesting there is some insulation.

We did receive a completion certificate as part of the house purchase, but doesn't contain anything in terms of plans or anything. I guess we can reach out to the previous owners who we're still in touch with and ask if they have drawings? 

I like the idea of flush fit lighting and will look into this as well to avoid punching big holes in the PIR. 

Have got a plasterer coming today to price it up. So just to confirm this shouldn't cause any issues with condensation assuming we seal it as best as possible? 

TELLIT01

That doesn't look like Warmroof construction to me.  We had a Warmroof installed about 4 years ago and it's about 150mm thick (possibly more) 

WIAWSNB

tunatuna said:

It was built in 2020. I checked the local planning portal and no documentation is available unfortunately. 
Good call to measure and compare the ceiling height relative to the height of the deck and roof level itself, I will look to do this and see if there is a difference between.
I just put a portable thermometer in the void, room temperature was reading 18 and void reading 15 so not a huge difference, suggesting there is some insulation.
We did receive a completion certificate as part of the house purchase, but doesn't contain anything in terms of plans or anything. I guess we can reach out to the previous owners who we're still in touch with and ask if they have drawings? 
I like the idea of flush fit lighting and will look into this as well to avoid punching big holes in the PIR. 
Have got a plasterer coming today to price it up. So just to confirm this shouldn't cause any issues with condensation assuming we seal it as best as possible? 

Interesting. If there's a completion cert, then surely there were building plans - enough to demonstrate to the BCO how the extension was going to comply - and that it did? Else, no CC.

Nothing on the Planning Portal under the address or the cert number?

Worth contacting BC to ask? 

We had a very similar-looking extension built at the same time - white walls, sky lanterns, column rads, bifolds! But...ours is cosy :-p

No, it won't cause issues. Adding an insulation layer to the underside of the ceiling is the single best way to both add effective insulation, and to prevent moisture percolating up into that void.  

ic

If its warm roof, you wouldn't have the insulation backed plasterboard, as the void is part of the warm side of the roof.  In my cold roofs, the electrician did a good job of hacking away big holes in the insulation for the spot lights - are you sure this isn't the same.  Many still work to the theory that the lights could be high wattage halogens, when of course we're on LED now - and such butchery just isn't necessary.

WIAWSNB

ic said:

If its warm roof, you wouldn't have the insulation backed plasterboard, as the void is part of the warm side of the roof.  In my cold roofs, the electrician did a good job of hacking away big holes in the insulation for the spot lights - are you sure this isn't the same.  Many still work to the theory that the lights could be high wattage halogens, when of course we're on LED now - and such butchery just isn't necessary.

The problem here is, the 'warm' roof void is draughty. It ain't 'warm'

On that point, I simply do not understand 'warm roofs'. They have a void above the ceiling, so presumably that void suffers from any moisture that percolates up. So, you either need to ventilate that void = bludy cold and pointless, or else you seal it off = wet and soggy.

tunatuna

I have a copy of the Final Certificate, issued by a company calls Midlands Building Control. Should I contact them, what should I be asking? Thanks so much for the feedback I really appreciate it all. 

ComicGeek

WIAWSNB said:

ic said:

If its warm roof, you wouldn't have the insulation backed plasterboard, as the void is part of the warm side of the roof.  In my cold roofs, the electrician did a good job of hacking away big holes in the insulation for the spot lights - are you sure this isn't the same.  Many still work to the theory that the lights could be high wattage halogens, when of course we're on LED now - and such butchery just isn't necessary.

The problem here is, the 'warm' roof void is draughty. It ain't 'warm'

On that point, I simply do not understand 'warm roofs'. They have a void above the ceiling, so presumably that void suffers from any moisture that percolates up. So, you either need to ventilate that void = bludy cold and pointless, or else you seal it off = wet and soggy.

The insulated plasterboard could have been installed to act as a vapour barrier, to reduce the amount of moisture entering the void. The main purpose of a warm roof is to prevent the need to ventilate (and thereby reducing heat loss), by keeping the structural deck warm. 

However, the insulated plasterboard could be causing an issue by reducing the temperature in the void, and actually increasing the risk of interstitial condensation on the underside of the structural deck above. That's why manufacturers don't recommend having layers of insulation either side of an unventilated void. Increasing the insulation below the existing ceiling, or stuffing the void with insulation, could also make the situation much worse in terms of interstitial condensation, as it's changing the way that the roof was designed to work.

The fact that the void is draughty is helping to mitigate this issue, but definitely not how it was (or should have been anyway) designed to work.

WIAWSNB

tunatuna said:

I have a copy of the Final Certificate, issued by a company calls Midlands Building Control. Should I contact them, what should I be asking? Thanks so much for the feedback I really appreciate it all. 

Yes. Ask for a copy of the plans and/or the spec. 

That sounds like an independent surveyor acting for the customer. They must be certified so that the LA's BC accepts their oversight.