New flat roof-Insulation

[NoNameNoNumber]

Evening all,
We're having an extension built, single storey using existing balcony steelwork as the new roof structure and maintaining the functionality of the balcony off the master bedroom.
200mm deep steels existing, so between the builders /building control and I we established it'll be a cold roof with 200mm of celotex PIR. GRP roof with 25mm abutment vents all round and plasterboard lined.

Now, my concern is that the combo of the cold bridging of the steelwork and the venting of the roof will make for a rather thermally inefficient design- albeit as good as could realistically be achieved.

Question is... Would it be worth asking them to put a layer of insulated plasterboard on now before it gets skimmed?

 

[Deleted User 298457]

I doubt you'd make any difference if you have 200mm pir.

 

[rangor gubbins]

I'm not an expert but have completely renovated a couple of houses. I'd always have as much insulation as possible.
Having said that if you have cold spots and put insulated plasterboard over the top, I believe there's a risk on condensation building up on the other side (cold side) of the insulation. I don't know how much of a risk it is. Probably depends how much moisture is being generated near by.

 

[rangor gubbins]

I doubt you'd make any difference if you have 200mm pir.

You're probably right. 200 mm is a lot so another few 10s of mm stuck to the plasterboard probably won't make that much difference.

 

[NoNameNoNumber]

I doubt you'd make any difference if you have 200mm pir.

Cold bridged by the steelwork though

 

[1pudding1]

Can you post a diagram/sketch of the build up and x-section?

 

[NoNameNoNumber]

I’ll drop a Paint masterpiece this eve

 

[Mason-]

So if I understand it you have a large steel, and wooden joists. With 200mm PIR in between, they’ll then PB over the top? If so like you said the part where the steel is will be a really bad thermal bridge. I wouldn’t be surprised if you got a damp condensation patch where the steel is.

I think whacking the thickest IPB you can find over the top will help. Foam/seal all the edges so no moist air can get between the IPB and the joists/steel.

As well you could batten off the ceiling and put 100mm more insulation and use normal PB. Depends how much ceiling height you’re prepared to lose.

 

[NoNameNoNumber]

Black is GRP roof
Blue is 25mm vent stipulated by building inspector
Grey is balcony steelwork
Beige is PIR
Pink is plasterboard that is fastened to wooden packers somehow attached to steels

 

[1pudding1]

Cant you put the 200mm on top of the steels? Really want to bring them inside the insulated envelope, otherwise they're a massive cold bridge and you're going to get huge cold spots and hence heat loss on the ceiling. I'm guessing as you said it's a balcony above, you're constrained by a door above. Any chance of a photo to help?

 

[NoNameNoNumber]

Yep, door above.
Plasterboard ceiling below is 2400mm high so I could lose some there without issue except I know the windows and doors are on order and I imagine dropping the ceiling height would have some implications.

 

[NoNameNoNumber]

Builder is going to fasten 50mm PIR under the steels.

 

[94JDH]

I recently upgraded two old flat roof extensions that had a central gutter with no insulation (built in the 80's by a previous owner).
All one roof now and no central gutter. 200mm beams with following :

100mm PIR
30mm air gap
40mm Superquilt
30mm air gap
40mm Superquilt
50mm battons
18mm OSB3
1.5mm EPDM

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