New unvented GRP cold roof... can this be fixed?

[e17jack]

Hi All.

We are having a typical side return infill to a victorian terrace built. This involves a new section of roof approx 1.5m wide x 6m long, in which there are 4 large Velux rooflights.

The architect specified a GRP roof with the velux fitted to upstands. The roof is not flat but at a gentle pitch. Architects drawings showed a warm roof construction with continuous insulation just under the GRP surface. However - the builders have ignored this and built a standard roof made of deep timber joists with OSB deck, GRP on top, and pieces of Celotex pushed between the joists. There is no ventilation to the spaces within the roof.

The building control officer (independent) has said that he can't pass this as there is a risk of condensation forming in the roof. The solution presented by the builders was installing a series of mushroom vents on the new roof, but this is not something i want to do. On further consultation building control have said they would pass the mushroom solution, but wouldn't recommend it as it essentially involves making a lot of holes in a new roof and they will inevitably leak at some point. They then suggested that a well installed vapour control layer between insulation and plasterboard could be passed, although that would not be certain to prevent condensation.

Given that demolition is not an option (currently living on site with 3 small kids) we need this resolved asap. I am trying to get building control to attend another site meeting to instruct builders on the detail required for vapour control layer installation.

In the meantime, does anyone have an opinion on a better solution, or whether any of this is necessary anyway?
The builders insist that since most of the roof is made up of rooflights, there is very little space left for condensation to form, and building control are being overly cautious.

Pictures attached showing structure from below (before Celotex went in), and a section of the roof from above.

Many thanks in advance for even reading this far!!

 

[tony1851]

Just tack a vapour control layer (eg thick polythene or one of the proprietory types) under the rafters, tape the joints, and apply foil-backed plasterboard.

 

[catlad]

The builders have just done it the easiest way for them and now they want you to live with it.
What does the architect think.

 

[e17jack]

I'm not totally confident in the (young) architects experience of this 'technical' issue. But building control essentially suggested what Tony has said (thanks Tony). So i am seeking reassurance that this should prevent condensation. But feel free to tell me it is a risk!

 

[oldbutnotdead]

I'm no expert on the VCL thing (though if that room is going to be a kitchen/living space then there will be a lot of water vapour kicking around). In your favour, GRP is a reasonable thermal insulator (much better than tin sheets) but it is a bit if a risk since any damage will be invisible til it becomes serious.

I'm not over impressed with the detail on the GRP roof- it looks as if they've just laid mat up against the parapet and slapped resin/topcoat on top (rather than flashing the edge properly). There will be movement between the timber and the block wall which will eventually cause the resin to crack and will give you leaks down the line- GRP is lovely stuff but not particularly flexible.

The detail round the Veluxes (not sure what the plural of Velux is- Velii?) is disturbing as well- the (admittedly brief) training course I did on GRP rooves advised using preformed trims for sharp angles like those for similar reasons.

 

[e17jack]

Thanks oldbutnotdead - the 'Velli' do have preformed L shaped trims embedded somewhere under all that GRP, and will of course have the Velux flashing over the top.

Your comments on the gutter detail are useful and i will see if they can improve that junction - thanks.

 

[John D v2.0]

To know whether you have a risk of condensation your designer needs to do a condensation risk analysis and suggest appropriate buildups to the builder which building control will check and pass.
This would be at the builder's expense.
We had a similar issue with a steel beam, the builder (kept) redesigning it to suit himself but it caused a thermal bridge which will risk condensation, but the builder refused to do it properly so we have to check in winter to see if there's water building up in there. Apparently not all builders understand what thermal bridging is.

 

[catlad]

Make sure they use insulated plasterboard with its own
vcl to minimise the condensation.