To fill or not to fill.

[MikeSharp01]

Our build has a false peak, a section of the west catslide roof oversails the east roof and forms a peak. The whole lot sits on 300mm I beams and the roof is covered with OSB, breather membrane (housewrap), 40mm of PUR, counter battens, tyvek, battens and slates.  The peak does not have the 40mm insulation on on the face but the oversail does have it. The breather wraps over the peak, runs down the face and continues down the east roof.

 

The challenge is what to do with the void in the peak - it is not ventilated although it is loosely connected, the OSB is not perfectly fitted in the gaps, to the 300mm insulation between the I-beams, see sketch. I can either fill it with thermo floc or not and I am not sure what the issues are either way. I am leaning towards filling it but as it is about 4m3 if I don't need it I will leave it. 

 

Any thoughts more than welcome.

 

[Mr Punter]

It looks quite unusual.  If you have loose / fibrous insulation and PIR the PIR normally goes inboard.  The VCL normally goes inboard of all the insulation.  I might be concerned that the unvented void would get cold and you may get condensation on the inside of the VCL on the pitched section.  Maybe do a condensation risk analysis for reassurance.

[saveasteading]

It is inside the vcl so should be pretty neutral conditions.

It will get hot in the summer (but not too bad as the tiles and wind will keep it out) and cold in the winter. But will that cause any harm? No worse than any other attic, albeit tiny.

 

Packing with insulation will help the room beneath a bit.

4m3 of the cheapest fibreglass £80?

But where are all your PIR offcuts? stack them in, no cost, save on skips and nothing to landfill or incineration.

 

I think this is a gut feeling decision.

[MikeSharp01]

Thanks both, @Mr Punter it is a bit unusual but it is what the architect specified and I am hopeful that it will all work, the idea of the external PUR is to break all the cold bridges. I will do a condensation risk analysis and decide.