Insulating loft space with warm roof

[peekay]

Hi All. 

 

We are mid way through a significant refurb, adding an additional storey to a bungalow.

 

We had planned for a warm roof space. 120mm PIR board between joists, with 50mm insulated plaster within the loft space, and no insulation at first floor ceiling level. 

 

This would allow the loft to be used for light storage, and all of the MVHR ductwork to be within the insulated zone of the house. 

 

The truss design doesn't really work with allowing the insulated plaster to be fitted around then. It would involve so much cutting and bodging that it would be infeasible. The loft area is about 150m² (15m X 10m), with joists every approx 600mm. The loft space is only 1200mm high, and has a huge amount of trusses in there, so not really useable space so we aren't particularly bothered about it being plastered in there, just getting the insulation correct.

 

I've attached a photo showing what it currently looks like. With the 120mm PIR between the joists, and the underside of the 150mm insulated flat roof section (above the OSB). I've also attached a section drawings.

 

Our builders have suggested putting 300mm of standard loft insulation roll above the ceiling level, over the top of the MVHR duct work as an alternative to the insulated plaster board on the roof line.

 

I'm not sure about this, as if the standard ceiling height loft insulation is effective, then it would mean the loft space becomes cold, and our MVHR extract plenum that is within the space is then extracting cold air from the loft. 

 

Would be great to have some thoughts on how best we could deal with this, so that I can have informed conversations with our builders and our engineer who is also doing our building regs drawings. 

 

Thanks

[DevilDamo]

Firstly, insulating between and under the rafters is not a warm roof. It’s a cold roof and would therefore require venting.

 

Have you already battened the pitched roof element because if not, can you not add insulation above the rafters?

 

A cold roof would have probably been better, especially if the roof void was just for access. So yes… as the builders had stated, 300mm of Rockwool or similar laid between and over the ceiling joists. Even those roof voids remain reasonably warm.

[peekay]

Thanks @DevilDamo

 

The pitched roof sections are already battened and tiled. 

 

So, having read a bit more, I agree with you that we do not have a warm roof. 

 

I think that we will keep the PIR board as shown in the photos, but add additional insulation at ceiling level. We'll make the new ceiling level insulation building regs compliant, making the pitched and flat roof sections 'bonus' insulation.