Insulation in Pitched Roof - Air gap required?

[Rob_Quads]

I'm building a pitched roof which has 125mm rafters. I am looking to use Kingspan K7/K18 insulation to get to the 0.2U value required. Looking at the documentation for the K7 it states its possible "Fully Filled Insulation Between Rafters & Under Rasfters".

But I have also seem mention of 'requiring' 50mm air gap above the insulation but I can't actually find anywhere stating what this requirement is from/for?

Anyone got any pointers?

Thanks

 

[freddiemercurystwin]

You will need the 50mm air gap if you have felt beneath the tiles. Only if you have a breathable membrane or NO membrane/felt beneath the tiles can you avoid the air gap. Obviously you need to vent at the eaves and the ridge if an air gap is req. If an air gap is required but your depth of insul required is deeper than the rafters its standard practice to fit say 75mm insulation between the rafters and then a continuous layer of 50mm beneath the rafters and fix the plasterboard with longer screws through the insulation into the rafters. A call to Kingspan technical dep will confirm whether you can reduce the overall insulation thickness slightly if fitting a layer beneath the rafters as you are removing the cold bridges that the rafters will create.

 

[Rob_Quads]

Thanks for the info. I'm doing a warm roof with breathable membrane so didn't think I needed this 50mm air gap.

As it is I will probably be needing some insulation below the rafters to get down to the 0.2U but not needing the 50mm gap obviously helps reduce the size needed.

 

[noseall]

Thanks for the info. I'm doing a warm roof with breathable membrane so didn't think I needed this 50mm air gap.

Correct. With a 'warm' roof construction the insulation sits entirely above the rafters so condensation issues are negated.

 

[Rob_Quads]

AH - My understanding was slightly wrong then. Sounds like I am doing a 'cold' roof. Even tho i will only have around 25mm gap between the top of the insulation and the tiles (well the breathable membrane will be draped down into that) will i need vented or unvents soffits?

Today when i mentioned breathabme membrane + insulation between + insulation below the rafters the building control guy was happy enough

 

[noseall]

Breathable membrane will require no other means of ventilation other than the air gap.

However, if the roof in question was connected to another roof that did not have breathable membrane, i.e. where a new extension cuts into an existing roof and it shares the air space with that roof, then additional ventilation is required.

It is only roofs in isolation with breathable membrane that (according to the wise folks a b.c.) will work.

 

[autolycusd]

Can i ask a related question in this thread?

I'm planning to fill with rockwool between rafters. The roof above is tiles and featheredge (no felt).

Should/can i put a breathable membrane between the rafters - below the featheredge and above the rockwool? This would be to allow the rockwool (and plasterboard beneath) to breath, but protect the insulation from any driven water ingress from above.

Thanks