Roof Ventilation

[BadgerBadger]

Hi all - I'd like to pick your collective minds on roof ventilation please!

 

The majority of our roof build up has rigid insulation placed between the rafters with a further layer placed under the rafters to reduce thermal bridging.  Our architect details show a 25mm gap on top of the insulation which is ventilated at both the eaves and the ridge.  There's a breather membrane over the rafters, which are then battened with plain clay tiles.

 

I haven't quite got my head around why I need both eaves/ridge ventilation and a breather membrane?

 

We have a fairly traditional looking roof so I'd really like to get away from dry-fix ridge ventilation systems.  Build is planned to be fairly airtight, with the airtightness layer on the underside of the roof insulation.  Could I use something like Klober Permo Air to remove the need for additional roof ventilation?

[nod]

I’d check the 25 It’s usually 50 mil

You will need the ridge ventilation Especially with a loose rafter type roof 

To give a continuous airflow 

Some building controls insist that the membrane is slit at the ridge 

I can never see the point of that 

[Thorfun]

we used Proctor Roofshield with battens and counter battens so had a 50mm gap. but I believe the Roofshield can be used with a 25mm gap (drapped 10-15mm) but PLEASE ensure you check your specific situation with the manufacturer and your BCO as it might not be compatible for you.

[Jilly]

My Building Control Officer told me if we didn't have the dry fix ridge ventilation we would have to have a tile vent in every single bay at the top of the roof to give free air circulation from the eaves.

[Temp]

On 16/10/2022 at 15:44, BadgerBadger said:

Hi all - I'd like to pick your collective minds on roof ventilation please!

 

The majority of our roof build up has rigid insulation placed between the rafters with a further layer placed under the rafters to reduce thermal bridging.  Our architect details show a 25mm gap on top of the insulation which is ventilated at both the eaves and the ridge.  There's a breather membrane over the rafters, which are then battened with plain clay tiles.

 

I haven't quite got my head around why I need both eaves/ridge ventilation and a breather membrane?

 

We have a fairly traditional looking roof so I'd really like to get away from dry-fix ridge ventilation systems.  Build is planned to be fairly airtight, with the airtightness layer on the underside of the roof insulation.  Could I use something like Klober Permo Air to remove the need for additional roof ventilation?

 

Our roof is similar and our BCO didn't object to mortared ridge tiles.

 

Make sure he knows you are using a vapour permeable membrane and show him articles like this from the membrane manufacturer you are using..

 

https://glidevaleprotect.com/roofing-membranes-and-ventilation-one-size-does-not-fit-all/#:~:text=Whether a cold or warm,related to cold roof construction.

 

Quote

 

Whilst an air-permeable underlay may involve higher initial material costs, this is offset by no additional requirement to ventilate, saving time and labour on installation. Whether a cold or warm roof, air and vapour permeable membranes can be used without additional ventilation. This is accepted by the NHBC in their Technical Requirements R3 Document related to cold roof construction. The only exceptions are where the underlay is installed on closed jointed sarking board or if an airtight roof covering is used, such as metal tiles or fibre cement slates. In this case, it would be necessary to ventilate the batten space with 25mm counter battens. The chosen manufacturer should always recommend a well-sealed ceiling and separate AVCL is used, where achievable.