breathable or non-breathable underlay

groober

Ventilated roof about to go on a low pitch lean-to single storey structure. Is there any advantage to having a breathable underlay such as tyvek given that it will be a cold roof system (insulation between the rafters and slabbed insulation on the ceiling side) - assuming the necessary 50mm gap between insulation and underlay?

sydthebeat

short answer no.
theoretically the air gap vents any build up of water vapour / condensation away, so theres no need to have a breathable felt.

groober

Thanks for the reply, its much appreciated. I know I would need a breathable underlay if I decided on a non-ventilated roof instead, do you know how much extra work is involved in structuring such a roof? Is it simply a matter of not having vents in the fascia and soffit?

RKQ

A low pitch lean-too roof can be difficult to ventilate at ridge / pararet.

A warm roof should be counter-battened.
Insulation above the rafters is preferrable but can be difficult to install on roof, health and safety etc.

Longer fixings, to go thru counter-batten, insulation, and into rafter.

Some Roofers like to lay osb or ply over rafters, then 25 - 50mm foil insulation, breather-felt,then counter-battens, slating battens and slates.

Extra cost = longer roof fixings and counter-battens.

groober

Thanks RKQ,
Thats a helpful outline but a warm roof is not going to be an option for me at this stage. Am I on the right track with my understanding of a cold non-ventilated roof (i.e simply not putting vents on the soffit)?

sinnerboy

You have to ventilate - every time .

Option 1 - non breathing felt . Condensation will form on its underside - you must vent below the felt min 50 airspace . 25mm continuous inlet strip at eaves and 5mm continuous outlet at top

Option 2 - breathing felt .
A little complex this - bear with me . Some felt manufacturers claim you need no vents BUT BEWARE . There have been failures . I have seen this with my own eyes . Condensation literally dripping of so called breather felt . Admittedly , new building , drying out , mid December .

When using breather felt you must provide a 50mm vent space ABOVE the felt . You achieve this by using counter-battens ( placed directly over and in line with the rafters ) and then battens across . AND STILL - 25mm continuous inlet strip at eaves and 5mm continuous outlet at top .

groober

Very helpful. I think I am happy at this stage to go the vented structure with the non-breather ensuring the 50mm gap.
Thank you both very much.