warm roof soffit vents.

[carlos21]

i have a warm roof, the cavity wall insulation goes right up to the underside of the roof deck and will be airtight sealed from the inside.

should i put in a few soffit vents in the roof overhang that will be soffited? i thought it a good idea to have a bit of air flow around the ends of the joists. it will have standard pvc fascia and soffit.

any thoughts thanks

carl

[Redbeard]

My first thought was that in a Warm Roof (note the initial capitals) you don't need vents - indeed there isn't anywhere to vent unless you want complete thermal by-pass. Then I wondered if by 'warm roof' you mean insulated between and perhaps also under the rafters, giving you a roof void which is warm (and a heated volume which is greater). A true Warm Roof has all the insulation above the rafters. BUT if you are simply talking about the oversail and the rafter ends then maybe there is some argument for ventilation there. However I have doubts as to whether that air will flow enough to purge the area of vapour. Ventilation generally needs 'ins' AND 'outs'. Diagonally-opposite is always good.

 

If I have completely misunderstood please ignore all of this!

[sgt_woulds]

You can have a hybrid warm roof if breathable insulations (e.g. woodfibre) is used over the rafters.  For these you need to allow a ventilation path above the insulation to draw away moisture exiting the structure.  min. 80mm on a flat roof & 40mm on pitch roof 30 degrees+ with slates/tiles)

 

 

[carlos21]

thanks for the replies, yes it is a true warm roof with all the pir board on top. the vents would be just for the ends of the rafters around the overhang area. i was on the fence about the vents then i opened up all the pvc fascias and soffit order to find that i didnt have any vents so that kind of answered that for me.

i could always retrofit some circular ones with a holesaw i guess.

another question, wide (350mm) 10mm thick fascia mounted onto a thick continues timber, what spaces for the nails?? i did 3 for the width of the board ie one top, middle and bottom at 800mm centers but not sure if thats a bit skimpy, i just didnt want it to look like it was peppered with nails.

thanks again

carl

[Redbeard]

4 hours ago, sgt_woulds said:

You can have a hybrid warm roof if breathable insulations (e.g. woodfibre) is used over the rafters.  For these you need to allow a ventilation path above the insulation to draw away moisture exiting the structure. 

 

Thanks @sgt_woulds. I was assuming VCL on sub-base and PIR or similar non-breathable ins'n. Good clarification though re WF.