Roof ventilation with loft conversion

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Just before Christmas, I created a thread about condensation under the tiles in my loft conversion. //www.diynot.com/diy/threads/shoddy-insulation-in-loft-conversion.468991/

I have since spoken to a roofer about the possibilities of installing some vents in the roof and he suggested 4 tile vents on each pitch together with 4 ridge vents plus a few soffit vents. I asked him if each rafter centre would need its own vent but he said no as air will definitely get to it.

My question is how exactly would the air get to the neighbouring rafters when the rafter centres are filled with rigid insulation and the felt laying taught over the top the timber rafter? But then again, I cannot say I've ever seen roofs with a row of continuous tile vents either.
 
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If you want to you can:
Have a continuous soffit vent,
Have a full length dry ridge kit,
Install rafter trays and lap vents in each bay.

For around the price of your four vented ridge tiles quite probably.

The air would get to the other rafters via the soffit, the tiles and the imperfections in the felt laps,assuming that is what is there. But I wouldn't really call it ventilated.
 
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Just reread your thread,
I feel sorry for you, as that work was poor to say the least.

As you have a breather membrane, get a full length soffit vent fitted and a full dry ridge kit fitted.
You could add some tile vents as well.
But your vcl needs to be complete to be effective.

As for your question on the other thread about moisture coming up through the eaves, well in an ideal build the vcl would envelope every room, be air tight and be ventilated mechanically like a passive house. But how far your willing to go is another thing.
You said there's a gap before the rigid insulation starts. but is there a good depth of wool over the entire ceiling of the room below, does it go right into the corner of the eaves, but still leave a path for air to flow from the soffit?
If not then remedy that as well.
 
Thanks for the suggestion of having a full-length soffit vent and a dry ridge kit. From my research, the vents on a dry ridge kit are very small so if that is used with a full-length soffit vent which has big grilles, what happens on windy days? As air will enter faster than it can escape, wouldn't there be a risk of uplift of the tiles as the trapped air tries to force itself out?

Another problem is that we wouldn't be able to have a full uninterrupted length of soffit vent as the front of the house has a bay window on the 1st floor and there is a flat roof extending outwards for half of the length of the roof.
 
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Air gets sucked out of the top, greater wind speed higher up or some such thing iirc, also your soffits face down ( and would be limited in size by the bay window) so it's not really going to be a problem.
It's a little different for you as you've got a breather membrane.but the principle is the same,
keep the moist air in the room from passing through the insulation to the roof via a complete vapour barrier,
keep the construction of your roof dry via ventilation,
and as much insulation as possible between the two.
 

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