Insulation - cold roof meets warm roof

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Hello,

We had a larger flat roof dormer extension built a couple of years ago. The ceiling and roof is made up of plasterboard - timbers- celotex - osb - rubber. They key fact for this topic being that the celotex insulation is above the timbers making it a warm roof.

At the back of the dormer's flat roof it meets the old pitched roof, which is a cold roof with the insulation sitting between the joists on top of the ceiling plasterboard.

I was about to top up the old loft insulation when I noticed that the two roofs are open to each other. Because no insulation is above the plasterboard in the flat roof, cold air from the loft in the old cold roof area can just flow into the space above the new ceiling in the dormer, bypassing the celotex insulation. Diagram below.

What's the appropriate way to deal with this situation? I don't want to make a condensation problem by putting insulation in the wrong place.

Thanks in advance!

1659558050872.png
 
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Sounds like the warm air can also leak into the attic from what I understand!
 
Well yes, it seems there's an open air path bypassing the insulation. I've added a diagram to my original message to try and make it clearer. I'm tempted to block the air path up, but that would mean restricting ventilation to the dormer roof timbers. I thought these warm roofs didn't need to be ventilated, but I don't know what I don't know and so I'm seeking advice :)

Thanks again
 
Yes need blocking off near your blue arrows, that will stop you loosing the heat from that room below.
 
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Thanks for your advice. I'm just concerned about blocking it in case it was left open for a reason, likely relating to ventilation.
 
Hi, I have exactly the same issue. Can I ask what you ended up doing? I've just topped up the cold pitched roof to 300mm of knauf insulation, and have managed to insulate up to where the warm roof starts on the dormer but there is still ab airflow in place. Can I safely fill the void with more insulation to stop the loss of hot air from above the dormer?
 
The dormer does not need ventilation and can be blocked up.
Hi, I have exactly the same issue. Can I ask what you ended up doing? I've just topped up the cold pitched roof to 300mm of knauf insulation, and have managed to insulate up to where the warm roof starts on the dormer but there is still ab airflow in place. Can I safely fill the void with more insulation to stop the loss of hot air from above the dormer?
 

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