Draughts in new loft conversion - possible causes?

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Our loft conversion is almost done. It's a full width rear dormer and 3 dormers to front with stairs in the middle one. It's now been insulated, plaster boarded and skimmed. There is no new insulation under floor but there is our original loft insulation there. The windows are double glazed with trickle vents and fitted by an excellent local company we have used many times. After the windows went in we noticed some draughts but the spotlight holes were there and we thought it was them. This week the electrics were finished and all the skirting and architrave is done. We still have draughts but can't work out where they are coming from.

There is an extractor in the en suite ceiling but you can't feel a draught right under it.
All the trickle vents are closed and again no noticeable draught.
There is no eaves storage or any built in storage.
The rads aren't in yet.
The draughts are so bad it's like a window is wide open. On the landing below you can feel the breeze coming down the loft staircase.


Surely this isn't right? Anywhere we should be looking/checking?
 
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The draughts are so bad it's like a window is wide open. On the landing below you can feel the breeze coming down the loft staircase.


Surely this isn't right? Anywhere we should be looking/checking?

Cold air dropping down out of an unheated room, due to warm air rising, might seem like a breeze - are you not feeling that? The smoke from Joss sticks is a good way to find air leaks, especially so on a windy day.
 
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Cold air dropping down out of an unheated room, due to warm air rising, might seem like a breeze - are you not feeling that? The smoke from Joss sticks is a good way to find air leaks, especially so on a windy day.

Yes we could be as rest of house is quite warm. Just didn't realise the cold air dropping could feel so breezy! Hopefully rads going in tomorrow so once they are on and running hopefully the draughts may go. I like the Joss sticks idea too.
 

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Do you know how the dormer walls have been closed under the floor?

Might not be explaining this well but assuming they have left the existing ceiling joists in place and ran new floor joists alongside which sit on the existing wall plate then the joists will run in to the roof space. This needs to be closed up or you will have cold air from the roof space blowing under your floors and creating draughts.
 
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Do you know how the dormer walls have been closed under the floor?

Might not be explaining this well but assuming they have left the existing ceiling joists in place and ran new floor joists alongside which sit on the existing wall plate then the joists will run in to the roof space. This needs to be closed up or you will have cold air from the roof space blowing under your floors and creating draughts.

I don't know how they have been closed but yes they left exciting joists and ran new ones alongside. The builder isn't great and I doubt I'd get an honest answer to your question. He didn't put the rockwool under the new floor that we requested which I assume would have helped reduce draughts too. It's been plaster boarded and skimmed now so I guess there is no way to tell ourselves either?

I'll add some photos we took during construction as they might shed some light.
DormerA.jpg
DormerB.jpg
DormerC.jpg
DormerD.jpg
DormerE.jpg
 
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Your builder could also have done with cutting the PUR insulation a bit tighter to the joists (it is gappy in places - so maybe foam the gaps?) and also taping across where cut pieces butt up to each other with silver insulation tape. Someone on here recently suggested using the smoke from a joss stick to ascertain where any draughts were - seems like an excellent idea to me to help pinpoint the issue(s)
 
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Was the insulation sealed up , taped? My loft below , so warm we don’t need any heating in there .

 
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Only issue is these photos are not current and he has since put plasterboard over (which was messily taped) and skimmed that. Can draughts still come through this? The builder keeps going on and on about how warm it is but while I agree it isn't cold (yet) the draughts are annoying. It's taken 6 months to get this far and it's been such an ordeal we really just want him gone. We do have another builder coming to look because there are unfinished items that we are struggling to get the current builder to do (like exposed water main and missing kitchen ceiling) so I will also see what he can suggest. Thank you for all your continued input in this.
 
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Only issue is these photos are not current and he has since put plasterboard over (which was messily taped) and skimmed that. Can draughts still come through this?
Potentially yes. Get some joss sticks to check for aur flow (draughts) first. A low cost infra red temperature sensor can be used to determine where cold spots are.

Without doing something to demonstrate where the problem is you'll get nowhere

The only differenve between joss sticks and the professional smoke generators used to check for airtightness in buildings is that the joss sticks are way cheaper
 
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