Cold roof cross ventilation

TheMilkyPirate

Got a new roof put on our extensions on a house we bought. Getting ready to insulate it now with 100mm PIR. Do I have to drill holes along the top of the joists to allow air to run through? How many holes and how big. I only have 150mm to play with in the majority of the joists. Worried about putting too many holes in and compromising the structure.

10-10-20

https://www.boards.ie/discussion/2058291393/cold-roof-cross-ventilation

OP, what is the intended use of that room? I see some ventilation issues there with that design as you have no continuous ventilation. You may need to reconsider some construction elements, especially if it's livable and even more-so if it's a kitchen.

Meet Steve Roofer:

https://www.youtube.com/watch?v=LL2-8RXSYKc&t=18s

https://www.youtube.com/watch?v=RixxqDfhRGU&t=23s

Lumen

If you're ventilating then the ventilation should run lengthways (and out...how?), don't be drilling holes in joists. It looks like the blocking has been installed to facilitate this with a gap at the top.

This is interesting though...

https://foursevenfive.com/blog/unvented-flat-roofs-a-technical-discussion/

Have you read the building regs?

TheMilkyPirate

https://www.boards.ie/discussion/comment/120450203#Comment_120450203

First room is kitchen second room is bedroom and third is dining room. Yeah I've seen the videos and in one he drills holes to help with ventilation. We have soffit vents all around the roof just with the bridging pieces in between joists it's blocking the air flow. Not sure how to rectify it if we cant drill a hole to allow air through.

TheMilkyPirate

https://www.boards.ie/discussion/comment/120450353#Comment_120450353

Yeah the gap at the top is only in one room for some reason the rest are level to the ply. If the gap was in all I'd be fine because I would have continuous air flow from one side to the other

10-10-20

Do you have the height on the exterior to flip it into a warm roof?

Other ideas are a facia ventilator, but still needs 50mm of continuous ventilation and the joists in the right orientation.

https://glidevaleprotect.com/faqs/how-do-i-ventilate-a-cold-flat-roof-and-which-product-do-i-use/

sydthebeat

You need "cross" ventilation, so from the eaves to where?

What's on all sides of this flat roof?

TheMilkyPirate

https://www.boards.ie/discussion/comment/120450648#Comment_120450648

We do have the height but we don't have the money the roof is finished on the exterior. It's something we could do in maybe 5 years or so if it came to it but not now unfortunately

https://www.boards.ie/discussion/comment/120450675#Comment_120450675

Soffit vents on 3 sides and then house on the 4th. I'm just learning tbh I don't know what I'm at so cross ventilation may be the wrong term

10-10-20

Did you plan to have downlighters in the ceiling? If you did, that's another serious concern as they can be as leaky as hell.

Your only way forward may be to completely cover the ceiling with a VCL, very well sealed and fully taped, and hope that the small amount of ventilation "just works". The smallest stream of warm/damp air is going to condense on the underside even on a mild enough day and you won't know about it until the exterior finish bubbles and starts rotting underneath. You need to take time now to consider all options, but this should really have been covered off in the design phase.

TheMilkyPirate

Yeah it really should but I was clueless and just needed a new roof not knowing anything. At least if this one fails I can rip it off and replace with a warm roof next time. Have been looking for a VCL for the last week but all seem to be in UK. Is this what I'm looking for?

https://www.screwfix.ie/p/cromar-vent-3-lightweight-breathable-roofing-membrane-dark-grey-50m-x-1m/2697P?tc=BI5&gclid=Cj0KCQjw_r6hBhDdARIsAMIDhV8nSsL6z2T5cFJOcboLFl2wwhf0bSJ4rvMTk8fBkwuAdDiN_LOCBfsaAk65EALw_wcB&gclsrc=aw.ds

Also what tape should I be using there seems to be a few different ones and wanting to get the right one.

Thanks

SC024

https://www.boards.ie/discussion/comment/120451160#Comment_120451160

Google Intello vapour control layer. you should find details on use & installation + tapes etc