Through Ventilation Cross Ventilation Explained
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- Опубликовано: 26 фев 2022
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Flat Roofs - Through Ventilation / Cross Ventilation questions I'm always asked
Through ventilation in a cold flat roof is essential. Without through ventilation, the air in a vented space will not move at the far end. Through ventilation means a vented space with vents on both ends of the channel, and the vented space has to be clear for the air to move though it. You cannot have restrictions. We like to use cross ventilation if possible; basically cross ventilation is through ventilation with a matrix of timbers over the top, to form a venting system that vents left and right as well as front to back. If you have eaves vents, soffit vents or even mushroom vents over a cross vented roof, you will have very good cross ventilation and any way the wind blows you will get movement of air in the vented cavity of your flat roof. Unvented flat roofs have all sorts of problems with moisture buildup; not only do you have to have a good Air Vapour Control Layer (AVCL) at ceiling level, but you also have to have good through and cross ventilation. The size of the vents you use in a flat roof really depends on the size of the roof you need to vent. Generally the more venting you can place on a roof, the better. However, you must make sure the holes to the vents are around 4mm in size to stop birds and small insects from entering the vented cavity. Adding through ventilation to a cold flat roof when you have a skylight is extremely difficult; this is when we like to use cross ventilation on a vented cold roof. The fact that it's crossed means that you will not get any spaces between the joists that have no ventilation.
When doing a loft conversion and a vented cold flat roof to the dormer, and you have a ridge with tiles, today we can fit a dry ridge roof system to vent under the tiles; this enables ventilation not only from the vented cold roof to the vented ridge, but also to vent the pitched roof at the front of the building. You must be careful not to put the vents of the cold roof too low, as water can be blown back into the vented area. Often we see the wrong vented ridges being fitted to the ridge; this means that the venting to the cold flat roof is not through venting, and then you may have problems.
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Steven Dickinson
London flat roofing
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Thanks for all these very informative videos.
Steve kindly helped set my mind at rest by answering a few technical questions that arose from awareness of potential problems based on researching this excellent information sharing. Outstanding work Steve.
Glad to help!
great information, I don't even work in the building trade but I find these videos fascinating and it almost makes me wish I was a roofer :) Steve helped me with an issue with a cold roof a builder did for me where it was condensing badly and I had to tell the builder myself what needed to be done after I'd watched some of these videos. Fast forward almost a year and my cold roof is now completely condensation free. Originally I had no ventilation at all, then I only had ventilation at the front, which still didn't fix the issue. Now I have through ventilation just using round soffit vents at each end section and its made a massive difference. All the timber is now dry on the deck above the insulation where previously it was sweating and mould forming. Keep up the great work with these fantastic videos.
Thanks for the comment
Thanks for taking the time & trouble to explain. 👍🏽
No problem 👍
No wonder so many get this detail wrong, there’s not much information about. Thanks Steve, never really contemplated just how ineffective ventilation is without through flow
I was amazed also
@@SteveRoofer every building inspector should watch this and advise builders accordingly
Using smoke to demonstrate your point? You’re a mad lad 😜 but also dedicated! Thanks for yet another super clear, informative video Steve!
Much appreciated
Best way to give a visual demonstration as to what’s happening, can’t see air 😊 Nicely done Steven 👍🏼
Just totally changed how I planned on insulating my garage roof, thanks dude!
Glad I could help!
Great video, I had seen cross battening before (probably one of your vids) but pretty much forgotten about it. Glad you reminded me of it's benefits as I need to build a little flat at some point roof this year.
Glad you enjoyed it!
Your lessons are pricless!
Yes, RUclips is great for many things
Thank you so much 👍🏽👍🏽
thanks for video , how to do if joists are fixed on wall hangers inside the building? is a cold roof , bcz of building hight ..thanks
Very good stuff. I always use a steel/aluminum head flashing from the sloped roof to the flat roof with perforations in the metal to ac lol posh two things.
First is your using the head flashing to provide waterproofing to the sloped roof, also then having the perforations (facing downward on a crated lip to avoid leakage) to have airflow coming in.
But this is some good stuff. So many people don’t vent flat roofs properly.
Also, now they use spray foam and say now you don’t have to vent….. stupid……
Yes spray foam works as insulation and vapour barrier. That is sweet.
But fibreglass bats act as insulation, and poly or similar acts as vapour barrier. Still need to vent either way…..
The spray foam is better R value per inch but still loses heat, and you’d rather that lost heat escape rather than form condensation.
A properly insulated and vented roof lasts so much longer.
What happens when you screw the drywall it's going to make a whole in the vapour barrier?
Any thoughts on plasterboard for the ceiling? I was planning on using 12.5mm tapered edge but is insulated plasterboard useful in a ceiling (flat warm roof) ?
How does the cross battening work with furrings on a flat roof with a sky light?
Hi Steve, I have cut through my membrane and installation to fit spotlights. Now I have sweating on the underside of my roof. As an option, I thought I could install surface mounted ceiling lights and fill the downlight voids in with installation, Sliver tape, and plaster board.
Do you think this would solve the sweating issues?
Kind Regards, Andrew
Hi Steve, when cross battoning...is it best to put the battons on top of the joist or ontop of the firings? Thanks
if I cant get that thick insulation board u have above plaster,, can i use styrofoam.. for my 10 by 14 ft extention
Brilliant
how can we make slop in flat warm roof ( for rain water ) ? Help
I have a room that i think was a patio that the previous owner closed in. I dont think there is any ventilation and im not sure what to do 😅
Where would you put those ridgeline vents?
Great video
So how does this work but on a pitched roof ?
Insulation. Vented soffit and so on
I will do a video on that in a week or so
I have a residential roof with four pitched shingled side and a flat roof.
The unfinished attic uses a few shingle vents. We are looking to finish the attic yet we do not know how to vent the roof or apply insulation.
If you'd like to send me some pictures I'll certainly have a look and give you an idea
Hi Steve very useful videos on your channel. What would you suggest for venting a flat roof which is enclosed by a building on one side? Thanks
We would use abutment vents there's always a way of doing it it's just a matter of detailing in cost
@@SteveRoofer thank you for your reply. I came across your video on abutment vents and it was most helpful. Thanks anthony
@@AntonWardo not a problem
Hi, i love your videos, but, i don't see any that applies to my case, can you help me out please. I want to put an a/c on my garage, now, i live in florida usa, summer are very hot here, my garage has a flat roof , the walls on both sides are made out of blocks and stucco, my rafters go from block wall to block wall, and on top of the rafters is the playwood roof, so no air movement it all ..how can i insulate my roof? I will hate to have moisture on the playwood. Can you please help me? I will appreciate it. Tks
If you'd like to send me more information by email info@londonflatroofing.co.uk perhaps I can help you
Thanks for your videos, your real world examples are very interesting.
From what I know, air moves only due to three reasons: stack effect (i.e height difference), wind, mechanical (fan). On flat roofs we don’t have any stack effect, we don’t install fans, and wind is unpredictable and maybe inexistent in some circumstances, so almost no air will flow through.
The experiment is explaining the theory nicely but I think it’s not indicative of reality. I’ve seen in some countries building regs venting being only suggested above a certain pitch. Curious to hear your opinion on this. Cheers
My opinion is don't do cold roofs, do warm roofs then you don't have to vent
I agree… a warm roof is the way to go for flat roofs. That would be a better suggestion as venting a cold flat roof which has no airflow makes no sense. Moisture will just stay there. Especially on balconies with parapet walls where you have zero airflow…
Balconies are alway a problem no height for a warm roof because of the door step
@@angelkj I've found even on a calm day when you think there is no wind at all, there is usually a small amount and it's enough to create a through draft.
Could you drill vent holes in joists to allow cross vent between dead spaces around sky light if it’s a new skylight in an existing cold roof without battens.
You can do but it's pretty much frowned upon I've shown people doing it in some of the videos but I would never do it vent the vent the roof properly
@@SteveRoofer only other way would in theory be to mushroom vent through the roof in each compartment on either side of the roof light. Ideally he would of done the battens to allow cross vent but roof finished now on top. Looking at doing multiple c.100mm vents through walls either side of extension into the space above insulation. Was gonna drill through joists to allow cross vent around roof. I don’t particularly want 10 vents up there penetrating the roof. 🥴
What are those two white (fascia) panels called? The one you're holding at 1:10 with the small lip and the one with holes for ventilation.
I just can't seem to find these panels available anywhere.
I think you are referring to the plastic sofit and fascia panels. These are often overlayed on to the existing wood soft and fascia.
Yes thats corect
Could you leave 25mm gap instead of a 50mm gap and use cross battening with 25mm batten to make up the 50mm air gap needed?
Yes I think that's the way to go. 50 mil is better than 25 mil, but when you've got cross battening you've got all sorts of flow of air
Skylight on a roof with all the insulation installed properly with adequate vapour barriers and a vent on all four sides does this pose a security risk?
Yes security is a problem however most skylights come with bolts that are temper proof
I wish I could send you some pictures I have 11 skylights. I’m not sure there is vents in there.
Hi Steve
What's the best way to ventilate a lean to flat roof which has got a glass lanter in the middle
As explained int he video cross batten the roof
@@SteveRoofer what would you suggest if there's not enough height to double batten the roof!
There's a balcony patio door cill accessing the flat roof which is only 80mm above joist level!!
Really appreciate the reply thanks
Something has to give! how about putting ventiationa round the skylight? ugly and hard to do
@@SteveRoofer really appreciate the reply
Keep up with the great information
i build a flat warm roof, but i want to close it off with plasterboard, do i have to ventilate the space between the beams ?(with inside air)
Hi Carl, the answer is definitely no, you must very tightly insulate the perimeter of your warm roof from the wall head upto the underside of the sarking. It's critical to make sure the roof is sealed. The warm void between the rafters is just that, a warm void that you can run cables for recessed lights. If your flat roof buts against an exiting pitched roof, you again must make sure that this junction is tightly sealed and that may include a soffit. Additional low level ventilation may be required to the existing roof. A warm roof is far superior to a cold roof which should be avoided where possible.
@@coldt1484 thanks, I'm happy with that
if I cant get plaster boards,, can I use chiprock walls
I don't quite understand, can you elaborate?
What if you have no roof vents at all (old house)?
Lots of old houses didn't have vents and they vented naturally because we double glaze get rid of our chimneys and so on. We now need to add ventilation so keep assessing your property to see whether you think you should add more ventilation.
Hi. Thanks for the videos. They are really helpful, so basically if you have a cold roof the only real option is a dropped roof.
I’m looking to convert a summer house into an office, with no room to drop the roof? Am I buggered.
I think buggered is a bit Strong but yes you are right why not do a warm roof
@@SteveRoofer just height issues is what’s stopping me doing a warm roof.
I have no sound after 2:40 the start of the smoke test. Is it like this for everyone???
I've tried it and no one else has complained try it again please and get back to me
Sound is only left channel at that moment
@@SteveRoofer Thanks - sound now working! RUclips is a mystery sometimes :).
Hi pal what if I use spray forum insulation
This is a real challenge, you need to concider the roof make up, foam usually needs to be sprayed to the underside of a breathable sub-strate and sheet sarking is not regarded as breathable. Check the BBA certification for guidance.
I'm not really up with spray insulation because of two things it's hugely expensive and the gassing off i.e. the smell the fumes and the health problems reported I've just not worth going there
Backward and forward. Backward and forward.
I mean it seems elementary. If you want to go through the wall, put a door in it. Same thing.
High Steve, I was watching this video from Robin Clevett ruclips.net/video/jaTWMUuTWnI/видео.html&ab_channel=RobinClevette and to me it appears he does mention an air gap but then pushes the PIR quite a way into rafters leaving a gap at the front. He then goes over that horizontally meaning there must be a gap between the 2 lots of PIR. I'm not asking you to bad mouth his work as I like that he went to all the trouble of making jigs and using that special tape, just trying to understand.
His video is not actually about the ventilation although he mentions it hopefully he will do through ventilation as regards more layers of insulation that's also acceptable as long as they are tightly secured together and that all of it is nicely secured with the appropriate through ventilation on the cold side of the roof
@@SteveRoofer ok thanks 👍