Can I save my sweating cold flat roof with no vapour barrier ?

I will post the link to the author of the drawing below when I find out who it was as this is a very good diagram.
This viewer sent me photos of his sweating cold flat roof that hadn't been build incorrectly and was now sweating so much that water was dripping through into the room below. He hadn't fitted ventilation to between the joists and he hadn't added an AVCL (air vapour control layer) he had cut lights into the ceiling and air was pouring into the cavity. Mould on the timbers and on the back of the roof decking was starting to grow. So the question is "Can I save my sweating cold flat roof" Using information from some ware on the internet I took some research that shows that plasterboard has a good vaper resistance and the fact that the ceiling is airtight should solve the problem. airtightness in flat roofs is always the biggest problem I come across. People get all hung up about the vapour control layer and forget that more moisture gets into the roof through the movement of air than through a vapour barrier.
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Steven Dickinson
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[@ivornotion6624]

Appreciate your time and effort Steve to educate the masses in how to do things properly. I so wish you were in Norfolk so I could get you in my big extension build. So many cowboys out there - they are in a majority rather than a minority I feel.

[@agussy3318]

Excellent video as per usual.

[@hazmat5749]

Its a pity no one has designed an air tight downlighter that can be sealed in place to prevent moisture leakage. It would solve a lot of problems.

[@SteveRoofer]

They there is a tight ones now, however, they have to be installed correctly

[@shanewiltshire5869]

Cold roofs strike fear into me

[@rhyswhitehead2167]

If adequate ventilation is present above the insulation it does not need a vapour barrier and there would be no moisture build up. Simples

[@tomf4547]

I'm creating a loft space in my garage to keep the space below warm n dry. If it works in the house it'll work in my garage.

[@104rubberducklanigan9]

Hi Steve great vid, we're Roofers in the Midlands and come across this all the time when the keen DIY'er has a go, we always try to go the warm deck route, gives space for lighting and especially downlights, vapour barrier with Alutrix 600, insulate and EPDM, Never had an issue, apart from How much !!! 😊 Keep up the good work. LANIGAN BUILDING & ROOFING LTD

[@jayraza1386]

stupid question. if he was to pur a vapour barrier under the insulation, how would you install down lights without cutting into the vapour barrier? but great video overall :)

[@SteveRoofer]

yes and no its properties of retaining heat are not as good so you would need a lot you shoud not stuff it hard as it needs to be fluffy to work properly you would still need a avcl

[@TurinTuramber]

My advice, never take a half measure when a full measure is needed.

[@clintm5591]

Hi Steve, appreciate all the videos. They where a great help getting the roof on my garden room. I went with a warm roof. There is one problem though, I was hoping you could give me some advice before I continue on with the build. Really loud bang noises coming from the roof, it's a 55 square metre warm roof. I'm hoping it can be fixed instead of starting again. It genuinely sounds like a wheelie bin being dropped on the roof a few times during the evening, I can send any pics or details needed

[@SteveRoofer]

Yes please email me with all the info and any recordings of the noise and I will see what I can do [e-mail]

[@clintm5591]

Highly appreciated, videos and details coming tomorrow, might be hard to catch a bang but I'll try. Thank you🤝

[@davidhill431]

Christ that's a big garden room

[@steveturnbull9869]

Wow many thanks for posting this video, it’s my building. Thanks for the advice so you think I can just stick with the current plaster board which has no foil backing and just cut out bigger holes around the lights to fix the insulation and then patch up the plaster? I’m going to replace the Dow lights with surface mount so only the cable is coming through the insulation and with foil tape around where the cable comes through the insulation to try to reduce the air escaping. I also wonder since there is only 50mm insulation board and there is enough space in the roof for 75mm, would it make a difference? There is a 110mm ventilation area between the insulation and ply roof at the front of the building and this reduces to around 75mm towards the back, so I could change insulation. To 75mm and still maintain at least a 50mm gap for airflow. Or would that not be worth doing as I wouldn’t see much of a difference between 50mm and 75mm insulation board.

[@SteveRoofer]

Just make sure there is no air leakage into the void from the room below. Make sure there is good though ventilation above.I cant see how you would get more insulation up there and fitted tight without taking down the ceilings. Would it make much difference? no not a lot 90% of the insulation quality will be coming from the first layer and the fact that its tight to all the joists and is stoping warm air from migrating up placing another on top that's not fitting tight wont make much of a difference

[@steveturnbull9869]

@@SteveRoofer hi steve, yes i meant if the ceiling was taken down and the 50mm insulation replaced with 75mm would there be much of a difference to heat retention and also reduce the amount of warm air going in to the void. If the void is also slightly smaller, will that also mean less warm air circulating around up there which may mean the building doesn't need to work as hard to remove it?, many thanks! I'm also seeing some loss around the corner front edge of the building like a L shape, i noticed it today as its snowing and this section at the front as missing snow! ~2foot section, this section is still having a little condensation in the front soffit too, where as the rest is now pretty much dry. I'm suspecting here that the insulation towards the front may have a slight gap in it, or not overlapping the cavity wall properly to seal it.

[@jeffreybresnahan]

@@steveturnbull9869 You could easily cut out 600mm x 600mm access openings in your ceiling area. You would have enough access to patch in your insulation bat, you could then make a feature of placing dropped 600 x 600mm flat panelled LCD light's? Like what is used in suspended ceilings, these panel's also have very little heat, which would reduce any ceiling void condensation?

[@steveturnbull9869]

@@jeffreybresnahan there is no way the Down lights are going back. I’ve been looking at similar led ceiling discs which just attach to the ceiling on the underside. One issue I have now (since it was snowing today) I can see some heat loss around the front right hand side of the building and interesting this is where I had still got two problematic sections of soffit between joists which are still slightly damp and a little condensed. The rest is all dry so it’s only that corner. Snow gave it away as it melted across that section even though the rest of the roof was covered with snow. Where the heat loss is happening there is not from the lights, I suspect it could be something like the ceiling insulation is not properly overlapping the cavity wall insulation to seal it properly and it might have a gap causing some heat to leak out. I’m ok with cutting out the lights and repatching, but what about this other section.

[@jeffreybresnahan]

You can purchase a cheap flexable mini micro video camera which has a light in it, you can drill a couple of 10mm to 12mm holes in the specific ceiling/wall void area requiring inspection? The video camera fits into a laptop or ipad USB port, you just move the inserted camera around the area to see if you can view the problem? If it is a specific cold spot, or void/cavity area due to no insulation, you can take a chance on spraying the area with expanding cavity spray foam at 200 x 200mm centres? I have used this technique on several job's I have been asked to help out on, and have had no call backs?

[@simondgrass]

Cut the insulation in, and have the builder pay for a re-skim, and decoration.

[@frikshun_]

I’m gutted. Think I’ve really messed up. I’m almost 80% through fitting a lowered ceiling in my outhouse. It has an un-insulated asbestos concrete pitched roof. I just built a new flat ceiling frame, fitted that foil coated bubble wrap stuff and plaster boarded it. I then cut holes for the Downlights. I went in there yesterday to start work and noticed condensation dripping off the inside of the roof. I assumed it was due the the rain the night before and that it would dry out, but it is getting significantly worse over time. That is what led me to this video. Now I’m thinking should I just stop, rip down what has been done so far, fix the issue and start again. 🥴😫😥

[@SteveRoofer]

send over photos and let me have a look

[@frikshun_]

@@SteveRoofer wow! Steve, I appreciate the reply. I’ll get something over to you. Many thanks.

[@frikshun_]

I think I might have sorted it. I watched a few more of your videos and realised I needed to add some ventilation. So I knocked out three bricks and replaced them with air bricks today. I’ve taken a look this evening and I can already see a reduction in the amount of condensation. Thanks for all the knowledge you share. 🙌🏽

[@alanmurphy3584]

If you fill the void where the lights are going with insulation how do you fit the lights in ? If you have good cross ventilation front to back do you need a vapour barrier ?

[@michaelhewitson2050]

Why is the vapour barrier needed if there’s enough ventilation above the insulation? Genuine question, just wondering why…

[@SteveRoofer]

The more you stop rising into the vented space the less work the vented space has to do basically it's a fail safe however we now know that most invented spaces don't actually work that well so therefore if they've got less to do you've got to be in a better situation

[@truckerdim]

Whats the best way to insulate an old concrete roof

[@allflapnofly8919]

Hi Steve, Great explanation on vapour transfer. I have a similar cold roof with no vapour barrier. Would it be worth using a vapour barrier paint (I think they are latex based) on the plasterboard ceiling?

[@stevejohnson4049]

No, dude You'll trap water this way

[@DavidOfWhitehills]

Yes. Since the moisture comes from the lived in space (cooking, showering), vapour barrier paint will stop any moisture from getting through to the cold underside of the flat roof, where it would condense.

[@energetykpower8648]

Hi Steve, I just want to ask how would you insulate metal box section roof panels ? I have 4x2 rafters and I was thinking at using 50mm pir kingspan and leave 1inch or 2inches gap between insulation and metal and also drill holes in rafters for the air flow. From the warm side Im thinking at putting pvc cladding. Do I still need to put vapour barrier and also is my way of thinking how to do it right ? Thank you for your reply. Regards

[@martincropper8675]

Hi Steve, thanks for the video. If fitted tightly would rockwool insulation in a cold roof have the same effect as kingspan-style insulation?

[@anthonyman8008]

Use hemp wool

[@baloo217]

Hi, i have a question, i live in florida, i have a one car garage with a flat roof like the one on this video, no soffit, so no air coming in or out of the roof when close wit drywall . my plan is to install a mini split in the garage, Summer is to hot here, i know i have to insulate my garage door, now, how should i insulate this roof and what to use, my rafters are 16" x 2" and like 15 or 16" a part, left and right walls are block and stucco, anynof the two walls are exterior, left wall has my kitchen on the other side, and right wall meets my neighbors garage, the rafters goes right thru the blocks, so no wood against any of the walls. I want to do this my self, and im scared of moisture on the roof and floors.. I hope this info its enough for you to help me out, if you can, thanks in advance...

[@granthobbs5179]

Hi sry to ask I have a shed attached to the side of my house with a concrete roof how should i proceed to make.it a livable space and prevent sweating in winter I have ventilation adequate enough but that's all I know if you have a video on this can you send me a link many thanks

[@bluevanmani]

Wrong insulation installed for starters should be 100mm in and 25mm over joists or 120mm i between. Hear what your saying about vapour membrane but how do you install downlights without compromising the membrane ??? We also use foil back plasterboard on ceilings

[@steveturnbull9869]

think the ceiling is going to have to come down and replace the 50mm insulation with 75mm as that's all can be squeezed up there and still leave a 50mm ventilation gap since the height between ceiling and ply roof won't allow for more. Height between ceiling and ply from the front is around 17cm but this lowers to around 13cm at the back. Lowering the ceiling is also not an option as they put the window right up to the ceiling..so no easy fix for that one. The building is 2450mm so changing the height to the full 2500mm would allow for 100mm insulation in there, although that would mean removing the rubber roof and re-doing the joists and not sure the builder will be too keen on that option. So I'm hoping 75mm insulation will be an improvement over 50mm..that together with foil backed plasterboard and ensuring all gaps are completely sealed and no gaps in insulation will hopefully be enough...

[@psymon25]

Would Aluminium tape work with this foil based insulating to creat the vapour barrier?

[@ConfusedFieldHockey-em6bt]

Iv had a garage roof put up 50ml insulated sheets iv still got terrible condensation any ideas anyone ??? Cheers

[@michaelgoodall8899]

I've got a 100m² Flat roof to make into a warm roof What is the best / affordable vapour barrier in your opinion steve? Thank you mick

[@dbiwatches1891]

How's best to stop heat going through a downlight and into a cold flat roof? Your highlighting the importance of no insulation and vapour barrier around the downlights being a bad idea but does that mean you can't have downlights? Downlights are generally fitted in the final stages of completion to extensions/outbuildings etc so how's best to thermally protect the downlights and without going through the vapour barrier also? 👍

[@SteveRoofer]

This is the most requested question and I will be doing a video on this in about three weeks

[@dbiwatches1891]

@@SteveRoofer Cheers Steve,can you not do it this afternoon? 😀👍

[@peterbrown954]

Im renovating my cold flat roof but am unable to create a warm roof due to external height problems. I’ll be ripping down the existing ceilings, so could I push celotex between joist, up to the underside of the flat roof deck. Then use insulated plasterboard for new ceilings and still use downlights? Venting the roof is going to be very difficult as there is no soffit, just fascia.

[@SteveRoofer]

No you have to leave a 50mm gap under the roof decking and above the insulation, this gap has to have thorough ventilation from one side to the other.

[@peterbrown954]

@@SteveRoofer thanks for the reply. I have no idea how I’m going to vent the space. Really don’t want a row of mushroom vents.

[@peterbrown954]

@@SteveRoofer I think I’m going to cut soffit vents into the fascia at one end and put mushroom vents into the other end nearest the house. Will I need to do that between every joist?

[@SteveRoofer]

@@peterbrown954 yes unless you put one in the middle of a joist so it vents both sides them you can put one mushroom every other joist

[@peterbrown954]

@@SteveRoofer thanks again. Do you mean a hole in the middle of the joist?

[@calbrock6302]

Does anyone know the calculation needed to calc what thickness of insulation board is needed in a cold roof? Googled it to death and can't find anything

[@atif6843]

What ventilation is required for a flat garden room rubber warm roof ?

[@danielhart3820]

Hi Steve, I got a question.. can you mix insulation? I have a cold roof that had rockwall in, it has a 50 mm air gap, and I can fit in 150mm pir boards with the 50mm air gap on top, can I say use 50mm pir boards flush to the ceiling and then use rockwall 100mm on top of the pir boards? Only suggesting because I have lots of wiring above and would be a nightmare to move all the wiring higher? Thanks

[@plumbertime]

Nice video. Question for you. I’ve just plastered so bit late but be nice to know if I’ve messed up. I’ve made a cold roof in 9x2, 100mm celotex. I left a gap of around 1” between the plasterboard and celotex so I could run the cables under the insulation. Didn’t use a vapour barrier as assumed celotex would do it. My worry is I’ve created a mini cold roof between plasterboard and insulation! Starting to think I should of put the insulation tight to the plasterboard. Rooms 6x5m

[@SteveRoofer]

No its on the warm side of the insulation so that cavity is not a problem

[@plumbertime]

@@SteveRoofer Ahh makes sense. Thank you very much

[@rafmo7413]

It's a bit like having a warm roof if you've done it this way. Good tip to leave a little void on the warm side for cabling.

[@cv6717]

Hi steve ive got 50mm cellulose with vapour barrier on top and 170mm fibreglass on top of that in between joists. Will that work and avoid condensation?

[@adamkingdon8063]

Does the mould in the roof created by moisture from the water vapour go away and what damage does it do ? Cheers

[@brianhewitt8618]

if the ceiling is resealed with no holes in it to prevent moisture going up enough for mold to grow and the airflow is adequate then yes it will dry out and go away, putting a drop false ceiling in for the lights and services etc is the best way to have no holes in your fire/moisture barrier ceiling, but if you cant and you have to put holes in the ceiling for spot lights etc then you should make sure the insulation is continuous and sealed and consider using the insulation coverable type spotlights and seal the rims on the finished surface

[@GANDHAM1978]

Hi Steve Hope ur well, I’m doing a cold roof, I’m at the insulation stage of my build of my garden house, is it a good idea to use 1.5m foil radiant barrier to the top deck first and then add 100mm PIR insulation? Or should I just use the 100mm of PIR insulation, ?

[@SteveRoofer]

I don't quite understand what you meen send me a drawing [e-mail]

[@freddiemoses467]

Steve, what do you think to plasterboard insulation like Celotex PL4000 which include a vapour barrier? Are they just as good as installing a polythene sheet behind plasterboard? Cheers

[@SteveRoofer]

Its Ok but no good as a vcl as you cant join the silver foil togther as its at the back

[@freddiemoses467]

@@SteveRoofer Thank you

[@rafmo7413]

In these scenarios how would people get around installing down lights? No matter where the down light goes it won't be airtight and it'll allow the warm air to pass through the gaps around the down light and hit the cold side causing condensation and mould growth? Or have I got it wrong?

[@SteveRoofer]

No you got it correct you have to do a lot of detailing to fix a down lighter correctly or you place a dropped ceiling with a cavity but people don't like doing that as it takes away ceiling hight

[@rafmo7413]

@@SteveRoofer thanks for the reply Steve. Good to know alot of detailing it required around the down lighter. I think a warm roof is the way forward, you always have a void for cables and downlighters. Or like you said have a cold roof but a small drop to accommodate lights and cabling etc. Love your videos, so many good explanations and good to know what not to do. Keep up the great work.

[@pavbahia6185]

What’s the best process to install down lights in a cold roof

[@SteveRoofer]

Build a void with another ceiling under the first one and fix the lights into that. But nobody douses this as it drops the ceiling so use slim LED spotlights and keep the insulation and vapour control layer intact

[@matthewbaxter5682]

With a 50mm gap maintained above the insulation and ventilation in the soffit and an abutment detail in line with your other videos. No downlights. Would this mean the vcl is not required?

[@SteveRoofer]

No you should still have a VCL but if you have forgot to put it in its not the end of the world

[@matthewbaxter5682]

Thanks for the response. In the planning stage at then moment so I'll add one.

[@kevinrae3299]

Hi Steve I’m a fellow flat roof contractor in Scotland, why are there so many cold roofs down your way ? They seem to have a lot of problems?

[@SteveRoofer]

Because we don't build to a good standard like you up there

[@dannybennett]

To stay under the 2.5m regulations for planning permission.

[@abbbee8918]

Hi Steve, I am half way through building a garden studio. I am restricted to 2.5m so felt I had no option but to build a cold roof. I have 6" joists and 4" noggins to allow a 50mm gap above. I am thinking of drilling some small holes (10mm at a depth of about 40mm) to allow ventilation between joist cavities as well. I am planning to tap some nails in so that when I insert the PIR insulation (90mm) it has something to stop it going too far. Do you think these could be a condensation risk? I will aim to cut the PIR for a tight fit, and chamfer the edges so I can seal with expanding foam from below. I am thinking that this will prevent foam from expanding inside the cavity above and provide the best insulation. I will then fit a vapour barrier to the underside before plaster boarding. I will vent the soffits but am concerned that 3m is quite a large span to vent. I am also concerned that saturated air on rainy days will naturally fill the cavity and without wind could be static there for quite a while. I see this as probably the main source of damp air and much more of a problem than small amounts coming from the room itself. Do you agree? Do you think my construction technique will work? Any suggestions? NB: I have seen a hybrid roof where the insulation is just pushed up against the roof deck, which is supposed to be okay for low occupancy rooms. It would be difficult to do this now my noggins don't meet the underside of the roof deck.

[@HampsteadBuildersLt]

Its all a bit limited so may work but you should try to increase the ventilation gap, hole sis not the way too you see people dining it but it really restrict the flow of air you need good flow of air

[@abbbee8918]

@@HampsteadBuildersLt Thank you Steve for your response. I’m afraid It would be difficult and expensive to increase the gap now. The roof is constructed and I have already got the 90mm PIR boards. I thought the thicker the better to prevent warming the cavity. I understand from your comment that you don’t recommend holes in joists - do they create a more turbulent flow of air, when what you are after is a laminar flow? Would it be worth painting the underside of the timber deck and joists with anti mould paint/stain as a precaution? I would select a breathable product. What about pushing the insulation against the roof, I’ve seen that done. Many thanks

[@HampsteadBuildersLt]

@@abbbee8918 They don't give enough ventilation but anything you do is enter then nothing

[@soarlikebird]

Hi Steve great video. I’m putting a cold roof on a small porch (6sqm) used as utility room on my bungalow. I have a question re upstand vents. I plan on venting the soffits all round , directing air flow back to the bungalow wall, where you would normally create a vented upstand. However if I were to do this, (As it’s a bungalow) there is not much room between the flat roof and the soffits/fascia of the main roof. In fact the vented upstand would sit behind/be covered by the soffit/fascia of the main roof. I was thinking of overcoming this by incorporating that section of main roof fascia in to the vented upstand of the flat roof. (In effect creating a bigger void as an upstand, which doubles as the main roof fascia). The result of that would be the airflow from the flat roof would also go into the main loft space of the bungalow (which in is vented at the soffits all round). Do you see any issues with this idea? Thanks.

[@SteveRoofer]

Normally I would not vent one roof into another but as this is a small roof and its a porch then I cant see a problem

[@johnmcdonnell5175]

@@SteveRoofer why wouldn't you vent one roof into another? I was going to do the same as this guys porch with the cold flat roof on my bungalow extension, it's 2.4 by 5.4 metres. I was going to install some vented double Roman tiles on the existing roof on that side of the property.would this work in your opinion?

[@rickywanless2239]

So how would u suggest for putting lighting into a roof space?

[@TurinTuramber]

Official answer for downlights is batten down after the under rafter insulation and install low profile down lights in the batten void. Often this isnt feasible. Edit: This is for a vaulted ceiling or cold flat roof. Ideally for a flat roof, in the first instance do a warm roof then you can just put them straight in, again not always feasible. Alternatively could always just install a low profile surface mounted light fixture similar to a bathroom light.

[@bucketandspade]

I'm the god of all roof knowledge. Here's a random photo I got from online. I dont know where it's from 😂

[@simonstones1918]

What do we do then Steve? It seems like an impossible situation 🫣

[@SteveRoofer]

Relistly the only way of dealing with this is to strip it and start again

[@ConfusedFieldHockey-em6bt]

Iv had a garage roof put up 50ml insulated sheets iv still got terrible condensation any ideas anyone ??? Cheers

[@SteveRoofer]

Is it constructed as a warm roof or a cold roof?

[@ConfusedFieldHockey-em6bt]

@SteveRoofer cold roof bud