Flat roof mould and bad ventilation in a new cold flat roof over a loft dormer.

What about the cold bridge steel beam it passes all insulation moisture builds up on the steel and you are fucked

What software are you using for the modelling

Sorry. Das ist seit 30 Jahren bekannt. Wie zum Teufel kann man heute noch so einen scheiss bauen? Die sollen sich alle ihr Lehrgeld wiedergeben lassen!!!👎👎👎🇩🇪🤨🤨

@steve roofer : love the rotatable graphic you created. What bit of software did you use?

How much room between roof osb and insulation is needed for airflow?

Hi Steve, I’ve just recently done away with the low ceiling in the kitchen which was installed purely to fit spot lights. The original ceiling is a vaulted lean to with a support beam midway which I favour as it’ll open up kitchen and looks far more interesting, the rafters are 4x2, no insulation as the original ceiling is old lath and plaster. It’s a slate lean to roof with a vapour barrier underneath with battens. Not ventilated.
I want to strip out the lath and plaster also. What’s the best way to vent and insulate it. I’m guessing after watching numerous videos as well as your own that a cold roof is the way to go?

Having an external vent in the eaves of the flat roof allows cold moist air to enter the roof void. When the sun shines on the top finish it warms the air/moisture inside. Then at night when the surface cools, so does the deck, the warm air trapped in the void then condenses. That is the cause of that mould. If the cross ventilation is suitable, from eaves to ridge, then you dont even actually need a vapour barrier.

By the model on the screen, You can't change to warm roof, unless of course you drop the joists. Because putting insilation on top of this joists would raise the ridge height, and planning very likely won't let you do that.

As a loft converter of 25 years I can fully explain the issue on this building. A plastic vapour barrier will not let any moisture through whatsoever. On top of this the celotex is totally moisture proof. I know it has joints along the timbers but every building built now is insulated this way with generally no problems. I know you mention the house ambient moisture levels a house sits at 9% but this did not effect this roof because of the plastic vapour barrier and celotex. This was purely air flow its Vital. Raising the ridge lines is a no no in most loft cases.

Based on the research I've done, it's important to consider the stack effect with your ventilation plan. Having the vent on one side of the joist cavity lower than on the other creates a passive, temperature-differential driven flow of air through the joist cavities.

I'm having an issue with moisture accumulation in the joist cavity for my cold, flat roof even though I do have continuous venting on each side of every joist cavity. I'm trialing making adjustments to my build to promote the stack effect to see if that helps.

A drawering

I'm designing a dormer cold roof now, will a dry fix ventilation ridge tile system be enough to vent at the ridge.

Also I will have a steel much like the above, will 37mm PIR underneath satisfy building control for cold bridging bearing in mind the steel will sit exposed to the air directly beneath the dry fix ridge tiles? Thanks.

If joist is running counter to vents from soffits are you best to counter batten with say 2 * 3 Inc batten so air can go to vents preferably on edge to promote really good 3 Inch gap also what vents shall I get how many?

Newbie here. Looking to batten out my internal block wall and put Superquilt over, along with a counter batten and insulated plasterboard on top. Do I need to worry about mould for this project? Many thanks

jeeze, thats so much water it looks more like the roof leaked. But then one year i noticed my outdoor plastic shed was dripping wet on the inside, with a pool of water in some plastic tubs that were up against the walls... I'd stupidly decided it was a good idea to tape up all the gaps in the structure, and evidently the gap between the roof and the wall panel was supposed to be to allow airflow in and to allow any condensation to flow out and into a sort of guttering system on the outside of the wall panels. Taping up the gap meant air couldn't circulate, and any condensation ran down the inside of the roof, down the wall, onto the shelving that was against the walls, into any boxes that were touching the walls. the whole shed had basically turned into a water condenser/collector.

Steve I am not a roofer but I am learning so good from you. I will show you the roof for my house before I close it in case you have some advice. Thank you so much for sharing you knowledge with us

Do you know that there are mushroom roof vents that can be used to achieve the through ventilation?

Hi Steve which application did you use to make these drawings? Thanks

I am not educated in roofing at all. Please tell me what is "RSJ"? Thank you

Overkill insulation to meet ridiculous eco mandates equals sweaty property’s!

Surely those downlights are compromising the vapour barrier? All downlights let air through - it's needed to cool the bulbs and this will bring air from the room into the ventilation gap. The answer, of course, is to NOT have downlights cut into the ceiling - find another lighting solution (there are plenty).So, in short, downlights letting warm moist air into the cold space and no through ventilation to take it away.

I freak out a bit.
I'm remodelling my bathroom. I'm making it as water tight as possible with foam backer board floor, walls and roof all taped up.

I've added a mechanical heat recovery unit for the very purpose to stop mould and give good air changes.

How comes you need to have a 50mm gap for ventilation on a cold roof but when you build a stud wall the insultation in the walls doesn't need ventilation?

Hope the builder sees this.you.never stop learning. Cant believe this.happened.in.a.year.i dont.blame builder he went too far .

Year after year we put up with poor standards of design and construction with regard to insulation in the UK. No one seems to be on site to check the final installation is correct and who understands the fundamental principals of good insulation and ventilation.

Unvented flow .. No chance it wouldn't rot

Agreed Warm roof is the best 👍

150mm would never be noticed just make it run back to front

Biggest mistake was no soffet vents in the dormer side

What a great video. Could the customer in this scenario put venitalition holes/turrets (sorry don't know what they're called) at the ridge end to allow a through movement of air, the flat roof will then have a series of ventilation pipes coming out of it but at least it'll stop the damp and rot.

Also warm decks are great and always the way to go, but it depends if the customer has the hight available to do a warm deck on a loft conversion?
Or is that not true?

Also on a warm deck would the void underneath not need to be vented as it'll be warm due to the insulation being above?

Love your videos, just discovered them.

How is it that whoever done that job (which looks to be a competent build) could overlook something so fundamentally important?

Nice clear video. One thing I have found to add to you story is this can be caused because the roof timbers were soaking wet just prior to being closed in so the water drying out of the joists has no were to go when there is little through draught . I have also found this on warm roofs. In many other countries structural timbers have to be stored under cover , Where as in the UK they are nearly always out in the weather and are soaking wet.

Oh, that's a typical sight for those conversions made by Romanian or Moldovan Building teams. 😂 The simple solution would be simply drilling holes in firrings and above that metal beam. This will get the moisture out of there. At least within a reasonable budget.

Customer must open windows daily. You cant expect the roof to do all the hard work. Open the windows man!

Just put ridge vents along

I was led to believe PIR insulation has a vapour barrier built in to it?

These sweaty Osb all shows EPDM covering. Is it the same for a torch on felt roof. Do they need an air cavity and cross venting? This is a cold roof from 2012

Hi
This is well explained thank you.
I am thinking about buying a timber house and totally expect to have to do some work so what insulation construction would you advice for this for the internal and external walls.
Many thanks
Steve

Well explained.

Is it worth using a wood preserver like Sikagard to help control damp and mould on the underside of a cold flat roof? Obviously as well as a 50mm+ well ventilated gap.
Would really appreciate some advice on this.

I don't get why anyone would do a cold roof in today's age.

hey, nice vid... particularly the use of CAD images to explain what you're seeing.

Just wanted to ask, if I have a bedroom where I have patches of black mould appearing on the ceiling that corresponds to the soffit vents behind...what's best to do?

I've recently been dealing with woodworm and, during this time, I had to strip out the existing 90s insulation to gain access through to the joists for preventative spraying.

As the new absolute minimum thickness of insulation is 250mm, I have decided to implement a raised frame to place my loft boards back onto giving me the space required for the new 200mm (the deepest I could find for a reasonable price 😔) insulation to sit under.

I have made the frame +50mm taller than the insulation which should give an air gap above it and, as such direct flow through to the soffit vents.

However, I'm wondering...should I be looking at doing anything like spraying a small amount of foam on the back of the plasterboard near where the soffit vents are?

Reason I wondered is because I was told it may be as a result of cold spots immediately surrounding the vents.

Also, when placing the new insulation in the loft space, would I push it down into the soffit area or leave it with ventilation around it?

Thanks for any feedback...I know I'm probably being a little cheeky asking you on here 😅

What is the best way to install down-lighters in this rigid cold roof insulation?

The Osb is not good to use eighter.

Hi Steve, watched a couple of your videos (not a builder, just a homeowner) and have learnt more about and the importance of roof ventilation then I'd ever known. Our dormer is being constructed and the builder was trying to figure out the venitlation as I've proposed a parapet style dormer so it has no sofit/fascia, which they've not had to do before, as the the standard dormers have a sofit/fascia, which is where the ventilation is. How would the ventilation work here and where would it be, while ensuring the parapet is kept to the minimum to avoid it going above the roof ridge (fyi the roof is slanted towards the ridge with drainage on the front of the house) - Appreciate any pointers.

Hi Steve. I’ve watched a few of your videos now and find them very informative.
I’m about to do a garden build with a pent roof and I’m going for a cold roof because of the height issues. I’ve just watched the video with the dreadful mould growth due to lack of ventilation for the second time now. After watching it the first time I decided not to install down lights.
Now after watching it again I now feel that it should be ok to install down lights so long as I have enough air flow through the roof. I just thought I’d ask you for the definitive answer.
Also, if I have plenty of air flow across the roof, do I still need to install a vapour barrier?
It’s not a case of trying to save money. If you feel it’s still necessary then that’s how it’ll be.
Many thanks in advance

Regards

Pete.

If the insulation would have been pushed all the way up to the top. Touching the OSB,PLY. Would this have happened?

Could it help if the insulation board was pushed tightly to the roof level and leave a gap between the insulation and plaster board in the room just like a hybrid roof construction?

Sadly I think I have this very issue :-( (thanks for the explanation)

fair play Steve your videos are well explained

When I go up inside my loft of a 1911 house I can see light at the bottom of the tiled roof...so think my loft has plenty of air whistling through