No ventilation in cold flat roof = condensation!

[Notch7]

ncluding firrings, the height from top of roof deck to bottom of joists goes from 183mm to 153mm...i.e. a void of between 63mm and 33mm if I pushed the existing 120mm Celotex up to the deck. So I guess I would then need to purchase some 30mm, 40mm, 50mm and 60mm Celotex and fit under the 120mm boards accordingly, to get it as flush as possible with the bottom of the joists

Do your firrings run in line with the joists?

To convert to a warm roof, you need the insulation to be pushed up so it is tight against the underside of the deck.
The key is that there is no gap between the deck and the insulation.
It doesnt matter what happens below the insulation, all that is needed is a vapour barrier on the warm side.

Your condensationnproblem has probably been caused because the roof is incomplete abd tgere is no vapour barrier in place. Weve had some really frosty days which has made your deck probably down to freezing, causing the problem.
Generally warm decks are better

 

[Ian H]

If you put your insulation in as you plan then once it’s complete that should stop and condensation shouldn’t it?

Can you not feel a draught where the joists end at the steel? Looks like a decent gap there.

Are your centre noggins touching the osb or is there a gap above them?

 

[jamie765]

Thanks guys. Have just had BCO round and he wasn't happy with the full fill option

Can't feel a draught where joists end at the steel, although the top of the steel is cold and a little wet to the touch from condensation. Centre noggings are same size as the joists and fitted in line with them, so do have a small gap above them but it's only as big as as the firrings are at that point, if that makes sense.

Re: vapour barrier, I will certainly be putting this under the insulation either way to help stop moisture getting in the void, but I'm aware they can't be 100% effective and would only help to a certain degree.

Discussed options to solve this issue with the BCO...bottom line he wasn't too pleased with the way the builder had done it! He said the steel should have been lower so air could flow on top of it.

We discussed all the options, including:
- Mushroom vents: he said it was an option to put 3 on top, but then even if I drilled holes between joists the cross ventilation wouldn't be great so not an ideal solution
- Full filling the void: he said Celotex themselves don't like hybrid roofs and he didn't agree to this option at all (however much I pushed the issue)
- Getting back on the roof and adding 50mm of insulation on top of the felt, then refelting on top of that...i.e. making it a warm roof. However he thinks this would then result in a height smaller than the required 150mm between top of ridge tile and top of flat dormer roof, so that option was also ruled out

He then called his manager (private BCO firm) who advised him to call a company called GlideVale, who specialise in roof venting products. BCO called them and they advised to get back on the roof and fit an 'FV250' vent all across the ridge, and open the soffit vents up on the other end too.

I asked BCO who was responsible and he said builder should have done it correctly from the start, and he'll be writing an enforcement letter saying it needs to be corrected within 90 days! Just called the builder who wasn't too pleased with BCO comments and will be coming round tomorrow...he thinks he could just drill into the steel ridge beam to provide airflow but I'm sure that would compromise it's integrity surely?!

Anyway just Googled the FV250 vent and looks like it's an over fascia vent so have no idea how it can be the solution, or how it can be fitted at the ridge tiles to provide ventilation to a flat roof! Product is here: https://www.roofingsuperstore.co.uk...a-ventilator-25-000mm-airflow-1m-pack-12.html

 

[Notch7]

Full filling the void: he said Celotex themselves don't like hybrid roofs and he didn't agree to this option at all

Celetex etc dont test for it because there are too many variables. VCL detailing needs to be done carefully to ensure its complete.

I can understand the BCO resistance to converting to a hybrid, because there wont be any insulation going over the joists, so there will be thermal bridging. Mind you Im not sure how much risk that creates with regard to condensation.

I really think testing should be done on hybrids roofs.
And further testing on cold roofs -I really wonder whether the ventilation is enough to stop condensation in all scenarios.

 

[jamie765]

BCO have just sent this drawing, forwarded from Glidevale, and advised this is the solution with the FV250 vents.

Basically that vent will be fitted to the top of the ridge, and an opening made on the inside to allow air to reach the vent, which will then be sealed with lead flashing.

Works in theory if done correctly but a lot of extra work, especially if scaffolding is needed...

 

[THarris]

Hi,

I've just come across a similar problem to this with our extension, where the BCO is happy with FV250 strips to provide ventilation, was he still happy with the drilling of the roof joists? It doesn't seem like a drastically different solution to the mushroom vents?