Had a builder do the shell/structure of my loft conversion earlier this year and I'm DIYing the rest.

Whilst putting insulation in I've discovered he hasn't left ventilation at either end of the cold dormer roof. It's felt on top, with 150mm (145mm) ceiling joists, plus firrings. Am using 120mm Celotex in between joists, with 25mm Celotex underneath. Taking the firrings and 120mm Celotex into account, the ventilation gap (between top of insulation and bottom of roof OSB) spans from 63mm at one end to 33mm the other end - I know, a little less than the recommended 50mm ventilation gap, but that particular detail is the least of my worries at the moment! Because...

I've already discovered severe condensation when I removed one of the roof insulation boards and found the dormer roof OSB wet to the touch! The top of the ridge beam is also wet to the touch.

I know it will be (relatively) easy enough to vent at the far end of the dormer roof - using soffit or fascia vents. However I know I need the air to circulate i.e. vented at both the 'far' dormer soffit end, as well as the ridge end. Am toying with the idea of mushroom vents on top of the dormer roof, but there are 13 openings and obviously I don't want 13 mushroom vents on my dormer roof! (would need one for each opening as there's currently no way for the air to circulate between them)

I'm also thinking I could notch and drill holes in each and every joist to the maximum regulations permitted, that would create some sort of airflow in between joists, and possibly result in needing far less mushroom vents? (could I get away with just 3 - one in the middle and one at each end?) I've attached a diagram showing joist holes and notches I've calculated. I'd have to batten 44mm x 44mm timber to the bottoms of the joists and bring the 120mm Celotex down to that level to allow for 30mm holes to still have the required 50mm height clearance from the hole to the top of the joist.

Also concerned about the roof leaking if I get a roofer to put mushroom vents into a felt roof, whether it's 1, 3 or 14...

Any other solutions to provide ventilation?

Pic of what I could potentially do to create cross ventilation between joist openings

Found it! See a few posts below.

Coming soon!

Vents sketch

Thanks for replies guys.

Nick - sorry I've just looked at the diagram I drew - not very clear is it! I've redrawn so it's a little more clear - yes you were right when you said it's plans, not just elevations. Hopefully second drawing will make more sense.

And yes those holes and notches would just be there to have cross flow ventilation between the joists. It's just an idea I have at the moment, as I can't think of any other way to provide a flow of air between the joists...and don't want vents in each joist opening as that will be 13 vents on my roof!

I'd still need to have an 'in' and 'out' you're correct. The 'in' I'm thinking is through the soffit or fascia vents, and the 'out' I guess would have to be through the flat roof mushroom vents like the ones you've posted Tony.

I don't really want to put those mushroom vents on the flat roof as I'm not sure how well they can be retrofitted to a felt roof? Would they not be prone to leaking? If it was GRP I guess it would make things a lot simpler but unfortunately it is what it is now!

I'd just chop sections out of the firring near the ridge, rather than make 18 mm notches. You can always put a metal strap over the top if anybody insists, but since you won't have touched the joist I don't see why there would be any problem.

Gosh, you must be busy just reading all the forum replies!

One of the posters on the diynot forum, in suggesting the full-fill lay-up, suggests that the water vapour won't get there *because of the VCL*. He/she must have more faith in VCLs than I do. I believe that it is rare for a VCL to be installed 100% tight. Therefore, in your position I would have a VCL ('for what it's worth' - don't get me wrong - I'd do my damnedest to get it tight) *and* a well-ventilated gap.

The full-fill option is not a common one, as far as I know.

Ventilate that gap! (And next time you do a flat roof you'll do a warm roof anyway, so you won't have any of this fuss!).

I hope it goes well.

Posted By: LoftDIY87However, I've been advised on another forum to push the existing 120mm Celotex all the way up to the underside of the decking, and put more insulation underneath until it's flush with the bottom of the joists...i.e. fill each and every joist opening with insulation, so there is no void at all for air to condense. And then cover with a further 25mm beneath the joists, vapour barrier, then plasterboard.

Seems a better way if it's scientifically correct and BCO agree?

I think the last little bit is the key. Whether the BCO agrees. When I did my roofs, regs said ventilation was required except for a true warm/inverted roof (I.e. no material that can rot on the cold side). I suppose that may have changed.

So I did my conservatory roof in the standard way ventilated with insulation underneath and a VCL at the bottom, much like your original scheme although I do have ventilation at both ends.

My house roof is not ventilated. It has (bottom up) skimmed plasterboard, VCL, panelvent, 450 mm warmcel in crossed rafters and purlins, 18 mm ply, 'metal' breather membrane and standing seam aluminium. My BCO (actually a private building inspector) accepted it on the basis of a condensation analysis done by the membrane supplier, who also warranted it for my benefit. To me the important differences from your proposal are: my insulation is hygroscopic and will buffer any moisture; the 'metal' breather membrane forms a small gap above the ply and below the aluminium to allow moisture to escape. Particularly with non-absorbent foam insulation and felt directly on top I'd be very concerned about condensation and mould in your OSB.

Thanks all, appreciate the assistance here - although am now even more concerned after having BCO round for a site visit today! 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
- 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/product/glidevale-over-fascia-ventilator-25-000mm-airflow-1m-pack-12.html

I often sit in my office, complaining about BSO's, and the system we have here in Scotland, whereby you have do prepare all the drawings, including sectional details like the one your BSO provided for you (very helpful of him), and get it audited/signed off before starting on site.

However, it makes total sense to have every junction/interface/fixing/etc detail fully thought through, put on paper, agreed with client/builder/BSO, before you start any work, and might have avoided this vent detail, and poss steel beam issue, and been clear as to what SHOULD have been provided by the builder and costs for correction.

Won't stop me sitting here cursing them though!!

When the client & builder get in a huddle, and decide they want to alter the design, and build it in a way that I know will be an issue, I usually tell them that's great, there's no problem doing it, and they are now assuming design responsibility for the project, and absolving me from any future issues. Usually gets the desired response

Trying to be helpful for a moment, your builder does have to comply with building regs, so if he has chosen to build it in contravention of building regs, he can have as many goes as he likes at his own expense, but it ultimately has to comply (and regs are just a minimum). Your BSO's sketch might also solve the the steel beam condensation issue, by venting around it. Just insulating a bit under the bottom flange, won't stop it sweating, and so likely to get water staining on the ceiling (which people often think is a roof leak, and spend months chasing rain ingress).

Looks fine to me but you have to convince building control

Just a though..apart from it being a pain, is there any reason you can't make more openings in the "firrings", along the length of each rafter?

It's not structural, just a spacer. If you drilled every let's say 150mm, or even 100mm? Youy might hit the odd fixing nail, but just stop, and move over.