Ventilate cold roof on bay projection or not?

[Grout Expectations]

I’m trying to work out the best approach for a small bay projection on our kitchen. The bay is roughly 2.5m by 0.6m and projects beyond the external wall only at ground-floor level. Inside, the ceiling in this area is lower than the rest of the kitchen, so the main ceiling steps down.

We’ve always had some damp marks on the bay ceiling, especially in the top corners. Outside, the bay is topped with a flat roof finished in fibreglass.

The original fascia was timber and beginning to fail. I planned to remove it, but it had been partly rendered into place. Since there was enough space, I ended up over-cladding it with a composite fascia instead.

While doing that, I drilled into the old fascia to see the roof build-up. From top to bottom it appears to be:

• Fibreglass roof covering on a deck • A void of about 100mm • A board forming the ceiling (timber or plasterboard)

So it is basically an uninsulated cold roof. The void has never had ventilation as far as I can tell.

I can add ventilation at the front by installing louvred vents in the fascia. The sides seem to be solid timber for at least 250mm or so, which means front-only venting is the only realistic option. Not ideal for airflow, but doable.

My concern is what happens if I do add vents. At the moment the void is sealed and presumably kept slightly warmer by heat loss from the room below. If I vent it, I’ll be introducing cold air into that space. That could cool the bay ceiling further and increase condensation risk on the inside surface, especially because this is a kitchen with higher humidity.

So I’m unsure what the lesser evil is. A cold roof is supposed to be ventilated, but will adding vents create a new condensation problem either inside the room or within the void itself?

Interested in any experiences or guidance.

 

[robinbanks]

Insulate it

Genuinely can't understand it when someone proposes having just a couple of square metres of plasterboard between their paid-for-heated envelope and the bitterest of winters

 

[Grout Expectations]

Completely agree and would have done if I’d put it on, but retro-fitting that doesn’t fill me with joy. Yet another thing on this house that comes under ‘why the hell didn’t they do it right in the first place?’

 

[noseall]

Completely agree and would have done if I’d put it on, but retro-fitting that doesn’t fill me with joy. Yet another thing on this house that comes under ‘why the hell didn’t they do it right in the first place?’

It's a poorly built structure, looing at it from outside. Who in their right mind builds a roof smaller than the building its covering below? The render looks like it breaches the DPC too. It makes you wonder about the lintel spanning (internally) and how that has been dealt with regards cavity tray and thermal protection.

At least the broad margin around the French door frame means you can at least do a fair bit of insulating and damp protection etc.

 

[Grout Expectations]

I know. It’s really oddly constructed.
The render does breach the DPC but it’s traditional lime render over single skin wall done by a heritage company under the previous owners. So ok to breach DPC I think?

 

[Grout Expectations]

Just bumping this for any more thoughts. Thanks

 

[noseall]

So I’m unsure what the lesser evil is. A cold roof is supposed to be ventilated, but will adding vents create a new condensation problem either inside the room or within the void itself?

Cold roof vents are there to vent the air space on the COLD side of roof.

 

[Grout Expectations]

Cold roof vents are there to vent the air space on the COLD side of roof.

Yep. The roof isn’t insulated so the void between the plasterboard ceiling and the ply and fibreglass roof covering is the ‘cold side’ it’s this I am suggesting ventilating.
My issue is that this would presumably make the ceiling of the bay section of the kitchen significantly colder than the rest of the kitchen ceiling therefore creating more condensation?

But… not ventilating the void feels like a recipe for condensation on the underside of the covering ply as some moist air may make its way through.

Having said that I couldn’t see any condensation issue though the holes I drilled in the fascia so perhaps not.

 

[^woody^]

Take the ceiling down, completely fill the roof void with insulation, leave no gaps, then a layer of insulation across the underside of the joists (25mm if possible) then foil backed plasterboard and reskim. No venting required.

Or leave it as it is and add a layer of thermal plasterboard to the underside of the ceiling

It's no good ventilating a cold roof and ceiling, that's high risk for condensation in the roof void and on the ceiling. And just venting from the fascia is waste of time in any case, that's not ventilation.