Flat roof: ambient roof system with ventilation on the warm side?

[thesheep]

I've now had a couple of builders suggest to me the idea of an 'ambient' roof system for a flat roof, where you ventilate on the warm side. The idea apparently is that you have your insulation tight underneath the deck, then a gap underneath that, then plasterboard ceiling. This cavity above the plasterboard is then ventilated to the outside.

I can't really get my head around this: why would you ventilate the warm side, isn't that just going to lose heat? I'm also struggling to find any resources or building guidance that suggests this as an option. Everything just talks about either warm or cold roofs.

From what I can work out, I think my builder likes this solution because it takes up less height than a warm roof, but doesn't require as much ventilation as a cold roof.

Has anyone else come across this and does it work?

 

[Swwils]

So the only barrier between the room and outside air is the plasterboard.

Not a good idea!

 

[^woody^]

Never heard of this. Im not sure if it's blarney or baloney.

Ca they reference any standard or certification for this?

You either have warm deck (insulated, unventilated) or cold deck (uninsulated ventilated).

 

[datarebal]

Blah blah

 

[thesheep]

Never heard of this. Im not sure if it's blarney or baloney.
Can they reference any standard or certification for this?
You either have warm deck (insulated, unventilated) or cold deck (uninsulated ventilated).

No standards. Doesn't seem to make a lot of sense to me as I said. But just struck me as strange that 2 separate builders mentioned the same idea. One said he had been building roofs like that for years...

 

[Swwils]

No standards. Doesn't seem to make a lot of sense to me as I said. But just struck me as strange that 2 separate builders mentioned the same idea. One said he had been building roofs like that for years...

Low occupancy spaces can get away with alot.

 

[^woody^]

One said he had been building roofs like that for years...

Every gypsy tarmacer could say exactly the same thing about their drives.

 

[datarebal]

What do you think the insulation is doing ?

 

[Swwils]

What do you think the insulation is doing ?

In this proposal it is just insulating outside air from outside air.

 

[thesheep]

In this proposal it is just insulating outside air from outside air.

He said to me the idea is that you have ventilation of the void above the plasterboard but not a lot of ventilation. So I am guess more of a trickle vent kind of arrangement, certainly less than you'd want in a cold roof.

As I said, it doesn't really make sense to me as a solution. I'm mainly curious about whether other people have come across this particular design, given that 2 separate builders mentioned it to me, and one even gave it a name 'ambient' roof system. Though I can't find anything on Google!

 

[Swwils]

It sounds like a hybrid roof with the important bits left out!

 

[^woody^]

It sounds like a hybrid roof with the important bits left out!

With a detrimental void added in more like!

It's like you posted earlier, plasterboard between the room and outside air is not a good idea .... and nor would be using those builders I'd add.

 

[noseall]

Warm deck flatties are the most practical solution, albeit a wee bit ugly from the outside. Flat roofs rarely contest beauty pageants, so no dramas.

Warm deck every time where practical.