Cold roofs without fresh air flow - how common is it?

[plockhart]

I have been looking at moving house so I’m doing a fair amount of viewings. Based in the north of Scotland, I keep stumbling upon cold roof houses built between the 60s-80s with no fresh air flow I can see. I have seen a few with cold pitched roofs with insulation at the joist level all the way to the eaves, and no soffit vents or roof tile vents. Then today I viewed a dormer with the eaves accessible with a hatch; where the 50mm rolls of insulation were between the rafters (resting on the room ceiling) but the floor of the hatch extended all the way to meet the roof with insulation on top. Bear in mind that all of these roofs have either plasterboard or some form of timber (not OSB) sheathing on top of the rafters so I cant see if there is a gap between the roof tile and the sheathing.

 

Am I missing something? I thought cold roof construction had to have fresh air flow? Evidently these properties were built like this from the start and haven’t had any mould/smells/damage. I appreciate it might be possible with a strict VCL on the warm side of the insulation but in the roofs I have been in I didn’t see any membrane or the like under the insulation. The houses are all older and so it's not like there is PIR backed plasterboard on the ceiling on inside of the room either to form some form of VCL.

 

Is it because the air flow is somewhere between the sheathing and the roof tile? But would that not mean the interrior part of the roof (beneath the sheathing) is still on the cold side and should be exhibiting condensation problems?

[Mr Punter]

I don't think you are missing anything.  The way buildings behave can be very complicated.  Building geometry, local climate, weather, occupancy lifestyle, heating, ventilation and insulation will all impact roof condensation.

[Iceverge]

Get up there with a penknife and poke at the timbers. 

 

The chances are that if it's survived so far it'll be fine long term. 

 

If you buy I'd upgrade the bathroom fans to dMev to ensure moisture gets extracted at source. 

 

If you upgrade attic insulation it'd be worth sealing all penetrations in the ceilings to,  to limit the amount of humid internal air making it's way into the attic as the he more insulation in there the colder the rafters and sheathing will be.