Log Cabin Condensation in roof space

stuartcoupe · Post by **stuartcoupe** » Tue Dec 29, 2009 9:51 pm

Hi,

Just putting up my new log cabin, 3m x5m. It is flat roofed, with a 300mm roof space into which I have put 200mm loft insulation the sort wrapped in plastic/foil.

The tongue and grooved ceiling is just about up, trapping the insulation, but where I have yet to seal it in, I have already noticed that the insulation plastic wrapping is getting moist where it faces the roof boards.

I'm thinking of making a few 'inspection' areas in the ceiling so that I can monitor it. I think that it might just be a case of trying to get more ventilation into the roof space space with added vents etc?

Any advice?
Any suggestions?

Deleted-user-3 · Post by **Deleted-user-3** » Thu Dec 31, 2009 4:23 pm

what you have here is a 'cold roof'...
a cold roof has the insulation at ceiling level, with a cold space above... consensation from the warm roof below will penetrate into the cold space borne on the warm air below which when it cools will give up its moisture whever it can on a cold surface as condensation...

a 'warm roof' has its insulation at rafter level i.e. tight up against the rafters...
any air gap above either needs to be ventilated properly, i.e. vents at both sides to encourage 'air flow' e.g. one at the ridge and one at the eaves...

ideally you should hav a 'vapour barrier' between your ceiling and your insulation / roofspace, this will stop the moisture pentetrating into the roofspace in the first place, although it will still need ventilation...

a good belt and braces solution to your problem would be thus -

remove youre tongue and groove (sorry)
then install 25mm of sheet insulation such as cellotex or kingspan to the bottom edge of the ceiling joists (the green cellular foam type with a foil covering) just tack them up with galvanised clout nails...
tape all the joints up, including the edges with gaffer tape or aluminium foil tape (this makes the insulation a vapour barrier in itself)
cross batten the insulation with 25 x 50 treated tile lath... (screw the lengths of timber through the cellotex into the ceiling joists at 90 degrees to them)
then refit youre cladding....

install 'soffit' vents between each roof joist either end of the structure (an 80mm round 'grille' that clips into an 80mm hole drilled with a holesaw)

a quick and easy solution besides the ventilation would be to install damp proof membrane as a vapour barrier between the ceiling joists and the tongue and groove...

simply put, wherever theres cold air, it needs ventilating, stopping moisture getting to that area requires a 'vapour barrier'...
combining the two keeps your roofspace bone dry and in a1 condition for years to come...

Deleted-user-3 · Post by **Deleted-user-3** » Thu Dec 31, 2009 4:42 pm

possible roof spec for a garage conversion, this is a 'warm' roof and would also have isowool loft insulation between the roof joists to acheive the required 'u' value, isowool being full depth of joist....

stuartcoupe · Post by **stuartcoupe** » Thu Dec 31, 2009 7:30 pm

Thank you: I'll get my head around this and set to.

Cheers.

Deleted-user-3 · Post by **Deleted-user-3** » Thu Dec 31, 2009 7:32 pm

http://www.planningportal.gov.uk/englan ... mentslist/
try here, you want the document governing moisture control and ventilation (think it might be approved document g but im not sure)