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Condensation in loft space

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pickle9000:
Sounds like no vapour barrier. That is a plastic sheet between the studs and drywall.

akis:

--- Quote from: Bud on November 27, 2023, 08:05:57 pm ---Google for Rafter Vents. They are channels that create air gap between the roof underside and insulation, providing unrestricted air flow from eaves vents to ridge vents. Once you install rafter vents between rafters, you can put all insulation you want under them.

--- End quote ---

That is the first resolution we tried to the problem. We fitted sections of plastic tubing over rafters, under membrane, to allow passage of air sideways from rafter to rafter wherever we saw a "blockage". A blockage is interruption to the flow of air due to roof construction, eg roof windows, dormers, porches, noggins, insulation too deep etc

The second step was to drill holes at the top ridge to allow passage of air from and to the the two main sides of the roof.

The third step was to remove the main ridge tiles, cut the section of membrane over lapping the ridge, and repositioning the ridge tiles. The ridge has a dry ridge system (breathable fabric type thing).

None of these measures worked. None oif these measures would be needed if Tyvek membrane did what it's supposed to do.

I forgot to mention this condensation has so far only happened on the north side of the roof, never the south.


--- Quote from: pickle9000 on November 27, 2023, 08:24:47 pm ---Sounds like no vapour barrier. That is a plastic sheet between the studs and drywall.

--- End quote ---

There is no vapour barrier in the plans, and I have never seen one used so far. The typical UK cold roof is made of : roof tiles, over battens, over membrane, over rafters, insulation in between and/or under rafters, (insulated) ceiling boards screwed into the rafters.

Which means condensing vapour under the membrane will run uninhibited down the rafters / into your ceilings.

Neutrion:
Forgot to mention: I meant the single  "metal box"  units to be installed somwhere with pipes, not the pseudo ones installed on the walls because those make almost no difference. If there is a bathroom it makes sense to have the extraction point there.

Even better is to have the ventillation system built into the fume extractor in the kitchen because if you use the fume extractor you need a lot of incoming air anyway, and you don't want to cool down the house completely.

Bud:

--- Quote from: akis on November 27, 2023, 08:41:50 pm ---
--- Quote from: Bud on November 27, 2023, 08:05:57 pm ---Google for Rafter Vents. They are channels that create air gap between the roof underside and insulation, providing unrestricted air flow from eaves vents to ridge vents. Once you install rafter vents between rafters, you can put all insulation you want under them.

--- End quote ---

That is the first resolution we tried to the problem. We fitted sections of plastic tubing over rafters, under membrane, to allow passage of air sideways from rafter to rafter wherever we saw a "blockage". A blockage is interruption to the flow of air due to roof construction, eg roof windows, dormers, porches, noggins, insulation too deep etc

--- End quote ---
Sorry what you describe is totally different. Rafter vents are installed along rafters, not across.

IanB:

--- Quote from: akis on November 27, 2023, 08:41:50 pm ---There is no vapour barrier in the plans, and I have never seen one used so far. The typical UK cold roof is made of : roof tiles, over battens, over membrane, over rafters, insulation in between and/or under rafters, (insulated) ceiling boards screwed into the rafters.

Which means condensing vapour under the membrane will run uninhibited down the rafters / into your ceilings.

--- End quote ---

The vapor barrier would go between the ceiling boards and the rafters. This is what stops moisture from inside the house diffusing through the ceiling boards and insulation and reaching the outer membrane where it can condense. This is what tggzzz meant in the post quoted below:


--- Quote from: tggzzz on November 27, 2023, 06:52:35 pm ---If it only condenses where there is no insulation at the moment, that isn't a problem per se.

When insulation is installed, there will be a thermal gradient across the insulation. If the air is humid, and the dew point is inside the insulation, then any water from the house will condense inside the insulation. Oops.
 
To prevent that, there must be a vapour barrier on the hot side of the insulation. Amateurs often put it one the cold side, which is disatrous.

--- End quote ---

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