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Condensation in loft space

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tggzzz:
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.

IanB:

--- Quote from: tggzzz on November 27, 2023, 06:52:35 pm ---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 ---

Yes. I think in general, that membranes on the outside behind roof tiles or wall cladding should block liquid water, but be permeable to vapour so it can get out. Membranes on the inside behind interior walls should be impermeable to prevent the problem mentioned above.

Neutrion:

--- 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 ---

Or if someone builds a new house at minimum a ventilation with heat recovery should be installed especially if you renew the heating anyway. It also spares a lot of energy.
If it is a small house one entry and one extracion point is enough if at the opposite end of the house.
And of course if you are installing a condensing boiler for heating make sure the minimum heating capacity where it can modulate down is as small as possible.  Ariston have some with 1-2 kW minimum capacity which is quiet rare.
But of course a water-air heat pump is a better thing especially in England with the "cold" 0 degrees in winter.

tggzzz:

--- Quote from: IanB on November 27, 2023, 07:10:13 pm ---
--- Quote from: tggzzz on November 27, 2023, 06:52:35 pm ---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 ---

Yes. I think in general, that membranes on the outside behind roof tiles or wall cladding should block liquid water, but be permeable to vapour so it can get out. Membranes on the inside behind interior walls should be impermeable to prevent the problem mentioned above.

--- End quote ---

Yes, exactly that.

Bud:
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.

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