How to ventilate a warm loft

19 Apr 2018
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United Kingdom
Hi, we are hoping to insulate the rafters of our loft and I am looking to sense check some facts a few of the builders have told us.

Our (currently cold) loft is a pitched roof, set on an A frame with purloins. Built about 1920/1930ish. It has ventilation in a few places at the eaves, but not at the top of the pitch on the ridge where you may expect. Ventilation currently seems to be fine as we have no condensation and you can feel the breeze coming in from the eaves in the loft on certain days.

We have already insulated between the joists (170mm mineral wool in a 190mm cavity, so not compressed) and placed boards over the top. We want to insulate the rafters and place 9mm plasterboard over them, creating a warm loft with a cleaner appearance. It would still only be used for storage.

My questions:

- If we insulate the rafters, I know we need to keep enough ventilation to avoid condensation. We have been told that mineral wool is breathable and therefore the 50mm gap recommended behind celotex isn't required. Is this correct?

- Some builders have said a vapour control layer (usually placed between the insulation and the plasterboard) is not required. Is this correct? I thought this was necessary to help eliminate condensation on the rafters.

- As there is no ventilation on the ridge, if we board over the rafters then airflow will only be in the runs where the eaves have ventilation up between the rafters. Should we place vents internally on the plasterboard where these opening are? I don't see how to circulate the air otherwise.

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a warm pitched roof does not require to be ventilated.
Your not really creating a true warm roof.
Condensation is formed when warm moist air rises and condenses into a liquid on contact with the colder surfaces above the insulation. The idea behind a vapour control layer is to install it on the room side of the insulation so it blocks the passage of warm moist air entering the structure.
Putting vent inside on the plasterboard will allow moist air to enter the void and condense.

You need high level vents on the outside at the ridge to create an airflow between the underside of the roof and top of insulation.

A vapour control layer beneath the plasterboard is a must.
Thanks @alastairreid. In regards to my first questoin, we have been told that mineral wool is breathable and therefore the usual 50mm gap recommended behind celotex isn't required. Is this correct?
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@datarebal So if I asked them to build a warm roof construction, they would fill the space between the rafters with mineral wool, cover with a VCL and then plasterboard on top, without any need for ventilation? The current vents to the eaves would be blocked by the insulation and VCL?
No, that's not a warm roof.
You are effectively creating a cold roof with lots of insulation .
You have vents at eaves level then vent the ridge too. But it's not a warm roof.

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