Loft insulation and boarding without condensation problems.

15 Apr 2007
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United Kingdom
We have moved into a 1950's house with minimal insulation.

There is a conventional loft space between a hipped roof and plasterboard ceiling with 4” rafters. There is 4” mineral wool insulation between the rafters, which I want to increase by about another 6 inches, either using Celotex, Steico wood fibreboard or similar, or wool insulation roll, sheepwool or mineral, and then part board over for access and storage. If I use wool then I expect I will have to use plastic loft legs. I have also found insulated loft boards available from B & Q with 100mm insulation glued underneath, so we might try wool for the non accessible areas and insulated board for the storage areas.

I have read that wool insulation over the ceiling joists in a cold loft should be covered by a membrane to prevent the circulating cold air from wind etc from “wicking” off warm air at the top layer of the insulation.
What kind of membrane should be used for this, as I have also read conversely that a non permeable membrane will cause condensation on the underside as water vapour rises from the house through the ceiling?

Also, for the boarded sections, will the board itself act to prevent heat wicking away?
If I use Steico or Celotex, laid across the joists, should I put conventional loft board above that, to protect it, or below it to stop it being compressed where it stands on the joists?

For what it's worth, currently the eaves, with fascia and soffit boards, are fully enclosed with no ventilation, so the only loft ventilation is by any accidental gaps between soffit and brickwork, fascia board and felt, and at joints in roofing felt under the concrete tiles.
We have white coloured but probably aluminium framed double glazing, cavity walls but no c.w.i.
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You do not need any membrane over or under the insulation.

You do need significant ventilation for the entire loft space, via soffit vents and possibly ridge vents.

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