Attic insulation - does it require layer of plastic sheeting underneath?

pokaface

Hi,

Has anyone installed plastic sheeting underneath their attic insulation to prevent moisture from the house coming up into the attic and condensing underneath the attic insulation?

Is there any recomendations of what's best and easiest to use for this?

Thanks

sydthebeat

is this a block build house and are you using a quilted attic insulation?

DOCARCH

It would be unusual to put a layer of plastic under the insulation.

Usually the ceilings of the house (under the insulation) would be foil-backed plasterboard for this reason. If you were really worried, and there is foil-backed plasterboard, you could consider applying a foil tape to all the joints to ensure the vapour control layer is complete.

Poor Uncle Tom

pokaface wrote: »

Hi,

Has anyone installed plastic sheeting underneath their attic insulation to prevent moisture from the house coming up into the attic and condensing underneath the attic insulation?

Is there any recomendations of what's best and easiest to use for this?

Thanks

This could be counter productive.

Assuming that you have foil backed plasterboard ceilings and timber ceiling joists, the plastic layer would be folded around the joists or laid above the joists with the insulation on top.

Any foilbacked ceiling is not fully protected at the sheet edges or at slab cut surfaces allowing a percentage of moisture through which would then be trapped with the timber joists under the plastic layer to condense and, over time, cause mould growth. Without the plastic layer any moisture vapours getting through to the attic would be dissapated with the attic ventilation, that's the theory part at least.

BryanF

pokaface wrote: »

Hi,

Has anyone installed plastic sheeting underneath their attic insulation to prevent moisture from the house coming up into the attic and condensing underneath the attic insulation?

Is there any recomendations of what's best and easiest to use for this?

Thanks

just clarify if this is a new build or an existing?

pokaface

It's an old (~ 40 years) block house, I laid down insulation between the joists when I moved in a couple of years ago but only head about the moisture theory this year.

BryanF

then take the advice above:)