Point in air tight membrane on ceiling if walls dont have it?

Idbatterim

Hey lads, the walls have been insulated, no air tightness membrane, the ceiling still has to be slabbed, is there a point in using this membrane behind the polyisio slabs, if no other surfaces will have airtight membrane? Thanks!

Gumbo

Has the external walls been sand and cement rendered internally?

Idbatterim

thanks for the reply, it has, there is now insulated plasterboard over this and it has been skimmed... Its a period house, external walls a few foot thick... (limestone)

BryanF

In a old house, you either use breathable construction methods or ensure a continuos vapor barrier (doubling as air-tightness) throughout - What you have done is neither.

who advised you to use polyiso slabs in a period house?

Idbatterim

BryanF wrote: »

In a old house, you either use breathable construction methods or ensure a continuos vapor barrier (doubling as air-tightness) throughout - What you have done is neither.

who advised you to use polyiso slabs in a period house?

we havent ordered ceiling slabs yet, should we use another slab type? The slabs on the wall are the ones with the aeroboard on the back...

sorry I should clarify, it is a period house,but the only original thing left is the walls. The ceilings and roof are now hollowcore concrete slabs...

BryanF

to Answer your original question. The external walls should have continuous insulation with continuos vapor barrier (air-tightness) on the warm side.

The roof (& fist floor) level hollow core slabs should have been installed with air-tightness in mind. The roof soffit should have air tightnesss taped and sealed to the walls.

I’d be concerned at the stage a lot of decisions would seem to have already been made...

1.Did you have an arch on board before you installed the PC slabs?
2. Assuming an engineer is envolved, What is he certifying - Just part A?
3. What is the ground floor detailing - are you tanking?

Idbatterim

Has the external walls been sand and cement rendered internally?

yes when the original ruin was turned into a small hotel...

sorry, this is the board we had in mind for the ceiling, is that ok?
Kingspan Kooltherm K16 is a premium performance rigid phenolic insulated dry-lining plasterboard for plaster-dab / adhesive dry-lining.

no architect or engineer involved...

yeah see, we are just unpealing work done in the 70's, knocked a few non load bearing walls to increase the size of some rooms. Ripped down the ceilings so that electrical contractor could install new wiring and fire alarms. At this moment in time, the only area I can still access, would be the ceilings, as the walls have been insulated and skimmed...

Wartburg

Idbatterim wrote: »

thanks for the reply, it has, there is now insulated plasterboard over this and it has been skimmed... Its a period house, external walls a few foot thick... (limestone)

What you´ve done there is the perfect base to create interstitial condensation. Due to the insulated plasterboard on the inside of your exterior walls, the original limestone wall will not get heated anymore. Unfortunately the humidity will still pass the insulated slab. If she meets the original structure and the surface temperature there is below the dew point, you´ll create condensation. You won´t see this difficulty in the beginning because it´s behind the insulated slab.