airtight membrane and spotlights?

ttowncat

I am going to be putting in airtight membrane on the ceiling joists of my bungalow with 50mm insulated plasterboard underneath, cold roof. In relation to the electrician's part, is there any way that I could avoid using batons and counter batons between the membrane and the plasterboard? For instance, would the following sequence work:

1)get walls chased
2)install windows
3)put airtight membrane around windows and to the underside of the joists. I plan not to tape the membrane to the block work at ceiling level just either side of where a chase comes down the wall so as to leave room for pushing wires down, then tape it.
4) electrical 1st fix whereby the electrician leaves the wires (lights,smoke,carbon alarms) down through small holes in the membrane. (Do attic wires run along over, under or drilled through the joists?)
5) I screw my insulated plasterboard to the ceiling and then carefully drill appropriate sized holes to meet the wiring hole for the light wire.
6) internal plastering happens.
7) Regarding Spotlights- Once the electrician has them installed, Do I come along and tape an airtight hood or box over them onto the membrane? I then let the attic insulation (fibre wool) come up alongside these hoods but not covering them?

Any suggestions or corrections to this sequence of events? Thanks.

bfclancy

ttowncat wrote: »

I am going to be putting in airtight membrane on the ceiling joists of my bungalow with 50mm insulated plasterboard underneath, cold roof. In relation to the electrician's part, is there any way that I could avoid using batons and counter batons between the membrane and the plasterboard? For instance, would the following sequence work:

1)get walls chased
2)install windows
3)put airtight membrane around windows and to the underside of the joists. I plan not to tape the membrane to the block work at ceiling level just either side of where a chase comes down the wall so as to leave room for pushing wires down, then tape it.
4) electrical 1st fix whereby the electrician leaves the wires (lights,smoke,carbon alarms) down through small holes in the membrane. (Do attic wires run along over, under or drilled through the joists?)
5) I screw my insulated plasterboard to the ceiling and then carefully drill appropriate sized holes to meet the wiring hole for the light wire.
6) internal plastering happens.
7) Regarding Spotlights- Once the electrician has them installed, Do I come along and tape an airtight hood or box over them onto the membrane? I then let the attic insulation (fibre wool) come up alongside these hoods but not covering them?

Any suggestions or corrections to this sequence of events? Thanks.

Run wires on top of joists easier put insulation between joists then. Why insulated plasterboard on ceiling. Can you do all your insulating in the attic space. Yeah there is domes you can fit over downlighters. I'd imagine no issue to insulate over. Led downlighters produce little heat so no issues there and the domes will.ensure no contact anyway

ttowncat

bfclancy wrote: »

Why insulated plasterboard on ceiling. Can you do all your insulating in the attic space.

I'm told the best way to insulate when dealing with those membranes is one third inside and two thirds insulation above it on the outside.

bfclancy

ttowncat wrote: »

I'm told the best way to insulate when dealing with those membranes is one third inside and two thirds insulation above it on the outside.

is that because of potential for condensation, i guess using the insulated plasterboard gives a nice continuous layer too, makes sense

ttowncat

bfclancy wrote: »

is that because of potential for condensation, i guess using the insulated plasterboard gives a nice continuous layer too, makes sense

Think so. The membrane I'm looking at says it can let water vapour up but not back down the way.

BryanF

ttowncat wrote: »

I'm told the best way to insulate when dealing with those membranes is one third inside and two thirds insulation above it on the outside.

Best way to insulate is to keep all insulation outside vapour barrier/ air-tightness membrane.

Calahonda52

BryanF wrote: »

Best way to insulate is to keep all insulation outside vapour barrier/ air-tightness membrane.

Outside which way?

Calahonda52

very messy, time consuming and difficult to get right.
IMO
If you want some insulation across the ceiling joist for thermal bridging then maybe fit 25mm.
put the rest above
Then airtight layer
Batten/counter batten, making the combined depth enough to house the d/lighters, ensuring locations that avoid the batten work.
Lay the wires for the lights, with one or two holes in the A/T layer for the cables
slab/drill holes for lights,
skim

Vote4Napoleon

I went with the baton and counter baton under the airtight membrane with 8" of metac between joists, insulated slab under the batons. Only now I had a thought that the fact my bedroom ceiling is laced with recessed lights does this mean the insulated slab is redundant?

Calahonda52

Vote4Napoleon wrote: »

I went with the baton and counter baton under the airtight membrane with 8" of metac between joists, insulated slab under the batons. Only now I had a thought that the fact my bedroom ceiling is laced with recessed lights does this mean the insulated slab is redundant?

This is hard to follow: what is under what?
Can you list out the layers
skim
plasterboard
etc.
.
etc
please

Vote4Napoleon

Apologies so its:
Skim
Insulated board
Baton
Counter baton
Airtight membrane up under 9" joists
8"metac between joists above airtight membrane

BryanF

Just use standard plasterboard

Wartburg

ttowncat wrote: »

I plan not to tape the membrane to the block work at ceiling level just either side of where a chase comes down the wall so as to leave room for pushing wires down, then tape it.

That´s not the best idea. The membrane should be continuously bonded to the adjoining walls, to prevent diffusion and convection. I always recommend air tight sealant for this important junction, because the sealant can level the movement between the timber structure (roof) and the blocks (wall) much better than any tape. It´s essential to leave a 2-3mm thick bead of sealant, once the membrane has been (gently) pushed into the sealant.
Don´t be mean with important details because they will protect your building structure on a long term.

Vote4Napoleon

BryanF wrote:

Just use standard plasterboard

Thanks for the reply, kinda came to the same conclusion myself a moment ago. Bit late for me, hopefully some1 else learns from my mistake/waste of money