Airtightness question.

893bet

My airtightness layer is the ceiling hoist of my first floor/attic floor. There will then be a service void of 6 or seven inches then my plaster board ceiling.

Given my insulation is in the attic between and on top the ceiling joists, is the airtight membrane sufficient to support this from underneath? Or is a second layer of plaster board needed either above or below this membrane for support?

BarneyMc

I heard of someone who planned on nailing ply to the underside of the joists before attaching the membrane (to the underside of the ply).

893bet

BarneyMc wrote: »

I heard of someone who planned on nailing ply to the underside of the joists before attaching the membrane (to the underside of the ply).

It makes sense to do it. Something like 3.6mm ply would be a similar price to plaster board and would do the job. Just not sure if it's needed. 400 quid thin ply, nails for pasload, plus labour is another 7-800 quid.

Any experts care to comment?

glashanator

Airtightness membrane is extremely strong and robust stuff. And would easily carry plenty of insulation. It may sag a bit,but this is taken care of by some counter battoning. Its all you'll need.

BarneyMc

glashanator wrote: »

Airtightness membrane is extremely strong and robust stuff. And would easily carry plenty of insulation. It may sag a bit,but this is taken care of by some counter battoning. Its all you'll need.

Does the counter batoning go under the airtight membrane at cross angle to the joists?

glashanator

BarneyMc wrote: »

Does the counter batoning go under the airtight membrane at cross angle to the joists?

Indeed it does. and can also be filled with insulation if you wish. but no more than a third of the amount of insulation thats above membrane.

thesteve

glashanator wrote: »

but no more than a third of the amount of insulation thats above membrane.

Mind if I ask why no more than a third?

glashanator

thesteve wrote: »

Mind if I ask why no more than a third?

my understanding of it is, it is to do with the dew point of any vapour released. Too much insulation below compared to above is a bad thing as could lead to droplets forming on the inside of th membrane. 2 thirds above membrane ,1 third below is recommended.

mfceiling

Thin mesh can also be used - have seen it done before.

BarneyMc

Hope I can ask here but what thickness is uninsulated plasterboard?

hexosan

12.5mm

893bet

Forgot I started his thread. Thanks for the response. Will be using just the airtight layer and counter battons to hold the insulation in place with a 6 or 7 inch service void.

BarneyMc

893bet wrote: »

Forgot I started his thread. Thanks for the response. Will be using just the airtight layer and counter battons to hold the insulation in place with a 6 or 7 inch service void.

Seems like a very generous void?

893bet

BarneyMc wrote: »

Seems like a very generous void?

May make it smaller but will be running pipe work for MVHR there.

BarneyMc

893bet wrote: »

May make it smaller but will be running pipe work for MVHR there.

Thought about doing that also but think I will keep my hrv ducting in the attic. It will be insulated and will also place insulation over it. That will vastly reduce the void size. It means more penetrations of the membrane but these can be easily dealt with.

BarneyMc

893bet wrote: »

My airtightness layer is the ceiling hoist of my first floor/attic floor. There will then be a service void of 6 or seven inches then my plaster board ceiling.

Given my insulation is in the attic between and on top the ceiling joists, is the airtight membrane sufficient to support this from underneath? Or is a second layer of plaster board needed either above or below this membrane for support?

Can I just ask what the calculated U value is for your ceiling? I have a calculation of 0.11 using 200mm mineral wool between and 200mm above ceiling joists.

893bet

BarneyMc wrote: »

Can I just ask what the calculated U value is for your ceiling? I have a calculation of 0.11 using 200mm mineral wool between and 200mm above ceiling joists.

Not sure. Will have a look at deap sheet when I am on the pc.

893bet

BarneyMc wrote: »

Can I just ask what the calculated U value is for your ceiling? I have a calculation of 0.11 using 200mm mineral wool between and 200mm above ceiling joists.

0.11 is down for mine also.

100mm between joists and 300 mm on top according to spec. I will how ever be going 200mm in between and 200 on top which makes more sense.

Curious Geroge

893bet wrote: »

0.11 is down for mine also.

100mm between joists and 300 mm on top according to spec. I will how ever be going 200mm in between and 200 on top which makes more sense.

150 Metac with a 62.5mm board works out at 0.15
180 Metac with a 50mm board works out the same.

Not sure how standard insulation would work out less but interested to know. I'll be putting 200mm in between and 200 on top also (metac is used for the slopes on the roof)..

893bet

Curious Geroge wrote: »

150 Metac with a 62.5mm board works out at 0.15
180 Metac with a 50mm board works out the same.

Not sure how standard insulation would work out less but interested to know. I'll be putting 200mm in between and 200 on top also (metac is used for the slopes on the roof)..

No idea so cant help. Just going off the BER spec sheet and files.

Why would you be insulating the slopes on the roof if you are insulating on the joist also?

BarneyMc

BarneyMc wrote: »

Can I just ask what the calculated U value is for your ceiling? I have a calculation of 0.11 using 200mm mineral wool between and 200mm above ceiling joists.

Missed something. Need to also add 50mm PIR board under joists to achieve 0.11!