Insulate warm side of airtight barrier.

cwboy

Hi,

I have just given the go ahead to insulate and make my timber house airtight to a company. Insulation being 140 fibre glass to the external walls. I plan to batten out on top of the airtight membrane for a service cavity. There will be 37.5 plasterboard going on the timber battens on external walls.

My question is this: should i insulate between the batttens with fibre glass/kingspan or leave it as a cavity?


Thanks
cw

Slig

Your vapour barrier should always be on the warm side of the insulation. I would personally reccomend if possible:
cross battening (horizontal)
insulate between
vapour check
batten
then service channel
plasterboard

cwboy

Thanks for the reply.

It is the insullated plasterboard that I have it came with the timber frame so this will automatically be on the warm side.

Carlow52

cwboy wrote: »

Hi,

I have just given the go ahead to insulate and make my timber house airtight to a company. Insulation being 140 fibre glass to the external walls. I plan to batten out on top of the airtight membrane for a service cavity. There will be 37.5 plasterboard going on the timber battens on external walls.

My question is this: should i insulate between the batttens with fibre glass/kingspan or leave it as a cavity?


Thanks
cw

Can u tell us a bit more how they intend to achieve airtightness and how will it be measured?

did u have a AT test done now, before the work starts?

cwboy

They are going to insulate first and then using siga membrane and tapes to make it airtight.

no airtightness test done yet. i dont think there is any point as still a few gaps around the windows to be filled and fascia and soffit still to go on.

plan to get airtightness test done after fixing of siga membrane

RKQ

Insulated plasterboard over the membrane will incease insulation values but won't have an effect on the airtightness.Its better if the membrane covers all the insulation.

The idea of airtightness is to basically to prevent air leakage, this helps increase insulation values as the air leakage between gaps in insulation is stopped or at least slowed.
So the insulation can achieve its ideal test condition rating. Putting insulation over the membrane doesn't help or lessen this process.

Carlow52

RKQ wrote: »

Insulated plasterboard over the membrane will incease insulation values but won't have an effect on the airtightness.Its better if the membrane covers all the insulation.

The idea of airtightness is to basically to prevent air leakage, this helps increase insulation values as the air leakage between gaps in insulation is stopped or at least slowed.
So the insulation can achieve its ideal test condition rating. Putting insulation over the membrane doesn't help or lessen this process.

Best practice is to always have the vapour membrane on the warm side of the insulation.
The reason for this is that there is a temperature gradient from room temp to outside temp through the (insulated) wall.
Where the dew point is reached along this gradient determines where any moisture laden water vapour will condense.

RKQ

Carlow52 wrote: »

Best practice is to always have the vapour membrane on the warm side of the insulation.

True.... but 25mm to 30mm with plasterboard over a airtight membrane won't be a problem as such. Best practise is "best to do" if possible.

Nobody is denying "Best Practice" but its not applicable in this case, as stated by the OP above. So we must make the best of it and advise the OP accordingly, by answering the question.

The vapour will still be prevented from entering the main wall structure by the airtight membrane. The extra insulation will also prevent vapour, assuming it is foil backed. The weakest points will be the joint in the boards - which will be minimal. Omitting the insulation will not help the vapour barrier and it will lessen the insulation value of the wall.

Carlow52

RKQ wrote: »

True.... but 25mm to 30mm with plasterboard over a airtight membrane won't be a problem as such. Best practise is "best to do" if possible.

Nobody is denying "Best Practice" but its not applicable in this case, as stated by the OP above. So we must make the best of it and advise the OP accordingly, by answering the question.

The vapour will still be prevented from entering the main wall structure by the airtight membrane. The extra insulation will also prevent vapour, assuming it is foil backed. The weakest points will be the joint in the boards - which will be minimal. Omitting the insulation will not help the vapour barrier and it will lessen the insulation value of the wall.

RKQ, no argument with you, I wrote what I wrote just in case someone has a peep at this thread and then decides to put the membrane behind sheeps wool or fibreglass or compressed paper etc as it is not crystal clear from the Op's original post what exactly is in train here.:)
Keep well.

cwboy

Thanks for replies lads. The opinions and advice here are first class.

The plan is to put 140 mm rock wool between studs then airtight membrane then batten for electrical work then insulated plasterboard. As the plasterboard is bought and sitting on site I dont really have much option but to put it on. I did see at one of the self build shows an A rated (i think) timber frame with rockwool insulation between the battens and thought it was a good way of getting extra insulation into the wall.

I know a kingspan type would be the best u value between the battens but would rockwool be better/safer? If using ks i would have 25mm between battens and 25mm on the plasterboard.

RKQ

Carlow52 wrote: »

RKQ, no argument with you, .:)
Keep well.

No argument Carlow52.... just making the best of it for Op:)

Carlow52

cwboy wrote: »

Thanks for replies lads. The opinions and advice here are first class.

The plan is to put 140 mm rock wool between studs then airtight membrane then batten for electrical work then insulated plasterboard. As the plasterboard is bought and sitting on site I dont really have much option but to put it on. I did see at one of the self build shows an A rated (i think) timber frame with rockwool insulation between the battens and thought it was a good way of getting extra insulation into the wall.

I know a kingspan type would be the best u value between the battens but would rockwool be better/safer? If using ks i would have 25mm between battens and 25mm on the plasterboard.

Due to the services in the service duct, it may not be possible to make it 100% airtight without due care and attention to detail, If it is not airtight then the insulated board is not working 100%.

If this was my gig and the money on the IB was already spent I would just grin and bear it, put it directly on the wall and create the servives duct outside with a layer of plain PB, easier to work with all round.

The services duct will need to be 'fire-breaked' with a continuous batten all the way around top and bottom: see Homebond manual: if u dont have one, get one best 50 euro u ever spent

williamsrk

cwboy wrote: »

They are going to insulate first and then using siga membrane and tapes to make it airtight.

no airtightness test done yet. i dont think there is any point as still a few gaps around the windows to be filled and fascia and soffit still to go on.

plan to get airtightness test done after fixing of siga membrane

how much did the sealing and tapeing cost per sq meter