Insulation in studs and insulated plasterboard

[revelation]

Hi all

 

I have a few questions regards to insulation.  

 

1. We have had a loft conversation, part of which is timber stud work filled with 90mm kingspan, we were looking to also add some insulated plasterboard ontop, do we need any kind of vapour barrier in between?  The ceiling of the dormer is a cold flat roof, we have 120mm kingspan between the rafters, what's the thinest insulation that I could use to get it to regs, we already only have 2300mm head space so don't want to lose too much more.

 

2.  We are having some insulation put on the party wall for sound and heat.  We are using Acoustic Rockwool for this, again would we need to use any kind of vapour barrier before boarding?

 

If we need to add vapour barriers, then does that mean on those walls we can't have any lighting or sockets as that may mean piercing the vapour barrier.

 

Thanks in advance. Regards

 

[nod]

It’s normal to use 15 mil duplex plasterboard (foil backed)

That will give you a vapor barrier without loosing headroom 

[PeterW]

Assuming you are in England and this is an extension then the ceiling of the dormer is borderline being at regs for a cold roof (assuming it as a ventilated space above it)

 

I would simplify this whole thing and use 25x50 battens at 400mm centres and infill between the battens with 25mm PIR, taping the joints and the battens over with foil tape. If you do this on all walls and ceiling you will end up with a very good airtight continuous vapour barrier, and then it’s a case of putting standard board over the top and skim. 
 

Sockets aren’t an issue with vapour barriers - if you do as above then you create a natural service void for the back boxes anyway in the insulation. 

[Onoff]

49 minutes ago, PeterW said:

Assuming you are in England and this is an extension then the ceiling of the dormer is borderline being at regs for a cold roof (assuming it as a ventilated space above it)

 

I would simplify this whole thing and use 25x50 battens at 400mm centres and infill between the battens with 25mm PIR, taping the joints and the battens over with foil tape. If you do this on all walls and ceiling you will end up with a very good airtight continuous vapour barrier, and then it’s a case of putting standard board over the top and skim. 
 

Sockets aren’t an issue with vapour barriers - if you do as above then you create a natural service void for the back boxes anyway in the insulation. 

 

So depending on the plasterboard thickness used, 12.5 / 15 you end up with nom. 37.5 to 40mm from face of new plasterboard to foil face of new PIR in between the joists (plus skim coat if used). You of course lose that 37.5 to 40mm off the room height/width.

 

Plenty for 35mm deep back boxes. Ditto on the ceiling where you can fit low profile LEDs in the same depth cavity.

 

With the sockets, would you take the time to break the vertical battens leaving a well taped gap to allow horizontal wiring between (sockets)? 

 

(Planning similar here for the lounge / diner knock through).

[ProDave]

I seem to be in the minority of electricians that routinely runs cables horizontally around the room at socket height.  As long as you have at least 1 socket on each wall you can run the cable all the way round at socket height meaning if you want to add another socket later at any point, the cable is there.

 

I even had a stand up finger wagging argument once with  a joiner working on the job who told me I could not do it like that and the cable must only come up or down to a socket.

 

The main thing to get right on a job like this is the detail.  As the back of the insulation will be open to a cold space, you need to be sure the insulation cannot fall out the back, and be sure there are no gaps in the insulation to let cold air in.  It is very disappointing even on a new build today to find a howling blast of cold air come out of a back box when you remove a switch or a socket due to poor detailing.

[Onoff]

16 minutes ago, ProDave said:

I seem to be in the minority of electricians that routinely runs cables horizontally around the room at socket height.  As long as you have at least 1 socket on each wall you can run the cable all the way round at socket height meaning if you want to add another socket later at any point, the cable is there.

 

I even had a stand up finger wagging argument once with  a joiner working on the job who told me I could not do it like that and the cable must only come up or down to a socket.

 

The main thing to get right on a job like this is the detail.  As the back of the insulation will be open to a cold space, you need to be sure the insulation cannot fall out the back, and be sure there are no gaps in the insulation to let cold air in.  It is very disappointing even on a new build today to find a howling blast of cold air come out of a back box when you remove a switch or a socket due to poor detailing.

 

I've run cables horizontally like this since, well forever. Last time was though without the wisdom since learnt on here (& eBuild before) ref air tightness etc. Next time I'll do it all different!

 

 

Edited December 25, 2020 by Onoff

[Alexphd1]

@Prodave you seam to be one of the few tradesman that uses common sense. Our sparkle run drops even with a socket a meter along the wall from the other socket  and large eaves behind. When I questioned this..... its the way we do it! 

[ProDave]

4 minutes ago, Alexphd1 said:

@Prodave you seam to be one of the few tradesman that uses common sense. Our sparkle run drops even with a socket a meter along the wall from the other socket  and large eaves behind. When I questioned this..... its the way we do it! 

Yes he has used more cable, introduced more chances of cold air getting in, and made it hard to add extra sockets.

[revelation]

5 hours ago, PeterW said:

Assuming you are in England and this is an extension then the ceiling of the dormer is borderline being at regs for a cold roof (assuming it as a ventilated space above it)

 

I would simplify this whole thing and use 25x50 battens at 400mm centres and infill between the battens with 25mm PIR, taping the joints and the battens over with foil tape. If you do this on all walls and ceiling you will end up with a very good airtight continuous vapour barrier, and then it’s a case of putting standard board over the top and skim. 
 

Sockets aren’t an issue with vapour barriers - if you do as above then you create a natural service void for the back boxes anyway in the insulation. 

 

Yes I am in England, I was advised that the 120mm Kingspan may not be enough, if it is I am happy to fill any gaps tape it up and then plasterboard it.  If I need go go for insulation then you think that battens + 25mm PIR would be better than insulated plaster board? Or is that just because of the cost?

 

Regards

 

[PeterW]

40 minutes ago, revelation said:

 

Yes I am in England, I was advised that the 120mm Kingspan may not be enough, if it is I am happy to fill any gaps tape it up and then plasterboard it.  If I need go go for insulation then you think that battens + 25mm PIR would be better than insulated plaster board? Or is that just because of the cost?

 

Regards

 

Insulated plasterboard is a pig to work with and expensive. You can do all the other stuff yourself and get back boxes in the right places etc easily with 25mm insulation. 145mm PIR will get you to Regs and you can detail it much easier if you want to do it yourself. 

[revelation]

On 25/12/2020 at 15:06, PeterW said:

Insulated plasterboard is a pig to work with and expensive. You can do all the other stuff yourself and get back boxes in the right places etc easily with 25mm insulation. 145mm PIR will get you to Regs and you can detail it much easier if you want to do it yourself. 

 

I think thats the route I will go down down for the ceilings and the walls too, if its going to be cheaper and easier to do, without reducing effectiveness of the insulation then it makes sense.