more insulation on new extension

[robbie77]

Hi All

I am in the process of getting a new extension built. The plans specified block and brick with 100mm cavity.
I asked the builder to reuse some bricks I had - this has resulted in around a 75mm cavity as the bricks are bigger than metric ones and the founds had trench blocks to start with so a fixed width...Bummer
The new extension with have wet underfloor heating bifolds etc so I want it as well isulated as possible.
So before the electrical install part I was thinking of either:

1. Adding PIR insulation board (dot and dab or similar to the blockwork), tape joints and then plaster board over - this would be better vapour barrier and maybe cheaper than thermal plasterboard as no gap?

2. Add insulated plasterboard straight onto block work

The reason I mention the electrical part is I want something reasonably solid for the back boxes to go into and I don't want the electrical wire to be stuck in insulation etc so do I add battens to option 1 or 2 to create a service void and extra noggins for back boxes? I don't want to hack into insulation if I don't have to.
The roof is 7 x 2 rafters so I can add plenty to this but the walls are concerning me!
What would you do?

Thanks

 

[robbie77]

Probably worth mentioning that the floor is concrete sub floor with celotex, pipes and screed so plumbing and electric will have to drop down from ceiling .

 

[John D v2.0]

I assume the reclaimed bricks are the outer leaf. Assuming it's not built, which I'm guessing isn't a safe assumption now, youc can just put 50mm celotex in the cavity then you won't have to mess around, put it tight up against the inner leaf.
If not and it's full of mineral batts already and you want to keep the external surface then internal insulation it is. Either dot and dab or timber battens with wool or celotex between. You can take the cables along the wall or along the plasterboard as appropriate to help with the cable heating.

 

[robbie77]

I assume the reclaimed bricks are the outer leaf. Assuming it's not built, which I'm guessing isn't a safe assumption now, youc can just put 50mm celotex in the cavity then you won't have to mess around, put it tight up against the inner leaf.
If not and it's full of mineral batts already and you want to keep the external surface then internal insulation it is. Either dot and dab or timber battens with wool or celotex between. You can take the cables along the wall or along the plasterboard as appropriate to help with the cable heating.

Hi

Yes. It's built with reclaimed brick as outside leaf with 75mm mineral insulation in.
Run cables along the wall do you mean leave a gap at top or bottom of boards?
The sockets need to be cut out of insulation board then? Could i dab pir insulation on block then batten the pir board and plaster board over battens using gap as service void or is this just a bad idea?
Thanks

 

[John D v2.0]

Run cables along the wall do you mean leave a gap at top or bottom of boards?

No i just mean don't put the cable completely surrounded by insulation, make sure the wall is on one side and the insulation on the other. Or alternatively plasterboard on the other

The sockets need to be cut out of insulation board then?

Yes, always cut the minimum depth possible, i used dry lining boxes where necessary, although they are not as good as wall fixed ones they avoid thermal bridging

Could i dab pir insulation on block then batten the pir board and plaster board over battens using gap as service void or is this just a bad idea?

You could, you don't even need to dab them if you are mechanically fixing the battens through the insulation. It does depend if you want to lose the space throughout the whole wall just for a few cables. I was going to do that to create a service void for our ensuite to hold the shower valve but in the end i did insulation between studs and put the shower valve etc on the stud wall instead.

 

[tony1851]

You could have overhung the brick skin by 25mm (the Code of Practice for brickwork allows you to overhang up to 1/3 the width of the skin).
This would have given you the 100 cavity (too late now unfortunately )

 

[robbie77]

You could have overhung the brick skin by 25mm (the Code of Practice for brickwork allows you to overhang up to 1/3 the width of the skin).
This would have given you the 100 cavity (too late now unfortunately )

"Not a lot of many people know that "(said in Michael Cane voice) well my bricky didn't.
What about just putting insulation straight onto block and butting some 6 "x 2 "or something similar vertically on the areas I am mounting sockets etc so they get a solid fix and run cables down the timber and plaster board over? Would I need to tape the timber edge to the insulation for vapour barrier or will it be ok with it being a cavity wall? I just want the warmest / solid best job I can do.
Thanks

 

[robbie77]

So plan is...
Dot and dab 50mm pir board to block wall. Use 2x4 inch timbers every 400mm. Tape pir board edges to timber.
Attach 12.5mm plaster board onto battens with cut outs for sockets lining up with 2 x 4 studs (i think 2 x4 will be sufficient to get a galvanised back box secure).

Does that sound feasible? Or I dot and dab insulation board onto block with fixings and also attach plaster board to insulation with fixings also to block and just fit pb back boxes or same as above but run 2 x 4 timber just where sockets are and use galvanised back boxes?

 

[robbie77]

Ok. More reading if anyone else is having this issue would suggest that a bit of adhesive on some foil insulation board stuck straight to wall first - just to hold it ,taping all joins and THEN adding battens that drill through the board straight to wall (maybe hammer fixings). The battens can then be used to mount plaster board and also provide a run for cables and pipes and maybe add extra support for radiators etc.
This keeps whole wall insulated and vapour barrier largely in tact.

Thanks for all suggestions and help.
Have a good one everybody!!