iamlegend2008 Registered User
#1

Hi

Im about to to arrange and sand/cement skim to inside of externals walls (as recommended by airtightness contract to make walls airtight). After this, the electrician will soon start first fix on the walls and Im struggling with a key question regarding installing the 50mm insulated slabs (37+12 .55mm plasterboard) that will be done after the electricians fix.

Elsewhere in the forum (and in the part L regs), the suggestion is to use a 50mm by 25mm baton around the perimiter of the wall and openings and screw the boards to the batons (I assume here the baton will need to be every 2 feet across the wall to ensure a good fix on the board) (not mentioned in the regs). Im also wondering whether these batons need to be treated at all ?

This approach leaves an 25mm airgap between the board and the wall which I also understand means that the electrician will not have to chase for wires as he can use the 25mm gap as a service cavity. There is 60mm xtratherm in the cavity.

However, I spoke to the insulation people and they said it simply depends on whether you mechanically fix the boards or whether you simply glue them to the wall. However, as i see it If the glue method is used, then there is no air gap and this also means the walls need to be chased.

Im a complete novice managing direct labour and would like to do this properly.

Any help would be appreciated.

Cheers

RKQ Registered User
#2

So many options, so little time to explain them all... but heres a few pointers.
Assuming your house is exempt from the latest revision of the Building Regulations, July 08.

It depends on your insulation and manufactures recommendations-

  • You could fix the insulation & plasterboard to the blocks with plastic mushroom fixings. They are about 75mm long with 45mm diameter heads - predrill hole push in fixing and hammer slightly below surface of slab. Skim slab to hide fixings and joints in boards. Only disadvantage is fitting curtain poles, shelves, kitchen units etc - you need to put battens in place to catch all these items.
  • You could fit battens at perimiter of wall & opes, also at 400mm vertical centres. Install insulation between battens, install airtight membrane, plasterslab and skim. Ensure electrics / socket boxes are taped and sealed. Disadvantage - technically each batten is a gap in insulation.
  • You could fit battens at perimeter of wall & opes and install insulation with plasterboard on plaster dabs, spread on the wall.
  • IMO the easiest way would be install airtight membrane to blockwork, install electrics and blow test house. Ensure airtightness. Then use plastic fixings or dabs, (to manufactures specs) to fit insulation with plasterboard onto wall. Checking socket positions and support battens would be hard but not impossible.

I'm sure others will have lots of ideas. You can only do so much so ensure its practical, viable and cost effective. A warm economical to heat home is the end goal. Good luck!

Carlow52 Closed Account
#3

IMO, using the 25mm airgap as a cable run is fraught with airtightness issue.
[In passing, please be aware that the perimeter batten, or continuous perimeter dab is a fire regs requirement.]

Whiile the following has a bit more cost involved, it will result in a better job: { this is a variant of an approach first floated by Sinnerboy some time ago.]
put one layer of 25mm insulation on the timber battens and tape all joints and perimeters.

now put on a 37.5mm insulated slab, with joints staggered vis a vis the first layer, and have the electrics, in conduits, in this outer layer and dont break the airtight seal on the inner layer

sinnerboy Closed Account
#4

careful here . never sandwich cables between insulation layers . they over heat . breakers will keep tripping - either that or cables will cause a fire

Good spec otherwise carlow - but move the cables into the battens cavity - mount the back boxes on the inner insulation layer . and seal the cable entry into the box using in tumescent silicone

Carlow52 Closed Account
#5

sinnerboy said:
careful here . never sandwich cables between insulation layers . they over heat . breakers will keep tripping - either that or cables will cause a fire

Good spec otherwise carlow - but move the cables into the battens cavity - mount the back boxes on the inner insulation layer . and seal the cable entry into the box using in tumescent silicone


Carlow52 .....and have the electrics, in conduits, in this outer layer,
was intended to address the /heating fire issue: there was NEVER any suggestion of sandwiching....
emphasis added

NovaGSi Registered User
#6



  • You could fit battens at perimiter of wall & opes, also at 400mm vertical centres. Install insulation between battens, install airtight membrane, plasterslab and skim. Ensure electrics / socket boxes are taped and sealed. Disadvantage - technically each batten is a gap in insulation.


I was going to do this and use an insulation backed plaster board, this would insulate each batten.

Can you run electical cables between the blockwork and insulation? I wasn't going to leave a gap between the blockwork and insulation, I was going to fit it flush to the blocks.

Cheers,
Mick

RKQ Registered User
#7

NovaGSi said:

Can you run electical cables between the blockwork and insulation? I wasn't going to leave a gap between the blockwork and insulation, I was going to fit it flush to the blocks.


Yes you could insulate between the battens and then over the batters. I was suggesting insulating between batten & finishing with plasterboard but if you have the space then you could insulate over the battens too.

There will only be services between a certain number of battens, these can be carefully insulated to avoid overheating the cables.

Maybe consider the overall thickness of insulation required, then maybe put 25mm between battens and 25mm + slab over battens.

NovaGSi Registered User
#8

RKQ said:

There will only be services between a certain number of battens, these can be carefully insulated to avoid overheating the cables.



Would you cut out/chase the back of the first layer of insulation?


I am building a sunroom onto a bungalow. It is built using 9" cavity block.

My plan is to stud/batten out the room
use 50mm insulation between battens
and a 35mm insulated plaster board over this.
Can the 50mm insulation go tight to the wall?
or
Should I leave a 10mm gap for electrical cable?

The plaster board has a vapour barrier built in but I was going to put an extra vapour barrier over the batten/insulation course.
Can you have too many vapour barriers?

As for the ceiling, I've a vaulted ceiling/roof. I am putting 80mm insulation between the rafters and 50mm over.
I am then finishing in T+G wood - how can I fix the T+G to the insulation?

Cheers,
Mick

Carlow52 Closed Account
#9

As listed elsewhere on this thread and also on the wider forum, internally insulationed walls can generally be done in one go using a composite board pinnned/dabbed on wall or in a number of steps using timber battens to provide an airgap followed by 2 layers of (say 50mm) insulation with staggered, taped, joints and then the PB.

There are other varients but will limit this to these 2 for now.

I have specified the multilayered approach and the builder , post signing the contract wants to do the all-in-one solution using 112.5 composite board. 100 insulation and 12.5 PB

He is critical of the air gap as it will
"allow vermin and insects to accumulate, as well as create condensation issues, as well as create cold bridging with the timbers"

He would expect the walls to be chased for cables and fill the gaps between composite boards, as well as the socket/switch cut-outs with airtight, not expanding, foam.

We know the multi layer approach has a lot more work but in my view is easier to get right in the long run.

All comments/thoughts welcome.
Thanks

RKQ Registered User
#10

Carlow52 said:

He is critical of the air gap as it will
"allow vermin and insects to accumulate, as well as create condensation issues, as well as create cold bridging with the timbers"

He would expect the walls to be chased for cables and fill the gaps between composite boards, as well as the socket/switch cut-outs with airtight, not expanding, foam.


Interesting question. But there wouldn't be a continuous air gap behind the complete walls as the perimeter of each wall, per room, would have a fire batter or continuous layer of dab! Also the internal walls would be connected to the exterior walls! - breaking the gap.

Vermin and insects shouldn't be a problem, as they couldn't travel from room to room along the air gap.

I do question in part about the real need for the air gap at all. Some suggest fixing the insulation directly to the wall, chase our for cables enclosed in conduit. A wide deep generous size chase in insulation, to limit contact with conduit, avoid over-heating.

Carlow52 Closed Account
#11

RKQ: tks for views: One reason I prefer the multilayer approach is for doing the corners for external walls and around the reveals.

The former requires the PB to be cut back from the edge of the composite board to 'half and half' join the insulation to insulation to avoid a thermal bridge along the PB.
The latter requires the insulation to be cut back to 'rebate' the reveal composite board into the wall fixed board and get a board to board corner. Usually they dont do this and just put scrim over the reveal insulation

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Carlow52 Closed Account
#12

Was looking at the specs for the kooltherm k17 and kooltherm k18 products form the Irish based 'king' of insulation and it seems to distinguish between the above fixing methods on the basis that if the wall structure is a cavity wall type, then the dab on fixing with k17 is okay, where as if there is no cavity then the timber battens/metal furrings seem to be the way to go with the k18.
The k18 has an extra layer, (over and above what the k17 is made from) of what looks like aluminium mesh on the back, which the builder telss me does not 'stick very well' when he dabs it on

Maybe this different application is not new but it is the first time I have seen it.

RKQ Registered User
#13

Interesting.... have you discussed this with their Technical Dept?
Always best to go with manufacturers recommendation / spec, but I can see the Builders point.

I'd ring the manufacturer.

Carlow52 Closed Account
#14

No, I have not called anyone yet but has asked the subcontractor supplier of the material for his views on the different applications: he has verbally told builder that to stick on a solid wall is okay but as to putting it on paper

Carlow52 Closed Account
#15

RKQ said:
Interesting.... have you discussed this with their Technical Dept?
Always best to go with manufacturers recommendation / spec, but I can see the Builders point.

I'd ring the manufacturer.


Just to close this off, having been told yet again by the contractor that 'it did not matter' I have had a discussion with the Tech dept and this is the answer
  • K17 is for Plaster Dab / Adhesive Bonding where there is a cavity in the wall construction
  • K18 for mechanical fixing to pressure treated timber battens or metal furrings where there is no cavity: eg old stone walls, solid walls
While a hollow block wall could be regarded is a hybrid, the preferred material/method is the K18

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