Cavity wall requirements around windows

[Adamantium]

Hi all,

 

Hopefully just a quick one here, but don't want to ask the officials yet.

 

I have blockwork walls with a 150mm cavity. I am having sash windows fitted from the inside that butt up against the outer leaf whose opening is obviously slightly smaller.

 

My windows are 126mm in profile so obviously the cavity extends beyond their depth.

 

I believe 150mm cavity closers may be an option, but the architects have designed in a returning block that closes the cavity immediately adjacent the window down to 50mm.

 

The architects are from NI, the builders on the mainland are concerned that this won't meet building regs requirement. I don't know if buildings regs are different in NI.

 

Just want to know if anyone has seen a reduced cavity thickness around the windows for this purposes before?

 

thanks in advance.

 

Adam

[bassanclan]

Back in the day they bridged the cavity completely with bricks. It sounds like bad detailing from the architect. I would just leave the 150 cavity and specify cleats for fixing the windows

[ADLIan]

Is this new build in England? Probably OK returning the blockwork but leave space for a 50mm (min) insulated cavity closer - not ideal but will meet BR minimum requirement. Better to use 150mm insulated cavity closer and use bracket/cleat to fix window frame - may be much simpler to install too, saves a lot of time cutting and laying blocks around each reveal. Cavity closer manufacturer may offer values for linear thermal transmittance - enhanced values can be used in SAP which can be advantageous.

[Declan52]

That's just how it's done in NI.  You put a length of dpc up the reveal on the inside of the outer skin.  Then you will have a strip of pir  insulation cut at a min depth of 50mm,75mm will be easier  to work with. Then the block work is returned.  You will need split brick or a block cut with a saw to 50mm if you go for 100mm insulation to fill the gap. The red arrow will be the  split brick. With 75mm insulation you can just use a normal concrete brick. 

The cavity in the pic is 100mm.

 

[Adamantium]

Thanks for the replies guys.

 

i just wondered if regs in NI allowed this but didn’t in England.

[Adamantium]

Builders decided to stick with architects plans and use NI design with a return. Cavity seems to have been dropped from 150 to 50 (might be 100 - I can’t tell). I’m happier with this as it gives the windows direct purchase on the block.

 

Building control have confirmed they are happy.

[Declan52]

Looks  100mm. 

Why are they jointing the block work mortar??? 

What are you putting in the cavity for insulation??? 

[Adamantium]

Celotex I guess.

 

why wouldn’t they joint the block work mortar?

[Declan52]

Easier to use a soft brush.  Gives the motar a rougher joint so wet plaster will key better to it. 

The back of the cavity wall will need scraped to take of the rough bits of motar or the insulation won't sit tight to the blocks. 

[Adamantium]

There’s no wet plaster going on any surface on that picture.

 

theres another layer of block work going on the inside which will be for and dabbed.

 

as for the insulation, I gues they are making life harder for themselves but that’s their call. They seem to know what they are doing.