Window sill airtightness options

[cwr]

Hi folks

Windows are in and now debating how to tape them to the blockwork at the sills. There is to be a ply board fitted, then an aesthetic windowsill board at a later stage. The cavity closure is 100mm thick PIR, width 130mm from frame to inner block skin.

 

Options include:

1. Tape frame to ply, and other 3 sides of ply to blocks

2. Tape membrane to frames and to block work. Need to bring down inside of inner wall. Would need to penetrate for fitting the ply which would be fitted on top of membrane

3. Tape frame to pir, and PIR to blocks. Need to scratch coat top of blocks.

4. Wide tape (200mm) to bridge pir. Also need to scratch coat top of block's.

5. Something else...

 

 

 

 

 

 

[Olf]

I'd go with no 2 (cause that's what I did )

What is the function of ply here? Can you not fit the windowsill board on glue/foam directly on the membrane? Especially if you put some sealant between the membrane and the block work (as you should, only that will guarantee airtight interface between both), when you spread it (with roller) it bonds and you have effectively a rigid surface.

If you do penetrate (can't avoid it in many places) then no drama, as the fitting itself should seal the hole, if you're paranoid squirt airtight sealant in the hole before fitting comes in.

[tonyshouse]

I can’t see what the tape or membrane is doing. It is just as easy or difficult to fix ply to the window as it is to fit the window board to the window,

 

are we on wet plaster? 
 

I would have had a window board slot in the frame to mark it easy to fit, could glue and pin on a thin bit of wood or screw a steel 10mmx10mm square rod on abs sir windowboard or ply on that 

 

are reveals blockwork? 

[cwr]

Thanks for the replies. 

Yes wet plaster. I can hardly remember the full rational behind the ply board, probably to give a solid and airtight base for final boards, which allow me more options in the future. And allows tape on frame to be hidden by final board. Either way it's option 1. as builders have now fitted the boards, lucky enough I was veering towards that anyway. 

 

Next question is now the under the headers which have similar detail with the PIR closures. I'm thinking of a 200mm wide tape over the PIR then plasterboard then skimmed.

[Andehh]

Sorry for the layman's question, but where is the air passing through? It looks like the window is silicone to the brickwork already? 

 

Just as ill be in a similar situation in a couple of months, I'm keen to be educated! 

 

Thanks