Render over compriband

[Mr Blobby]

This week I am mostly worrying about reveals and windows.  I would ideally like to install our windows pressed against the outer leaf with compriband as the weather seal on the front face of the window frame.

 

Reading through the various threads on here about check reveals it seems that most have done this with a brick facade but we will have rendered blockwork.  Can I install the windows like this with a render facade?

 

I took my kid's markers (we called them felt-tip pens when I was a kid) to draw this masterpiece:

 

 

 

 

I have omitted air-tightness detail and foam for clarity. 

Rendering to the window frame is, I think,  standard practice here in NI.

 

So, questions are:

 

1.  Can I install windows as above? If not why not?

2. Assuming the block reveals are square and vertical, then what thickness compriband tape, the 3-7mm? 

3. Will the window be pushed back into the cavity about 5mm if a 3-7mm tape is used?

4. Can the compriband be rendered over?

5. If rendering over the compriband/gap to the window then should a plaster stop bead be installed on the window frame?

 

Have I missed anything?  Do I need to think about anything else?

Edited May 17, 2023 by Mr Blobby

[ProDave]

We did this (but with wood fibre board not block)  We used a render stop bead and that covered the compriband nicely.

 

I found it easier to fit the compriband first. it will start expanding and then squash it with the wood fibre board as it was fitted.  Make sure your brickies know to position the blocks so they squash the compriband again.

[Mr Blobby]

This has become a bit of a hot topic this week between myself, builder and architect.

 

The architect recommends the cavity closer to be supalux board but at a U value of 0.17 W/m K is not great as it will bridge the cavity.

Supalux does however carry an 'A' fire rating.

 

The alternative, bosig phontherm has a much lower u value at 0.08 and is marketed as a cavity closer.

 

https://www.ecologicalbuildingsystems.com/product/phonotherm-200#technical-details

 

... but phontherm is, AFAICS, not fire rated.

 

 

The builder would usually push some PIR into the cavity with a block return.  Great u-values but hardly fire-rated.

 

 

(and how would this be plastered over)

 

I guess this is, in part, a building regs question around fire-rating of cavity closers.  Do build regulations allow the use phontherm board / PIR to close the cavities?

 

 

Edited May 18, 2023 by Mr Blobby

[Dave Jones]

good question, never had a bco ask about cavity closers.

 

Ive used in last house and this the kingspan cooltherm closer but it doesnt have any info on  fire rating.

 

Guess the question is why are they selling it if it cant be used ?

 

https://www.kingspan.com/gb/en/products/cavity-closers/kooltherm-cavity-closer/?s=d