Fitting a upvc window in single skin block

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Hey all,

I’m converting a single skin detached brick built garage into a gym / insulated workshop.

current plan is to brick up the car entrance with blocks (finish will be roughcast) - forming a window opening to a predefined size as I already have the upvc window and cill.

I had planned to install the window straight onto the block work - flush with the inside edge of the blocks (which are 100mm) - which would leave c40mm external reveal. The cill I have is 150mm - so would still protrude around 25-30mm past the finished roughcast when done.

I had then intended to build an internal stud wall around the new window creating the internal reveal (if that’s the right terminology) and an internal cill / window board.

however a couple of builders I’ve spoken to say the stud walls need to be built first and the window should be excesses further - effectively fitted into the timber studs.

is there a right / wrong way or is this just preference?

cheers!
 
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Not a window expert, but my windows are fitter to the external leaf leaving a small 25mm reveal.
Never had a problem.
 
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Not a window expert, but my windows are fitter to the external leaf leaving a small 25mm reveal.
Never had a problem.

cheers!

my other consideration is the cill I have is 150mm which I think is ok if the window is fitted to the ‘external’ (in fact only ) leaf.
But might me too short if the window is recessed further.
 
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You can set the frame forward if you deal with the potential damp and thermal bridge at the reveal.
 
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You can set the frame forward if you deal with the potential damp and thermal bridge at the reveal.

thanks Woody.

How would you typically do that?
Bellcast bead above the window to divert rain out from the wall above?
 
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By set forward, this means set forward of the back of the external leaf.

Normally frames are fitted as your two builders mentioned so that the frame covers the gap between the external and internal leaves, and this covers the vertical DPC and prevents the cold part of the external wall being in the warm room side.

So if the frame is set forward, you need to ensure the DPC is correct placed and insulate the jamb to remove the cold surface of the external leaf.

A bellcast to the render is no related to this, but is needed to prevent rain running down the wall and along the head of the opening.
 
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Thanks again woody that makes sense.

given my internal leaf will be a cls stud wall, in a non building regs garage ‘conversion’ and that my cill is only 150mm deep.

What’s the minimum cavity (between external and internal leafs) I can get away with?

Would 25mm be ok?
 
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10mm wil do.

Ensure you use a breather membrane on the outside of the frame and vapour check membrane on the inside face with no gaps and properly jointed.
 
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10mm wil do.

Ensure you use a breather membrane on the outside of the frame and vapour check membrane on the inside face with no gaps and properly jointed.

thank you so much, that’s super helpful
 
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