Windows sizing to fit openings

[akist]

I have received two slightly contradicting statements from window suppliers when I asked how tight the window frames fit into the window openings. One supplier said that they typically size them 10mm narrower and another supplier said that they size them down to the mm and there are practically no visible gaps to be filled in.

I lean towards the latter, ie make the window as close fit as practically possible, because a 5mm gap (5+5) appears too much, is this correct?

 

[Notch7]

I have received two slightly contradicting statements from window suppliers when I asked how tight the window frames fit into the window openings. One supplier said that they typically size them 10mm narrower and another supplier said that they size them down to the mm and there are practically no visible gaps to be filled in.

I lean towards the latter, ie make the window as close fit as practically possible, because a 5mm gap (5+5) appears too much, is this correct?

The answer is 'it depends'

Going into new openings with cavity closers -I would go for 10mm less overall or a tad less than that, when measuring at the narrowest between brickwork.

Height -Id go 10mm less

Also depends on window size -huge windows need slightly larger deductions.

 

[23vc]

Agree with notch, allow 10mm. You’ll generally have a bead of silicone which more than covers any gap. Right fiddle if you get it too tight and the opening is unlikely to be perfectly square.

 

[akist]

Does a 10mm (5mm + 5mm) mean that I will see a 5mm gap between frame and bricks?

Could you please explain "cavity closers" ? My builder also mentioned it but did not explain. I'd be grateful.

 

[Harry Bloomfield]

10mm smaller than it measures, it needs some space to be filled - you cannot effectively seal too narrow a gap and it needs to allow for the brickwork not being perfectly true.

 

[^woody^]

It depends on how good the builders are and how confident you are that they can build an opening dead square, with flush plumb jams and level cills and heads.

You may need to allow 100mm on your openings.

 

[Dereekoo]

I assume you mean 10mm Woody or is this just another one of your facetious comments, I never can tell. And is that 10 (or 100) overall or per gap.

 

[23vc]

I think he facetiously means 100.

 

[Notch7]

Does a 10mm (5mm + 5mm) mean that I will see a 5mm gap between frame and bricks?

Could you please explain "cavity closers" ? My builder also mentioned it but did not explain. I'd be grateful.

Modern construction of walls is a cavity which is left open at window and door reveals. Cavity closers are used to close the cavity -they have a lip of a couple of mms or so that sits over the brick ends -if you measure tight there wont be room to get past the lip.

 

[akist]

You may need to allow 100mm on your openings.

We will measure soon and I hope they have not messed it up too much.

 

[akist]

Cavity closers are used to close the cavity

Aha, I just checked at Travis Perkins, I see now what they look like and what you meant.

 

[Harry Bloomfield]

Aha, I just checked at Travis Perkins, I see now what they look like and what you meant.

I had only a vague idea about them, what did they do/ use before these please?

 

[stuart45]

You used to return the brickwork or blockwork on the inner skin to close the cavity and form the inner reveal. 6 inch DPC was tacked to the frame and went between to stop damp.

 

[Harry Bloomfield]

You used to return the brickwork or blockwork on the inner skin to close the cavity and form the inner reveal. 6 inch DPC was tacked to the frame and went between to stop damp.

Mmmm - I wondered because my house is a mid 1950's, DG'ed with CW, yet I still get a slight draft when it is very windy, between the wooden window ledge at the base and the edge of the plaster. Shrinkage since it was all done, means there is a tiny gap around 0.5mm. Hardly worth bothering, but I was thinking to try to tease the ledge up and squirt some flexible sealer into the gap, then weight it down until it sets.

 

[stuart45]

The cavity wasn't always closed at the bottom. The window board covered the open cavity.