Window rough opening

[Russell griffiths]

Ok so I have read various topics all covering problems between the sizes of the openings and the actual window sizes. 

 

so can anybody tell me why I can't do this. 

If I have a window that is 1000mm x 1000mm why can I not make the opening 1030mm x1030mm so 15mm clearance on all 4 sides,

in this 15mm clearance I will have a sheet of 12mm rigid insulation cut down to the same width as the timber frame, that will be stuck to the rough opening prior to window instillation allowing for 3mm all around for final sealing. 

 

In in the event of slightly inaccurate rough opening or to allow for a bit out of square the insulation layer can be trimmed or tapered to take out any inaccuracies, and then sealed in the opening. 

 

Just a thought, tell me why this is either stupid or not necessary. 

Cheers. 

[Crofter]

It's much easier to seal up a 15mm gap than a 3mm one.

[MikeSharp01]

Sounds like a plan but you will need to check that the insulation, which will be structural in this application, can take the strain!

[ProDave]

Our windows were fitted in openings deliberately made 50mm bigger in both dimensions. The frame opening were first lined with 20mm PUR insulation and any remaining gap then filled with expanding foam.

[craig]

I've worked with builders who have erected steel frames and basically worked to the window.

 

Doesn't work with all builds obviously but how's the window being fixed?

 

Fitting with brackets requires a certain tolerance (the brackets we use require 6mm space). You also need to insulate and need additional space (12mm all around is what we recommend).

[Russell griffiths]

  On 22/06/2017 at 20:23, MikeSharp01 said:

Sounds like a plan but you will need to check that the insulation, which will be structural in this application, can take the strain!

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Why would you class it as structural @MikeSharp01 If any fixings pass through into the timber frame is it not just a packer or filler just the same as expanding foam would be ?  

[jack]

Isn't at least the bottom layer structural though? It's supporting the weight of the window.

[Russell griffiths]

So if a 25mm gap was left all around and a 15mm strip of insulation fixed to the timber structure that would still allow 10 mm for a sealing strip or expanding tape/ foam. 

 

Im just trying to think that any out of squareness or slight miss measure can be taken up in this packing area rather than trying to shoehorn in a window into a Miss made opening, 

 

[Russell griffiths]

  On 23/06/2017 at 07:06, jack said:

Isn't at least the bottom layer structural though? It's supporting the weight of the window.

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I suppose it is @jack could this bottom piece be eps300 if it is dense enough to put under a foundation it should hold up a window. 

[jack]

It's pretty strong stuff - I suppose it would work. Just curious about what the advantage is of having it under the window.

[ProDave]

In my case the bottom piece of standard PIR / PUR type insulation comfortably supported the weight of the windows until the fixing plates were screwed home.

[Crofter]

Given that there are no point loads involved, a piece of PIR/PUR may well be perfectly fine underneath the window. In my case there are a couple of wooden packers that gave me the ability to level up the window during fitting.

As I said earlier in the thread, don't try to get carried away with making everything a tight fit. I left a 15mm gap all round and that was just about perfect for getting the nozzle of the expanding foam gun into. Anything less would have been a right PITA.

[MikeSharp01]

  On 23/06/2017 at 07:00, Russell griffiths said:

 

  On 22/06/2017 at 20:23, MikeSharp01 said:

Sounds like a plan but you will need to check that the insulation, which will be structural in this application, can take the strain!

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Why would you class it as structural @MikeSharp01 If any fixings pass through into the timber frame is it not just a packer or filler just the same as expanding foam would b

 

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Structural because you need to meet a number of constraints keeping the frame fixed in place mechanically. The longer the fixings are through insulation the more flexible they will be as the insulation offers no support to them in all three planes - as you tighten the fixings you will crush the insulationand bend the frame as the insulation deforms rather than tighten. Wind pressure will cause, at the outset, slight bending of the fixing and, as the suporting insulation deforms, more movement. Also ensuring you meet the fire regs which I suspect might depreciate too much non fire rated material supporting, or not, the frame.

[Russell griffiths]

@MikeSharp01 can you elaborate on the fire regs, if the main structure is wood with a wooden window fitted how would having a 15 mm layer of insulation impede on fire regs? 

 

Are you you saying the 15mm layer could provide a gap for fire to pass through rather than a solid 190mm stud for arguments sake, assuming the window frame was fitted tight to the studs in the first place and no gap left and filled with expanding foam. 

What am I missing here. 

[Russell griffiths]

  On 23/06/2017 at 07:34, jack said:

It's pretty strong stuff - I suppose it would work. Just curious about what the advantage is of having it under the window.

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I don't think having it underneath is a requirement I'm just thinking of how to leave a larger tolerance around the window, be it at the top bottom or wherever, just to allow for proper squaring up and height adjustment. 

 

Imagine a room with 4 windows where you would want the heads to all be exactly in line having 20 mm to play with could be a godsend when it come to lining them all up. 

 

[Russell griffiths]

  On 22/06/2017 at 21:48, ProDave said:

Our windows were fitted in openings deliberately made 50mm bigger in both dimensions. The frame opening were first lined with 20mm PUR insulation and any remaining gap then filled with expanding foam.

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This @ProDave is exactly what I am thinking about. 

Can you say why you did this? 

Do you think the insulation on the stud/jamb surface will help with cold bridging from inside to out. 

Any pics. 

[ProDave]

  On 23/06/2017 at 09:00, Russell griffiths said:

This @ProDave is exactly what I am thinking about. 

Can you say why you did this? 

Do you think the insulation on the stud/jamb surface will help with cold bridging from inside to out. 

Any pics. 

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It was the builders suggestion, the ones that build my frame, to make the window openings over size and line them with rigid insulation to reduce any cold bridging.

 

It really was simple, make the openings too big, fix 20mm Celotex strips around the sides then fit the windows. fill in any gap left with expanding foam.

I later tidied up the packing detail to fit a continuous strip of ply packing, and countersunk the screws into the fixing plates

 

Then the airtightness membrane wraps around and gets taped to the window frames

 

 

 

[Russell griffiths]

Perfect that's what I was thinking. 

 

What i I would like to know is why shouldn't I do this?

 

thanks dave. 

[MikeSharp01]

  On 23/06/2017 at 08:49, Russell griffiths said:

@MikeSharp01 can you elaborate on the fire regs, if the main structure is wood with a wooden window fitted how would having a 15 mm layer of insulation impede on fire regs? 

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Sorry for the delay. It's about the use of materials to form walls and the use of Thermoplastic. I see no reason to think you cannot do what you want. The relevent sections are part B2 section C. Table 1 para 3.2, 3.4 & 3.8