Window strap positioning, inside or out?

[cwr]

Hi folks,

It appears to be convention for window fixing straps to be fixed to the inside skin of a cavity wall. Is there a reason for this, or more specifically is there a reason why I shouldn't attach to the outer skin? Outer skin will be block with render over it so no aesthetic reason not too.

Reason I ask is that I have a 240mm cavity with the windows positioned just inside the cavity nearer the outer skin.

Thanks.

[joe90]

I fitted my windows in check reveals (fixed to the inside of a brick outer skin) so I fixed mine to the outer skin with no problem.

[nod]

2 hours ago, cwr said:

Hi folks,

It appears to be convention for window fixing straps to be fixed to the inside skin of a cavity wall. Is there a reason for this, or more specifically is there a reason why I shouldn't attach to the outer skin? Outer skin will be block with render over it so no aesthetic reason not too.

Reason I ask is that I have a 240mm cavity with the windows positioned just inside the cavity nearer the outer skin.

Thanks.

There are several reasons for putting the straps on the inside 

First being ease of fixing 

Far easier to be stood on a solid than on a scaffold or ladder 

Second is if you are fixing to block you are likely to put packers under the straps which would be a pain to try and render over and likely to crack

 

[joe90]

10 hours ago, nod said:

There are several reasons for putting the straps on the inside 

First being ease of fixing 

Far easier to be stood on a solid than on a scaffold or ladder 

Second is if you are fixing to block you are likely to put packers under the straps which would be a pain to try and render over and likely to crack

 

 

Although mine were fixed to the outside skin it was from the inside so not from scaffold. NOTE my reveals are angled, the inside blockwork hole was bigger than the outside so it was easy to do.

[cwr]

Ok, that makes sense. I've also been told that fixing to outer skin may breach the DPC, be a potential corrosion corrosion issue, and invalidate the window guarantee.

 

I've also got angled reveals on the inside, and a rather large cavity (240mm). All makes it that little bit more complicated...

[joe90]

51 minutes ago, cwr said:

Ok, that makes sense. I've also been told that fixing to outer skin may breach the DPC, be a potential corrosion corrosion issue, and invalidate the window guarantee.

 

I've also got angled reveals on the inside, and a rather large cavity (240mm). All makes it that little bit more complicated...

 

Actually I found it easier, my cavity is 200mm, inner blockwork 200mm wider per side, windows 40mm wider than brickwork “hole”, 20mm expanding foam tape between window and brickwork, stainless straps and stainless screws, vertical DPC stuck to back of brickwork before fixing window straps. Angled reveal made of plasterboard fixed to angled button on side of window and plasterboard adhesive to blockwork after its internal corner was rounded. NOTE I asked the plasterers to use a stop bead at the joinfrom plasterboard to window but they said it was not needed OH YES IT WAS. I have suffered cracking plaster at this junction (so have others on this forum including @JSHarris,) if a stop bead was used the small gap between stop bead and wooden window could be filled with non setting caulk to accommodate any shrinkage or movement. I know this because I did this on the porch I built and I have no cracking.

 

p.s. On the air test the small leaks were mainly at these cracks.

 

 

[cwr]

Hi Joe90. Thanks for sharing that. Can you explain to me about the 'angled button on side of window', is that a bracket off the frame to fix the plasterbaord too, or something simalar?

 

Good point re the stop bead, I'll make a note of that.

[joe90]

19 minutes ago, cwr said:

is that a bracket off the frame to fix the plasterbaord too, or something simalar?

 

No I cut a piece of timber the height of the window, screwed to the side of the window but with an angled edge to screw the plasterboard to, hang on I feel a diagram coming........

[cwr]

Ah, I get it. thanks. That looks good.

I'd be keen to do the same, though we are being forced by the mortgage lender to involve an architect to oversee the build who is not too keen on bracketing the windows to the outer skin....

[Pete]

On 09/07/2019 at 20:56, joe90 said:

 

No I cut a piece of timber the height of the window, screwed to the side of the window but with an angled edge to screw the plasterboard to, hang on I feel a diagram coming........

I do like your drawings!!

[joe90]

11 hours ago, Pete said:

I do like your drawings!!

 

Ha, I don’t do cad, bit of a Luddite me!

[joe90]

11 hours ago, cwr said:

Ah, I get it. thanks. That looks good.

I'd be keen to do the same, though we are being forced by the mortgage lender to involve an architect to oversee the build who is not too keen on bracketing the windows to the outer skin....

 

Why?

[Pete]

1 hour ago, joe90 said:

 

Ha, I don’t do cad, bit of a Luddite me!

I have an aerospace engineer son to do that for me

[cwr]

Reasons given are it would be bridging the cavity, fixings would be bridging the vertical DPC, may invalidate guarantee of windows or fitting of them, and its just tradition and everyone does it that way....

 

With stainless steel fixings and given it'll be a full fill cavity and PVC window frames I'm struggling to see a major problem, though i'm no expert...

[Mr Punter]

I would not want the straps on the edge or the outside face of the outer leaf and rendered over. Would be near impossible to change the windows without wrecking the render. Why not have them fix to the inner leaf with longer straps?

[Oz07]

@joe90 no cavity closers then ? Just plasterboard up against insulation?

[joe90]

44 minutes ago, Oz07 said:

@joe90 no cavity closers then ? Just plasterboard up against insulation?

 

What I omitted in my drawing was a vertical DPC on the brickwork, (belt and braces) the rock wall batts don’t wick water anyway, also spray insulation against the back of the plasterboard.