Box sash windows in cavity walls

[marcus]

Hi there

I'm trying to figure out you fit box sash windows into new cavity walls—in particularly walls built under the new regs which have a 150mm cavity. Excuse my ignorance about this. Also please excuse by builders ignorance—he doesn't want to fit sash windows anyway (waste of money mate!) and is being no help whatsoever.

In the old days, obviously, there would have been a recess in the solid brick wall and the windows would fit neatly into that with the boxes hidden behind the front course of bricks.

I assume that with a cavity wall they have normally been done with a similar arrangement, except with the recess is formed by the inner block work and the windows bridging the gap? Or am I wrong about this?

With these new walls, though the cavity is so wide that the thickness of the windows won't bridge the gap. What are you supposed to fix them too?

I realise I may be barking up completely the wrong tree, but any help appreciated.

Thanks

Marcus

PS @Jacob I expect you will have some understanding of this?

 

[rogxwhit]

If your cavity's insulated then thermally speaking a window should be in the plane of the insulation and / or somehow contiguous with it. But a weight box will constitute a thermal bridge so to me your window choice is curious ...

Offset fixing can be achieved with straps / brackets.

 

[marcus]

Choice is simply an aesthetic one, it’s an old house and all existing windows are box sash and I want them to match …

 

[rogxwhit]

How is that squeezed past building control?

So brackets, possibly, and you still have to close the cavity ...

Note that I love box sash windows and making them seems to honour generations of craftsmen who have gone before.

 

[marcus]

I don’t know re building control, I assumed there would be a way, are you saying that it’s no longer possible to fit box sash’s to new buildings because the regs won’t allow? That would be sad

 

[Dibs-h]

I don’t know re building control, I assumed there would be a way, are you saying that it’s no longer possible to fit box sash’s to new buildings because the regs won’t allow? That would be sad

Either speak to your local authority Building Control and\or (private) inspectors. IIRC - you can choose a private inspector.

Most are likely to be bothered about the emissivity of the glass\units and thermal performance of the windows (although I suspect most wouldn't have a clue about the thermal performance of the frame\sashes), and trickle vent requirements.

 

[Doug71]

Do you have an architect on the job, probably worth running it past him so the openings can be designed to take the windows, that's if it's not already been built!

 

[rogxwhit]

are you saying that it’s no longer possible to fit box sash’s to new buildings because the regs won’t allow?

No, I'm not placed to assert that, I was essentially just wondering aloud how a sash frame having a hollow box could achieve current whole window u-value requirements. For a spring-balanced version, though, the hollow box is no longer an issue. But the balances themselves are rather tinny affairs and less, shall we say, emotionally satisfying?

 

[HOJ]

In the process of planning for the building works did you not submit a Building Regs application, which I would have thought should have included details of your
windows.

 

[Dibs-h]

Do you have an architect on the job, probably worth running it past him so the openings can be designed to take the windows, that's if it's not already been built!

Don't you mean a Structural Engineer?

 

[Jameshow]

Wouldn't the manufacturing company be fensa registered and therefore self certify or are you making them yourself and therefore defending your own wicket?!!

 

[Dibs-h]

just wondering aloud how a sash frame having a hollow box could achieve current whole window u-value requirements.

Maybe using some expensive stuff - Spacetherm® Aerogel Insulation | A. Proctor Group Ltd. ?

You'd have to do the U value calcs for the whole lot, but specifying low emissivity glass (argon filled if DG) might help you get closer to a pass than a fail. For new installs that is.

I think refurbs\replacements don't have quite the same hoops to jump through.

 

[Ollie78]

As long as there is a cavity closer in there you could just fix a board accross the cavity over the top of this and wedge it in as normal this could be cement board or timber, sort the reveals out after.
It would be simpler to just brick up the last bit of the cavity but it will create a "thermal bridge" unless there is an insulated break in between the walls.

I normally do 80mm boxes with 100mm outer cheek so depending on the amount of cheek you want visible then 100mm overlap on the outer brick should be fine.
In the case of new extension work on an old building you will need the heritage people to trump the building regs people.

Ollie

 

[Doug71]

Don't you mean a Structural Engineer?

Sorry, I wasn't really clear, I meant more how the windows sit in the opening.

I'm presuming the Windows aren't the problem, Marcus just needs to get some that have the correct U value or energy efficiency rating whatever it is these days.

If he wants them to sit behind the brickwork that is a bit of a break from the norm (but a much better look), there must be a standard way of doing this in new builds which I would hope his architect could advise on.

Regarding building control I find with some things they give advice but with others they just want you to prove it meets the regs so they can tick the box.

 

[XTiffy]

Having built four houses in the past 17 years and am about to start number five. I can atest to the fact that each one gets more complicated by BRs, Planners and a host of other constantly evolving rules and regulations. I have downloaded this guide LABC ( Local Authority Building Control, though it is rather comprehensive). This should answer any question. The short answer is that you need "Cavity closers" around the opening, which prevents the thermal bridging, The windows can be placed anywhere (in reason!), the window reveal is then usually plasterborded etc. Hope that helps

 

[marcus]

Thanks for your thoughts, and thanks XTiffy for that link, which is very useful. I've been making enquiries of building control and others, and getting closer to a so;llution, will report back in a bit...

 

[marcus]

Turns out to be pretty straightforward. Building Control happy for us to make up the check reveal using the cavity closers, or to do a block work return and simply foam up to the cavity closer or block work to hold them in place.

 

[Bristol_Rob]

The modern way is to use cavity closers:

https://www.toolstation.com/ybs-easi-close-eps-white-cavity-closer/p60951

 

[Adam W.]

Turns out to be pretty straightforward. Building Control happy for us to make up the check reveal using the cavity closers, or to do a block work return and simply foam up to the cavity closer or block work to hold them in place.

You don't need architects or structural engineers for this kind of stuff and your builder sounds like he's the wrong man for the job, if he's not willing to listen to what you have to say.

 

[marcus]

You don't need architects or structural engineers for this kind of stuff and your builder sounds like he's the wrong man for the job, if he's not willing to listen to what you have to say.

You're right about that for sure, but stuck with them now I'm afraid ... !