size of casement window elements

[argoman]

Hi all,

I'm building windows for my new build and I am trying to find out what size timber I need. Its a new build house so needs to be build regs compliant so at this point thinking 18mm glaze panel with standard void not fancy argon or krypton. Build regs wouldn't express an opinion - just said as long as its 'compliant'. But for now I'm assuming 18mm. Agree - disagree?

So on an 18mm glaze panel what size timber do i need for a traditional flush opening casement? The house is pure 1670's cotswold so I'm trying to work out what size timber to use and what size rebates. My guess is that for the width of the casement it needs to be 18mm plus moulding either side of 15mm squared which makes 38mm plus glazing tape / silicon so 44mm thick. How does that sound?

For depth I've noticed that most radtional windows have a deeper bottom piece than the other 3 pieces. What size depth should these be? The bottom looks like 63mm and the sides and top are 44mm.

Casements are made in oak and left unfinished to silver. Really appreciate your comments on this please.

Thanks

Gary

 

[oakfield]

I think if making your own windows you need a u value of 1.2 for the glass.
You will struggle to get that without some form of gas filling.
The last units I got were 4-12-4, low e glass, argon filled with a warm edge spacer and this got them down to 1.2.

 

[richarnold]

As far as I know, current regs need 24mm argon filled, planitherm units, and working on this basis you would need a section of about 2 1/4" deep. There is a new form of unit available that claims to comply to new regs, but they are not cheap. info here http://www.timbalite.com/.
Cheers, Richard

 

[jimmy_s]

You would probably need to have a look in your building regs and check the maximum U value permissible for a window - I don't have to work to the English Regs very often so don't know what the value is. In Scotland the average U value of the windows in a house has to be 1.8 w/sqm k or less.

You would need to do the calcs I would think, certainly in Scotland here we would do the calcs for all windows in the property and issue to building control to prove compliance.

 

[RobinBHM]

The section sizes depend on your window design.

24mm thick dg is generally the min for compliance.

Sash section sizes may typically be 60 x 58 thick for stile and top rail. Bottom rail may be 90mm. (60mm for bot rail of sashes above a transon)

Sash stile section width is determined by depth required for espag lock.

Frame section could be 93 x 58 jambs and head. 93 x 68 cill with plant on cill.

Strictly speaking all windows should be tested for whole window u value, although Ive not seen BC ask for this.

 

[argoman]

Hi thanks for the comments! I've checked building regs approval and it is asking for 1.8 U value. I've also spoken to technical at Saint-Gobain and they have confirmed that Planitherm Total Plus with 90% argon fill on 4-10-4 will give 1.5 U value with total thickness of 18mm so thats adequate officially and in writing! But it does mean I have to use argon - although i gather this is now the default. So I'll accept the argon.

Given that the 18mm sealed unit is ok with argon, what size sash section should I use? I think it all comes down to the moulding and sealant either side of the sealed unit. I'm thinking to use clear silicon to seal the dg unit in the frame as glaze tape i've used isn't waterproof. This works out about 1mm either side when fitted so total 20mm.

The question then is how wide and how deep should the moulding either side be? If I look at howdens site their external doors are 44mm thick and rated for up to 20mm glaze unit so they must think a 10mm x 10mm moulding / bead is adequate? Do you agree for windows or should it be 15mm?

I'd prefer to keep the thickness as narrow as possible to keep it looking as traditional as possible.

In terms of visible frame depth i.e. the vertical bit I mocked up a 65mm (height) x 44mm (thickness) section and the 65mm just looked too heavy when set into the subframes. Was ok for the bottom rail but for the sides was just too much. If I'm on 10mm x 10mm bead what is the thinnest section x 44mm I could get away with whilst retaining strength? As I mentioned above it is solid pippy grade oak not softwood.

Thanks

 

[argoman]

Actually looking more at doors most external doors are 44mm thick and yet have double or triple glazing. Surely then this IS adequate for windows?

Am i missing something?

Thanks

 

[jimmy_s]

The U value of your glazing unit is probably going to be better than your oak so the less oak you have the better.

To work out the overall U value you would need to calculate the U value for the oak as follows;

Thermal conductivity of oak 0.16m2k/w
outside and inside surface resistances 0.06 and 0.12

the u value of the oak is given by 1/(0.06+0.12+(0.044/0.16) - based on it being 44mm thick which is the 0.044 in the equation

this works out at a U value of 1/0.455 or 2.2 w/sqm K

so the U value of your glazing unit is 1.5 w/sqmK and 44mm oak is 2.2w/sqmK

If your door or window has a frame area of say 10% leaving the 90% as glazing then the U value of the combined frame window is 0.1 x 2.2 + 0.9 x 1.5 which works out at an averaged U value of 1.57w/sqm K

Hopefully this helps

Jimmy

 

[argoman]

Jimmy,

Thats fantastic. Thank you. I hadn't dared ask how to calculate the overall U-value as it is so complicated but I can follow your maths!

So for the thickness all I need to do now is to confirm the size of the beading either side of the panel and then I'm good to go. right now I'm thinking with 18mm glaze unit that for the top and sides it will actually be 48mm / 48mm made up of:

2 - 12 - 1 - 18 - 1 - 12 - 2
lip int bead silicon DG Unit silicon ext bead lip

For the bottom rail I'll go for 64mm / 48mm.

Using Jimmy's approach that will keep me well within 1.8 overall.

Is there any sense that this won't be strong enough? I've quickly knocked up a prototype with a nail gun and it looks right so I am happy with the dimensions but before I order £1,500 of timber stock does anyone think it won't be strong enough?

Thanks all

 

[jimmy_s]

PM sent

 

[Nicky T]

Remember to leave enough room for stays and fasteners!
I expect you'll lose 10mm of your sash style behind the jamb rebate and possibly more if considering a mould detail ie.. ovolo/lambs tongue.
You also not only have to apply with 'u values' but toughening of any glass units below 800mm and some first floor windows should have a clear opening of 350mm for fire escape access.