"tertiary" glazing question

[piper]

This is really more a question for the green building forum - but people here know a lot about windows, so here goes.

Due to what was probably misunderstanding, rather than actual incompetence on anyone's part, the big floor-to-ceiling window in our extension has not been passed by building control because the toughened glass double-glazing is not deemed strong enough to prevent anyone smashing their way through it and falling out (it's at first-floor level) - it should apparently be 6mm glass, and it's not. (We changed the original specification of laminated glass to toughened mainly because I had primed the wood with linseed oil, prior to painting with Holkam paint, and there can be an issue with the oil seeping in between the lamination - I have subsequently been told by a friend who is another architect that it should always be toughened glass at first floor anyway; professional opinions seem to differ on the matter!)

Of the options to correct this (short of completely replacing the three big DG units) one idea is to put half height panes on the inside, just as a protection barrier, with a gap at the bottom so dust and spiders don't fall down and stay there.

However, an idea which appeals to me is to put complete extra panes, sealed all round, thus making triple-glazed windows of which the innermost pane is like "secondary glazing", but I have been told that condensation will probably form on the inside.
Does anyone have any experience with a similar arrangement? Is condensation a serious problem? Other secondary glazing we have around the house doesn't seem to form condensation, but probably the outer sashes are too draughty and it is ventilated away. I'm loath to have holes drilled to the outside in our nice new big window - we have escaped having to have trickle-vents in it so far.
The air-flow window described on Biff's website is another idea, but perhaps difficult to do at this stage, and would also seem to induce too much cooling draught into what is otherwise a very well-insulated room (There is already the statutory air vent in the wall for the wood-burning stove we have in the room).

 

[Penners]

Here's a completely inexpert view.

If you're installing the tertiary glazing primarily on safety grounds, then presumably you're not too concerned if its efficacy as thermal insulation is slightly compromised. That being the case, you could have some modest ventilation holes at the top and bottom inside, allowing a flow of interior air between the secondary and tertiary panes. As the secondary pane is protected from the external low temperatures anyway, by the primary pane, I doubt whether you would get condensation with this arrangement.

But I repeat, those are inexperienced amateur musings.

 

[piper]

Thanks, but purely for safety the option would be for the low level half pane - just a transparent barrier. The idea is also to gain an extra layer of thermal and sound insulation. The existing DG is very good, and the rest of the room is very well insulated, so although the window is huge (it's south facing) the new room keeps its heat well. However - if we HAVE to have the extra protection layer, it seems to make sense to get the absolute maximum insulation benefit, and not lose some of what we gain by bringing in cold air.

 

[biffvernon]

I wonder if anybody has ever managed to fall through a double glazed unit made with 4mm toughened glass. I'd guess you'd have to take quite a run at it.

To comply with Jobsworth your third layer will have to be 6mm toughened?

Can't you just fix a waist-height rail in front of the window?

 

[Pford75]

The fact that it is on the first floor is a bit of a red herring.


Low Level Glazing

(Excluding guarding) not covered by (1) or (2) (Glazing in doors and glazing adjacent to doors which have more stringent requirements). Glazing which is wholly or partially within 800mm from floor level shall be:

Minimum Class C to BS 6206 and marked according to BS 6206

The only exceptions are:

1. Panes having the smaller dimension, less than 250mm and of area less than 0.5m², may be minimum 6mm (nominal) thick glass not complying with BS 6206.

2. Panes forming parts of fronts (but no other locations) to shops, showrooms, offices, factories and public buildings, supported on all edges, may be of equivalent robustness in glass not complying to BS 6206

3. Panes protected by a suitably designed carrier

I would check with Building Control as they may insist that even with tertiary glazing, the original window still does not meet the minimum requirement and will need to be replaced.

 

[Penners]

I would check with Building Control as they may insist that even with tertiary glazing, the original window still does not meet the minimum requirement and will need to be replaced.

I certainly think that's good advice. It would be awful to install your tertiary glazing and then find that you still have to replace the original units.

 

[piper]

Thanks to those who gave advice and opinions back in early November when I posted this question. I thought you might be interested to know that today we heard that Building Control have accepted the existing windows and are not going to insist on replacement or any extra protection.
Our architect went to the trouble of getting calculations from Pilkington Glass to prove that the existing spec. met British Standard - so it just goes to reinforce what people on this Forum have often said. Building Control regulations require compliance with the principles - not necessarily with the first advice of the inspecting officer. And you need to persist.

 

[Penners]

Good news, piper - well done.