Windows - Single Glazing, Underfloor Heating & LBC

[swoodley]

We have just applied for LBC for a number of (essentially) repairs & renewals to our C.17th cottage. A large amount of work was undertaken in the 80's which isn't in keeping with the age or style of the property (unfortunately this included stripping out all the original windows and doors and replacing with modern examples).

We've applied to replace the modern softwood double-glazed units with hardwood ones. In initial consultations with one of the CO they indicated we could get 'like for like' (i.e. retain the double glazing element). Having applied for LBC on this basis we've now been told that we'll only be allowed single glazing, which appears to be the case in a number of authorities - has anyone else come across this?

We did intend to put in underfloor heating, but if we can only get single glazing will this be as efficient/up to the job? Any help would be greatfully appreciated.

 

[Pford75]

Double glazed units will naturally have thicker glazing bars (which in my opinion do not look as nice).

If however, you already have double-glazed windows..I am surprised that you are being asked to replace with single-glazed units.

When was the property listed? When were the double-glazed units fitted? If this was after listing, did they have LBC?

 

[Nemesis]

How large are the windows?

 

[swoodley]

Property was listed in the 80's, pic in listing shows the double glazed windows.

A number of changes were made in the late 80's which did not have LBC, this was however given retrospectively when previous owners applied (included the windows).

Windows aren't very big at all (hence very gloomy inside!), largest ones are in the new extension. All are a similar style with either 2 or 3 panes - 1 or 2 opening lights with 1 or 2 fixed. Don't have exact measurements to hand, but they're probably (max) 1.5 metres long by 0.75 metres high, with some noticeably smaller. All windows have modern applied diamond leaded effect, which we would like to replace with something more suitable.

 

[Pford75]

I'm not saying this is the best solution, but if the property was listed with the double-glazing, it should be possible to make a like-for-like replacement without LBC.

I would suggest that you discuss this with the CO, point out that the current windows are not originals. If LBC was granted for the current windows (even if retrospectively) I can't understand why they would oppose

 

[skier-hughes]

I was told by my CO that even if our house was listed we wouldn't get permission to change double glazed windows to single glazed, as we (and he) wanted to.
He fought our case in a battle against the Building control officer, who eventually won.

Graham

 

[Nemesis]

I would suggest that the DG issue would make not a lot of difference to the heat retention, the windows not being huge, and the opportunity has now arisen to put something better, more sympathetic, in their place, which should be considered.

 

[KN]

We have new single glazed large sash windows to the floor and underfloor heating (wet) and it works fine here. Haven't even got as far as curtain/shutters yet either, so I wouldn't be put off.

Karen

 

[biffvernon]

But your heating bills will be higher than they might otherwise be.

Not all double glazed windows are equal.

 

[Penners]

I've installed several new windows in our listed building. We wouldn't have been allowed to have DG units (not that I wanted to), for the reason mentioned above: the glazing bars would have been too thick.

But we might have been allowed to install secondary glazing inside, provided the glazing bars aligned with those on the primary windows.

 

[Flyfisher]

the glazing bars would have been too thick.

Does 'thick' refer to width or depth (or perhaps both)?

I've seen details of DG casement windows with 18mm wide glazing bars. Obviously they are a bit deeper than 'trad' windows but they don't have those disproportionate-looking wider glazing bars.

 

[biffvernon]

They are probably fake - stuck on to the glass surface both sides.

 

[Flyfisher]

Nope. Proper wood frames, 18mm glazing bars and thinner, white spacers within the DG units.

 

[biffvernon]

White spacers within the DG units. So the glazing bars a fake - the wood does not go all the way through, dividing the window into small, individual dg units. Instead there is one big dg unit divided up with stuck on glazing bars.

 

[Flyfisher]

No. A normal wooden casement window with 18mm glazing bars but made a bit deeper to accommodate individual DG units.

The DG units had white 'spacers', 'beading' (or whatever its called) inside their edges (for white painted frames, of course).

Definitely NOT a big DG unit with fake, add-on glazing bars, inside or out!

 

[biffvernon]

OK, so how, with a glazing bar only 18mm wide do you get a rebate sufficiently wide to cover the dg unit's seal. These are usually 10mm wide plus you need a couple of spare mm because the glass edge will never butt tightly to the bar and frame (it's better if it doesn't, to stop a capillary bridge), so you really need rebates of at least 12mm. One each side of the bar makes 24mm and then the bar needs to have some finite thickness. Even with oak I'd want 6mm. That adds up to 30mm absolute minimum.

So how does this 18mm bar work?

 

[Penners]

Does 'thick' refer to width or depth (or perhaps both)?

Our CO stipulated that the glazing bars on our new (softwood) windows could not be more than 21mm wide. He made no stipulation about depth. In fact they are 20mm wide and 41mm deep. They're single glazed.

Theoretically, I suppose you could make a glazing bar very narrow - much less than 21mm - but still make it deep enough to accommodate a DG sealed unit. However, it would not, of course, have the strength to support the glass.

Biff will have a much more expert view on this.

 

[Nemesis]

dg unit's seal.

Which I gather, from past discussion with Biff, needsto be well rebated in order that it doesn't degrade.

 

[middi]

The main stumbledlock in having proper "trhough" wood glazing bars on DG units, regardless how very thin the glass seat recess, is the beading, putty or any other form to hold the glass in position and at the same time cover the beading seal of the " proper" DG unit. As this is 12,15 or more wide, then whatever beading is used any of the above distances have to be doubled. Add to this the " very thin" glzing bar ( even 5 mm) then the min. visual cross section of the finished glazing bar will be 30mm .

Even at this very minimum level the finished product will be larger than " traditional". Also due to the DG gap, unless this glazing bar is viewed totally at horizontal, it will look larger as the angle of vision is increased. There has to be a trade off I am afraid.

It would be much better to use stick on thin glazing bars.

 

[Penners]

It would be much better to use stick on thin glazing bars.

Visually, I suppose you're probably right, Middi. However, I think you would have trouble persuading the "that's-not-the-way-it's-traditionally-done" lobby.