Part L and Building your own Windows

It's the glass that has the u-value, and I don't think you'll be making the units yourself! when you order the DG units, they will have k-glass or equivalent on the inside pane....you can get building control to pass them off for a set fee per house, or it may be per visit, so you might want to make them all and get him out once. If any of your window cills are below 1 metre from the floor, you'll also need toughened glass.

I think you should look carefully at the regs. ISTR that you may repair windows without having building regs and planning permission etc. I think common sense would say that your newly repaired windows would be more efficient than the current ones. It think it would be very difficult to define exactly what 'repaired' means. I would suggest it is quite likely that many 'original' windows might have had a few new frames in their life as well as some new panes...a bit like Trigger's broom from Only Fools and Horses.

IIRC that should be 800 mm. I don't recall when it changed.

Read Part L here:

I made and fitted a complete set of windows for a chapel conversion, specifying 4/16/4 sealed units with a u value of 2.0 and proving they would meet the ventialtion requirements (based on opener to floor area ratio) as part of a "full plans" application.

You probably only need to make a short statement on a building notice to your local BCO.

Alternatiely, if it's like for like, replace (most of) each window as you make it and it may well be considered repair - which requires no interaction with planning or building control.

Over the past three or more years I've built & fitted 12 sash windows in our

The U value is a composite of the whole window and the frame.

For steel windows, for example, even double glazed panels can be worse than single glazed windows in wooden frames, as the frame conducts so much heat.

However, though timber isn't a great insulator, it's not a bad one.

U values for windows should be 1.8 or below.

If we assume that the frames are 4m long, and this is a 1m^2 pane, then with 50mm square timber, this is .2m^2 of timber. Timber has a k value of around 0.14, or for 1/20th of a meter thickness, that's a U value of 2.8.

Divide by 5, as it's only the contribution of the window frame that you're interested in, and it's probably around 0.6 of the 1.8 U value for double glazed wooden framed windows that comes from the wood.

(needed to do this sum anyway - as I'm considering making my own windows).

That was my initial understanding, but if you look at Table 2 on Page 20 of the Regs it says U-value=2.0 or centre pane U-value=1.2. So from this it would seem as longer as your double glazed units manage 1.2 then you are ok, or am I miss understanding something?

"Repair" the frames this year re-using the old sliding/opening bits and then "repair" those next year.

MBQ

The building inspector who looks at your new windows isn't interested and neither should you be. The glass will be K-glass and it will be checked, likewise if it's toughened or not...they don't have a thermometer to check inside and outside temps of framje material - wood is better than PVC, PVC is better than metal, that's all he knows and that's all he needs to know.

The message from "Phil L" contains these words:

Out of interest, how do they check?

AIUI, point an IR thermometer at it, and it's very different between glass and not.

Look for the Pilkington logo?

It's written on the inside of the unit!

(sometimes)

Every one I've done, he's asked me to leave the stickers on until he's seen it...these usually have unit measurements, date, type of glass, customer name, house number and manufacterer printed on them, hard to forge, although I wouldn't be surprised if someone hadn't tried...

The message from Andy Hall contains these words:

It's on a sticker - which of course you remove 'cos it's in the middle of the glass when they're delivered.

Wood falls within a predictable range as does plastic and metal..there are tables for calculating U values...if the BCO is unhappy he will require YOU to spend YOUR money on a report by a heating consultant who will do the calcs and present a report.