Interesting question. I presume the glass looks clear, but slightly wavy compared to modern float glass?

I don't know the answer. I suspect if anybody does then probably these people https://histoglass.co.uk/period-glass/genuine-hand-drawn-glass/ might.

I suspect the value will be something like 0.85 but you could try putting in slightly lower and higher values to see how much difference they make. Are the U-values similar to modern single glazing? Do let us know if you find out

Thanks both.

I'm actually talking to Histoglass about a separate project so can ask them for input.

Posted By: djhAre the U-values similar to modern single glazing? Do let us know if you find out

I found a range of figures, but decided to trust this document from Historic England which puts it at 5.4W/ m²K for 4mm glass:

https://historicengland.org.uk/images-books/publications/eehb-secondary-glazing-windows/heag085-secondary-glazing

Posted By: fostertomYou are talking about just the glass, not the composite glass/iron frames?

I am talking about the glass, but ultimately will have to figure out the Uf of the frame as well to fill out the PHPP fully. Hoping I can work something out from THERM psi-value if I can't find anything online.

Posted By: fostertomIf 'wavy' as Dave says, that would make the glass irreplaceably historic/precious. Is it Listed or anything?

The building is Grade II listed, but the project is only theoretical - our practice's office - since we have a good point-cloud survey and can re-measure anything missed!

The glass is remarkably flat (so may not be original) but the frames certainly look old (I'm fairly certain they're iron rather than crittall steel for instance) and I can't see much sign of later re-glazing.

Posted By: fostertomIf 'wavy' as Dave says, that would make the glass irreplaceably historic/precious. Is it Listed or anything?

Historic looking glass is fairly easy to recreate. The hard part usually is finding someone with a kiln who will run the glass through a 900deg. cycle with an annealing period on the way down. If the glass is put in the kiln on a bed of chalk powder then a reasonable facsimile of historic glass is produced.

I would go with 5.4W/ m²K for 4mm glass. a different glass composition may have a (slightly) different value and of course the thickness will also affect the numbers but in the scheme of things I don't think it will make much difference considering the area of the glass compared to the rest of the structure.

So I spoke to the manufacturers, who were very helpful.

Whilst they didn't have a figure for genuine historic glass, as Peter suggests they do offer a recreation historic glass called MONO which is made using the 'hand-drawn' sheet method. That has a g-value of 0.74 ( or 74%).

For comparison the guy I spoke to also looked at their laminate products (which offer a choice between float glass and hand-drawn outer layer) and the g-value was 0.68 for both options. So in other words the PVB interlayer has more of an effect on the g-value figure than the 'waviness' of the glass itself.

There's every chance the recipe of modern recreation glass is a bit different to the original Victorian stuff, since it will be made under modern factory conditions with less impurities, but I think it's probably the closest I'll get to a figure for now.

I've also contacted Historic England in case they are aware of any research into the topic.

Posted By: fostertomMay be polished plate glass? Expensive alternative, for shop fronts and prestige hotels, to wavy cylinder glass. How thick?

Could be, but I don't think they'd splash out on that for a rural Brewery building. I'm assuming 4mm thick for now until I can dig out some vernier callipers!

Posted By: djhI don't understand that. How can you laminate two layers where at least one of the layers is not flat?

Not sure, although several manufacturers now offer this.

My guess would be that since hand-drawn glass is already fairly flat (unlike crown glass), the surface distortion is low enough that the squishy PVB interlayer takes up the difference.

Laminated or toughened is mandated by building regs in some cases. And maybe just prudent in others?

Yeah but why would wavy conservation/restoration glass ever be needed in safety-reg locations - surely only as replacement in extg or replacement trad small-pane (say 400sq) locations?

The news that wavy glass is still available is dramatic. Does anyone know if it can be made into double glazed units? Would do wonders for the look of reglazed windows in old buildings, making double-glazing same look less 'un-natural'. Not least because it would kill that smooth curved reflection which is OK in new buildings but a dead give-away in old ones.

You can get a safety /security film to stick on the inside of the glass that will hold the glass in place in the event of a breakage or attempted forced entry