Green Building Forum - g-value of historic glass Tue, 19 Dec 2023 08:28:07 +0000 http://www.greenbuildingforum.co.uk/newforum/ Lussumo Vanilla 1.0.3 g-value of historic glass http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17937&Focus=303057#Comment_303057 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17937&Focus=303057#Comment_303057 Fri, 01 Sep 2023 13:54:46 +0100 Doubting_Thomas
I'm currently modelling a Victorian industrial building in PHPP to assess retrofit options. It dates to the 1870's and has single glazed iron frame windows.

Can anyone recommend a resource where I might find g-values of historic glass from this era?

There's a few numbers online for estimated u-values, but surprisingly little on the other performance figures. Given there must have been fewer types of glass back then (no coatings etc.) I assumed there might be a reasonably standard figure to assume.

All suggestions welcome!]]> g-value of historic glass http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17937&Focus=303059#Comment_303059 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17937&Focus=303059#Comment_303059 Fri, 01 Sep 2023 15:20:41 +0100 djh
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 :bigsmile:]]> g-value of historic glass http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17937&Focus=303061#Comment_303061 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17937&Focus=303061#Comment_303061 Fri, 01 Sep 2023 15:49:19 +0100 fostertom
If 'wavy' as Dave says, that would make the glass irreplaceably historic/precious. Is it Listed or anything?]]> g-value of historic glass http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17937&Focus=303062#Comment_303062 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17937&Focus=303062#Comment_303062 Fri, 01 Sep 2023 16:47:16 +0100 Doubting_Thomas
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.]]> g-value of historic glass http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17937&Focus=303064#Comment_303064 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17937&Focus=303064#Comment_303064 Fri, 01 Sep 2023 22:55:46 +0100 fostertom g-value of historic glass http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17937&Focus=303068#Comment_303068 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17937&Focus=303068#Comment_303068 Sat, 02 Sep 2023 19:31:04 +0100 Peter_in_Hungary 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.]]> g-value of historic glass http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17937&Focus=303069#Comment_303069 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17937&Focus=303069#Comment_303069 Sat, 02 Sep 2023 21:17:12 +0100 fostertom Posted By: Peter_in_HungaryHistoric looking glass is fairly easy to recreateDidn't know that - nor does the Listed building regime, I'd say!]]> g-value of historic glass http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17937&Focus=303086#Comment_303086 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17937&Focus=303086#Comment_303086 Mon, 04 Sep 2023 10:03:30 +0100 Doubting_Thomas
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!]]> g-value of historic glass http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17937&Focus=303088#Comment_303088 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17937&Focus=303088#Comment_303088 Mon, 04 Sep 2023 12:20:04 +0100 djh Posted By: Doubting_ThomasFor comparison the guy I spoke to also looked at their laminate products (which offer a choice between float glass and hand-drawn outer layer)I don't understand that. How can you laminate two layers where at least one of the layers is not flat?]]> g-value of historic glass http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17937&Focus=303091#Comment_303091 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17937&Focus=303091#Comment_303091 Mon, 04 Sep 2023 13:01:55 +0100 Doubting_Thomas 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.]]> g-value of historic glass http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17937&Focus=303099#Comment_303099 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17937&Focus=303099#Comment_303099 Mon, 04 Sep 2023 15:14:07 +0100 fostertom g-value of historic glass http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17937&Focus=303106#Comment_303106 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17937&Focus=303106#Comment_303106 Mon, 04 Sep 2023 17:08:21 +0100 djh g-value of historic glass http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17937&Focus=303108#Comment_303108 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17937&Focus=303108#Comment_303108 Mon, 04 Sep 2023 17:42:17 +0100 Doubting_Thomas Posted By: fostertomWhat would such laminated glass be for, and why considering it in this case?

It's often for either security or safety - depending on whether it's been heat treated (or if it has impurities) a broken glass pane can shatter into varying sizes. If used at height this can be dangerous, so a laminate layer can help to retain the broken pieces until the glass can be replaced.]]> g-value of historic glass http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17937&Focus=303111#Comment_303111 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17937&Focus=303111#Comment_303111 Mon, 04 Sep 2023 18:53:20 +0100 fostertom g-value of historic glass http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17937&Focus=303121#Comment_303121 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17937&Focus=303121#Comment_303121 Tue, 05 Sep 2023 08:37:36 +0100 Peter_in_Hungary g-value of historic glass http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17937&Focus=303122#Comment_303122 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17937&Focus=303122#Comment_303122 Tue, 05 Sep 2023 09:04:02 +0100 fostertom g-value of historic glass http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17937&Focus=303127#Comment_303127 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17937&Focus=303127#Comment_303127 Tue, 05 Sep 2023 15:06:24 +0100 Doubting_Thomas Posted By: fostertom 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.

Yes, the same supplier also do double glazed units. They are called TDG with the T standing for 'Thin' because they can make them slim enough to be rebated back into original timber. Most units range from 10-13mm so not all windows will be suitable, but a lot will be.

The inner pane is usually modern float glass with low-e coatings etc., while the outer pane is the 'historic' looking one.


Posted By: fostertomYeah 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?


As above, we're looking at the potential to refurbish first floor windows to a historic industrial building by a busy road, and in another location a pub fronting onto a back alley with glazing down to a low level. In both locations it is sensible for the glass to remain in place if breakage did occur, either through stone chips or intentional vandalism.]]> g-value of historic glass http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17937&Focus=303130#Comment_303130 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17937&Focus=303130#Comment_303130 Tue, 05 Sep 2023 17:25:43 +0100 Peter_in_Hungary