Not exactly DIY - but Double Glazing Question

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Geektastic

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#255848 3-Sep-2019 11:00

We are replacing the windows in our house with double glazing.

At present, to do the job with Pacific Architectural frames and Low E Max sealed units with Argon the cost will be around $43,000 including GST for the windows supply and deliver only.

To upgrade the windows to thermally broken frames, it adds $15,000 + GST or thereabouts.

I have waded through screeds of information (unhelpfully, often presented in different formats or using different measurements etc) trying to calculate the cost/benefit of paying for the thermally broken frames. It is not an easy thing to do!

It seems that thermal breaking an aluminium frame will save you about 20% of the heat lost through the window. A window typically loses 10% of the heat lost from the house (I think) so you are looking at saving 20% of 10% - or 2% of the total.

Does anyone have experience of this? Does it sound about right? The glazing supplier is very much not what I would call a technical person and to be honest probably doesn't actually know how to make these calculations etc so his advice is a bit vague and generic.

There is a paucity of choice out here in terms of suppliers and, to be honest, most of them seem to get their stuff from the same place anyway, so shopping around is hard.

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#2309893 3-Sep-2019 11:43

Surprised windows only lose 10% of the heat from a house. They also sometimes have drafts which let cold air in, don't seal and let noise in. $43K pays for a LOT of power.

How many / how large are your windows? We have a fairly old house, replaced 11 windows with PVC double glazing, total cost including fitting was around $15K. Doors cost about $3K each.

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#2309899 3-Sep-2019 11:53

It sounds like you are chasing the top of the Curve,

43K to go Low-E Double glazing, but an extra 17K for a thermal break, that's a 40% premium, - will it improve the windows heat rating by 40% , I doubt it....

Use the money to buy good long thick thermal curtains,

https://www.eboss.co.nz/detailed/keith-huntington/thermal-drapes-do-they-work

neb

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#2309921 3-Sep-2019 13:22

It's not worth it. The R-value of the most expensive, exotic low-E argon-filled double-glazed window you can buy is slightly less than that of a plain uninsulated wall, so gib on the inside, no insulation whatsoever, and cladding on the outside. You would need a solid metre-thick slab of glass to get an R-value of 1.0, same as the most basic insulated wall. Thermal drapes will add more insulation value than argon-filled low-E glass.

If you want the best bang for buck, get generic double-glazing and uPVC frames, which are better than thermally broken alu. Then draw the curtains when it gets cold, making sure you have a good seal around the window to eliminate chimneys.

If you don't mind blinds, honeycomb blinds have good insulation properties because of the air cavities they provide.

neb

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#2309923 3-Sep-2019 13:25

Geektastic:
The glazing supplier is very much not what I would call a technical person and to be honest probably doesn't actually know how to make these calculations etc so his advice is a bit vague and generic.

Another reason for that will be that the ultra-pricey low-E argon-filled etc stuff is principally there for goldilocks pricing, it's the maximally expensive stuff that no-one ever buys, makes everything else look reasonable in comparison. So the fact that they don't know the technical details may be because they rarely if ever sell any of that stuff.

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#2309930 3-Sep-2019 13:36

We've just replaced some windows too. We went for the thermally broken option. In the end, we did upgrade to the low e + argon option since we got offered a deal that was relatively minimal cost, but if I had to choose one or the other, I would have opted for the thermally broken frames over the fancy glass. You can reglaze later; you're note likely to re-frame.

We also have some existing non-thermally broken double glazing. During a recent cold snap, I noticed condensation forming on the non-thermally broken frame. Some approximate measurements showed that the inside of the non-thermally broken frame was about 5 deg, the thermally broken frame was about 15 deg (IIRC). So the thermal breaking definitely does work. Whether it is sufficient to justify the additional costs?

We used High Performance Windows in Wellington and they were great to deal with. I think they supply Vantage windows. My builder also recommended Fairview in Greytown, which I think is geographically convenient for you?

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#2310249 3-Sep-2019 19:41

The argon low E is comparatively cheap in this matter.

Only $2500 extra.

It's the frames that are costly.

Yes I'd have upvc if it was locally available, but it's not and we've learned that sticking to local options saves hassle later.

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bfginger

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#2310269 3-Sep-2019 20:26

I have waded through screeds of information (unhelpfully, often presented in different formats or using different measurements etc) trying to calculate the cost/benefit of paying for the thermally broken frames. It is not an easy thing to do!

Many of the numbers are out of date or misleading which is more confusion for the average person.

At present, to do the job with Pacific Architectural frames and Low E Max sealed units with Argon the cost will be around $43,000 including GST for the windows supply and deliver only.

To upgrade the windows to thermally broken frames, it adds $15,000 + GST or thereabouts.

I do not recommend you get Low E Max. That's a second rate product so if you're going to spend all that money on new windows spend a couple of thousand more and get Low E XCel or an equivalent like Viridian Planitherm XN specified with the thermal spacer.

Pacific Architectural as the name suggests is mostly aimed at architects. It's a 40mm platform versus 35mm for their standard Pacific Residential. You'll pay more for something thicker in many cases for no benefit beyond an architect's fashion sense. There may be advantages like being able to hold a wider glazing unit but hardly anyone in New Zealand cares about that.

The additional $17,250 for Pacific Thermal is crazy. Thermally breaking adds little to manufacturing costs so a massive 60% step up in profile costs is hard to justify based on a mildly wider platform. You may be able to import triple glazed timber from Europe for that. Try a quote for Nulook AllSeasons, it's from the same factory but should be more affordable. Omega does what looks like a good thermally broken profile too.

It seems that thermal breaking an aluminium frame will save you about 20% of the heat lost through the window. A window typically loses 10% of the heat lost from the house (I think) so you are looking at saving 20% of 10% - or 2% of the total.

How much heat thermally broken saves depends on what glass is in there, the quantity of exposed aluminium relative to glass and the outdoors temperatures. In an otherwise well insulated house single glazing can lose up to 50% of the heat. How many windows, doors and m2 of area are you getting, how well insulated is the house, how many degrees C does the room temperature sits above outside's, how much direct sunlight do the windows receive and where in the country is it?

The glazing supplier is very much not what I would call a technical person and to be honest probably doesn't actually know how to make these calculations etc so his advice is a bit vague and generic.

Metroglass should know this stuff inside out. The average aluminium supplier is unlikely to know much more than you.

It's not worth it. The R-value of the most expensive, exotic low-E argon-filled double-glazed window you can buy is slightly less than that of a plain uninsulated wall, so gib on the inside, no insulation whatsoever, and cladding on the outside. You would need a solid metre-thick slab of glass to get an R-value of 1.0, same as the most basic insulated wall. Thermal drapes will add more insulation value than argon-filled low-E glass.

If you want the best bang for buck, get generic double-glazing and uPVC frames, which are better than thermally broken alu. Then draw the curtains when it gets cold, making sure you have a good seal around the window to eliminate chimneys.

Aluminium joinery should last decades longer than PVC which should be taken into account with value for money.

The step up cost of low e glass is small relative to the cost of joinery and installation. Glass is a poor insulator so the still air between double glazing, and low e coatings are what provides most insulation value whereas increasing glass thickness makes little differences. Double glazing with a good low e coating really can achieve R1.0 from a thickness of 24mm inclusive of a 16mm argon air gap. The whole window won't achieve that much as PVC and thermally aluminium window frames lose more heat than good double glazing units. R0.9 glass like XCel in a non thermally broken frame can make a solid aluminium window mildly exceed the insulation value of a PVC framed window with standard glass. Standard double glazing glass without low e is only R0.35 (reference 12mm air gap) or R0.4 (best with 16mm argon gap) so quite weak although it does let through more solar heat.

Only very good thermal drapes will have a better insulation value than XCel. It's a good idea but most drape products aren't that good.

The AllSeasons profile supports multipoint locking like PVC frames as does Fairview Thermal. If you don't want to pay extra for it, the "Avon" handle is screwed in with 4 screws instead of the 2 used by others which has to count for something.

timmmay

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#2310277 3-Sep-2019 20:38

Geektastic:

Yes I'd have upvc if it was locally available, but it's not and we've learned that sticking to local options saves hassle later.

By locally do you mean in your little town? That's a small area. Thermalframe make uPVC in Lower Hutt, which is 45 minutes drive away, and others make it in Bulls.

Geektastic

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#2310292 3-Sep-2019 20:48

"How much heat thermally broken saves depends on what glass is in there, the quantity of exposed aluminium relative to glass and the outdoors temperatures. In an otherwise well insulated house single glazing can lose up to 50% of the heat. How many windows, doors and m2 of area are you getting, how well insulated is the house, how many degrees C does the room temperature sits above outside's, how much direct sunlight do the windows receive and where in the country is it?"

I'll try and answer that...! ;-)

There are 8 French Doors, 1 entry door and 16 assorted windows.

The house is concrete block, filled with concrete in the voids, on solid concrete foundations. Roof is butynl over steel with pink batts but not to current specs probably.

The front of the house faces the sun all day, the back of the house never sees it. It's 80km NE of Wellington.

timmmay

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#2310303 3-Sep-2019 21:09

I think you can expect that a house (or castle) that size has a lot of windows and will be expensive to heat or reglaze! How many hundred square meters is it?!

Geektastic

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#2310386 4-Sep-2019 08:24

timmmay:
I think you can expect that a house (or castle) that size has a lot of windows and will be expensive to heat or reglaze! How many hundred square meters is it?!

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#2310391 4-Sep-2019 08:52

That's a pretty decent sized house. Not surprised it costs a bit to heat / reglaze!

Geektastic

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#2310397 4-Sep-2019 09:11

It isn't so much the overall cost, it's more the fact that changing from an aluminium frame that does not have a thermoplastic spacer in to one that does adds 50% to the bill even though, in every other respect, the frames are identical.

Based on the helpful info above, it seems that paying that 50% is not even close to being worth it in terms of the cost savings in heating that it will create.

Even if you said our heating and cooling averaged $200 a month over the year, if the frame saves 2% of that annually, it still only amounts to $50 or so. That means it would take 300 years to recoup the expenditure in cost savings...!

There are some other benefits - well, one - which is that the inside of the frame would have little or no condensation on it, but the conditions for that probably only occur a few times in the year really.

timmmay

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#2310414 4-Sep-2019 09:51

PVC tends to cost a little more than non thermally broken aluminium frames but less than thermally broken. We've never had condensation on the frames. On the coldest Wellington days, maybe 5 - 10 days a year, we do get some condensation on the windows. I could reduce that by running the ventilation system to blow cold air in overnight, but I just have it running during the day before the heating comes on.

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#2310783 4-Sep-2019 20:49

The biggest factor from what I have experienced with thermally broken frames is where the break happens in relation to the wall's insulation line.

Imagine, for arguments sake, that the wall center line is the optimal position to continue the insulation line up through the line of the window opening. Now also consider that the window itself (due to the building code) is sitting out on top of the external wall cladding. This puts the thermal break you've just paid ~20k for out in the cavity line (a cold spot) or at best near the external line of the framing/insulation line. The cold can easily travel to the internal face of the window frame if it's so far out in front of the walls insulation line.

I would not recommend the extra cost of thermally broken window frames unless you really care about a slightly less amount of condensation (it will still be present with thermally broken frames), or you have designed the position of the frames in a way that allows them to be placed more centrally in the insulation line.

Some light reading is below, but there are plenty more studies from the likes of Proclima on more local conditions.

https://web.ornl.gov/sci/buildings/conf-archive/2013%20B12%20papers/006-Misiopecki.pdf

https://issuu.com/detail-magazine/docs/bk_passive_house_design_issuu_30/11

https://www.ecohome.net/guides/2261/how-to-install-windows-for-the-best-performance-and-durability/

Source: I am an architect.

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