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Double glazing recommendations please

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Started 2 years ago by Annette
Latest reply from TonyT

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tom
Member

Thanks Cava,

Our house is a lot smaller - 27 squares including garage, 30 if including verandas. Ceilings will be 3.3m, but as it will be a Victorian style house, the windows aren't floor to ceiling, or wall to wall. Biggest window will be 3.6m wide, and 1.8m high. So hopefully a lot less than your house. I was expecting/hoping $50K would buy us high quality/efficient windows.

Have you investigated buying/importing your own windows? I am planning on contacting http://www.seriouswindows.com/ as they have U values around 0.7 and 0.8, even after converting from the US units. Some other kind soul mentioned them here a while back.

Posted Tuesday 7 Feb 2012 @ 8:36:07 pm from IP #
ghostgum
Member

TonyT said:
Examples of calculations that can be used to estimate the amount of heat that is lost through single glazing, through "smart" glass and through double glazing are recorded at
http://www.ata.org.au/forums/topic/999#post-12539
and in several of the subsequent posts.

The figures in that posting are not correct for where I live in Melbourne. Using "heating degree days" figures, the average temperature difference between inside and outside is about 7C during the heating period (May to September), not the 15C used in those calculations. We also close curtains on most windows at night, further reducing heat loss through the windows.

I am considering using low-e film on some south facing windows 4.8m2 in area, that are double glazed with wood frame, and that will never have curtains. That means the current heat loss averages 4.8m2 * 7C * U3 = 100W, or 2.4kWh per day, 1300MJ per year, $23 natural gas per year.
From the WERS data, it looks like the difference between clear double glazing and low-E double glazing is about 10% improvement (U value 2.8 down to 2.5), so the gain is about $3 of gas per year. US prices suggest the low-E film will cost about $1200 for that window, making the payback time too long. Clearly I should have got low-E coated glass when I installed the windows, rather than as a retrofit.

Anyone got some hard figures to contradict this?

Posted Tuesday 7 Feb 2012 @ 11:04:08 pm from IP #
cava
Member

tom said:Have you investigated buying/importing your own windows?

There are many things that I would consider importing, however due to breakage, glass windows is not one of them. Good luck if you go this path.

Posted Tuesday 7 Feb 2012 @ 11:41:22 pm from IP #
dlvb19
Member

Annette,

I'm surprised you have not tried Stegbar.
looking at the other manufacturers I surmise you don't want wooden frames that require maintenance.

Anyway They were the second cheapest manufacturer I approached 12.5m^2 U = 1.9-2.0 SHGC=0.35-0.4 and the visible transmittance escapes me right now. Cedar Frame.
They were about 110% price of cheapest (EuroTech) and the most expensive (Parhammer of course) was ~280% of cheapest.
Trend of EuroTech would have been my next two choices.

IMHO argon filling is overcomplex and overpriced for the small gains made and may simply leak out over time (unsubstantiated).

Remember that visible transmittance is a very important metric: If your after views then god speed to you however if you are looking to brighten your rooms and your super-low U-value windows have a Visible transmittance of only 10% then perhaps you would be better with a smaller, cheaper plain single pane window?

Lastly remember no matter how insulating a window is it will always be the Achilles heel of your buildings thermal performance. I wanted to leave windows out of a few rooms in new house however Aust building standards state windows must be 10% floor space.
Try calculating heat loss per element of build (walls, roof, floor, windows, doors .etc) and see what the figures are, may be an enlightening exercise. Also windows are a very expensive element of build, try not to go crazy, unless you enjoy your day job and large mortgages.

Good Luck to you Old Bean

Posted Wednesday 8 Feb 2012 @ 4:22:00 am from IP #
Bushwalker
Member

cava said:
There are many things that I would consider importing, however due to breakage, glass windows is not one of them. Good luck if you go this path.

I disagree.

We imported a bunch of custom made double glazed windows (10 large panes) and doors (2 doubles and 1 large entrance with large sidelight) from Germany.

They arrived from the factory packed in a single custom crate. Nothing was broken and the fit and finish was better than we have ever had locally.

Would do it again. Superior product from a part of the world that understands window insulation and weather sealing.

Posted Wednesday 8 Feb 2012 @ 5:58:06 am from IP #
tom
Member

According to WERS ratings Sovereign triple glazed windows perform the best. Yet I have never heard any mention of them on this site. Are they poor quality, to expensive, or just not known about?

Posted Wednesday 8 Feb 2012 @ 8:34:27 pm from IP #
TonyT
Member

The heat-loss numbers below can be used to compare the cost of the heat energy that will be lost from

(a) single glazing with standard glass
(b) single glazing with "smart" glass
(c) double glazing with standard glass
(d) double glazing that includes one sheet of "smart" glass (inside)

For glass that is 4mm thick, the heat-loss values (U) in watts per square metre per degree C of temperature difference across the glass are

(a) 5.9 Watts/sqm per degC
(b) 3.6
(c) 2.7
(d) 1.9

These figures come from

http://www.viridianglass.com/products/downloads/Viridian%20Glass%20Performance%20Data%20-%20Energy.pdf

It is difficult to obtain firm prices to buy glass.
Some prices quoted in 2011 to buy 4mm glass are

(a) $85/sqm
(b) $160/sqm
(c) $170/sqm
(d) $245/sqm

Compared with glass, the heat energy that is lost through a wall is much less.

For example, for a timber-frame wall (costing about $30/sqm) built from
weatherboard, 25mm reflective air-space, 75mm fibreglass insulation, gyprock
the heat loss (U) = 0.4 watt/sqm per degC

Posted Wednesday 8 Feb 2012 @ 9:50:46 pm from IP #
Bushwalker
Member

What gap in your double glazing, Tony?

Ours are 18mm Argon filled.

Also consider frames and air ingress around seals.

Posted Thursday 9 Feb 2012 @ 12:17:27 am from IP #
TonyT
Member

Bushwalker

In the Viridian Table (see the link in my earlier post) the spacing that is used for double glazing is 12mm.

Within a post in one of the threads on this Forum, there is a link to a chart that suggests that 12mm is the optimum air gap to minimise the transmission of heat.

Can anyone help me to find that post ?

An air gap wider than 12mm is needed to minimise the transmission of sound.

What is the additional cost for Argon filling and by how much does the Argon reduce heat transmission ?

For how long will the Argon remain sealed in the air gap ?

Posted Thursday 9 Feb 2012 @ 9:26:50 am from IP #
TonyT
Member

The chart that shows that for double glazing the optimum air-gap is 12mm ( = 0.5 inch) can be seen by clicking on the link [ img ] near the bottom of the post at
http://forum.homeone.com.au/viewtopic.php?f=23&t=17758

Here is that chart

Posted Thursday 9 Feb 2012 @ 11:07:10 am from IP #
gazmo88
Member

Tony T asks:

What is the additional cost for Argon filling and by how much does the Argon reduce heat transmission ?

For how long will the Argon remain sealed in the air gap ?

---------------

By coincidence I read the following yesterday:

http://www.sovereignwindows.com.au/resources/brochures/Argon%20gas%20brochure.pdf

none of it anything to do with me

Posted Thursday 9 Feb 2012 @ 12:06:50 pm from IP #
Bushwalker
Member

TonyT said:

What is the additional cost for Argon filling and by how much does the Argon reduce heat transmission ?

For how long will the Argon remain sealed in the air gap ?

Thanks Tony,

18mm was the minimum spec from Germany, as was the Argon fill. They must know something over there we don't?

No idea how long the argon stays there. Aren't the window capsules sealed?

From your linked thread, P1

Studies show leakage is around 15% in 20 years. But the Argon concentration starts out 75% - 90%. Of course some double glazed units leak more which shows up as condensation and they get replaced.

Also from that thread, 2nd page:

I still think that manufactures (even low cost) should look at air tightness of the system. Even the NFRC rating and the computer modeling for the energy rating in Australia does not reflect the air infiltration as a problem, which is a major issue from my point of view. I think air infiltration if crucial to all other aspects, noise insulation, convection, conduction and finally U Value.
By not considering increased air movement, due to air infiltration, systems with high conductive frames and weak glazing, will always look better than they’d perform in real life. A thermal scan would easily proof that.

Couldn't agree more!

BW

Posted Thursday 9 Feb 2012 @ 1:39:26 pm from IP #
stib
Member

I'm currently building some timber casement windows for a room in an old house. Currently the room has windows right along the two exterior walls, about 12sqm in all, none of which open. I'm building the frames out of wood, and hadn't actually thought of double glazing until a friend suggested it. Looking at the frames – which I've built but not yet glazed – there is room for two sheets of glass with a 12mm gap, so it would be a small increase on the overall price of the project – basically just double the cost of the glass.

What I'm interested in finding out is whether just using two sheets of glass with a wood spacer is good enough - does it have to be totally air tight? Also, can you buy cut sheets of low-e glass, and how does it compare to regular glass in price?

Posted Saturday 14 Apr 2012 @ 12:46:18 pm from IP #
TonyT
Member

stib

Air in the atmosphere contains water.
If air enters the gap between the panes of glass so does the water.
When the temperature drops, air can hold less water so the excess water forms drops on the surface of the cold glass.
A compound like silica gel can be included within the sealed air-gap to absorb the water from the trapped air.

Some prices are at
http://www.ata.org.au/forums/topic/471/page/2?replies=44#post-24680

Posted Saturday 14 Apr 2012 @ 9:00:13 pm from IP #
Bushwalker
Member

Stib, depending on how you have designed your windows, you may find it easier and more effective to order a double glazing unit for each window. You can specify low-e, and on which face(s) and the unit will arrive ready to fit into your window.

Shop around for pricing as some glazers are getting competitive whilst others still think double glazing is an expensive luxury and price accordingly.

Posted Sunday 15 Apr 2012 @ 7:23:04 am from IP #
termite
Member

Yes, if you want double glazing, I would also recommend buying/fitting sealed glazing units, to avoid the contamination in the gap. No matter how clean it starts out, it would eventually show annoying spots. You would need to build them with one of the panes hinged, to enable periodic cleaning: complicated and with ongoing maintenance, and make an ongoing seal difficult.

Congratulations on building your own windows - what timbers are you using? Usually the sill needs to be a more sturdy/harder timber than the rest (traditionally a low-rot timber such as western red cedar which is very soft), to cope with additional wear and tear.

Posted Sunday 15 Apr 2012 @ 9:55:41 am from IP #
stib
Member

Thanks for the advice. Now the thing about this house is that it was owner-built, and by the looks of it it was made from whatever fell off the back of a truck at the time. So none of the windows are actually the same size. Do the double glazing units come in standard sizes or would I be able to get a dozen differently sized? Are there any recommendations about suppliers for the inner north of Melbourne?

Termite, I'm not building the sills, I'm just replacing panes of glass which don't open with casement windows. To my shame I'm just using Tas Oak from Bunnings (I never get time to go to a proper timber merchant and Bunnings is open late). I've tried to pick pieces which are close to quarter sawn, and I'm actually using two layers of 19x65 glued together to make a 48x65 frame, with the outer one rebated to fit the glass. I'm hoping that having two different pieces of timber will make it less prone to warping. Also, it makes doing the mortice and tenon joints easier - to make my mortice I just cut a dado out of each side before gluing them together.

Posted Sunday 15 Apr 2012 @ 12:51:07 pm from IP #
Bushwalker
Member

Stibb, the double glazing units are made to order. You can specify every one as a different size.

Sorry, cannot help with a supplier in Melbourne.

Posted Sunday 15 Apr 2012 @ 10:05:38 pm from IP #
jeffjj
Member

I have looked into double glazing several times, but the numbers don't seem to stack up. We are in a 3 BR brick veneer in Melbourne with aluminium framed, single glazed windows. Our total heating bill is around :$1100 annually, with a hydronic gas heater. If double glazing halved the heat loss through the windows ( which is maximum 20% of house heat loss according to Your Home, 3 rd edition, page 1.6a), we would save $110/ year which will take a ridiculous length of time to pay back-- surely you would get way more bang for your buck with improved insulation in the roof space, extermal shutter for east and wet facing windows etc.

Posted Sunday 6 May 2012 @ 6:09:43 am from IP #
TonyT
Member

jeffjj

By using town-gas hydronic heating your cents-per-kWh cost of heating would be hard to beat.

How does your heating cost compare with the 0.5kWh of heat per 24hrs per square metre of floor area that is being achieved in snow country at Mt Best ?

http://www.ata.org.au/forums/topic/3352#post-26759

Using
http://www.ata.org.au/forums/topic/471/page/2#post-24680
your potential savings in (heating-kWh and hence) heating-$ from double glazing would be in the ballpark with what you have estimated.

Sealing gaps, increasing insulation, fitting curtains and pelmets will probably give you the best return on dollars invested.

Posted Sunday 6 May 2012 @ 6:41:21 am from IP #
ghostgum
Member

jeffjj, I think the payback time is about 50 years for a 5 star rated house.
It does improve comfort in the house because you don't get cold spots near the windows (except first thing in the morning when the hydronic heater is running hard and you have just opened the curtains).

We installed double glazing when we built 5 years ago in Melbourne. Heating bill is about $600 annually, with hydronic gas heating. With R4 insulated ceiling and R2 insulated walls, my calculation is that about half our heat loss is through the windows, even with double glazing.

Posted Sunday 6 May 2012 @ 11:20:36 pm from IP #
TonyT
Member

ghostgum

How does your heating cost compare with the 0.5kWh of heat per 24hrs per square metre of floor area that is being achieved in snow country at Mt Best ?

http://www.ata.org.au/forums/topic/3352#post-26759

Posted Monday 7 May 2012 @ 1:06:56 am from IP #
ghostgum
Member

A five star house in Melbourne uses 149MJ/m2 per year.
http://www.nathers.gov.au/about/starbands.html
If the heating period is 150 days, that's 1MJ/m2 per day, or 0.28kWh/m2/day. Where does that 0.5kWh per 24hrs per m2 for Mt Best come from? Are we talking about average energy consumption over winter, or the consumption for the worst day of winter?

Our house is about 4.5 stars.

Mt Best is not snow country. It's 10-15km inland, on the south edge of the Strzlecki Ranges.

Posted Monday 7 May 2012 @ 2:01:38 am from IP #
TonyT
Member

gg

Thanks for your correction about the location of Mt Best.

At Mt Best they are using a heat pump with COP approx 4 and electrical input of one kW to heat a floor area of 200sqm.

I calculated that their heat pump can deliver
1kW x 4COP x 24hrs/day = 96kWh of heat over 24hrs.

If the heat pump runs for 24 hrs,

96kWh of heat to heat 200sqm is 0.5kWh/sqm

In fact, at

http://mtbest.net/heat_pump.html

it says

" ... house needs 16.3 Watt per square metre of living area to maintain the interior temperature 12°C higher than outside."

16.3W x 24hrs = 391Wh = 0.39kWh per day

Has the software at

http://www.nathers.gov.au/about/starbands.html

been used to publish a table that displays the target heating kWh/day value for each star rating ?

Posted Monday 7 May 2012 @ 3:23:52 am from IP #
ghostgum
Member

TonyT, have you looked at "heating degree days" figures? For Melbourne, the figure is closer to 7 degrees over the heating period, so the Mt Best heating requirement is probably closer to 0.39 * 7 / 12 = 0.23kWh per m2 per day.

My house is closer to 40000MJ / 150 days (heating period) / 220 m2 / 3.6MJ/kWh = 0.336kWh per m2 per day. That's a 4.5 star house, and a house built now should be better than that. A 7 star house in Melbourne would average 98MJ/m2/year / 150days / 3.6MJ/kWh = 0.18kWh per m2 per day.

That starbands table was the "starbands.pdf" link on that page. The figures are in MJ per m2 per year, so you need to know how much of the year is heating and cooling, and then convert to kWh. MJ is a simpler energy unit. The climate for Mt Best is probably closer to East Sale than Melbourne.

Posted Monday 7 May 2012 @ 5:15:10 am from IP #
jeffjj
Member

TonyT

I am totally confused now, the 2nd link you provided (to viridian glass) seems to show fabulous reductions in heat loss which I cannot correlate with the table I referred to (page 1.6a of 3rd edition Your Home), I read the Your Home table as showing maximum 20-30% reduction achieved with optimal glazing (at huge expense!)which is much less than Viridian seem to be saying. Or have I misunderstood it? In any case, in principle of course I am all in favour of anything that reduced energy use in the house....but at what price? And also how long does it take to recover the embedded energy required to produced double glazed windows?

Posted Monday 7 May 2012 @ 5:40:43 am from IP #
jeffjj
Member

I also forgot to mention that if you have your curtains closed all evening (in winter) and overnight, and pelmets (as I do), there are probably marginal gains only from double glazing at all, except during the day when hopefully there is a little sunshine and hence passive solar heating. At least that is what I understand from the Your Home technical manual.

Posted Monday 7 May 2012 @ 5:43:42 am from IP #
TonyT
Member

From
http://www.ata.org.au/forums/topic/471/page/2#post-24680

Heat loss through single glazing
5.9W/sqm per degC
Heat loss through double glazing
2.7W/sqm per degC
Potential saving (5.9 - 2.7)=3.2W/sqm per degC

Inside temperature ? 22degC
Average outside temp when not covered by curtains ? 15degC

Save 3.2 x (22-15) = 22.4W/sqm

Hours of heating when not covered by curtains ? 5hrs/day

Save 22.4 x 5 = 112Wh/sqm per day = 0.112kWh/sqm per day

Multiply by your cost per kWh of heat(? 15cent/kWh for town gas) and by your number of heating days/yr (? 150days/yr) gives your annual saving

0.112kWh/sqm per day x $0.15/kWh x 150days/yr = $2.52 per sqm per year

Compare the cost for a second sheet of glass at ? $85/sqm.

Double glazing can be useful when:

no curtains (because want night views e.g. of city lights or of lighted garden)
low outside temperature
high cost per kWh of heat
room heated for many hours per day

Posted Monday 7 May 2012 @ 6:30:59 am from IP #
dlvb19
Member

Embodied energy of Double Glazed windows:

Approx 10kg/m^2 (mass) for DG @ Approx 3.5 kW.h/kg (embodied energy).
Therefore 10 x 3.5 = 35kWh/m^2 of double glazing.

With 15 square meters of glazing = 525kWh, it would take a while to pay back but probably a year or two for most people. A colorbond roof for instance (just the sheet metal, not the structural bits) embodies approx 10,000kW.h for 200sqm roof. Just to put it in perspective.

Posted Monday 7 May 2012 @ 8:40:18 am from IP #
jeffjj
Member

I just worked out that we consumed about 73000MJ (? Megajoules) of gas for heating purposes over the past 12 months... How does that relate to kWh- I am confused.
We run the hydronic heater 6-9am and 5-11pm except on a few REALLY cold days especially on the weekend where it stays on all day. Hence the curtains would be open maximum 2 of those hours on 90% or so of the days. The thermostat is set to 20 degrees C but if ambient temperature drops to 15C during the day it turns on automatically, this happens occasionally ( Melbourne!).

Posted Monday 7 May 2012 @ 10:40:20 am from IP #

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