U values and solar gain

refurb15

Hello
I am planning to replace the windows in my house with double or triple aluclad windows and have some questions about U values and solar gain.

The house faces south and gets a fair amount of sun, and even the November sunshine heats the south facing rooms nicely, though of course the windows are a mix of single glazing and old double glazing. Because of this, it seems to me sensible to try to go for windows with good solar gain.

Here are my questions
1. The example WEP on the NSAI page on Window Energy Performance gives a Solar Factor and the Solar Energy Transmittance figure. Which of these do window suppliers usually quote as a g figure?
2. If I aim for windows of a Uw value of around .9 to 1.0, what g values can I reasonably expect in a double or triple glazed aluclad window?
3. In order to raise the solar gain, does that necessarily mean that the U value has to increase as well? If so, what kind of trade off makes sense between them?

Thanks in advance!

fatty pang

refurb15 wrote: »

Here are my questions
1. The example WEP on the NSAI page on Window Energy Performance gives a Solar Factor and the Solar Energy Transmittance figure. Which of these do window suppliers usually quote as a g figure?

The ‘Solar Factor’ given on the WEP is for the whole window
The ‘Solar Energy Transmittance’ on the WEP is for the glazing.

The ‘Solar Factor’ is only useful for entering into DEAP to calculate a BER. You will only find it on NSAI WEP certificates so it’s effectively useless for comparative purposes.
The ‘Solar Energy Transmittance’ is for the glazing only. The relevant standard - EN 410 ‘Glass in Buildings. Determination of luminous and solar characteristics of glazing’ – refers to ‘the total solar energy transmittance (solar factor)’ . This is the glazing industry standard definition.

refurb15 wrote: »

2. If I aim for windows of a Uw value of around .9 to 1.0, what g values can I reasonably expect in a double or triple glazed aluclad window?

The framing material and design of the windows will have a bearing on the Uw value, i.e the same glass will give varying Uw depending upon the frame. To get Uw of anything less than 1.2 you will need to go to triple-glazing. Triple-glazing comes in different widths. Ideally the cavities should be 16-20mm wide. Then Ug (u-value of the glazing) would typically vary from 0.5 to 0.7. The Solar Factor – per EN410 – could be anywhere from 0.37 up to 0.63. It very much depends on the coatings, their location and if low-iron glass is used.[/QUOTE]

refurb15 wrote: »

3. In order to raise the solar gain, does that necessarily mean that the U value has to increase as well? If so, what kind of trade off makes sense between them?

Typically yes. A low-emissivity coating by definition restricts the transmission (emission) of energy.
Low E coatings, depending on their composition are designed to limit long wave (internally radiated heat) and/or short wave (solar energy) energy. Changing one will have an effect on the other.
Trade off is the $64,000 question. Personally I would advocate a ‘fabric first’ approach. Restrict your heat losses. Any heat to be gained would be an additional bonus. High solar gain glazing also can cause overheating issues during spring/summer with large south facing windows.

refurb15

Thanks very much, fatty pang.

Do you mean that what window suppliers quote as a g value corresponds to the Solar Factor? And in that case, you are saying that the g value (i.e. Solar Factor) is not useful in comparing the solar gain in e.g. two different windows with similar U values? If so, how would two windows be compared in terms of solar gain?

Maybe solar gain is a minor issue for another reason. At present, I have some large south facing windows that are either single glazed or with double glazing that is maybe 20 or more years old. Presumably they let in a lot more solar heat than a new double or triple glazed window would-- is that right? If so, even if I try to achieve windows with higher solar gain, it will be so much less than from the existing windows that it will seem much less of an advantage? But my interest in solar gain is more in perception than in the actual heat gained--i.e it's quite pleasant to sit by a warm sunny window: presumably new windows with U values of 0.9-1.0 will be just as sunny but a lot less warm, or not?

fatty pang

refurb15 wrote: »

Do you mean that what window suppliers quote as a g value corresponds to the Solar Factor? And in that case, you are saying that the g value (i.e. Solar Factor) is not useful in comparing the solar gain in e.g. two different windows with similar U values? If so, how would two windows be compared in terms of solar gain?

What I am saying is use the ‘g’ value/ solar factor/Total solar transmission figure that is provided for the glass only. What is described as the ‘Solar Factor on the WER cert is no use for comparative purposes.

refurb15 wrote: »

Presumably they let in a lot more solar heat than a new double or triple glazed window would-- is that right?

Yes

refurb15 wrote: »

If so, even if I try to achieve windows with higher solar gain, it will be so much less than from the existing windows that it will seem much less of an advantage?

Qualified yes. Its relative

refurb15 wrote: »

presumably new windows with U values of 0.9-1.0 will be just as sunny but a lot less warm, or not?

Qualified yes again. It will still be sunny but not quite so as the daylight transmission will be reduced from circa 90% with single pane glazing to circa 70% with coated triple-glazing

refurb15

thanks again, fatty pang. still confused about the g value/solar factor I'm afraid.

if g value quoted by suppliers is not useful in comparing windows, is there an alternative measure that is, and if so what is it?

RITwing

refurb15 wrote: »

Here are my questions
1. The example WEP on the NSAI page on Window Energy Performance gives a Solar Factor and the Solar Energy Transmittance figure. Which of these do window suppliers usually quote as a g figure?
2. If I aim for windows of a Uw value of around .9 to 1.0, what g values can I reasonably expect in a double or triple glazed aluclad window?
3. In order to raise the solar gain, does that necessarily mean that the U value has to increase as well? If so, what kind of trade off makes sense between them?

1. In my experience - suppliers will tend to give the apparently better figure. In this case the Solar Energy Transmittance figure states the amount of heat that will transmit through an unframed sheet of glass / glazing unit which is faced at an angle which places it perpendicular to the rays of the sun. The solar factor takes account that the glass is in fact placed perpendicular to the ground, not the suns rays and that the glass is placed within a frame which will cast shadows, both realities reducing the amount of heat gain.

2. There is no direct relationship. Cavity fill gases , cavity widths and coatings on the glass panes will all have an affect.

3. Reduce the U value then reduce the U value then reduce the U value then reduce the U value then reduce the U value then reduce the U value then reduce the U value . Then look at solar gain.

refurb15

thanks, RITwing-- especially for your decided view on U vs g!

BarneyMc

RITwing wrote: »

3. Reduce the U value then reduce the U value then reduce the U value then reduce the U value then reduce the U value then reduce the U value then reduce the U value . Then look at solar gain.

I agree. I had to decide between 2 types of glazing, one with poorer U value but better g value versus better U value but poorer g value. The differences were minimal but went the U value route in the end.