Does window G value matter

fclauson

I thought I would share this with you - many windows are sold on U-value but there is a mutch more subtle g value (not Ug which is the U value of the glass). Although U value is important its only part of the storey

SF = Solar Factor, commonly used in Europe sometimes known as ‘g’ value is the same as Solar Heat Gain Coefficient (SHGC) used in the USA. The Solar Factor is the ratio of the total solar energy entering through the glass compared to the incident solar energy. This total energy is the sum of the solar energy entering by direct transmittance and the part of the energy absorbed by the glass and re radiated to the inside.

U value = the relevant insulation value of the glass as per EN673 calculated as Heat transfer through the glass

For a build it can make an amazing difference

Glass A : 0.6 U value – 37% g value
Glass B : 0.7 U value – 61% g value

Note: For DEAP changing values is harder to do as you have to also update the overall window U-value – so I have just altered the g value for purposes of demonstration
Glass A :

• PHPP: 19Kwh/M2
• DEAP: 3667 Kwh/Annum heat use

Glass B :

• PHPP: 14Kwh/M2
• DEAP: 2205 Kwh/Annum heat use

So a 66% reduction from DEAP calcs and 35% reduction via PHPP – WOW!!!!!!!

BryanF

if we assume that where large window units are installed there will be shading from the mid day sun, i wonder how important the G factor actually is in Ireland, given our generally overcast weather and often the need to capture every last ray... In northern Germany they install shutters on the west facing (evening) windows, you'll never see that here!! I guess what I'm asking is - should we be a little sceptical of the software packages figures and allow allow for more solar gain, even if that means a little over heating for the one week a year where the temp goes over 25c? (especially in thermal mass friendly homes)

sinnerboy

fclauson wrote: »

Glass A : 0.6 U value – 37% g value
Glass B : 0.7 U value – 61% g value

Where did you get these value from fc?

Typically one expects to find values like

Triple glazing Ug 0.7 and g value 0.5 ( or 50% )

Double Glazing Ug 1.2 and g value 0.6 ( or 60% )

Single Glazing Ug 5.6 and g value 0.87 ( or 87%)

In very basic terms the more glass the less solar gain we get and the greater thermal insulation we get.

Triple gazing tips the balance - the improved heat loss benefits us more than the reduced solar gain ( assuming of course that the building DESIGN is correct) .

But there is an over arching principle that you cannot see reflected in a spread sheet. The surface temperature of a triple glazed window will always be 2-3 degrees warmer than a double glazed window. Our human bodies are very sensitive to this temperature difference.

If one was to build two identical rooms facing the same direction and heated in exactly the same way to produce exactly the same internal ambient air temperature. One room with a triple glazed window and one with a double window. i.e. if this was absolutely the only difference between the two rooms.

Then persons using the room with the double glazed window would feel colder.
The lower surface temperature of the double glazing would cause this.

So anyone building a new house ( or replacing windows ) should fix hard on triple glazing. You building will feel nicer. In a way spreadsheets can't demonstrate.

fclauson

sinnerboy wrote: »

Where did you get these value from fc?

Typically one expects to find values like
...

Sinnerboy - Carey (and other fine makers of triple glazing units ) are now producing a range of units - see attached for an examples

0.7 U / 61%g
0.5U / 61% g

Eoghan Barra

From what I understand, the g-value (solar transmittance, or percentage of solar energy - ie heat - transmittance) of a window is not only dependent on whether it's single, double or triple glazed, but on the type of glass used.

It seems the lead content is what makes the difference. As using 3 panes of standard, high lead content glass (ie triple glazing) leads to a very serious reduction in g-value, some manufacturers are making triple-glazed windows with low lead glass. There may be more to it than that, but that's as much as I know.

The importance of this aspect is demonstrated by the fact that passive windows should have a minimum value of 0.5 (ie 50%). I found that few window makers, even those making high-end stuff, advertise the g-value of their products; if you ask, you will sometimes be told a value, but how reliable that might be is anybody's guess.

fclauson

Now have an update - the better glass adds costs (obviously ) so its about rerturn on investment

Using PHPP you can tweek it so that some windows are of one format and another group are of another

doing this I have managed to just increase my Kwh/M2 by just 1 Kwh/M2 - but reduce the additional glazing costs by 30 to 40%

needs refning - to check this out - but all interesting stuff

gears

For the less learned among us, would a lower or higher g% be better

fclauson

Higher is better - lets in more energy (unless of course your are so well insulated and there is an overheating problem)

Juantorena

Is there an argument that solar gain - and therefore concentration on a windows g rating - is not hugely relevant in Ireland (cloud cover during heating months)?

BryanF

Juantorena wrote: »

Is there an argument that solar gain - and therefore concentration on a windows g rating - is not hugely relevant in Ireland (cloud cover during heating months)?

yes (that's why when we say 'passive house', in Ireland at least, it still requires some heating) But its sunny today in Cork, so if I had good windows maybe my house would retain the heat...

fclauson

Juantorena wrote: »

Is there an argument that solar gain - and therefore concentration on a windows g rating - is not hugely relevant in Ireland (cloud cover during heating months)?

Sunny here in wexford too - wish my current house was passsive 'cause its also cold and the thermal bridges are working overtime to get rid of my heat:mad:

See the post I did above for the difference it can make - there is a piece of work done by a Cork Arch which shows just moving around the county of Cork can make 20% difference to the solar gain you get.

Juantorena

BryanF wrote: »

yes (that's why when we say 'passive house', in Ireland at least, it still requires some heating) But its sunny today in Cork, so if I had good windows maybe my house would retain the heat...

We have 10.4m of glazing (kitchen/dining/sittingroom) orientated sw, 23.4 degrees in the kitchen right now. Clear skies in Galway too. But a rare occurance. I wonder at the energy balance of such glazing (heat gain vs heat loss) as well as the financial balance all things considered.....again, in Ireland.

fclauson wrote: »

Sunny here in wexford too - wish my current house was passsive 'cause its also cold and the thermal bridges are working overtime to get rid of my heat:mad:

See the post I did above for the difference it can make - there is a piece of work done by a Cork Arch which shows just moving around the county of Cork can make 20% difference to the solar gain you get.

If you can, could you link or point to that study fclauson?

fclauson

PM sent (I don't want to be "muffled" :eek:)

Juantorena

fclauson wrote: »

PM sent (I don't want to be "muffled" :eek:)

Thanks.

muffler

fclauson wrote: »

PM sent (I don't want to be "muffled" :eek:)

fast learner

seanhenno

Solar gain . I work for a window manufacturing company. Look at it this way, how much sun is there in the winter? How much heat will you gain in the winter when you actually want heat? So yeah, you can look at it in a theoretical way, but in reality heat retention far surpasses heat gain. =D

fclauson

Not sure of your point here - I would have to go back to my maths but the glass defiantly gets warm in winter and that is g value

sinnerboy

No fc that is a function of U Value.
Glazing units which have better ( lower) U Values will have higher internal surface temperatures.

fclauson

Nope it's actually a function of bothUvale will stop heat leaving the building but g value will determine how warm the glass gets from absorption of energy from the out side

seanhenno

the g value of standard triple glazed is .34 and .4 for double glazed. the u-value is 1.1 for the double glazed and .6 for the triple glazed glazing unit. So there's only a .06 difference which is pretty small , and your trying to tell me that this little difference combined with the fact that no matter where you are in ireland, we don't get that much direct or even strong sunlight during winter, would exceed the far superior heat retention that triple glazed has? note that the lower the u-value the better and the higher the g value the better.

sinnerboy

fclauson

seanhenno wrote: »

the g value of standard triple glazed is .34 and .4 for double glazed. the u-value is 1.1 for the double glazed and .6 for the triple glazed glazing unit. So there's only a .06 difference which is pretty small , and your trying to tell me that this little difference combined with the fact that no matter where you are in ireland, we don't get that much direct or even strong sunlight during winter, would exceed the far superior heat retention that triple glazed has? note that the lower the u-value the better and the higher the g value the better.

You can different types of glass which have different g vales - see my original post

I optimised my build so that I had good u values (triple glazed units ) AND high g value

In the winter the internal glass can often be at a higher temp than the room air temp thus providing solar gain to the room while he goodby value provides low verbal loss