IGU's are advantageous if you are looking for increased value from your dollar. An IGU is a complex appliance, which if constructed and installed correctly, will give many years of trouble free service.
IGU's are two panes of glass, bonded to a spacer and separated by a sealed air space. Because this space is fully sealed, it acts as an insulator, keeping your home at a comfortable temperature in both Summer and Winter. The air gap between the glass is a critical factor in achieving good insulation values, the larger the air gap better the U value. Dargavel Windows Pty Ltd use a 12 mm air gap in all their windows and doors except for the sliding window which has an 8 mm air gap. It is also important to have well designed window and door frames to accommodate the insulating glass to achieve the best results. The most cost effective and efficient window is the double glazed MK15 Awning.

Ordinary windows may not insulate as well, allowing warmth to escape in Winter and admitting heat in Summer. A modern home with ordinary glass can lose up to 50% of its heat through windows.
Insulating Glass Units significantly reduce heat transfer through windows in two ways:

• They trap a layer of sealed dry air, which acts as a natural insulator.
• They reduce the transfer of heat by radiation, where the glass itself warms and passes heat.

Because the sealed air in the IGU is dry, there is no condensation in the unit. Condensation on the inside of the window is also dramatically reduced.

By reducing the transfer of heat from the warm to the cold side of the window, IGU's can significantly reduce loss of warmth in cold weather. For even greater performance, Low Emissivity (Low-E) glass can be incorporated into IGU's. This invisible, metallic coating on the glass enhances its insulating properties, providing greater energy efficiency and comfort.
However, Low-E glass and use of argon filling within the units comes at a considerably higher cost.
Heat gain through windows in Summer depends on many factors, such as; shading, direction of window, outside temperature, even wind speed.
Where windows are in direct sun, or subject to high radiated heat, tinted (toned) or reflective glass in an IGU can greatly reduce heat gain by in effect shading the window.
Incorporating Low-E glass as the inside pane not only improves performance but also helps retain warmth in the cooler times.

U-value measures how well a product prevents heat from escaping. It is a measure of the rate of non solar heat loss or gain through a material or assembly. U-value ratings generally fall between 2.0-10.0 W/m2.K for Australian products. The rate of heat is indicated in the terms of the U-value of a window assembly which includes the effect of the frame, glass, seals and any spacers. The lower the U-value, the greater a window's resistance to heat flow and the better its insulating value. The U-value for a window takes account of the various U-values for the components making up the window, so you may see these in technical literature; UW is the value for whole window and because of its importance is usually abbreviated to U.

R-value and U-value are essentially two sides of the same coin. R-value is usually cited when discussing things such as wall and ceiling insulation values. The term does not translate well to windows and other fenestration products. That industry prefers U-values. The two are actually inversely related. The higher the R-value, the better insulated are the walls and ceilings. The lower the U-value, the better job a window does in keeping out the heat and cold.

SHGC measures how well a product blocks heat caused by sunlight. The SHGC is the fraction of incident solar radiation admitted through a window, both directly transmitted, and absorbed and subsequently released inward. SHGC is expressed as a number between 0 and 1. The lower the figure the heat gains, the higher the figure the more heat gain. Grey or tinted glass has a lower shgc, clear glass has a higher shgc.

Visible transmittance measures how much light comes in through a product. It is an optical property that indicates the amount of visible light transmitted. VT is expressed as a number between 0 and 1. The higher the number, the more light is transmitted.

Reference Notes:

1. Uw is the whole window U-value
2. SHGCw is the whole window solar heat gain coefficient
3. Tvw is the whole window visible (light) transmittance
4. Percentage improvement figures are compared with using base-case Generic Window 1 (3mm clear in standard aluminium frame)
5. A negative percentage improvement figure indicates performance worse than the base-case window
6. A positive percentage improvement figure indicates performance better than the base-case window
7. Maximum air infiltration is 5.0L/s.m2 at a positive pressure difference of 75 Pa as measured according to AS 2047
8. Static performance (Uw SHGCw Tvw Tdw) calculated using Window 5.2 and Therm 5.2 software (LBNL), 2000-2003
9. Annual energy performance (stars and % improvements) calculated using Nationwide House Energy Rating Software (AccuRate) according to procedures of WERS 2008.
10. Results disclosed at National Fenestration Rating Council (NFRC) regulations.

Window ID	Glazing	Cooling Stars	Heating Stars	Cool %	Heat %	UW	SHGC	TVW	Air Inf.
Capral Artisan Bi-Fold Door - Single Glazed
CAP-042-01	6Clr			31%	16%	6.0	0.55	0.57	0.33
Capral Artisan Bi-Fold Door - Double Glazed
CAP-043-01	6/12/6			47%	35%	4.2	0.47	0.49	0.33
CAP-043-02	6Gy/12/6			58%	26%	4.2	0.30	0.24	0.33
Mk7 Aluminium Sliding Window - Single Glazed
CAS-004-01	3Clr			9%	16%	6.5	0.75	0.77	2.10
CAS-004-04	6.38DLam			17%	16%	6.3	0.69	0.76	2.10
CAS-004-05	5Gy			30%	8%	6.4	0.54	0.43	2.10
Mk7 Aluminium Sliding Window - Double Glazed
CAS-004-07	3/10/3			33%	42%	4.1	0.64	0.66	2.10
CAS-004-10	5Gy/8/3			49%	32%	4.1	0.43	0.37	2.10
Mk15 Aluminium Awning Window - Double Glazed
CAS-006-04	3/12/3			39%	60%	4.1	0.57	0.58	0.29
CAS-006-05	5Gy/12/3			53%	51%	4.1	0.39	0.33	0.29
CAS-006-08	3/12Ar/4EA			45%	67%	3.3	0.54	0.53	0.29
CAS-006-09	5Gy/12Ar/4EA			59%	57%	3.3	0.34	0.30	0.29
Mk10 Aluminium Awning Window - Single Glazed
CAS-012-01	3Clr			20%	31%	6.6	0.65	0.65	0.29
CAS-012-04	6.38DLam			25%	32%	6.4	0.60	0.64	0.29
CAS-012-05	5Gy			35%	24%	6.5	0.47	0.36	0.29
Mk5C Aluminium Sliding Door - Single Glazed
CAS-016-01	5Clr			15%	19%	6.2	0.71	0.75	1.47
CAS-016-05	5Gy			32%	11%	6.2	0.52	0.42	1.47
41mm Aluminium Casement Window - Double Glazed
CAS-019-01	3/12/3			43%	59%	4.1	0.53	0.53	0.09
CAS-019-04	5Gy/12/3			55%	50%	4.1	0.36	0.30	0.09
CAS-019-06	3/12Ar/4EA			48%	64%	3.4	0.49	0.49	0.09
CAS-019-07	5Gy/12Ar/4EA			60%	55%	3.4	0.32	0.27	0.09
Platinum Sliding Door - Double Glazed
CAS-020-05	5/12/5			37%	42%	3.9	0.60	0.64	2.81
CAS-020-06	5Gy/12/5			51%	33%	3.9	0.41	0.36	2.81
CAS-020-07	5/12Ar/5			38%	43%	3.8	0.60	0.64	2.81
CAS-020-08	5Gy/12Ar/5			52%	34%	3.8	0.41	0.36	2.81
Platinum French Door: Open-in - Single Glazed
CAS-034-01	5Clr			28%	19%	5.8	0.60	0.62	1.91
CAS-034-05	5Gy			40%	11%	5.8	0.44	0.35	1.91
Platinum French Door: Open-in - Double Glazed
CAS-035-05	5/12/5			44%	40%	3.8	0.52	0.56	1.91
CAS-035-06	5Gy/12/5			55%	32%	3.8	0.36	0.31	1.91
CAS-035-07	5/12Ar/5			44%	41%	3.7	0.52	0.56	1.91
CAS-035-08	5Gy/12Ar/5			56%	33%	3.7	0.36	0.31	1.91

All above figures and references have been sourced from the WERS website.