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Better Windows for Efficient Florida Homes

Windows are almost always known to be the largest source of cooling loads in Florida residential buildings. Most know that a low Solar Heat Gain Coefficient (SHGC) is a must to keep out the sun's heat, and a low conductance or U-value (Btu/hr-sqft- F) is important to reduce design cooling load. The most common windows used in Florida homes hardly meet these needs. Typically, windows are single pane clear glass with aluminum frames (SHGC= 0.875, U= 1.1).

For the PVRES home, we desired to locate the very best performing product available for the residential market. However, simply a low SHGC could not be the sole objective; very low solar heat gain values can indicate windows with a dark tint that transmit very little visible light. This can reduce daylight to such a point that occupants have to use electric lighting to supplement -- certainly not reducing energy use. And physical appearance is also important; this generally excludes either dark tints or reflective products which give the outside of the building a mirrored look. We desired a spectrally-selective product. This means a glazing which transmits much of light in the visible portion of the solar spectrum, but limits transmission in the infrared and ultraviolet portions which cause overheating and fading of interior materials.

We selected PPG Industries' Sungate 1000 solar control, low-E glass product with Argon gas fill. As shown in the table below, Sungate 1000 has a SHGC of only 0.38 and center-of-glass U-value of 0.24. We reduced heat transmission through the window frame by specifying white thermally broken vinyl frames (overall U-value= 0.35). What does this mean for the required size for the air conditioning system? Plenty. With 384 square feet of glass in our floor plan, calculation with Manual J showed a 7,700 Btu/hr difference (0.64 tons) in the required size of the air conditioning system — a large reason why we are able to use an air conditioner half the size of that in the standard building. And the Sungate windows are spectrally selective, providing a daylight transmittance of 73% against 90% for single glazing as well as a color neutral appearance.

Air Space (inches)	Glass Thickness (mm)	Unit Construction	Daylight(%)		Shading Coeff- icient	Solar Heat Gain Coefficient	U-Value Center-of-Glass		R-Value Center-of-Glass		Indoor Glass Temperature Winter-oF
			Trans- mittance	Reflec- tance			Winter	Summer	Winter	Summer
-	3.0	Clear, monolithic	90	9	1.00	0.85	1.11	1.03	0.90	0.97	16
1/2	3.0	Clear & Clear	81	16	0.87	0.75	0.49	0.55	2.04	1.82	45
1/2	3.0	Sungate 1000(2) & Clear	73	12	0.45	0.39	0.30	0.30	3.33	3.33	54
1/2	3.0	Sungate 1000(2) & Clear with Argon gas fill	73	12	0.44	0.38	0.24	0.23	4.17	4.35	57

 

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