Related Resources: heat transfer

Heat Transfer Coefficients U-Factor for Windows

Heat Transfer Engineering
Thermodynamics

Overall U-factors (heat transfer coefficients) for various windows and skylights in W / (m² · °C) (from ASHRAE Handbook of Fundamentals, Ref. 1, Chap. 27, Table 5)

Aluminum frame
Glass section (glazing) only					(without thermal break)				Wood or vinyl frame
Type	Center of glass		Edge of glass		Fixed	Double door		Sloped skylight	Fixed		Double door			Sloped skylight
Frame width	(Not applicable)				32 mm (1 1/4)	53 mm (2 in.)		19 mm (3/4 in.)	41 mm (1 5/8 in.)		88 mm (3 7/18 in.)			23 mm (7/8 in.)
Spacer type	—	Metal		Insul.	All		All	All	Metal	Insul.		Metal	Insul.	Metal	Insul.
Glazing Type
Single Glazing
3 mm (1/8 in.) Glass	6.30	6.30		-	6.63		7.16	9.88	5.93	5.57		-	5.57	7.57	-
6.4 mm (1/4 in.) acrylic	5.28	5.28		-	5.69		6.27	8.86	5.02	-		4.77	-	6.57	-
3 mm (1/8 in.) acrylic	5.79	5.79		-	6.16		6.71	9.94	5.48	-		5.17	-	7.63	-
Double Glazing (no coating)
6.4 mm air space	3.24	3.71		3.34	3.90		4.55	6.7	3.26	3.16		3.20	3.09	4.37	4.22
12.7 mm air space	2.78	3.40		2.91	3.51		4.18	6.65	2.88	2.76		2.86	2.74	4.32	4.17
6.4 mm argon space	2.95	3.52		3.07	3.66		47.32	6.47	3.03	2.91		2.98	2.87	4.14	3.97
12.7 mm argon space	2.61	3.28		2.76	3.36		4.04	6.47	2.74	2.61		2.73	2.60	4.14	3.97
Double Glazing [ ε = 0.1, coating on one of the surfaces of air space (surface 2 or 3, counting from the outside toward inside)]
6.4 mm air space	2.44	3.16		2.60	3.21		3.89	6.04	2.59	2.46		2.60	2.47	3.73	3.53
12.7 mm air space	1.82	2.71		2.06	2.67		3.37	6.04	2.06	1.92		2.13	1.99	3.73	3.53
6.4 mm argon space	1.99	2.83		2.21	2.82		3.52	5.62	2.21	2.07		2.26	2.12	3.32	3.09
12.7 mm argon space	14.53	2.49		1.83	2.42		3.14	5.71	1.82	1.67		1.91	1.78	3.41	3.19
Triple Glazing (no coating)
6.4 mm air space	2.16	2.96		2.35	2.97		3.66	5.81	2.34	2.18		2.36	2.21	3.48	3.24
12.7 mm air space	1.76	2.67		2.02	2.62		3.33	5.67	2.01	1.84		2.07	1.19	3.34	3.09
6.4 mm argon space	1.93	2.79		2.16	2.77		3.47	5.57	2.15	1.99		2.19	2.04	3.25	3.00
12.7 mm argon space	1.65	2.58		1.92	2.52		3.23	5.53	1.91	1.74		1.98	1.82	3.20	2.95
Triple Glazing [ ε = 0.1, coating on one of the surfaces of air spaces (surfaces 3 and 5, counting from the outside toward inside)]
6.4 mm air space	1.53	2.49		1.83	2.42		3.14	5.24	1.81	1.64		1.89	1.73	2.92	2.66
12.7 mm air space	0.97	2.05		1.38	1.92		2.66	5.10	1.33	1.15		1.46	1.30	2.78	2.52
6.4 mm argon space	1.19	2.23		1.56	2.12		2.85	4.90	1.52	1.35		1.64	1.47	2.59	2.33
12.7 mm argon space	0.80	1.92		1.25	1.77		2.51	4.86	1.18	1.01		1.33	1.17	2.55	2.28

Notes:

(1) Multiply by 0.176 to obtain U-factors in Btu/h · ft² · °F.

(2) The U-factors in this table include the effects of surface heat transfer coefficients and are based on winter conditions of -18°C outdoor air and 21°C indoor air temperature, with 24 km/h (15 mph) winds outdoors and zero solar flux. Small changes in indoor and outdoor temperatures will not affect the overall U-factors much. Windows are assumed to be vertical, and the skylights are tilted 20° from the horizontal with upward heat flow. Insulation spacers are wood, fiberglass, or butyl. Edge-of-glass effects are assumed to extend the 65-mm band around perimeter of each glazing. The product sizes are 1.2 m x 1.8 m for fixed windows, 1.8 m x 2.0 m for double-door windows, and 1.2 m x 0.6 1 8m for the skylights, but the values given can also be used for products of similar sizes. All data are based on 3-mm ( 1/8 -in.) glass unless noted otherwise.