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Today we answer the age-old question that has plagued humanity for centuries: Should east and west-facing windows have vertical or horizontal louvers? We used COMFEN software to test a 20' x 15' room in Austin, Texas with a 10' ceiling. Each room had a floor-to-ceiling west-facing window with a low-E IGU and one of three differently proportioned louvers. Those three louver spacing to louver depth ratios were 1:1, 1:0.67, and 1:0.33. Both vertical and horizontal louvers were tested for heat gain, HVAC energy costs, daylighting, and glare.The results: Horizontal louvers out-perform vertical louvers in almost every category! Unless they are sufficiently deep (or operable), vertical louvers are not as effective at preventing the high-angled summer sun from reaching the window. West-facing horizontal louvers offer similar benefits to south-facing horizontal louvers: They allow relatively low heat gain during warm months, and relatively high heat gain during colder months. Compared to a controlled scenario with no louvers, the average vertical louver saves about 12 kBtu per square foot each year, and horizontal louvers save almost twice as much energy at 23 kBtu/ft2-yr. This energy reduction translates to $410 in savings each year in HVAC costs with horizontal louvers, and $145 a year with vertical louvers. Horizontal louvers slightly out-perform vertical louvers in daylighting, providing an average of 112 footcandles more than the control, compared to 103 footcandles for vertical. Vertical louvers are a slightly better option for overall glare reduction, though both are much better than having no louvers at all. Additional tests revealed similar results for east-facing windows and for different locations around the country, proving that over the course of a year, horizontal louvers are typically the best option for east and west-facing windows.