Block the Sun Before it Enters Your Windows

In recent years, sun shading has become a primary consideration to reduce energy consumption in homes and buildings. Since windows are the largest consumer of energy in a structure, blocking direct sunlight from entering windows can dramatically improve the performance of any cooling system. Conversely, allowing the sun to enter during the winter months can improve heating performance. A shading system that does both is the key to lowering energy consumption and cost.

Interior vs. Exterior Shading

Interior Shading

Interior window shades provide a benefit to occupants by reducing glare, direct sunlight, and heat gain.  However, interior shading provides a minimal energy reduction. This is because interior shades allow the sun to enter the home or building (Figure 1).

Figure 1

Interior Sunshade

Once the sun breaks the plane of a window, heat gain is generated.  This heat is captured by floors, furnishings, walls, etc.  By shading a window on the building interior, we are blocking the heat from hitting these locations.  Instead, the heat is captured by the shade.  But since the sunshade is located within the structure, this heat must ultimately be overcome by the cooling system.

Therefore, shading the interior of a window provides comfort for the occupants, but little energy cost benefit.

Exterior Shading

Exterior window shades block the sun before it penetrates the windows. Any heat gain is captured by the shading device and is dissipated by outside air (Figure 2).

Figure 2

Exterior Sunshade

By utilizing an exterior shade, we are provide the benefits to occupants while adding energy savings due to reduced heat gain within the building.  Blocking the sun prior to the window is the best way to improve cooling performance.

Summer and Winter Savings

Sun entering through windows during the summer months is always an energy consumer due to increased cooling costs. But did you know that in most climates, sun entering through windows during winter is a natural energy saver? That sunshine heats the interior of the building or house and reduces heating costs.  So how do we block the summer sun and let it enter during the winter?

The simple solution is to let nature to it for you.  The sun angle during summer is much steeper than it is during winter.  By properly sizing and locating sunshades (see article), the sun can be blocked during summer and allowed to enter during winter months (Figure 3).

Figure 3

Minimal Shading in Winter

Full Shading in Summer

Best Sun Shading Practices

Window sunshades provide occupants with comfort in all seasons. But blocking it before it hits window surfaces is far superior to any interior shading. Also, by blocking the summer sun and allowing the entry of winter sun, energy savings can be dramatically improved.  Proper sizing and locating of exterior sunshades is the key to capturing these benefits in one device.

How Louvers Work at www.archlouvers.com

louver is a ventilation product that allows air to pass through it while keeping out unwanted elements such as water, dirt, and debris. A number of fixed or operable blades mounted in a frame can provide this functionality. The basic considerations for selecting louvers are Free Area, Water Penetration, and Resistance to Airflow (Pressure Loss). Once these concepts are understood, they can be used to properly apply a louver.

Free Area

Free area is derived by taking the total open area of a louver (after subtracting all obstuctions - blades and frame) and dividing by the overall wall opening. This gives a comparison of a louvered opening to an unobstructed opening. Common louver free areas range from 35% to 60% of the wall opening (65% to 40% obstructed). A high percentage free area is beneficial because more air can enter into a smaller wall opening, reducing the cost of the wall opening and louver.

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Obviously some obstruction is required in order to keep undesirable water out. A fully obstructed opening would allow no water in, while a totally unobstructed opening would allow water to enter unimpeded. A properly designed louver will maximize free area while allowing a minimal amount of water to enter. back to top

Water Penetration

First Point of Water Penetration is the point at which a louver first allows the passage of water through the louver. It is a threashold measurement of air intake velocity at which the louver will begin leaking (in feet per minute or fpm).

The typical method of testing for water penetration is to intake air through the louver while applying a measured water content into the airstream. The velocity of airflow through the louver free area is increased until the louver allows water to enter. The result of this test is the first point of water penetration - ranging from 300 fpm (a very poor resistor to water entrainment) to 1250 fpm (a very good resistor to water entrainment). back to top

Resistance to Airflow

Every obstruction in the airstream creates resistance - louvers, ductwork, filters, coils, building structure, etc. The resistance of the louver can be measured by running air through the louver and measuring the pressure differential at various free area velocities (measured in water gauge or wg). Every louver will create resistance based on the frame and blade shapes. Lower blade angles or more aerodynamic shapes create less resistance. We must know the free area velocity through the louver in order to properly evaluate the overall resistance to airflow. For a majority of applications, we can calculate the pressure loss of the louver at the required free area velocity and determine if it is acceptable. The resistance created can be detrimental to the application of fans and other air movement equipment, so we should attempt to minimize it.

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Applying the Principles

To properly evaluate a louver's capability, we must have a method to include both the Free Area and the First Point of Water Penetration in a meaningful way. Since the overall objective is to get as much air as possible through the louver, we want to evaluate the allowable volume of air through the louver (cubic feet per minute or cfm). Test methods for these principles are covered in AMCA Standard 500-L Laboratory Methods of Testing Air Louvers for Rating. The following example compares two louvers for a wall opening size of 48" wide x 48" high with different performance caracteristics:

Louver	Free Area (percentage)	Free Area for 48" x 48" (square feet)	First Point Water (fpm)
1	45%	7.2	1190
2	53%	8.5	750

Since our objective is to get more air through the louver, we might assume that Louver 2 is better than Louver 1, since it has a higher free area. However, more evaluation is required. The real question is, "How much total air can I get through the louver without entraining water?"

Louver	Free Area (percentage)	Free Area for 48" x 48" (square feet)	First Point Water (fpm)	Design Velocity (fpm)	Volume of Air (cfm)
1	45%	7.2	1190	893	6426
2	53%	8.5	750	563	4781

View performance of Architectural Louvers

Louver 1 has a free area of 45% for a size 48" wide x 48" high wall opening. The total square feet of free area is 7.2 ( = 45% x 16 sq ft of wall opening). The tested First Point of Water Penetration for this louver is 1190 feet per minute free area velocity. We should build in a safety factor for some variation in our airflow through the louver - we have chosen 25% safety factor. The design velocity including the safety factor would be 25% less than 1190 fpm, or 893 fpm (1190 x .75). We can now determine how much air can safely be run through the louver by multiplying the louver free area by the design velocity ( 7.2 sq ft x 893 fpm). The resulting Volume of Air for Louver 1 is 6424 cfm.

If we go through the same calculations for Louver 2, the result (with a 25% safety factor) is only 4781 cfm. This is 25% less air through the same opening size. Louver 1 is a better choice - IF we can live with the pressure drop from the higher airflow rates! back to top

Most Manufacturers publish air flow resistance for their louvers. Each louver will have slightly different resistance based on the blade and frame shapes and angles. These charactoristics can be expressed by a formula and graphed, such as:

Louver 1	Louver 2

Simplifying things a bit - most louvers do not fluctuate dramatically from these graphs (about 15% in this example), unless the louver is designed for very high air velocities, like wind driven rain louvers. However, if you have the data, use it! Here we can calculate the resistance at the design velocities for each louver and determine that:

Louver 1 - at 893 fpm free area velocity, will create 0.090 inches w.g. of static pressure

Louver 2 - at 563 fpm free area velocity, will create 0.055 inches w.g. of static pressure

Both of these values should be acceptable for HVAC system design, and would mean that our Louver 1 is the better choice, even though the free area is lower. A good rule of thumb is to stay below 0.2 inches w.g. static pressure for most applications. If your values exceed this rule, we recommend you increase the opening size or select a louver model with higher free area, higher first point of water penetration, lower pressure drop, or a combination of these factors.

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The following table represents the performance capability of all louvers by Architectural Louvers. Table is based on a test size of 48" wide x 48" high for comparison purposes:

Product Model	Free Area	First Point of Water Penetration (free area velocity)	Overall Performance (cubic feet per min.)	Pressure Loss at this velocity (inches water gauge)
E6JP	58.9%	872 fpm	8214 cfm	0.11
E6KP	58.9%	872 fpm	8214 cfm	0.11
E4JP	57.2%	840 fpm	7686 cfm	0.09
E4KP	57.2%	840 fpm	7686 cfm	0.09
E4DS	52.5%	898 fpm	7543 cfm	0.13
E6DP	51.2%	1155 fpm	9560 cfm	0.15
E6WH	51.4%	>1250 fpm	10275 cfm	0.21
E4DP	50.6%	1000 fpm	8100 cfm	0.11
E2JS	48.3%	797 fpm	6153 cfm	0.10
E2KS	48.3%	797 fpm	6153 cfm	0.10
E4JS	46.8%	975 fpm	7303 cfm	0.16
E4WH	46.5%	1250 fpm	9300 cfm	0.25
E4KS	46.3%	814 fpm	5324 cfm	0.10
E2DS	43.1%	952 fpm	6569 cfm	0.11

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