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Green Building Design Guide for Construction: Envelope and Lighting

This guide will show you envelope and lighting design strategies for saving energy in your building when trying to achieve LEED certification or designing a green building.

Building Envelope

The building envelope is the most important part of your green building design.  An effective envelope can highly reduce heating and cooling energy cost due to its ability to reject the sun’s radiation during summers and prevent heat from leaving the building during winters.

High Performance Windows

Windows are the most important element in the building envelope.    A double pane, low-e coating, window with a low shading or solar heat gain coefficient will provide your building with the proper insulation and solar radiation heat rejection to keep your heating and cooling HVAC equipment working at a minimum.  Place overhangs over your windows in the western façade of the building to shade the hot afternoon sun.  Your building window to wall ratio (WWR) should be less than 40%.  This means that you should have 40% or less in area of windows in a given wall.  If your building requires you to have a high WWR, it is highly preferable that those windows are facing north.  The sun has the least radiation heat effect to your building on the northern façade.

Roof

Add roof insulation with a resistance value of R-13 or better depending on the type of climate where the building is located.   The cooler the climate, the higher the resistance of the insulation should be.  Have roof skylights over hallways and atrium’s to take advantage of natural sunlight.

Cover the top layer of the roof with a solar heat reflecting paint, such as white, or a reflective material like aluminum.  The solar reflective properties of the top layer on the roof is the most important factor for rejecting solar heat.  The amount of insulation used becomes irrelevant if the roof itself can prevent heat from entering the building.

The following table specifies the solar radiation reflective index of the exterior surface of a roof.  The higher the number, the better the roof is at rejecting solar heat.

Material	Reflective Index	Paint	Reflective Index
Tin surface	0.95	White, lacquer	0.79
Aluminum, polished	0.88	White, gloss	0.75
Brick, white glazed	0.75	Aluminum paint	0.6
Iron, white galvanized	0.74	Green, light	0.53
Gravel	0.71	Blue, medium	0.49
Felt, aluminized	0.6	Yellow	0.43
Brick, buff, light	0.45	Orange, medium	0.42
Marble, white	0.42	Red, oil	0.26
Concrete, uncolored	0.35	Gray, light oil	0.25
Wood	0.22	Rust, medium	0.22
Asphalt pavement	0.18	Green, lacquer	0.21
Concrete, brown	0.15	Brown, medium	0.16
Slate, blue-gray	0.13	Blue, azure lacquer	0.12
Brick, red	0.12	Brown, dark brown	0.12
Felt, bituminous	0.12	Blue, dark	0.09
Concrete, black	0.09	Black, lacquer	0.08

Walls

You should only add wall insulation in buildings where the weather is cold during the winter.  Wall insulation is not going to do much to keep the sun’s radiation heat out of a building during summer months, since the wall itself will do a good enough job.  Insulation for walls should have a resistance value of R-11 or higher.

Lighting

Design your building lighting to have a lower lighting density, measured in watts per squared feet (W/ft²), than the density listed in the standard ASHRAE 90.1.  For office buildings this number is 1 W/ft².  Use occupancy sensors in conference rooms, bathrooms, kitchens, hallways, store rooms and any rooms that are not normally occupied.  Use skylights in hallways for natural lighting and use daylight sensors for lights near perimeter windows and skylights to automatically turn of the lights when there is enough sunlight to maintain the proper level of lumens per square foot in a room.

If you follow the above envelope and lighting strategies, you will be well on your way at designing a sustainable building and achieve LEED certification.  Remember that the building envelope is the most important element when designing for energy conservation.