The Passive Solar Exterior Envelope

is a critical component of any building, since the envelope both protects the occupants, and plays a major role in regulating the interior environment. The envelope consists of the building's roof, walls, windows, and doors. Its design and construction quality determine the flow of energy between the interior and exterior. The envelope is a selective pathway for the way a building performs in its climate – designed to respond to heating, cooling, ventilating, and natural lighting needs.

For a new building project, envelope optimization opportunities begin during the pre-design phase. A superior building envelope can provide significant reductions in heating and cooling requirements (and even “Zero Energy”), which in turn can allow downsizing of mechanical equipment and external energy source expense. When the correct design patterns and strategies are integrated through HOLISTIC SYSTEMS ENGINEERING, the additional initial construction cost for a high-performance building envelope may be paid for through savings achieved by installing smaller heating, ventilating and air conditioning (HVAC) equipment. From that point on, the lower overall building operating costs will provide long-term lower total cost of ownership.

With existing buildings, facility managers have much less opportunity to change most envelope design components. Reducing outside air infiltration by improving building envelope weatherization is usually very cost effective. During re-roofing, extra insulation can typically be added with little difficulty. Windows and insulation can be upgraded at higher cost during more significant building improvements and renovations.

The building envelope, "skin" or “shell” consists of structural materials and finishes that enclose space, separating the outside environment from the inside. This includes walls, windows, doors, roofs, and floor surfaces. The envelope must balance requirements for ventilation and daylight while providing thermal and moisture protection that is appropriate for the climatic conditions.

Envelope design is a major factor in determining the amount of energy a building will use in its operational lifetime. Also, the overall environmental life-cycle impacts and energy costs associated with the production and transportation of different envelope materials vary greatly. Local availability will influence envelope design decisions.

In keeping with the Holistic System Engineering approach, the entire design team must integrate the envelope elements with other design elements, including: the purpose of the building, budget, material selection, day lighting, passive solar heating and cooling strategies, HVAC, electrical requirements (computer servers, product refrigeration, etc.) and overall construction and long-term project performance goals. One of the most important factors affecting envelope design is climate. Hot/dry, hot/humid, temperate, cold and windy climates will suggest different design strategies. Specific designs and materials can take advantage of, or provide solutions for, any given climatic/environmental conditions.

A second important factor in envelope design is what occurs inside the building. If the activity and equipment inside the building generate significant heat (as do computers), the thermal loads may be primarily internal (from people and equipment) rather than external (from the sun or cold wind). Internal and external sources affect the rate that buildings gain or lose heat.

Building volume, orientation and site location all have significant impacts on the efficiency and requirements of the building envelope. Careful study is required to arrive at a building footprint that works well with the envelope design to maximize energy conservation.

Openings are located in the envelope to provide physical access to a building, create views to the outside, admit daylight and solar energy for heating, and supply natural fresh air ventilation. The form, size, and location of the openings (windows, doors, vents) vary depending upon the role they play in the building envelope. Window glazing can be used to affect heating and cooling requirements and occupant comfort by controlling the type and amount of light that passes through windows as seasons change. Solar gain is often desirable in the winter and undesirable in the summer. Many design opportunities exist, but they must be integrated with location-specific, place-based engineering that is based on the local annual path of the sun.

Decisions about construction details and quality assurance strategy play a crucial role in the design of the building envelope. Superior buildings require superior construction supervision AND more-detailed subcontractor acceptance criteria specifications. (Contractor’s Checklist)

Building materials conduct heat at different rates. Components of the envelope such as foundation walls, sills, studs, joists, beams, connectors and roofing materials, among others, can create paths for the transfer of thermal energy, known as thermal bridges, that conduct heat in-or-out of the envelope. Wise detailing decisions, including choice and placement of insulation materials, are essential to assure thermal efficiency. Innovation in insulation types and strategies is long overdue.

Your Passive Solar Energy information source is our comprehensive 800+ page Zero Energy DesignTM ebook on CD-ROM(PDF) with hundreds of full color illustrations, photos, and diagrams.

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Passive Solar and Your Windows

Glazing systems have a huge impact on energy consumption, and glazing modifications often present an excellent opportunity for energy improvements in an existing building. DOE ENERGY STAR® certified windows are highly recommended in most applications (except NOT the south windows of a passive solar sunroom). Appropriate glazing choices vary greatly, depending on the location of the building, its uses, and (in some cases) even the glazing's placement on the building. Movable window insulation can have a very large impact on thermal performance, comfort, and energy requirements. (ZED has over 100 ways to reduce heat transfer through windows.)

In hot climates, the primary window strategy is to control heat gain by keeping solar energy from entering the interior space, while allowing reasonable visible light transmission for views and day lighting. Solar screens that intercept solar radiation, or spectrally-selective window films that prevent infrared and ultraviolet transmission while allowing good visibility, are useful retrofits, especially in hot climates.

In colder climates, the focus shifts from keeping solar energy out of the space to reducing heat loss to the outdoors and (in some cases) allowing desirable passive solar radiation to enter. Windows with two or three glazing layers that use low-emissivity coatings will minimize conductive energy transmission, but they may not be desirable on south-facing solarium windows. Filling the spaces between the glazing layers with an inert low-conductivity gas, such as argon (67% as conductive as air), can further reduce heat flow.

Much heat is also lost through a window's frame, which is often made of highly-conductive metal, such as aluminum. For optimal energy performance, specify a low-conductivity frame material, such as wood or vinyl, or a frame with a built-in thermal break. In addition to reducing heat loss, a good window frame will help prevent condensation - even high-performance glazings can cause in condensation problems, IF those glazings are mounted in inappropriate frames or window sashes. Long-term condensation can cause many serious problems, some of which may be hidden from view.

Windows and doors (fenestration) can be a source of discomfort when solar gain and glare interfere with work station visibility, or increase contrast and visual discomfort for occupants. Daylighting benefits will be negated if glare forces occupants to close blinds and turn on electric lights, for example, to perform detailed visual tasks, or simply read computer displays. In some cases it may be much better to use translucent insulation (such as R-20 silica aerogel), which allows controlled natural daylighting, without harsh solar glare (or views). 

Facility managers should choose appropriate window technology that is cost-effective for the climate conditions. Computer envelope modeling may help determine which glazing system is most appropriate for a particular climate. In mild California near the coastline, single glazing may be all that is economically justified, while in both hotter and colder climates, more sophisticated glazings are likely to be much more cost-effective, with the lowest total cost of ownership.

Your Passive Solar Energy information source is our comprehensive 800+ page Zero Energy Design ebook on CD-ROM(PDF) with hundreds of full color illustrations, photos, and diagrams.

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Walls and Roofs

For buildings dominated by cooling loads, it makes good sense to provide exterior finishes with high reflectivity or wall-shading devices that reduce solar gain. Reflective roofing products help reduce cooling loads (by up to 70o F), because the roof is exposed to the harsh summer sun for the entire operating day. Specify roofing products that carry the DOE ENERGY STAR® roof certification. These materials have an initial reflectivity of at least 65%. ENERGY STAR roofing products are widely available with single-ply roofing, as well as various other roofing systems.

Wall shading can reduce solar heat gain significantly—use roof overhangs, window shades, awnings, a canopy of mature trees, or other vegetative plantings, such as trellises with deciduous vines. Shade screens can help block two thirds of solar radiation, which may be particularly value on the west side of a building.

To reduce cooling loads, wall shading on the east (morning) and west (hot afternoon) is most important. It is especially beneficial for buildings with year-round cooling loads. Shading the south side of the building has almost no value in southern climates in the summer, when the sun rises in the northeast, sets in the northwest, and may never be on the south side all day long. In new construction, providing architectural features that appropriately shade walls and glazings should be considered with location-specific design models, based on the seasonal path of the sun (which is a significant 47o difference summer versus winter in all locations).

With new buildings, adding more wall and roof/ceiling insulation than normal can be done for a relatively low-cost premium. Also, consider thermal bridging, which can significantly degrade the rated performance of cavity-fill insulation that is used with steel framing. With steel framing, and concrete construction consider adding a layer of rigid foam insulation (such as 5 inches of expanded polystyrene).

Boosting wall insulation levels in existing buildings is difficult without expensive building modifications. One option for existing buildings is adding an exterior insulation finish system (EIFS) on the outside of the current building skin. With EIFS, use only systems that include a drainage layer to accommodate small leaks that may occur over time - avoid barrier-type systems that can cause serious moisture problems.

Roof insulation can typically be increased relatively easily during re-roofing. At the time of re-roofing, consider switching to a protected-membrane roofing system, which will allow reuse of the rigid insulation during future re-roofing - greatly reducing landfill disposal.

While we think of insulation as a strategy for cold climates, it makes sense in cooling climates as well. The addition of insulation can significantly reduce air conditioning costs and should be considered during any major renovation project. Roofs and attics should receive priority attention for insulation retrofits because of the ease and relative low cost. In hot climates, radiant barrier roof insulation can be very valuable for minimal cost. The Zero Energy DesignTM goal is to have a peak summer attic air temperature that is never as hot as the peak outside air temperature (something that no conventional building can do). A cool roof with a cool attic GREATLY reduces air conditioning requirements, at a surprisingly low cost.

Your Passive Solar Energy information source is our comprehensive 800+ page Zero Energy Design ebook on CD-ROM(PDF) with hundreds of full color illustrations, photos, and diagrams.

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Your Passive Solar Energy information source is our comprehensive 800+ page Zero Energy Design ebook on CD-ROM(PDF) with hundreds of full color illustrations, photos, and diagrams.

Climate Considerations

Assess the local climate (using typical meteorological-year data from NOAA) to determine appropriate envelope materials and building designs. The following considerations should be taken into account, depending on the climate type.

Assess The Site's Solar Geometry  
Solar gain on roofs, walls, and the building interior through window openings can be either a benefit or a hindrance to heating, cooling, and occupant comfort. A thorough understanding of solar geometry specific to the site is crucial to proper envelope design.
Hot/Dry Climates
Use materials with high thermal mass (like concrete with significant insulation on the OUTSIDE). Buildings in hot/dry climates with significant diurnal temperature swings have traditionally employed thick walls constructed from envelope materials with high mass, such as adobe and masonry. Openings on the north and west facades are limited, and large southern openings are detailed to exclude direct sun in the summer and admit it in winter (where passive solar heating is desired).

A well-insulated building material with high thermal mass and adequate thickness will lessen and delay the impact of temperature variations from the outside wall on the wall's interior. The material's high thermal capacity allows heat to penetrate slowly through the wall or roof. Because the temperature in hot/dry climates tends to fall considerably after sunset, the result is a thermal flywheel effect - the building interior is cooler than the exterior during the day and warmer than the exterior at night.

Hot/Humid Climates
The design criteria for climates with large air conditioning requirements and minimal winter heating requirements result in envelopes and mechanical systems that are significantly different from other climates. Everything is done to reject solar gain most of the year. This includes landscaping, shading devices, porches, overhangs, light (bright white) colors, radiant barriers, insulation, and high-volume nighttime ventilation of areas that build up heat, like the attic.
(See Chapter Twelve for significant ZED summer cooling design details)

Temperate Climates
Select materials based on location and the heating/cooling strategy to be used. Determine the thermal capacity of materials for buildings in temperate climates based upon the specific locale and the heating/cooling strategy employed. Walls should be well insulated. Envelope openings should be shaded during hot times of the year and unshaded during cool months. This can be accomplished by roof overhangs sized to respond to the local seasonal solar geometry, or by the use of awnings, which can even be motorized and automated with sensor-based controls.

Cold Climates
Design wind-tight and well-insulated building envelopes. The thermal capacity of materials used in colder climates will depend on the use of the building, and the heating strategy employed. A building that is conventionally heated and occupied intermittently should not be constructed with high mass materials because they will lengthen the time required to reheat the space to a comfortable temperature. A passive or active solar heating strategy may necessitate the use of massive materials, if not in the envelope, then in other interior building elements. Where solar gain is not used for heating, the floor plan should be as compact as possible to minimize the surface area of the shell. A complex exterior with more than four basic corners, decreases thermal efficiency (and increases opportunities for weatherization construction flaws).


Your Passive Solar Energy information source is our comprehensive 800+ page Zero Energy Design ebook on CD-ROM(PDF) with hundreds of full color illustrations, photos, and diagrams.

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Doors, Windows, and Openings

Size and position doors, windows, and vents in the envelope based on careful consideration of daylighting, heating, and ventilating strategies.

The form, size, type, and location of openings may vary depending on climate and how they affect the building envelope. A window that provides a view need not open, yet a window intended for ventilation must do so. Window screens allow intake of dust, dust mites, mold, pollen and allergens. Controlled fresh air ventilation through high-quality air filtration, and air-to-air heat recovery systems is often a far superior alternative to manually opening and closing windows for ventilation during certain seasons. Carefully design window placement for maximum beneficial daylighting to minimize east/west summer heat gain and eliminate undesirable glare in locations (around televisions and computer displays).

Air-lock vestibules at building entrances should be designed to avoid the loss of cooled or heated air to the exterior. The negative impact of door openings on heating or cooling loads can be reduced with careful design, where both doors are normally not open at once. Members of the design team should coordinate their efforts to integrate optimal design features. For passive solar design, this includes the professionals responsible for the interactive disciplines of building envelope, daylighting, orientation, architectural design, thermal mass, HVAC, electrical and plumbing systems.

Shade openings in the envelope during hot weather to reduce the penetration of direct sunlight to the interior of the building.

Use overhangs or deciduous plants where appropriate. Be aware, however, that deciduous southern plants can cut solar gains in the winter by 20 percent. Shade window openings or use light shelves in work areas at any time of year to minimize thermal discomfort from direct radiation and visual discomfort from glare.

In all but the mildest climates, select double-or-triple-paned windows with as high an "R" value as is cost effective, and proper shading coefficients within the project's financial guidelines.

The "R" value is a measure of the resistance to conductive heat flow across a wall or window assembly (with higher values representing a lower energy loss). Shading coefficient is a ratio used to simplify comparisons among different types of heat reducing glass coatings. The shading coefficient of clear window glass is 1.0. Glass with a shading coefficient of 0.5 transmits one-half of the solar energy that would be transmitted by clear double-strength glass. One with a shading coefficient of 0.75 transmits three-quarters.

Select the proper glazing for windows, where appropriate. Spectrally selective glazing uses metallic layers of coating or tints to either absorb or reflect specific wavelengths in the solar spectrum. In this manner, desirable wavelengths in the visible spectrum that provide daylight are allowed to pass through the window while other wavelengths, such as near-infrared (heat) and ultraviolet (which can damage fabric), are partially reflected. Thus, excess heat and damaging ultraviolet light can be reduced while still retaining the benefits of natural lighting and exterior views. More advanced windows use glazings that are altered with changing conditions, such as windows with tinting that increases under direct sunlight and decreases as light levels are reduced. Research is being conducted on windows that can be electronically adjusted by the building occupant (or an automation system) to allow more or less radiation into a building.


ZED has all of your future energy answers – What are YOUR questions

Larry Hartweg

Your Passive Solar Energy information source is our comprehensive 800+ page Zero Energy Design ebook pdf on CD-ROM with hundreds of full color illustrations, photos, and diagrams.

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