Building Envelope

Building Envelope

Humans first created shelters to provide thermal comfort and protection from natural elements, and this still remains a primary objective of buildings. The building envelope is the physical separator between the interior and exterior of a building.

Components of the envelope are typically: walls, floors, roofs, fenestrations and doors. Fenestrations are any opening in the structure: windows, skylights, clerestories, etc. When designing the building envelope, knowing some fundamentals of building materials and heat transfer will help you make the right trade-off decisions.

Envelopes for Climate Types

A well-designed envelope responds to the local climate. As described in the Climate page, there are many classifications of climate around the globe, but the summary below shows four common extremes that people design for. Milder climates can use milder versions of these strategies, or mix and match.

Arid Climate Envelope

Arid climates are very dry, and usually hot, but they often have large swings of temperature from day to night. Thus thermal mass on the outside of the building is the most crucial design strategy to even out such temperature swings. For consistently hot locations, it also helps to have high ceilings, shaded breezeways, light colors, and daylighting via reflected light (not direct sun), such as in this audience hall in the Jaipur city palace. Courtyards with natural ventilation and pools or fountains can provide  evaporative cooling as well.

The City Palace in Jaipur, India. Photo: Jeremy Faludi

Tropical Climate Envelope

Tropical climates are hot and humid. Therefore, keeping the heat of the sun off is the top priority, as well as maximizing ventilation—essentially a reflective insulated roof with walls that pass breeze but not rain is ideal. This traditional Papua New Guinean home’s thick light-colored thatch roof keeps out the sun’s heat, while open eaves and porous bamboo slats for walls and floor maximize natural ventilation. The materials are all low-mass to avoid condensation and mold growth, which can happen with high-mass materials in humid climates. (Note: Jalousie windows are found in the tropics, but are not as common elsewhere, because they are so porous to breezes.)

Cold Climate Envelope

A vernacular-design cabin in Finland.

Cold climates have many more heating degree days than cooling degree days. Thus maximizing insulation is the key to keeping warm, as well as using windows for solar gain on thermal mass inside the building envelope (not outside as in arid climates). Part of having effective insulation in cold climates is an air-tight envelope, avoiding infiltration. This Finnish cabin has very few and very small windows except on the south side, to maximize solar gain while minimizing losses elsewhere. Before modern insulation, thick solid log walls such as these provided better insulation than board walls could.

Mixed Cold / Hot Climate Envelope

Many “temperate” inland climates actually have two extremes–cold in winter, hot and humid in summer. Flexibility is the key to designing for these climates. The Aldo Leopold Center in Wisconsin, the first building to be LEED certified as carbon-neutral, uses deep overhangs to allow low winter sun in through the windows to heat up a high-mass concrete slab inside, while blocking high summer sun. It also uses a light roof and darker walls to repel summer sun but absorb winter sun. Extra insulation retains heat in winter, but operable windows passively cool it in summer.

The Aldo Leopold Center in Baraboo, Wisconsin. Photo: Jeremy Faludi

Envelope Energy Flows

From an energy flow perspective, the envelope is a composition of layers with varying thermal and permeability properties. The envelope may be composed of membranes, sheets, blocks and preassembled components. The choice of envelope is governed by the climate, culture, and available materials. The range of choices in envelope design can be illustrated by two opposite design concepts: the open frame and the closed shell.

In harsh climates, the designer frequently conceives the building envelope as a closed shell and proceeds to selectively punch holes in it to make limited and special contact with the outdoors. This may also be true where there are unwanted external influences such as noise or visual clutter.

When external conditions are very close to the desired internal ones, the envelope often begins as an open structural frame, with pieces of the building skin selectively added to modify only a few outdoor forces.

The flow of heat through a building envelope varies both by season (heat always flows from hot to cold and generally flows from a building in winter and to a building in summer) and by the path of the heat (through the materials of a building’s skin, or by outdoor air entering). These complexities must be considered by a designer who intends to deliver comfort and energy efficiency.

The following links provide more detailed information on building envelope components and the minimum thermal requirements of envelopes for different climates as defined by ASHRAE.

Walls

Understanding and optimizing the heat transfer through the walls is important in high performance building design.  Using thermal mass and insulation to your advantage with passive design strategies can help reduce the amount of energy that active systems need to use.

Insulation

Thermal insulation is a material that blocks or slows the flow of heat through the building envelope. Insulation is vital to most green building design because it allows spaces to retain what heat they have, while avoid gaining excess heat from outside.

It’s important to understand Heat Energy Flows in a building to understand insulation.Insulation primarily is designed to prevent heat transfer from conduction and radiation.

Resistance to conduction is measured by R-value (high thermal resistance = high R-value); Resistance to radiative heat transfer is measured by emissivity (high resistance = low emissivity and high reflectance). Conduction is the dominant factor when materials are touching each other; when there is an air gap between materials, radiation becomes important. Convection usually only becomes an issue when significant air pockets are involved.

Materials used for insulation fall into two broad categories:

(1) Fibrous or cellular products – These resist conduction and can be either inorganic (such as glass, rock wool, slag wool, perlite, or vermiculite) or organic ( such as cotton, synthetic fibers, cork, foamed rubber, or polystyrene).

(2) Metallic or metalized organic reflective membranes – These block radiation heat transfer and must face an air space to be effective.

R-values and Insulation (Conduction)

Below is a table of R-values for some common building products.

Material, 1″ (2.5cm) thickness	m2•K/W	ft2•°F•h/BTU
Vacuum insulated panel	5.28 – 8.8	R-30 – R-50
Polyisocyanurate spray foam	0.76 – 1.46	R-4.3 – R-8.3
Polyurethane rigid panel	0.97 – 1.2	R-5.5 – R-6.8
Closed-cell polyurethane spray foam	0.97 – 1.14	R-5.5 – R-6.5
Extruded polystyrene (XPS), low-density	0.63 – 0.82	R-3.6 – R-4.7
Expanded polystyrene (EPS) high-density	0.65 – 0.7	R-3.85 – R-4.2
Air-entrained concrete	0.69	R-3.90
Fiberglass batts	0.55 – 0.76	R-3.1 – R-4.3
Cotton batts (Blue Jean insulation)	0.65	R-3.7

Open-cell polyurethane spray foam	0.63	R-3.6
Cardboard	0.52 – 0.7	R-3 – R-4
Rock and slag wool batts	0.52 – 0.68	R-3 – R-3.85
Cellulose wet-spray	0.52 – 0.67	R-3 – R-3.8
Straw bale	0.26	R-1.45
Softwood (most)	0.25	R-1.41
Hardwood (most)	0.12	R-0.71
Brick	0.03	R-0.2
Glass	0.025	R-0.14
Poured concrete	0.014	R-0.08

Steel stud

5.3×10-4

R-0.003

Table of R-values for 1″ thickness of common building materials. From Wikipedia and Klepper, Hahn & Hyatt.

Reduction in heat loss does not follow R-values linearly, but in an inverse logarithmic curve.

Because R-values are 1 / conductance (U), doubling the thickness of insulation will not cut heat loss in half. Rather, there is an exponential decay of heat flow, where the difference between no insulation and one inch (or one cm) of a particular insulation may save 80% of heat loss, while going from one inch to two inches of that insulation saves an additional 9%, and going from 9 inches to ten inches only saves an additional 1%.

Low-Emissivity Insulation (Radiation)

There are many situations where radiative heat transfer is important to avoid–for instance, attics or warehouses where the sun heats the building’s skin excessively. In conditions like this, just a thin sheet of reflective material can make as much difference as adding many inches of conventional insulation. These are usually called “radiant barriers”.

Radiant barriers must have a low emissivity (0.1 or less) and high reflectance (0.9 or more). Thus they are shiny reflective or white materials.

Low-emissivity insulation is reflective foil-faced.

They only reduce radiative heat transfer. Because of this, reflective insulation is only useful on the surface of insulation facing a cavity or the outside air.

Convection and Insulation

Convection through fluids (like air) can also transfer heat. Unwanted convection through the building envelope can cause unwanted heat gains or losses through infiltration. Also, suppressing convection within the materials of the building envelope is often what makes insulation effective.

Convection within the building envelope hurts insulation as well. Still air is an excellent insulator, so good insulation often uses small pockets of air. The main reason that foam insulation is a better insulator than batt insulation is that there is less convection of the air within foam. Aerogel is such a high-performance insulator because it is mostly air, but the micro-scale structure of the aerogel prevents convection of the air held in it.

Fibrous or cellular products prevent conduction by keeping air still (preventing convection). Here’s how:

• Batt insulation traps air in a mat made from a low conductivity solid, such as glass or organic fiber (wool or polyester).
• Open-cell foams trap tiny bubbles of air or other gas in a poor conductor, such as polystyrene or polyurethane. However, gas can still migrate through open-cell foams.
• Closed-cell foams, where gas cannot travel from cell to cell, are the best way to avoid convection.

Insulation Materials

Although insulation can be made from a variety of materials, it usually comes in five physical forms: batting, blown-in, loose-fill, rigid foam board, and reflective films. Each type is made to fit a particular part of a building.

Batting / Blankets

Form Factor & Installation: In the form of batts or continuous rolls that are hand-cut or trimmed to fit. Stuffed into spaces between studs or joists.

Material: Fiberglass is manufactured from sand and recycled glass, and mineral fiber (“rock wool “) is made from basaltic rock and/or recycled material from steel mill wastes. Even recycled cotton fibers from jeans are used. Available with or without vapor and flame retarding facings.
Benefits: Common and easy to install. Available in widths suited to standard spacings of wall studs, ceiling or floor joists.

Blown-in/ Loose-Fill

Form Factor & Installation: Loose fibers or fiber pellets are blown into building cavities using special pneumatic equipment. The best forms include adhesives that are co-sprayed with the fibers to avoid settling.

Material: Fiberglass, rock wool, or cellulose. Cellulose is made from recycled plant material (such as newspaper) treated with fire retardant chemicals.

Benefits: Can provide additional resistance to air infiltration if the insulation is sufficiently dense.

Foamed in Place

Form Factor & Installation: Sprayed directly into cavities within the building, where it expands as it sets to fully seal the cavity, filling all nooks and crannies.

Material: Polyurethane or polyisocyanurate. Some brands are partially made from bio-plastic rather than fossil-fuel-derived polyurethane. However, the percentage of bio-material is generally no higher than 10 – 15%, as there are currently not yet viable bio-based alternatives to the bulk of the polyurethane polymer.

Benefits: It can fully seal the cavity, helping to reduce air leaks. Spray foam, once set, is rigid and can even provide some structural shear strength. It generally has high R-values, and also provides acoustical insulation.

Rigid Board

Form Factor & Installation: Plastic foams extruded into boards, or fibrous materials pressed into boards. Can also be molded into pipe-coverings or other three-dimensional shapes. Rigid board provides both thermal and acoustical insulation, strength with low weight, and few heat loss paths if it fits the installation location well.

Material: Polyisocyanurate, polyurethane, extruded polystyrene (“XPS”), expanded polystyrene (“EPS” or “beadboard”), or other materials. May also be faced with a low-E reflective foil.

Benefits: Lightweight, provide structural support, and generally have a high R-values. Can be used in confined spaces such as exterior walls, basements, foundation and stem walls, concrete slabs, and cathedral ceilings.

Reflective

Form Factor and Installation: Roll of foil, integrated into housewrap, or integrated into rigid insulation board. These “radiant barriers” are typically located between roof rafters, floor joists or wall studs.

Material: Fabricated from aluminum foil with a variety of backings such as craft paper, plastic film, polyethylene bubbles or cardboard.

Benefits: Resists radiative heat transfer. The resistance to heat flow depends on the heat flow direction–it is most effective in reducing downward heat flow². Radiant barriers are installed in buildings to reduce both summer heat gain and winter heat loss. They are most effective in hot climates rather than in cool climates.

Working Together

Different forms of insulation can be used together. For example, you can add bat or roll insulation over loose-fill insulation, or vice versa. Usually higher density insulation should not be placed on top of lower density material that is easily compressed. Doing so will reduce the thickness of the lower insulation and thereby reduce its R-value.

Since hot air rises by convection, hot air generally pools at the undersides of surfaces and conducts heat upwards into those materials. Radiant barriers only resist heat transfer by radiation, they cannot resist conductive and convective heat flow. Thus they are more effective at preventing heat from flowing downward through spaces than upward.

Movable Insulation

Windows often provide valuable heat gain during the day but cause problematic heat loss during the night. One way to prevent this is movable insulation, in the form of insulated shutters or movable walls, insulated internal or external roller-shades, or–most commonly–thick curtains.

Curtains often have a fatal flaw of allowing air convection around them. Cold air between the curtain and window sinks, falling under the bottom of the curtain into the room, and warm air from the room is sucked from above the curtain into the space between it and the window, to repeat the cycle. This is solved by having a closed cornice board or “pelmet” sealing the top of the curtains.

Closed cornice board over curtains

Adding layers of insulation in series

Adding R-values In Series

When materials are sandwiched together, perpendicular to the direction of heat flow, it is called adding “in series”. An example of this is a cavity-brick wall, with two layers of brick, an air gap, and 1/2″ (1.2 cm) of plasterboard, all in a row.

The heat must pass all the way through one material before it gets to the next material, so any heat flow blocked by one material is blocked the rest of the way. Mathematically, adding in series is easy: simply sum all thermal resistances (R-values).

Adding layers of insulation in parallel

Adding R-values In Parallel

When materials are sandwiched parallel to the direction of heat flow, it is called adding “in parallel”. The heat being transferred does not need to pass all the way through one material before it gets to the next material; instead, it can take the path of least resistance. An example of this would be a standard window in a well-insulated wall.

Mathematically, adding in parallel means the overall R-value will be one divided by the sum of the reciprocals of all the individual materials’ R-values. A highly conductive material can completely short-circuit other insulative materials and cause the total R-value to be low.

Calculating the Total R-value

The total insulation of an assembly includes all of the resistances of its individual materials, whether in series or in parallel or both. If some materials are in parallel while others are in series, each section of materials in parallel should be treated as a layer, and its overall R-value calculated. Then all layers can be summed for the total R-value.

Air velocities are near zero at the surface of an object. This insulating layer of air “attaches” itself to the surface is an air film.

Resistance from Air Films and Air Spaces

Air on the surface, and between, building constructions add insulating properties. In addition to the insulation due to the materials themselves, this air provides a slight additional insulation value and should be considered when you’re calculating the total R-value.

Air films are layers of air that are assumed to be static on each side of a building envelope, and air spaces are volumes of air within building constructions. They are both interesting thermal components because although they are actually void of material, they have potentially useful thermal properties. They can contribute substantially to the insulating capabilities of some construction assemblies.

Air film position and climate	RSI (K•m²/W)	RUS (hr•ft²•°F/Btu)
Horizontal (flat ceiling) in winter	0.11	0.61
Horizontal (flat ceiling) in summer	0.16	0.92
Vertical (wall) anytime	0.12	0.68
Any outdoor surface, anytime, in significant wind (15mph)	0.030	0.17

Cavities and Air Spaces

An air space is a planar volume of air contained on two sides by some elements of an envelope assembly (drywall, brick, insulation, etc.). As mentioned in insulation, air spaces are commonly built-into wall constructions to help reduce heat transferwhen multiple layers are in series.

Air has a high resistance to heat conduction, but it has almost no resistance to heat radiation, and little resistance to heat convection outside of the thin air film touching surfaces. When conduction, convection, and radiation all occur at the same time, the overall thermal resistance of air spaces becomes virtually independent of gap width when it is greater than around 1″ (2.5 cm).

The resistance of a thick air space can be increased by subdividing it into several thin layers. The resistance of the whole space is then the sum of the resistances of the thin air spaces, plus the resistances of the separators. Triple-pane or higher multi-pane windows use this strategy. (See Glazing Properties)

Dividing air cavities is most effective when low emissivity materials like aluminum foils are used to subdivide the space because they can also block unwanted radiation as well as convection. It is most effective when adding layers in series, not in parallel, because in parallel the materials used to divide air spaces might cause what are called “thermal bridges”.

Thermal Bridging and Thermal Breaks

Metal window frames often create thermal bridges around well-insulated windows.

A thermal bridge is an unwanted path for heat flow that bypasses the main insulation of a building envelope. This happens when a good conductor is put in parallel with the insulation.

Placing a good conductor in parallel with good insulation is often referred to as “thermal bridging” because it provides a path for heat flow that bypasses the main insulation. Steel studs and metal window frames are common thermal bridges. A window’s total insulation value can sometimes be only half as good as center-of-glass insulation values.

Thermal bridging can be avoided by placing insulation in series with conductive material, rather than in parallel. For instance, you can place insulation outside a stud wall instead of only between the studs. This is sometimes called “exsulation” as opposed to “insulation”.

Thermal bridging can also be avoided by looking for the lowest R-value in an assembly and improving it. For instance, replacing metal window frames with fiberglass frames.

Thermally-broken window frame

Thermal breaks

A thermal break is when an assembly that would normally be a thermal bridge is broken up into separate pieces that are isolated by a more insulative material. Assemblies like this are called “thermally broken”. “Thermally improved” assemblies do the same thing, but with less of a thermal break.

For example, many metal window frames are broken up so that one piece of metal faces the outside of the building, a separate piece of metal faces the inside of the building, and in between are pieces of rigid plastic. The plastic is not as good an insulator as proper insulation, so some thermal bridging will still occur, but the plastic is more structural than insulation could be.

Infrared photograph of a house, showing good insulation defeated by thermal bridging in the framing.

Framing Factor

The extent to which a wall, roof, or floor’s framing reduces the R-value of its insulation is called its “framing factor”. It is simply a percentage reduction in R-value. For instance, a wall with R-20 insulation and a framing factor of 25% would have an overall insulation value of R-15. The more framing members, the higher the framing factor. Steel stud assemblies often have framing factors of 50% and above, while wood framing is usually closer to 25%.

As with any thermal bridging, framing factors can be eliminated by placing insulation in series with the framing rather than (or in addition to) between framing members.

Windows

The design of fenestration (windows, skylights, etc) requires special attention because of the huge variety of available building components and the several important roles that windows play.

Perhaps thermally most important, they admit solar radiation. This is often advantageous in the winter and disadvantageous in the summer. Also, despite dramatic improvements, glazing still usually has the lowest R value (highest U-factor) of all components of an envelope. Windows and skylights also admit daylight to buildings and often provide desired ventilation.

Glazing Properties

Good glazing properties are important because they control the amount of daylight, quality of light, and amount of solar heat gain let into the building, along with other factors. They very much determine the thermal comfort and visual comfort of a space.

Certifying window thermal performance

Fenestration is any opening in the building envelope. When that opening is covered with a translucent or transparent surface (like windows or skylights), that’s called glazing.

Three of the most important properties of the materials, coatings, and constructions that make up windows, skylights, translucent panels, or other products used to let sunlight into a building include

1. Thermal conductance (U-value)

2. Solar Heat Gain Coefficient (SHGC)

3. Visible Light Transmittance (VT)

Appropriate values for glazing properties vary by climate, size, and placement of the aperture. There is no one best kind of glazing to use. It’s not unusual for a single building to have three, four, or even five different kinds of glazing for apertures in different sides and at different heights on a building.

Thermal conductance (U-factor)

As with opaque envelope components, sensible heat flow due to temperature differences through windows and skylights is a function of the U-factor (See Heat Energy Flows). It measures how well glazing insulates or rather, how poorly glazing insulates.

U-values for various glazing constructions

U-factors measure thermal conductivity, the rate of heat transfer per unit area, per unit temperature difference from the hotter side to the colder side. This is W/(m²K) in SI units, BTU/(h°F ft²) in Imperial unitsIt is important to know which units you’re working in. R-values are 1 / U-factor.

U-factors are either measured for the glazing only (“center of glass”) or for the entire window assembly (including framing and spacers). There are significant differences in heat flow rates between the center-of-glass, edge-of-glass, and frame portions of a unit, so the U-Factor for the entire window assembly is the value most-often referenced. The NFRC (National Fenestration Rating Council) is a trusted source for this information.

The size of the air gap between glazings, the coatings on the glazings, the gas fill between glazings, and the frame construction all influence the U-factor.

In cold climates, a low U-value is usually the most important window property and a rule of thumb is to look for windows with a U-value of 0.35 or less (Imperial units). In warmer climates, low U-values are often less important than the Solar Heat Gain Coefficient because gains from direct solar radiation are more important than conduction through the window.

Solar Heat Gain Coefficient (SHGC) 

Solar Heat Gain Coefficient (SHGC) measures how much of the incoming heat from sunlight gets transmitted into the building, versus how much is reflected away. Heat from the sun is long-wave radiation (infrared and other non-visible light).

This thermal property is also generally based upon the performance of the entire glazing unit, not just the glass. The SHGC depends upon the type of glass and the number of panes, as well as tinting, reflective coatings, and shading by the window or skylight frame.

Heat transmission and radiation from a window

The SHGC is a dimensionless number between zero and one. SHGC can theoretically range from 0 to 1, with 1 representing no resistance (all of the heat from the incoming sunlight comes through) and 0 representing total resistance (none of the sun’s heat reaches the inside). SHGC values for real products typically range from about 0.9 to 0.2.

Choosing the right SHGC depends on the size and placement of the apertures as well as climate and other design factors. The SHGC is especially important in hot sunny climates (where cooling is the dominant thermal issue), and you should generally use glazing with lower SHGC  (below ~0.4). Buildings in cold climates should generally have higher SHGC to enable passive solar heating and to reduce heating loads.

The SHGC for ordinary uncoated, un-tinted glass can be .9, while values can be as low as .25 or even .15 for some specialized glazing units. In spectrally selective glazing, the SHGC can be independent of visible light transmission. The Light to Solar Gain ratio is used to measure the effectiveness of spectrally selective glazing and is visible light transmittance divided by solar heat gain coefficient.

Visible Light Transmittance (VT)

The point of windows is to let light pass through. The percentage of visible light that passes through a window or other glazing unit is called the Visible Light Transmittance (VT). Also known as  Tvis, VLT and LT. An opaque wall would have a VT of 0%, while an empty opening would have 100%; many un-tinted glass and plastic materials have a VT of 90% or more. VT does not measure shorter-wavelength light like UV or longer-wavelength light like infrared only visible light.

More light is often not better, as it can cause glare and overheating. Tints, frits, and coatings can be chosen to produce any VT; common values are often 30 – 80%.

VT is influenced by the color of the glass (clear glass has the highest VT) as well as by coatings and the number of glazings.

VT may be expressed relative to the glass portion of a glazing unit only or relative to the glass and frame. The appropriate expression will depend upon the nature of an analysis; in any case, non-comparable values should not be compared. All NFRC-certified VT values are directly comparable.

LCD films are switched on or off to provide clear or diffusing coatings. Demonstration from the Pacific Energy Center.

Adaptive Properties

Some advanced glazing systems can change their visible light transmittance, solar heat gain coefficient, and other properties.

• Liquid crystal windows change from clear to frosted or dark when a voltage is applied by a control system, improving their privacy but not changing their solar heat gain.
• Thermochromic coatings turn from clear to dark at high temperatures (generally when struck by direct sunlight), reducing their VT and SHGC.
• Photochromic coatings turn from clear to dark when struck by light; many sunglasses use this feature.
• Electrochromic coatings change from clear to dark when a voltage is applied by a control system, also reducing their VT and SHGC.

Other Considerations

Some other significant variables to consider with windows or other apertures are infiltration rates, light distribution angles, condensation, and acoustics.

Infiltration is air leakage through the framing of a glazing unit. Tightening a unit can improve effective U-values by 10% or more. Standard leakage rates are .3 CFM/ft² (0.0015 m/s) while tight units can be as low as .02 (0.0001) or even .01 CFM/ft² (0.00005 m/s).

Light distribution angles are the direction that light is transmitted into the building. Ordinary windows let light travel straight through, while advanced glazing units may bounce the light to different angles, or spread it diffusely through the room. This is usually especially important for skylights.

Condensation can occur in glazing units when there is a large temperature difference from inside to outside. In addition to being unsightly, this can cause mold and mildew, which is detrimental to indoor air quality. Good glazing units control condensation.

Acoustic damping is valuable, since glazing generally transmits more sound than walls. Noise can be problematic for buildings in noisy locations. Some glazing units have better acoustic damping than others, particularly multi-pane constructions that use different glass thicknesses and layers of different material in their framing.

High Performance Windows

Window configurations that use low-E coatings, selective transmission films, inert gas fills, and thermal breaks can lead to higher energy performance. The net effect of these measures is to reduce the U-factor, and the right choice of these features depends on the application.

Window constructions with air gaps and inert gasses to reduce convection between window panes

Air Gaps and Inert Gases

U-values can be decreased by reducing the convection within the glazing unit. The simplest way to do this is to subdivide the air space by adding more panes. Interior panes are often merely thin films, since only the outer panes need to be structural materials like glass.

Air between panes can be replaced by denser gases like Argon or Krypton, which both reduce conduction and convection. Filling an enclosed air space with argon or krypton has thermal benefits. These less-conductive gases greatly reduce heat transfer by convective currents, producing lower U-factors. As a result, the inner surface of the glass is maintained at a temperature closer to that of the indoors, with greater comfort (because radiant heat flow to or from the window surface is reduced) and less chance of condensation on the inside surface. To preserve this gas fill over the life of the window, a very reliable edge seal is required.

Some constructions fill the space between panes with cellular materials or batting to reduce convection, though these units are no longer transparent, so they can no longer be used for views.

Low-Emittance (low-E) Coatings

What are Low-E Coatings

The performance of windows and skylights can often be improved by using Low-Emittance (low-E) coatings on their glazing surfaces.

• Hard-coat : durable, less expensive but less thermally effective
• Soft-coat : better thermal performance but more expensive and subject to degradation by oxidation in the manufacturing stage

These coatings are typically applied to one glass surface facing into the air gap between multiple glazings. A low-E coating blocks a great deal of the radiant transfer between the glazing panes, reducing the overall flow of heat through the window and thus improving the U-factor. Indeed, one such coating is almost as effective as adding another layer of glazing. An important added benefit of these films is their reduction of UV transmission, thus reducing fading of objects and surface finishes in rooms.

Low-E Coating Configurations and Heat Transfer

Low-E coating glass works like a thermal mirror with the low-E material on the back of the pane that reflects the heat radiation. The surface that is treated by the low-E coating material is selected based on the climate.

The intended use of the window in the building should dictate which surface the low-E coating should be applied to. There are quite a range of configurations. Some manufacturers only put the low-E coating on the #2 surface because of the concern in seal failure of the window.

Low-E Coating Configurations and Heat Transfer 

Low-E coating glass works like a thermal mirror with the low-E material on the back of the pane that reflects the heat radiation. The surface that is treated by the low-E coating material is selected based on the climate.

In multi-pane windows, thermal radiation from warmer pane to other pane is the main mechanism of heat flow. By applying low-emittance material to the interior or exterior pane, depending on the side that heat is meant to be kept, a significant amount of radiant heat transfer is blocked.

Three common types of low-E coatings are:

1) High-transmission low-E: In heating-dominated climates where you want to block heat radiation from inside, the low-E coating should be deposited on the outer surface of the interior pane of glass (surface #3). This is good for passive solar heating applications, where a low U-factor is combined with a high SHGC. In this case the coating traps outgoing infrared radiation that otherwise would be lost. Summer overheating can be avoided with external shading devices.

2. Selective-transmission low-E: Where winter heating and summer cooling are both important, requiring low U-factor and low SHGC, but with a relatively high VT for day lighting. The coating is on the outer glazing, where it blocks incoming infrared radiation, which as heat is then convected away by outdoor air.

3. Low-transmission low-E: In cooling dominated climates, where the sun is the enemy, low U-factor, low SHGC, and even low VT seem warranted. Again the low-E coating is placed on the outer glazing, where it rejects more of the solar gain. With a tinted exterior glazing, even lower SHGC and VLT could result.

Note that the exact placement of the low-E material is not as critical as using low-E materials in the first place.

The following table shows the U-values for various glazing constructions. As it can be seen, adding low-E coating improves the U-Factor  of the window which results in better performance.

Glazing Type	U-value
	W/m2K	BTU/(h℉ ft2)
Single pane	4.8	0.85
Double pane, air filled	2.5	0.49
Double pane, low-E	2.1	0.37
Triple pane	2.1	0.37
Triple pane, low-E	1.4	0.25

Thermal breaks 

U-values can also be reduced by reducing the conductivity of the materials in the glazing unit. As mentioned above, denser gases like Argon conduct less heat than air. More significantly, the window frame should not conduct heat around the glass.

Metal framing should be “thermally broken” to separate interior metal elements from exterior elements. Wood or fiberglass conducts much less heat than metal window frames. The framing of windows and other glazing can cause so much heat loss that a unit’s overall U-value may be double or triple the “center of glass” U-value. Thus it is important to use U-values that include framing.

Thermally-broken window frame

Selective Transmission Films

You can also control which wavelengths of light that you transmit into the space. “Spectrally selective” windows allow visible light in while blocking most other wavelengths, such as infrared and/or ultraviolet. They have a high Tvis without a high total light transmission. Ultraviolet light can fade and otherwise deteriorate interior finishes and furnishings. Infrared light is heat, and is often undesirable in warm climates.

Spectrally selective windows can block certain wavelengths of light

These films admit most of the incoming solar radiation in both the visible and near-infrared (short) wavelengths. Warm objects within a room emit far-infrared (long-wave) radiation. This long-wave radiation is reflected back into the room by the selective film.

These selective films typically are available as separate sheets that can be inserted between sheets of glazing as a window is fabricated. As a separate sheet, a selective film could also be applied to existing windows—for instance, between storm windows and the ordinary windows they protect.

Spectrally selective windows can block certain wavelengths of light

Aperture Placement & Area

“Aperture” refers to any daylight source, including windows, skylights, openings, and any other transparent or translucent surfaces. Aperture placement and area are important because strategic use of windows and skylights can help you achieve thermal and visual comfort passively, saving both energy and money.

Be sure to visit the passive heating, cooling, and daylighting pages for more specific strategies.

Side Light Aperture Area

Bigger apertures are not necessarily better. They can cause too much heat loss or heat gain, or too much brightness and glare.  Choosing just the right sizes for apertures (“right-sizing”) is key.  One way of measuring side light apertures is the Window-to-Wall Ratio (WWR):

Window Wall Ratio = Net Glazing Area / Gross Wall Area

Here, the “net glazing area” refers to only the transparent part of the window, not mullions or framing (usually net glazing area is around 80% of gross window area), and the “gross wall area” uses the full floor-to-floor exterior height of the wall.  A common rule of thumb states that the window to wall ratio should be 40% or lower for adequate insulation in cold climates, though more advanced windows with higher R-values (lower U-values) allow higher ratios.

In warm climates, higher ratios can be acceptable even without well-insulated windows, as long as the windows are well shaded from the sun’s heat.

Different window-to-wall ratios and the resulting illumination

Another metric to pay attention to for proper glazing from side windows is the Window-to-Floor Ratio (WFR).  A rule of thumb¹ for side-lighting thresholds is that the Window-to-Floor ratio, multiplied by the visible light transmittance (VLT, or Tvis) of the windows, should be:

0.15 < VLT • WFR < 0.18

Top Light Aperture Area

Top lighting apertures are much brighter than side lighting apertures, so less area is required.  Similar to the Window-to-Wall-Ratio, there is a Skylight-to-Roof Ratio (SRR) that is the net glazing area divided by the gross roof area.  A rule of thumb¹ is that the SRR should be between 3% and 6%. Tubular skylights require a much lower SRR than traditional skylights, approximately 1-2%.

To size a rectangular skylight, you can use this simple formula:

Area of one skylight = (Floor to Ceiling Height x 1.5)² • target SRR

To choose a size that is appropriate, start with 5% SRR and modify depending on climate and building use. For example with a 12′ ceiling and 5% skylight to roof ratio the right size skylight would be approximately: (12 x 1.5)^2 x 5% = 16.2sf. Therefore the project should use 4’x4′ or 8’x2′ skylights for good light distribution.

Aperture area is not the entire story, however.  The right size for apertures depends on their placement in the building, the building’s orientation, and the glazing properties.

¹ From the LEED rating system. IEQ Credit 8.1, LEED BD+C, 2009 edition

Shading & Redirecting Sunlight

Shading is an important set of strategies for visual comfort and thermal comfort.  As such, successful shading is measured by the overall success of visual and thermal comfort.

Shading strategies include overhangs, louvers, and vertical fins.  Light redirection strategies include light shelves and baffles.  All of these strategies can be external to the building or internal, and can be fixed-position or adjustable.  Some elements both shade and redirect light at the same time.  Both thermal comfort and visual comfort should be considered simultaneously when designing these elements, as they affect both.

Shades can keep the heat and glare of direct sun from coming through windows, while still allowing diffuse light and views to enter. They can also keep direct sunlight off of walls or roofs, to reduce cooling loads.  Interior shades do not block solar heat gain, but can block glare and even-out light distribution.

Interior shades can improve visual comfort, but do not block out solar heat gain

The most common form of shade is an exterior fixed horizontal overhang.  These are used on the side of the building facing the sun’s path, sometimes including east and west faces.  However, east and west faces often have more need of vertical fins to avoid low-angled sun.

The side of the building facing away from the equator needs no shading, except near the equator where the sun may be on the north or south side depending on the season.

There are many variations on fixed external shades, to reduce the profile and/or let more diffuse light in.

In hot climates, it can be especially useful to shade the building’s roof to avoid solar heat gain.  Rooftop solar panels, if placed right, can act as shades and thus perform double duty as energy generators and energy load reducers.

Adaptive Shades
Shading can be designed to allow the sun’s light and heat into the building at some times of day or year, while rejecting it at other times. The simplest method for this is to use a fixed horizontal overhang whose width is calculated to shade during summer months when the sun is high, and allow the sunlight in during winter months when the sun is at a lower angle.

An overhang shades in summer but lets heat in during winter

You can visualize the sizes for such overhangs for your location with this shading angle tool.

Shading can also be adapted by making it movable–either manually operated by occupants or automatically controlled.  Such systems can be much more responsive and finely tuned, but they are also more expensive, and require more maintenance and repair over the years.  User-operated systems may require occupant training, and are often not properly used.

Infiltration & Moisture Control

Water also moves through building envelope assemblies—in both liquid and vapor states. Unwanted infiltration can be a major cause of this. The focus here is upon water vapor movement. Water vapor will often need to be handled by a climate control system through the use of energy (termed latent heat).

Infiltration causes surprisingly large heat loss because unwanted moisture (latent heat) often must be removed from the air.

Infiltration causes surprisingly large heat loss because unwanted moisture (latent heat) often must be removed from the air.

Moisture Control

Dehumidification requires the removal of the latent heat and is an important function of HVAC systems.

The information below on latent heat flows and envelope design are excerpted from Mechanical and Electrical Equipment for Buildings By Walter T. Grondzik, Alison G. Kwok, Benjamin Stein, John S. Reynolds. Page 197.

In the summer, moisture will typically flow into an air-conditioned building, increasing humidity and requiring dehumidification. In the winter, it is not unusual to add water vapor to the air in a building to keep the relative humidity from dropping too low. This is often accomplished by evaporating water by adding the latent heat of vaporization. In some building types and climates, dealing with latent heat may be as big a problem as dealing with sensible heat. 

Moisture Control 

A difference in vapor pressure is the driving force behind moisture flow through components of an intact building envelope assembly, while gaps in the envelope can provide a route for airflow that carries water vapor. Vapor pressure difference is to latent heat flow as temperature difference is to sensible heat flow. The permeance of the materials of construction is the latent equivalent of sensible conductance.  The less permeable a material is, the greater the resistance to water vapor flow. Materials with low permeance are termed vapor retarders, and are incorporated in envelope constructions as a means of reducing the flow of water vapor and subsequently the risk of condensation of the vapor within the envelope assembly. From an architectural design perspective, reducing water vapor flow is accomplished using very thin materials (membranes) that must be carefully located to ensure that they work as intended. Although placement within an envelope assembly is critical, vapor retarders take up virtually no space—in drastic contrast to the thickness requirements of sensible heat retarders (insulations). The specific location of a vapor retarder within a wall, roof, or floor cross section will vary with climate and construction types. The fundamental principle, however, is for the retarder to stop the flow of water vapor before the vapor can come in contact with its dew point temperature within the assembly. 

Cold Climate Moisture Control

Most common building materials, including gypsum board, concrete, brick, wood, and glass fiber insulation, are easily permeated by moisture. Most surface/finish materials are also permeable. In cold climates, the winter outside air contains relatively little moisture, even though the RH may be high. By contrast, inside air contains much more moisture per unit of volume, despite its probably lower RH. The resulting differential vapor pressure drives the flow of water vapor from high to low vapor pressure. 

Hot, Humid Climate Moisture Control

In hot, humid conditions, cool inside surfaces are often encountered—for example, a radiant cooling panel containing chilled water, a water pipe, or a supply air diffuser. Hot and humid air contacts such a surface and condensation can occur. The moisture vapor in the air condenses to form visible droplets of water on the cool surface. The result can be mildly annoying if droplets of condensation fall on occupants, or serious if water stains occur and, eventually, mold grows on damp surfaces.