Climate Responsive Architecture

Building EnvelopeBY CHANDAN K B

Building EnvelopeHumans 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.  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

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

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.

Tropical Climate Envelope

A traditional home in Papua New Guinea. Photo: Jeremy Faludi

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 says 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

The Aldo Leopold Center in Baraboo, Wisconsin.

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, 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.

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. 

WallsUnderstanding 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.

InsulationThermal 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. Total R-Values and Thermal

BridgingIn order to know the building's true thermal performance, you must calculate overall R-values for assemblies like walls, roofs, floors, and glazing. The total R-value (or "overall" R-value) of an insulated assembly may be higher or lower than the R-value of the insulation, depending on the assembly's construction. Thermal bridging is when the overall R-value is lower than the insulation's R-value.

InsulationIt’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: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).

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

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

Insulation MaterialsAlthough 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 / BlanketsForm 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.  

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 flow.   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. 

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. 

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. 

Total R-Values and Thermal BridgingBuildings are rarely built of a single material, so to determine the total R-value you need to factor-in all of the individual components. Thermal resistance adds differently if it is in series or parallel. For high performance buildings, you usually want high R-values (good insulation).

Adding R-values In SeriesWhen 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. 

Total R-Value

Adding R-values In ParallelWhen 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. 

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

Thermal Bridging and Thermal Breaks

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".

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

Thermal breaks

Thermally-broken window frame

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.  

Framing Factor

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

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.

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. 

Windows

Glazing PropertiesGood 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.  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 include1. Thermal conductance (U-value)2. Solar Heat Gain Coefficient (SHGC)3. Visible Light Transmittance (Tvis)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. 

High Performance WindowsWindow 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.

Low- Emittance (low-ε) Coatings

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

Low-E coatings are invisible thin layers of metal or metallic oxide particles deposited on the glazing surface of windows and skylight. 

Selective Transmission Films

Spectrally selective windows can block certain wavelengths of light

Spectrally selective windows can block certain wavelengths of lightThese 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.

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.

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)Top Light Aperture AreaTop 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.

Different window-to-wall ratios and the resulting illumination

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.

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.