Information regarding the types and technologies of building envelopes for a building.

1. Climate Responsive Architecture
Building Envelope
BY
CHANDAN K B

2. Building Envelope
Humans first createdshelters to providethermal comfort and
protection from natural elements, and this still remains a primary
objective ofbuildings.
Thebuilding 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 areany 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.

3. Envelopesfor climatetypes
A well-designed envelope responds to the localclimate.
The summary below shows four common extremes that
people design for. Milder climates can use milder
versions of these strategies, or mix and match.

4. Arid Climate Envelope
The City Palace in Jaipur, India. Photo: Jeremy
Faludi
Arid climatesarevery dry,andusuallyhot,butthey
oftenhavelargeswings oftemperaturefromdayto
night. Thusthermalmasson theoutsideof thebuilding
is themostcrucial design strategytoeven outsuch
temperatureswings. Forconsistentlyhotlocations,it
alsohelps tohavehigh ceilings, shaded
breezeways,light colors, anddaylightingvia reflected
light (not directsun),such asin this audiencehall in the
Jaipurcitypalace. Courtyardswithnatural
ventilationandpoolsorfountainscan
provide evaporativecooling aswell.

5. Tropical ClimateEnvelope
A traditional home in Papua New
Guinea. Photo: Jeremy Faludi
Tropicalclimatesarehotandhumid. Therefore,keeping
the heatofthesun offis thetoppriority,as well as
maximizingventilation—essentially
areflective insulatedroofwithwallsthatpassbreezebut
notrainis ideal. This traditionalPapuaNew Guinean
home’s thicklight-coloredthatchroofkeepsoutthe
sun’sheat,while openeaves andporousbambooslats
forwallsandfloormaximizenaturalventilation. The
materialsareall low-masstoavoidcondensationand
mold growth,which canhappenwithhigh-mass
materialsin humidclimates. (Note:Jalousiewindows
arefoundin thetropics,butarenotas common
elsewhere, becausethey areso poroustobreezes.)

6. Cold Climate Envelope
A vernacular-design cabin in Finland.
Cold climateshavemanymoreheating
degree saysthancooling degree
days. Thusmaximizinginsulationis the
keytokeeping warm,as well as using
windowsforsolargainon thermal
mass inside thebuilding envelope (not
outsideasin aridclimates). Partof
having effectiveinsulationin cold
climatesisanair-tightenvelope,
avoiding infiltration. ThisFinnish cabin
hasvery few andvery small windows
excepton thesouthside, tomaximize
solargain whileminimizing losses
elsewhere. Beforemodern insulation,
thicksolid log walls suchas these
providedbetterinsulationthanboard
walls could.

7. Mixed Cold / Hot ClimateEnvelope
The Aldo Leopold Center in Baraboo,
Wisconsin.
Many“temperate”inlandclimates
actuallyhavetwoextremes--coldin
winter,hotandhumid in summer.
Flexibility isthe keytodesigning forthese
climates.TheAldoLeopoldCenter in
Wisconsin,firstbuilding tobeLEED
certifiedascarbon-neutral,usesdeep
overhangsto allowlow wintersunin
throughthe windowstoheatup ahigh-
massconcreteslabinside,while blocking
high summersun. Italso usesalight roof
anddarkerwalls torepel summersunbut
absorbwintersun. Extrainsulation
retainsheatin winter,butoperable
windowspassivelycool it in summer.

8. Envelope Energy Flows
From anenergyflow perspective, the envelopeis a composition of layerswith varyingthermaland permeability
properties. Theenvelope maybe composed of membranes,sheets, blocks and preassembled components. Thechoice of
envelope is governedbythe climate, culture,and available materials. Therangeof choices in envelope design can be
illustrated by two opposite design concepts: the open frameand the closed shell.
Inharsh climates, the designer frequently conceives the building envelope as a closed shell and proceeds to selectively
punchholes in it to makelimited and special contact with the outdoors. Thismay also betrue wherethere areunwanted
external influencessuchas noise or visual clutter.
When external conditions are veryclose to the desired internalones, the envelope often begins as an open structural
frame, with pieces of the building skin selectively added to modify only a few outdoor forces.
Theflow of heat througha building envelope varies both by season (heat always flows from hot to cold and generally
flows froma building in winter and toa building in summer) and bythe path of the heat (throughthe materials of a
building’s skin, or by outdoor air entering). Thesecomplexities must be considered bya designer who intends to deliver
comfort and energyefficiency.

9. Walls
Understanding and optimizing the heat transfer through the walls is important inhigh performance building
design. Using thermal mass and insulation to your advantage with passive design strategies can help reduce the
amountof energythat active systems need to use.
Insulation
Thermalinsulation is a material that blocks or slows the flow of heat
throughthe building envelope. Insulation is vital to most green building
design because it allows spaces to retain what heat theyhave, while avoid
gaining excess heat from outside.
Total R-Valuesand Thermal Bridging
Inorder to knowthe building's true thermalperformance,youmust
calculate overall R-values for assemblies likewalls, roofs, floors, and glazing.
Thetotal R-value(or "overall"R-value)of an insulated assembly may be
higheror lower than the R-valueof the insulation, depending on the
assembly's construction. Thermalbridging is when the overall R-valueis
lower than the insulation's R-value.

10. Insulation
It’s important tounderstand Heat Energy Flows in a building tounderstand insulation. Insulation primarilyis designed toprevent
heat transfer fromconduction andradiation.
Resistance toconduction is measured by R-value (high thermal resistance =high R-value); Resistance to radiative heat transfer is
measured by emissivity (high resistance =lowemissivity andhigh reflectance). Conduction is the dominant factor when materials
aretouching each other; when there is an airgapbetween materials, radiationbecomes important. Convection usually only
becomes an issue when significant airpockets areinvolved.
Materials used forinsulation fallinto two broadcategories:
Fibrous or cellular products –These resist conduction andcan be either inorganic (such as glass, rockwool, slag wool, perlite, or
vermiculite) ororganic ( such as cotton, synthetic fibers,cork, foamed rubber, orpolystyrene).
Metallic or metalized organic reflective membranes - These block radiation heat transfer and must face an airspace tobe
effective.

11. Convection and Insulation
Convection through fluids (like air) can also transfer heat. Unwantedconvection throughthe building envelope
can cause unwanted heat gains or losses throughinfiltration (see Infiltration & Moisture). Also, suppressing
convection within the materials of the building envelope is often what makesinsulation effective.
Insulation Materials
Althoughinsulation can be made from a varietyof materials, it
usually comes infive physical forms: batting, blown-in, loose-fill,
rigid foam board, andreflective films. Each type is made to fit a
particular part of a building.

12. 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.

13. 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
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.

14. 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.

15. Total R-Valuesand Thermal Bridging
Buildings 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-valuesIn 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.
Total R-Value
Adding R-valuesIn 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.

16. 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.

17. 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.

18. 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.

19. 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.

20. 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

21. 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.
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 (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.

22. 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.
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.

23. Selective TransmissionFilms
Spectrally selective windows can block
certain wavelengths of light
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.

24. 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 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.
Different window-to-wall ratios and the
resulting illumination

25. 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.

26. 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.