MMC 52pp M10 WEB

External Building Envelope
What goes into a building envelope?
In layman terms, a building envelope is the physical separator between the conditioned and unconditioned environment of a
building including the resistance to air, water, heat, light, and noise transfer. MMC Editor Joe Bradbury breaks the building
envelope down in detail to help specifiers better understand this crucial area of a construction project:
The building envelope encompasses all of
the aspects of a building’s outer shell
that work together in creating a
comfortable, dry indoor environment and
facilitate climate control for its occupants.
Building envelope design is a specialized area
of architectural and engineering practice that
draws from all areas of building science and
indoor climate control.
The building envelope serves many purposes,
but can broadly be split into three general
categories:
1. Support (to resist and transfer structural and
dynamic loads)
2. Control (the flow of matter and energy of all
types)
3. Finish (to meet desired aesthetics on the
inside and outside)
Effective control is at the core of all well
performing building envelopes. For a building
to operate safely and efficiently, it needs to
offer adequate rain control, air control, heat
control and moisture control.
Rain and moisture control
In Britain our obsession with the weather is
world-renowned, and for good reason. On
average it rains for 156.2 days per year, so we
feel a strong desire to be prepared for it. Being
prepared for rain doesn’t just mean carrying a
brolly; control of rain is fundamental for the
protection and continuation of our built
environment. To this end, there are numerous
strategies that can be implemented within a
structure to protect it from the Great British
drizzle.
These include barriers, drained screens, and
storage systems for rainwater.
Arguably, the main purpose of a roof is to resist
water. Flat roofs actually slope up to 10° or 15°
but are built to resist standing water. Pitched
roofs are designed to shed water but not resist
standing water which can occur during winddriven
rain or ice damming. Typically
residential, pitched roofs are covered with an
underlayment material beneath the roof
covering material as a second line of defense.
Domestic roof construction may also be
ventilated to help remove moisture from
leakage and condensation.
Walls do not get as severe water exposure as
roofs but still require suitable protection.
Barrier walls are designed to allow water to be
absorbed but not penetrate the wall, and
include concrete and (some) masonry walls.
Drainage walls allow water that leaks into the
wall to drain out such as cavity walls. Drainage
walls may also be ventilated to aid drying such
as rainscreen and pressure equalization wall
systems. Sealed-surface walls do not allow any
water penetration at the exterior surface of the
siding material. Generally most materials will
not remain sealed over the long term and this
system is very limited, but ordinary residential
construction often treats walls as sealedsurface
systems relying on the siding and an
underlayment layer sometimes called
housewrap.
Air flow
Poor air quality is proven to negatively impact
on everyone’s health, increasing the risk of
serious illnesses and making existing
conditions, like respiratory disorders, worse.
Control of air flow is important to ensure
indoor air quality, control energy consumption,
avoid condensation (and thus help ensure
durability), and to provide comfort.
Control of air movement includes flow through
the enclosure (the assembly of materials that
perform this function is termed the air barrier
system) or through components of the building
envelope itself, as well as into and out of the
interior space, which can affect building
insulation performance greatly. Hence, air
control includes the control of windwashing
(cold air passing through insulation) and
convective loops which are air movements
within a wall or ceiling that may result in 10%
to 20% of the heat loss of a building alone.
The physical components of the envelope
include the foundation, roof, walls, doors,
windows, ceiling, and their related barriers and
insulation. The dimensions, performance and
compatibility of materials, fabrication process
and details, connections and interactions are
the main factors that determine the
effectiveness and durability of the building
enclosure system.
Temperature
The thermal envelope, or heat flow control
layer, is part of a building envelope but may be
in a different location such as in a ceiling. The
difference can be illustrated by understanding
that an insulated loft floor is the primary
thermal control layer between the inside of the
house and the exterior, whilst the entire roof
(from the surface of the roofing material to the
interior paint finish on the ceiling) comprises
the building envelope itself.
40 MMC
Nov 2018 M10