16394 Design: Form and Fabric Nick Kelly

Form and fabric: overview

• far reaching consequences with regards to energy and environmental performance • fabric: – insulation – thermal mass – moisture transport • form: – solar access: heating and daylighting – ventilation: wind driven and stack driven ventilation

Fabric

•

U-value

U-value (W/m 2 K) is a measure of how readily heat will flow through a material or structure: k 1 k 2 Q h o

U

   1

h i



i n

  1

k l

 1

h o

   1 h i

Q



UA

(

T i



T o

) l 1 l 2 • • the

lower

the U-value the better a surface is insulated U-value is used to calculate the steady state heat flow through a construction

Fabric problems[1]: thermal bridges

• common problems with constructions are ‘thermal bridges’ inside outside If Tsurf < Tdew condensation occurs window conc. lintle – low resistance heat flow path – low U-value • solution – disrupt high conductivity flow path with insulation concrete insulation

U-value problems

• the constructions below have identical U-values but will give rise to totally different thermal performances

inside outside outside inside concrete insulation insulation concrete

Dynamic characteristics

• the U-value approach is actually a simplification of how heat flows with time through a material • need to consider ‘dynamics’ • for the concrete layer: themal capacity heat storage 

c



T



t



k

 2

T



x

2 1-D heat flux

Thermal mass

• or more accurately ‘exposed thermal’ mass has a profound impact on how a building behaves heat outside inside outside inside heat concrete insulation low thermal mass insulation concrete high thermal mass

Thermal mass

• can be used to ‘damp’ oscillations in the internal air temperature or limit peak temperatures when used in conjunction with ‘night’ flushing • problematic in buildings that require fast thermal response low thermal mass high thermal mass midday

Thermal ‘lag’

• heat transfer through an opaque material does not occur instantaneously – it is always associated with a time delay

v

 2 (

π λ n α

) 1 / 2 

t



l

/

v

 

k



c p

Passive Solar [1]: Trombe wall

• the Trombe-Michelle wall is a passive solar component that makes used of thermal mass and time lags to absorb and transmit solar radiation to the interior of a building heat travels through wall in a temperature ‘wave’ arriving several hours after solar radiation was absorbed by outside wall glass admits s.w. solar radiation and traps l.w. radiation from wall surface

Passive Solar [1]: Trombe wall

temperature increasing at wall surface solar radiation decreasing solar radiation flux moves through wall (lags temperature) mid afternoon middle of the day heat flux solar radiation decreasing flux moves further through wall late afternoon evening heat flux reaches internal surface

Vapour transport

• in addition to heat, moisture can also travel through the building fabric • rate of moisture transport is related to the partial vapour pressure inside and outside the building • just and in the case of different materials can have different moisture ‘conductivities’ – –

permeable

materials allow moisture to pass through easily

impermeable

materials act as a barrier to vapour

Fabric problems [2]: interstitial condensation

• in certain cases this can lead to condensation inside the construction when the local temperature falls below the local dew point temperature inside dry bulb temperature dew point temperature (related to vapour pressure in material) outside condensation risk

Form

Form: solar energy

• solar energy can be used in a building design to displace the need for both electric lighting and heating • form of the building can be designed to maximise the use of beneficial solar energy • the most basic renewable energy device is the window!

Form: glazing and orientation

• glazing is a transparent high conductivity, high speed energy flow path • wall U-value 0.3W.m

2 K double glazing 2.0 W.m

2 K – admits energy as s.w. solar radiation – opaque to l.w. radiation – loses energy through conduction/convection/radiation (l.w. & s.w.) • south facing glazing is has a net energy

gain

• north-facing glazing has a net energy

loss

Form: glazing and orientation

• orientation affects when solar energy is available West solar energy p.m.

south -maximises solar energy East solar energy a.m.

Form: glazing and orientation

• • • • so … quantity and orientation of glazing dictates how much and when solar energy is admitted into a building traditional solar buildings have (in temperate climates) significant glazing facing south and little glazing on the north spaces with high glazing areas are subject to significant temperature swings and so are not good performers for comfort often used in conjunction with a ‘buffer space’

Form: overheating and shading

• poorly designed glazing admitting too much solar radiation at the wrong time of day or period of the year leads to overheating • can lead to

increased

energy consumption – need for mechanical cooling • also .. can lead to uncomfortable environment temperatures – high daytime temperatures, cool night time • increased heating load in winter

Form: shading

• shading can improve the temporal characteristics of glazing admits low angle sun in the morning or winter when energy is needed screens sun in middle of the day and in summer when overheating is a risk overhang shading device

Form: ventilation

• the form of the building has a significant impact on the ability to ventilate a building using natural means • natural ventilation is achieved by two mechanisms: wind driven pressure and stack effect • when considering natural ventilation it is important to remember that the ventilation level is dependent upon wind speed and/or temperature and is therefore variable

Form: wind driven ventilation

• air flowing over a building gives rise to natural pressure differences low pressure high pressure low pressure • creates pressure difference across the building façade – this is the driving force for air flow • judicious placement of ventilation opening creates a natural ventilation scheme

Form: wind driven ventilation

• the available pressure difference between two surfaces of a façade is given by 

P

 (

c pa



c pb

) 1 2 

V f

2

Form: stack ventilation

• this is driven by internal and external temperature differences • occurs between openings at different heights • as with wind pressure ventilation schemes the amount of ventilation is variable

Form: stack ventilation

• stack ventilation calculation: 

P

  

g

273

h

  1

T ext

 1

T

int   h

Form: natural ventilation

• in low energy buildings the form is often engineered to make best use of stack and/or wind driven ventilation – shallow plan (low driving force resistance) – atrium (high/low openings + high low pressure) – ventilation chimney (high/low openings + high low pressure)

Examples: Glasgow

• solar residences – passive solar heating • lighthouse building – passive solar heating

Combining effects

• finally it is worth pointing out that sustainable form and fabric features are rarely used in isolation – very often combinations are required to achieve the desired effect: – thermal mass and night time ventilation – mixed mode ventilation – wind driven + stack – trombe wall + shading devices

Review

• looked at how fabric properties can affect the sustainability of a building – fabric: insulation, dynamic performance, thermal mass, condensation, thermal bridges, trombe walls • and the impact of form: – form: glazing, solar access, passive heating and daylighting, overheating and shading – form: air flow, wind pressure, pressure differences, stack effect, temperature differences, ventilation opening