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Special | A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z | ALL

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A

Adiabatic

Not diabatic, occurring without gain or loss of heat.
For thermal bridge modelling, the cut-off planes of the model are assumed to be adiabatic. This can be controlled by checking the isotherms of the model: if they are perpendicular to the cut-off plane some centimetres before they meet the cut-off plane, no heat loss will occur through the cut-off plane.


Anisotropic

An=not iso=equal tropic=directed.

Not isotropic; having different physical properties in different directions.

Used for materials where the thermal conductivity varies depending on the direction of heat flow; for example timber has an approximately 2.2 times higher thermal conductivity, when the heat flow is in line with the grain, than when it is perpendicular to the grain. End grain boards used as flooring will thus have a significantly lower thermal resistance than for example plywood boards, even when exactly the same timber is used. While most building materials can be viewed as isotropic, i.e. having a thermal conductivity not depending on direction, building materials other than wood may behave anisotropic as well, for example vertical coring lightweight bricks, or certain forms of reinforced concrete. When a material is not uniform, or extensively structured, anisotropy is likely.


C

Chi-value

The chi-value applies to a point thermal bridge. This can be a bolt or fastener - anything with not much of an extension in two of the three dimensions. Greek symbol χ. Unit: W/K. A single point thermal bridges can be neglected for the energy balance, but recurring point thermal bridges, as they occur e.g. in curtain wall facades, need to be accounted for.


E

Emissivity

Emissivity, symbol ε, is the ratio of energy radiated by a particular material to energy radiated by a black body at the same temperature, which corresponds to a value of 1. Surfaces with a modest emissivity emit little, those with high emissivity radiate much energy. Emission levels of common construction materials vary from 0.80 to 0.95. An exception are coated glasses and polished metal surfaces, whose values may be considerably lower. If the emission level of a building material is unknown, 0.90 can be assumed.


F

Final Energy

Energy supplied to consumers, where it can be converted into useful energy, e.g. electricity into light or warmth, using respective devices.

Final energy is derived from primary energy, usually taking in transformation and distribution loss in the process.


G

g-Value

The g-value gives the total solar energy transmittance, i.e. the fraction of solar energy (direct and indirect) that enters the building through a transparent element. The direct gains equal the total short wave transmissivity of the element. This fraction is zero with opaque elements. Indirect gains are obtained via absorption of solar energy, which is then radiated as heat. This fraction is > zero with both, opaque and transparent building elements. In steady state calculations, the g-value is usually only used with transparent elements, nevertheless. As input value for PHPP, the g-value needs to be assessed using EN 410.

The solar heat gain coefficient (SHGC) used in the United States and some other countries, for example New Zealand, describes the same physical property, but is assessed using deviating boundary conditions. Thus, g-value and SHGC can not be used interchangeably.


Glazing for Passive Houses

Physical properties for window glazing of interest for planning Passive Houses are total solar energy transmittance and U-value, predominantly.
NFRC 100 and 200 are the standards used in the United States and some other countries, while in Europe and some other parts of the world EN 410 and EN 673 standards are used to assess these values.
Using NFRC standards will result in U and g-values that differ significantly from those derived from the relevant European standards.
Certification criteria for Passive Houses require the use of EN 410 ( g-value) and EN 673 ( U-value) as assessment standards, and PHPP results may be inaccurate, when NFRC values are used instead.
Total solar energy transmittance ( g-value, Europe) or solar heat gain coefficient (SHGC, USA) give the proportion of solar energy that passes through the glazing.
Ug for glazing U-value gives the thermal transmittance of the glazing, in other words: the amount of heating energy that can escape through a square meter of this glazing unit. This is however determined at the centre of glass; with multi-pane units the influence of the material of the edge spacer can significantly add to the heat loss for the glazing unit.


Gross density

The formal definition of density is mass per unit volume. In some contexts the density is expressed in grams per mL or cc. In the building sector, kg/m3 is more commonly used. Mathematically a "per" statement is translated as a division. Density = Mass/Volume, in the building sector most commonly expressed in kg/m3.


H

Heat

Heat is the least "noble" energy form. According to the second law of thermodynamics, heat cannot be transformed into higher forms of energy - like mechanical or electrical energy - without a loss, whereas the reciprocal process can happen loss free. While heat is a scalar and thus undirected, heat transfer always occurs from warm to cold, and never the other way around, unless work is added. As heat is at the end of a chain of energy transformations, it can be the tip of an iceberg of energy usage. Using "nobler" forms of energy to generate heat, e.g. using electricity to generate space heating, should be minimised, and waste heat from mechanical or electrical processes should be utilised to preserve energy sources.


Heat flow

Heat flow describes heat per unit of time in J/s or W; it is also a measure of power. Heat flow is a scalar, therefore not directed and not affected by changes of the co-ordinate system.

Applied to a building element, however, heat flow turns into a heat flow rate in W/m². It looses is undirectedness, becoming a vector. The heat flow rate is proportional to the temperature difference at both ends of the system, for example at building element surfaces.


Heating Demand

QH. What remains after usable heat gains (QS and QI) have been added to heat losses (QT and QV) in the energy balance of a building.


Heating Load

Heating load describes the required size of a heating system for providing comfortable indoor temperatures even on the coldest design day of the year. As this is sometimes confused with heating demand, one way of differentiating the two with an analogy might be thinking of the heating load as the power of a motor in horse power or kW, and heating demand as the energy you need to feed this motor for it to do what you want it to do, e.g. taking you from A to B. In PHPP the heating load is assessed for two weather variations: a cold, clear day and a moderate, overcast day. On a cold, clear winter day you can expect temperatures to be in the lowest range, as nothing (no cloud cover) stops the earth's surface from radiating back heat to the sky. On the plus side, solar radiation will be higher than on an overcast day. Whatever scenario turns out to result in the highest heating load will be applied for further considerations. For a Passive House, the heating load should not exceed 10W/m2 treated floor area. Example: with a treated floor area of 100 m2, the heating load should be no more than 1,000 W or 1 kW. How is the maximum heating load explained? By multiplying the specific fresh air requirement (30W/person) with the specific heat capacity of air at 20℃ and the difference of maximum supply air temperature (at about 52℃) and the minimum temperature at which air leaves the heat exchanger (16.5℃). The approximate result of this calculation is 350W/person.

\frac{30m^3}{h*person} * \frac{0.33Wh}{m^3*K} * (52-16.5)K \approx \frac{350W}{person}

Divided by the average space requirement per person (assumed  as 35m²) a maximum heating load of 10W/m² results:

\frac{350W}{person} * \frac{person}{35m^2} = \frac{10W}{m^2}


K

k-Value


L

Lambda value


Low-e glazing

Low-e stands for low emissivity. Microscopically thin, nearly invisible, metal or metallic oxide layers are coated to the outside of the interior pane (thus located in the interstitial cavity) of a multi-pane Insulating Glass Unit (IGU) to reduce the heat conductance by minimising radiative heat flow. A low-e coating is usually transparent to the solar spectrum (visible light and short-wave infra-red radiation) and reflective of long-wave infra-red radiation. In triple glazed IGUs there sometimes are two low-e layers.


P

Passive House

A very comfortable and healthy house that needs very little energy.

Quantitative performance benchmarks include:

  • heating energy demand below 15 kWh per year or
  • heating load below 10 W per square metre treated floor area
  • airtightness below 0.6 air changes per hour at 50 Pascal pressure differential
  • primary energy demand requirements depending on certification class

PHPP

PHPP is the Passive House Projecting Package, a spreadsheet based tool for planning and optimising highly energy efficient buildings. It can be obtained from the Passivhaus Institut in Germany and various national bodies.


Primary Energy

Primary energy is energy contained in raw fuels or other naturally occurring forms of energy before it undergoes transformation or is put to a use. It is a measure of the energy potential of a source, what could be used if conditions were ideal - which they are however not, for the largest part. Electricity derived from fossil fuels e.g. loses around 2/3 of its energy potential in the transformation process. In other words: it needs roughly 3 parts fossil fuel to generate one part of electric energy.
After transformation, energy is delivered to consumers as consumer or final energy, with additional losses due to transportation. Primary energy can be re-calculated from consumer or usable energy with the help of primary energy factors, if these are known. PHPP contains a list of primary energy factors for non-renewable energy services. The new certification categories are among other things concerned about renewable primary energy.


Psi-value

Coupled 2D thermal bridge coefficient, used to gauge the numeric impact of thermal bridges. Ψ, upper case psi (English: sigh), is the most commonly used symbol. The unit for psi is W/(m K). It applies to a length, i.e. the length of the thermal bridge. It denotes the thermal conductivity of an assembly of materials. The higher the value, the higher the additional heat loss through that joint.
When external dimensions are used to assess the heat loss through the thermal envelope, psi values can be negative (=calculatory heat gain), as with external dimensions convex corners are considered twice (or even thrice, if assessment is made in 3 dimensions). When internal dimensions are used for heat loss calculations (like it is done by default in NZ), every convex corner has to be analysed for thermal bridging effects. Using external dimensions overestimates heat loss, and thus compensates for some geometrical thermal bridge effects. Thus, when external dimensions are used, an assessment of thermal bridge effects at convex corners is usually not necessary.


R

R-value

Is the measure of thermal resistance, used in the building and construction industry. The higher the value, the better the insulation effectiveness of a material layer. The R-value is always the property of a material layer in m2 K/W, derived from dividing the material layers thickness (m) by the material's thermal conductivity. The R-value is therefore not intelligible without the material layer thickness.

Several indices indicate more detailed information about what sort of R-value is considered. RT for example indicates the total R-value of the sum of all material layers in a building element. If calculated in accordance with international standard ISO 6946, it also includes repeating thermal bridges.


S

SHGC

Refer to g-value.


T

Thermal Conductivity

Thermal conductivity is a material property. It indicates the heat flow (W or J/s) occurring in 1m of material length at a temperature differential of 1 Kelvin on both ends of this length.
The Greek symbol commonly used for thermal conductivity is lower case lambda λ. Sometimes, lower case k is used instead. The unit for thermal conductivity is W/(mK). The parentheses are not optional, as this example might illustrate: A material might conduct 30 Watt over the length of 1 meter at a temperature differential of 30 Kelvin. Without parentheses the calculation goes: 30/1*30=900; with parentheses it's 30/(1*30)=1.
Usually, a material would have a measured thermal conductivity (from a sample) and a design thermal conductivity fit for calculations, containing a safety margin, thus acknowledging inconsistencies in the production process.
The smaller the thermal conductivity, the lesser the amount of heat that is conducted, or in other words: the better the insulation properties of the material.
Metals usually have a high thermal conductivity (aluminium for example of at about 220 W/(mK)).
Wood usually has a low thermal conductivity, but can be anisotropic: having different thermal conductivities in line with the grain and perpendicular to the grain (lower). By default the lower value perpendicular to the grain is given, thus if the piece of wood is used with the grain in line with the heat flow (e.g. end grain boards), adjustments have to be made.
As a rule of thumb, thermal conductivity will be relative to the gross density of a material: high gross density = high thermal conductivity. Water content and temperature of the material are furthermore relevant for its thermal conductivity.
ISO 10456:2007 and NZS 4214:2006 give tabulated values for the thermal conductivity of generic materials. Where ever possible, more accurate, specific values should however be used for calculations.


U

U-value

U-values gauge how well a building element allows heat to pass through. The lower the U-value, the greater a building element's resistance to heat flow and the better its insulating value.
The U-value is the reciprocal of the total R-value, inclusive of external and internal surface resistance.
Note: neither a material nor a material layer has a U-value. As U-values encapsulate surface resistance, they only make sense for building elements.
With the building element "window" the U-value is the weighted mean of the U-value of the glazing unit, the frame, and the linear thermal transmittance of the glazing unit's edge spacer, multiplied by the length of the glazing unit's perimeter.


Λ

λ


Χ

χ

Refer to chi-value.


Ψ

Ψ

Refer to psi-value.



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