Building Heat Losses

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Building heat losses

Table of Contents


Dwelling Dimensions2



Store rooms and utility rooms4


Heat Losses4

U-values of opaque elements5

Window U-values5

U-values of elements adjacent to an unheated space6


Stairwells and access corridors in flats8



Building heat losses


In buildings, our interest in heat loss is in sizing our HVAC equipment, sizing windows for solar gain, and of course estimating what a building's yearly energy cost will be. Since the exact conditions are complex and changing, we ignore all the subtleties of heat loss and generate approximations using approximate U values and historical weather conditions: for example the typical worst case cold day in any year (Carlson, 1972, pp. 41).

However, keep in mind that the actual energy used for heating a building is very dependent on occupant behavior: what temperature the house is kept at, and whether, for example, they leave bedroom windows open all night in the winter. Energy consultants and energy code officials sometimes seem to treat the heat loss values as gospel, when in fact they are just very good guesses. It would not be surprising to find, for example, that after an energy retrofit, the amount of energy used didn't go down as much as predicted, because the occupants are now more comfortable.

Dwelling Dimensions

The boundary of the heated accommodation consists of all the building elements separating it from external environment or from adjacent buildings or unheated spaces. Except where otherwise indicated, linear measurements for the calculation of wall, roof and floor areas and dwelling volume should be taken between the finished internal faces of the appropriate external building elements. Space taken up by any internal elements (internal partition walls or intermediate floors within the dwelling) is disregarded for the purposes of establishing the total floor area of the dwelling. Linear measurements for the calculation of the areas of external door, window and roof light openings should be taken between internal faces of appropriate cills, lintels and reveals (Grau, 2009, pp. 23).

“Volume" means the total volume enclosed by all enclosing elements and includes the volume of non-usable spaces such as ducts, stairwells and floor voids in intermediate floors. Dimensions refer to the inner surfaces of the elements bounding the dwelling. Thus floor dimensions are obtained by measuring between the inner surfaces of the external or party walls, disregarding the presence of any internal walls. Storey height is the total height between the ceiling surface of a given storey and the ceiling surface of the storey below.

For a single storey dwelling, or the lowest floor of a dwelling with more than one storey, the measurement should be from floor surface to ceiling surface. Floor area should be measured as the actual floor area, i.e. if the height of a room extends to two storey's or more only the actual accessible floor area should be entered. However, as an exception to this rule in the case of stairs, the floor area should be measured as if there were no stairs but a floor in their place at each ...
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