4.3.3.1.3
Target U-value Method for new dwellings
The Target U-value Method gives more flexibility than the Elemental
Method in selecting areas of windows doors and roof lights and in
the insulation levels of different elements of the building envelope
for the design of new dwellings. It can be used for any heating system
and can take solar gain into account.
A dwelling complies by the Target U-value Method if the Target U-value
is not less than the average U-value, where the average U-value is
defined as the areaweighted average U-value of all exposed elements
of the dwelling. Exposed elements include walls, roofs, floors, windows
and doors, including elements
adjacent to unheated spaces.
The Target U-value ( UT) may be calculated using the equation:
UT = [0.35 – 0.19(AR / AT) – 0.10(AGF / AT) + 0.413(AF / AT)]
where UTis the Target U-value prior to any adjustment for heating
system performance or solar gain, ARis the exposed roof area, AGFis
the ground floor area, AFis the total floor area (all storeys) and
ATis the total area of exposed elements of the dwelling (including
the ground floor).
If appropriate a solar gain term may be added  S = 0.4[(AS –
AN / ATG]
Where:
AS = area of glazed openings south facing
AN = area of glazed
openings north facing
ATG = total of all glazed openings in the building
Suppose the wall construction is to be a cavity wall with a brick
outer leaf, a cavity partially filled with 17mm of Celotex and a
115mm Aircrete inner leaf finished with 13mm dense plaster internally.
It has a U- value of 0.4 W/m K, higher (ie. poorer) than would be
required in the Elemental Method but the designer wishes to use
it.
The U-values of the remainder of the elements are as in the following
table. The heating system based on a gas boiler with an efficiency
of 85%. The area of windows and doors is equal to 25% of the total
internal floor area.
The total area of North-facing glazed openings is 6.82m and the
total area of South-facing glazed openings is 8.88m.
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The average U-value = 100.94/191.3
= 0.528
The Target U-value is
UT = [0.35 – 0.19(44.4/191.3) – 0.10(44.4/191.3) + 0.413(88.8/191.3)]
= 0.474 W/mK
which is multiplied by fe, the ratio of proposed boiler SEDBUK divided
by reference boiler SEDBUK (which is 78% for heating by mains gas),
giving a Target
U-value of
UT x fe = 0.474 x (85% / 78%) = 0.517 W/mK
Since the average U-value is slightly greater (worse) than the target
U-value modifications should be made to the design by reducing the
average U-value or increasing the Target U-value (or both).
As alternatives to changing the wall construction, some ways of modifying
the proposed design to use the proposed Aircrete wall construction
and comply with
the Target U-value are:
a) Using a higher performance heating system and adding solar gain
A boiler efficiency of 88% of the value of fe, the ratio of proposed
efficiency to base efficiency, increases the Target U-value above
the average U-value.
Since the area of glazed openings on South elevations exceeds that
of the North, the benefit of solar heat gains can be taken into account
to ease the target U-value by adding ?S to the target U-value.
Final Target U-value
UT = [0.474 x (88% / 78%)] + 0.04 x [(8.88 - 6.82) / 18.4]
= 0.539 W/mK
Since the final Target U-value, 0.539 W/mK, is greater than the average
U-value, 0.528 W/mK, compliance with the Target U-value Method is
achieved.
or
b) Reducing the U-value of the roof
Reducing the U-value of the roof from the standard elemental U-value
of 0.20
W/mK to 0.15 W/mK reduces the average U-value by
[44.4 x (0.20 – 0.15) / 191.3], i.e. by 0.011 W/mK.
This reduces the average U-value to less than the Target U-value and
satisfies the Target U-value requirement.
Final Average U-value = 0.528 – 0.011 = 0.517 W/mK
A similar result could be obtained by reducing the U-value of the
ground floor
or
c) Reducing the total area of the windows and doors
Reducing the total area of windows and doors from 25% of the floor
area to 19% of the floor area reduces the average U-value by
(6% of 88.8) x (2.2 – 0.40) / 191.3, i.e. 0.050 W/mK.
This reduces the average U-value to well below the Target U-value.
Final Average U-value = 0.528 – 0.050 = 0.478 W/mK
or
d) Using insulated doors and reducing the area of the windows
Doors with a U-value of 1.9 W/m2K instead of 2.2 W/mK reduce the
average U-value by (2.2 – 1.9) x 3.8 / 191.3, i.e. 0.006 W/mK.
This is not sufficient on its own to reduce the average U-value to
below the Target U-value. If, however, the total window area is also
reduced by 20% the average U-value falls by an additional amount equal to (20%
of 18.4) x (2.2 – 0.40) / 191.3 = 0.035 W/mKand the dwelling satisfies the Target U-value Method.
Final Average U-value = 0.528 – 0.006 – 0.035 = 0.487 W/mK
Since the final average U-value, 0.487 W/mK, is less than the final Target Uvalue, 0.521 W/mK, compliance with the Target U-value Method is achieved.
e) Specifying windows with lower U-value
Using windows with a U-value of 1.7 W/mK instead of 2.2 W/mK will reduce the average U-value by [(2.2 – 1.7) x 18.4 / 191.3], or 0.048 W/m2K. This is sufficient to reduce the average U-value to below the Target U-value. Reducing the
window U-value to 1.7 W/m2K is therefore sufficient to achieve compliance.
Final Average U-value = 0.528 – 0.048 = 0.480 W/mK
Since the final average U-value, 0.480 W/mK, is less than the final Target Uvalue, 0.521 W/mK, compliance with the Target U-value Method is achieved. |
