L2 Elemental Method (ADL2 paragraphs 1.7–1.68)
"To show compliance following the Elemental method, the building envelope has to provide certain minimum levels of insulation, and the various building services systems each have to meet defined minimum standards of energy efficiency..." (ADL2, paragraph 1.7).
E 1. Fabric Requirement (ADL2 paragraphs 1.8–1.16)
E 1.1 Pure Elemental Method (ADL2 paragraphs 1.8 & 1.12)
E 1.1.1 U-value (Table 1) Requirement (ADL2 paragraph 1.8)
"The requirement will be met if the thermal performances of the construction elements are no worse than those listed in Table 1." (ADL2, paragraph 1.8).
For each heated room, all heat-loss elements are subjected to the U-value checks set out in paragraph 1.8 and Table 1.
The Table 1 category ‘Pitched roof with insulation between rafters’ is applied to ceiling elements linking to rooms of type ‘Unheated space with pitched roof and insulation between rafters’.
The Table 1 category ‘Pitched roof with insulation between joists is applied to ceiling elements linking to rooms of type ‘Unheated space with pitched roof and insulation between joists.
Ceilings linking to other types of unheated space, and roof elements, are treated as ‘Flat roof or roof with integral insulation’.
In the case of windows, roof windows and personnel doors the check is applied to an area-weighted average U-value for the whole building. The U-value requirement in Table 1 depends on the type of frame (metal or wood/PVC). In cases where there is a mixture of frame types the required U-value is set to the value appropriate to the type of frame accounting for the greater overall area, as stipulated in a reply posted on the BRE FAQ website[10].
Display glazing, as specified by the check-box in APcdb, is excluded from the Table 1 checks.
Vehicle access and similar large doors, as specified by the check-box in APcdb, form a special category in Table 1 with U-value limit 0.7 W/m2K
Elements defined as walls in ModelIT are treated as roofs if they have a slope less than 70 degrees, and elements defined as roofs in ModelIT are treated as walls if they have a slope greater than or equal to 70 degrees. The same principle applies to windows and rooflights.
Table 1 note 2 is applied to elements with a slope of between 10 and 70 degrees in rooms of type ‘Room-in-the-roof as material alteration’.
For link-exposed elements option b) in paragraph1.8 (the less stringent requirement) is always applied.
E 1.1.2 Heating Systems Adjustment Factor Requirement
Compliance by the Pure Elemental Method (that is, without trade-offs) is only valid if the adjustment factor for required average U-value derived from the heating systems carbon intensity (see Section E 4.1) is not less than 1. The Construction Trade-off Method may be used to show compliance in other cases.
E 1.1.3 Maximum Window/Door/Rooflight Area (Table 2) Requirement (ADL2 paragraph 1.12)
"Provision should be made to limit the rate of heat loss through glazed elements of the building. One way of complying would be to limit the total area of windows, doors and rooflights so that they do not exceed the values given in Table 2." (ADL2, paragraph 1.12).
In accordance with paragraph 1.12 and Table 2, checks are performed on the total areas of heat-loss windows, doors and rooflights in heated rooms. A heat-loss element is any environment-exposed, link-exposed or earth-contact element.
The percentage figure for windows and doors is calculated as the total area of heat-loss windows (excluding display windows) and doors (excluding vehicle access and similar large doors and any doors used to represent wall or roof elements) divided by the overall wall area above ground level, which is calculated as the sum of the areas of all heat-loss walls, windows and doors excluding earth-contact walls but including all types of window and door. Earth-contact walls are excluded from the area calculation in accordance with a ruling posted on the FAQ website10. Earth-contact walls are identified as walls with adjacency type ‘Outside air with offset temp.’ or ‘Temp. from profile’. Any windows and doors with these adjacency types are also classified as earth-contact walls.
The percentage figure for rooflights is calculated as the total area of rooflights (including any link-exposed ceiling windows) divided by the overall roof area, which is calculated as the total area of roofs and rooflights excluding earth-contact ceilings but including any link-exposed ceiling windows and display rooflights. Earth-contact ceilings are identified as ceilings with adjacency type ‘Outside air with offset temp.’ or ‘Temp. from profile’. Any rooflights with these adjacency types are also classified as earth-contact ceilings.
Any rooflight assigned a ‘display window’ glazing type is treated as an ordinary rooflight.
Buildings of type ‘Hospital’ are placed in the category ‘Residential buildings (where people temporarily or permanently reside)’.
Buildings of type ‘School’ are placed in the category ‘Places of assembly, offices and shops’.
E 1.2 Construction Trade-off Method (ADL2 paragraphs 1.14-1.16 & 1.32)
"In order to provide greater design flexibility, the U-values of construction elements and the areas of windows, doors and rooflights may vary from the values given in Table 1 and Table 2 provided that suitable compensating measures are taken." (ADL2, paragraph 1.14).
E 1.2.1 Overall Building Heat Loss Requirement (ADL2 paragraphs 1.15a & 1.32)
"Compliance with the provisions of Part L would be achieved if ... the rate of heat loss from the proposed building does not exceed that from a notional building of the same size and shape that meets the criteria set out in Table 1 and Table2....." (ADL2, paragraph 1.15a).
"As further constraints in these methods, however:
a) if the U-value of the floor in the proposed building is better than the performance given in Table 1 with no added insulation, the better performance standard is to be adopted for the notional building: and
...
c) no more than half of the allowable rooflight area can be converted into an increased area of window and doors." (ADL2, paragraph 1.16).
The example in ADL2 Appendix E makes clear that this last requirement is more stringent than a literal reading might suggest, and is to be interpreted as
“c) no more than half of the allowable rooflight area can be converted into an increased area of window and doors or traded off in any other way (for instance against glazing U-values or heating system efficiency)”.
It is thus equivalent to the following constraint on the notional building:
"The area of rooflights in the notional building should not exceed that in the proposed building by more than half the Table 2 allowance (that is, by more than 10%)".
The constraints on floor U-value and glazing/door area are applied by the software when setting the properties of the notional building, and their effect can be seen in the tables presented in the results file. Paragraph16b is dealt with as a separate requirement (see below).
The area of vehicle access and similar large doors in the notional building is set equal to the area of such doors in the proposed building, since Table 2 imposes no area limit in this case. The same principle applies to display glazing. In accordance with Table 1, the U-value for vehicle access doors is set to 0.7 W/m2/K in the notional building. There is no such requirement for display glazing, and the U-value of display glazing in the notional building is set equal to the average U-value of display glazing in the proposed building.
The apportioning of area between personnel doors and windows in the notional building makes no difference to the heat loss characteristics. However, the rule applied is that the area of personnel doors in the notional building is normally set equal to the area of such doors in the proposed building, any adjustments being made to window area. The only exception to this rule would be in the unlikely event that personnel doors alone accounted for more than the Table 2 allowance.
The comparison between the proposed building and the notional building appears in the results file as a table showing areas, average U-values and heat loss coefficients for each category of heat-loss element. Average U-values for the proposed building (Uact ) and for the notional building (Uref ) may be read off from the bottom of the table.
A separate table displays, for the proposed and notional buildings, the window/door area percentage for the walls and the rooflight area percentage for the roofs.
Before the design can be tested by the Construction Trade-off Method, allowance must be made for trading off average U-value against heating system efficiency.
"In order to allow greater design flexibility, there can be a trade-off (in either direction) between the average U-value of the envelope and the carbon intensity of the heating system provided that the rate of carbon emissions is unchanged. A way of complying would be to adjust the average U-value of the building fabric such that it is no worse than the value determined from the following equation
Ureq = Uref . (εref / εact )
where
Ureq = the required average U-value.
Uref = the average U-value of the building constructed to the elemental standards of Table 1.
εact = the carbon intensity of the actual heating system at an output of 30% of the installed capacity.
εref = the carbon intensity of the reference heating system at an output of 30% of the installed capacity." (ADL2 paragraph 1.32).
As explained in section E 4.1 Heating Systems Carbon Intensity Requirement, a straightforward extension to the above definition of the required U-value adjustment factor, εref / εact , allows a trade-off to be applied to the performance of the system at maximum output as well as at the 30% level. The permissibility of this extension is implied by ADL2 paragraph 1.26.
The required average U-value, Ureq , is obtained by multiplying Uref by the adjustment factor obtained by this modified definition.
Finally, Ureq is compared with the average U-value for the proposed building (Uact ).
E 1.2.2 Maximum Traded-off U-value (Table 3) Requirement (ADL2 paragraph 1.15b)
"Compliance with the provisions of Part L would be achieved if ... the U-value of any part of an element is no worse than the values given in Table 3." (ADL2, paragraph 1.15).
These checks follow the same pattern as those described in section E 1.1.1 U-value (Table 1) Requirement, but apply only to roofs, walls and floors.
E 1.2.3 Average Traded-off U-value Requirement (ADL2 paragraph 1.16b)
"...if the area of openings in the proposed building is less than the values shown in Table 2, the average U-value of the roof, wall or floor cannot exceed the appropriate value given in Table 1 by more than 0.02 W/m2K." (ADL2 paragraph 1.16b).
A calculation is carried out to determine the maximum allowable area of heat-loss openings (windows, personnel doors and rooflights) permitted by the Table 2 requirements. If the actual area of heat-loss openings is less than this figure, the average U-values of the roofs and heat-loss walls and floors are checked against the appropriate Table 1 values plus 0.02 W/m2K.
In the table displaying element categories failing this test, the category ‘Flat roof or roof with integral insulation’ includes any earth-contact ceilings, and the category ‘Wall’ includes any earth-contact walls.
E 2. Thermal Bridging and Air Leakage Requirements (ADL2 paragraphs 1.9-1.11 & 1.17-1.19)
"The building fabric should be constructed so that there are no significant thermal bridges or gaps in the insulation layer(s) within the various elements of the fabric, at the joints between elements, and at the edges of elements such as those around window and door openings..." (ADL2 paragraph.1.9).
"Buildings should be reasonably airtight to avoid unnecessary space heating and cooling demand and to enable the effective performance of ventilation systems..." (ADL2 paragraph.1.17).
These requirements cannot be analysed in software, but reference is made to them in the results file for reasons of completeness.
E 3. Solar Overheating Avoidance Requirement (ADL2 paragraphs 1.20-1.23)
".Buildings should be constructed such that: a) those unoccupied spaces that rely on natural ventilation should not overheat when subject to a moderate level of internal heat gain and b) those spaces that incorporate mechanical ventilation or cooling do not require excessive cooling plant capacity to maintain the desired space conditions." (ADL2 paragraph.1.20).
E 3.1 Glazing Area Limitation (Table 4) Method (ADL2 paragraph 1.22)
"A way of achieving compliance for spaces with glazing facing only one direction would be to limit the area of glazed opening as a percentage of the internal area of the element under consideration to the values given in Table 4." (ADL2 paragraph.1.22).
For each heated room all the heat-loss elements are classified according to the orientation bands in Table 4. The total area of solar-exposed windows and rooflights in each orientation band is then calculated and converted to a percentage by dividing by the total heat-loss façade area for the band (including all types of windows, doors and rooflights). For rooms with glazing in one orientation band only, this percentage is then compared with the maximum allowable area in Table 4 for the orientation band. Any failures are reported on a room-by-room basis.
Solar-exposed windows and rooflights are those which are either directly exposed to the external environment or linked to it via a space of type ‘Glazing cavity’. Glazing linking to other types of unheated space is not classed as solar-exposed and is not subject to the Glazing Area Limitation checks.
Glazing openings linking to glazing cavities are assigned an orientation derived from the mean orientation of the external glazing in the glazing cavity (calculated from the mean vector area of the external glazing).
Under the terms of paragraph 1.22, this method applies to rooms with glazing facing only one direction. Any rooms with glazing falling into more than one orientation band will be listed among the rooms failing by this method.
E 3.2 Solar Load Estimation (Appendix H) Method (ADL2 paragraph 1.23a)
"Alternative approaches to achieving compliance include... a) showing that the solar heat load per unit floor area averaged between the hours of 07:30 and 17:30 would not be greater than 25W/m2 if the building were to be subject to the solar irradiances for the particular location for the month of July were not exceeded on more than 2.5% of occasions during the period 1976 -1995. The procedure given in Appendix H can be used to do this." (ADL2 paragraph.1.23).
This test relies on the model being constructed in such a way that perimeter zones extend no farther than 6m from the external wall. If this is not the case the method may give a false ‘pass’ result.
The test is applied to any rooms that fail by the Glazing Area Limitation (Table 4) Method or which have multiple glazing orientation and are therefore not covered by that method.
The method set out in Appendix H is applied as follows.
Each solar-exposed window and rooflight (with the exception of display windows) is assigned to one of the orientation bands in Table H1. A contribution to solar load is then calculated from equation H1 or H2, which for a single glazing opening both reduce to
Q = (A /Ap) q fc (1 – fr)
where
Q is the solar load per unit floor area (W/m2)
Ap is the floor area of the room (m2)
A is the area of the glazed opening (m2)
q is the solar load for the particular orientation of opening (W/m2 of opening) – Table H1
fc is a correction factor for glazing/blind combination
fr is the framing ratio for the window.
The framing ratio fr is taken from the APcdb parameter percentage frame.
The correction factor fc is derived from data input in APcdb using equation H3 for fixed shading:
fc = Sc / 0.7
or equation H5 for a combination of fixed and moveable shading:
fc = (Scf + Sctot) / 0.7
where
Sc is the shading coefficient including purely fixed shading
Scf is the shading coefficient including fixed shading when moveable shading is present, and
Sctot is the shading coefficient including fixed and moveable shading.
Values of Sc, Scf and Sctot are calculated from the glazing-only shading coefficient displayed in APcdb, modified as necessary by the following shading devices if present:
External shutters/louvers
Internal blinds/curtains
For these devices the glazing-only shading coefficient is multiplied by a shading factor for the effect of the shading device (averaged over incidence angle in the case of an external shutters/louvre). Either equation H3 (fixed shading) or Equation H5 (combination of fixed and moveable shading) is then applied depending on the parameters governing the operation of the shading device – namely its operation profile and its radiation thresholds. Equation H3 is applied if the device always operates, and Equation H5 is applied if it operates conditionally. If the parameters are such that the device never operates, equation H3 is used with Sc set to the glazing-only shading coefficient.
In the case of link-exposed glazing linking to a glazing cavity, an overall shading coefficient is calculated by multiplying the shading coefficient for the glazing cavity’s environment-exposed glazing (a weighted average where appropriate) by the shading coefficient for the link-exposed glazing linking the spaces.
The analysis does not take account of local shading forming part of the construction (recesses, overhangs, side-fins and balconies) or shading calculated by SunCast. Shading of this sort can in some cases be approximated using one of the other forms of shading. Alternatively the Detailed Overheating Calculation Method can be used.
E 3.3 Detailed Overheating Calculation Method (ADL2 paragraph 1.23b)
"Alternative approaches to achieving compliance include... b) showing by detailed calculation procedures such as those described in chapter 5 of CIBSE Guide A, that in the absence of mechanical cooling or mechanical ventilation, the space will not overheat when subjected to an internal gain of 10W/m2." (ADL2 paragraph.1.23).
The Detailed Overheating Calculation Method is not implemented as part of the automated Part L checks. However, the <Virtual Environment> provides suitable tools (Apache-calc, Apache-sim) for applying this method.
