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Thermal Information
Following a great deal of apprehension, the 2002 edition of Approved
Document L1 entitled "Conservation of Fuel and Power in Dwellings"
came into effect on 1st April 2002.
Many rumours had been circulating in the press saying that the introduction
of this new regulation would sound the death knell for traditional masonry
build. Not so.; please read on.
There are 3 ways of complying with the new regulation.
1. The elemental method: Suited to alterations
and extensions to existing building stock and new build where
minimal calculation is desirable.
Below, see table detailing the minimum U-values required for the
said external components of the structure needed to meet the regulations
by this method.
| Elemental Method: U-values (W/m2K) for construction elements. | |
| Exposed Element | U-value |
| Pitched roof with insulation between rafters 1,2 |
0.2 |
| Pitched roof with integral insulation | 0.25 |
| Pitched roof with insulation between joists | 0.16 |
| Flat roof 3 | 0.25 |
| Walls, including basement walls | 0.35 |
| Floors, including ground walls and basement floors | 0.25 |
| Windows, doors and rooflights 4 (area-weighted average), glazing in metal frames 5 |
2.2 |
| Windows, doors and rooflights 4 (area-weighted average), glazing in wood or PVC frames 5 |
2.0 |
| Notes: 1 Any part of a roof having a pitch of 70° or more can be considered as a wall. |
|
| 2 For the sloping parts of a room-in-the-roof constructed as a material alteration, aU-value of 0.3 W/m2K would be reasonable. | |
| 3 Roof of a pitch not exceeding 10° | |
| 4 Rooflights include roof windows. | |
| 5 The higher U-value for metal-framed windows allows for additional solar gain due to the greater glazed proportion. | |
Please note also that, in conjunction with the tabulated U-values, the minimum SEDBUK boiler ratings required for each of the specified fuels to comply using this method are shown in Table 2:
Table 2:Minimum boiler SEDBUK to enable
adoption of the U-values in Table 1 and reference boiler SEDBUK
for use for the Target in the U-value Method |
|
| Central heating system fuel | SEDBUK 1 % |
| Mains natural gas | 78 |
| LPG | 80 |
| Oil | 85 2 |
| Notes: 1 For boilers for which the SEDBUK is not available, the appropriate seasonal efficiency value from Table 4b of the SAP may be used instead (see paragraphs 0.17-0. 18.) |
|
| 2 For oil-fired combination boilers a SEDBUK of 82%, as calculated by the SAP-98 method, would be acceptable. | |
2. Target U-value method for new dwellings: This is by far the most popular
and preferred method for new build, owing to the flexibility it permits
by “trading off “ one component against another.
It works as follows:
The areas of each exposed element are multiplied by their design U–values
to give a heat loss per m2. The calculated heat loss figures
for each of the exposed areas are added to give a grand total for all
of the exposed fabric. This total heat loss figure is then divided by
the total area of the exposed elements to give an average design U-value
for the building.
This average design U-value is then compared to the result given by the
target U-value calculation,
0.35 – 0.19 (AR / AT)
– 0.10 (AGF / AT) +
0.413 (AF / AT).
Where:
AR = Area roof
AT = Area total exposed elements
AGF = Area ground floor
AF = Total area all floors
If the average design U-value of the building is equal to or better than
that figure provided by the target U-value calculation, the building “passes”.
Conversely, if the average design U-value of the building is worse than
the figure provided by the target calculation, then the design U-value
of one or more of the exposed elements has to be improved to bring the
average design U-value in line with the target.
Example:
Element |
Exposed surface area (m2) |
U-value W/m2K |
Total heat loss W/k |
| External wall | 99.63 |
0.41 |
40.848 |
| Roof | 44.49 |
0.15 |
6.674 |
| Exposed ground floor | 35.26 |
0.34 |
11.988 |
| Windows | 12.59 |
1.9 |
23.921 |
| Doors | 3.78 |
1.26 |
4.763 |
Total exposed surface area (m2) |
195.75 |
Total heat loss (Wk) |
88.194 |
Average heat loss = 88.194 / 195.75 = 0.451 W/m2K
In this example case the target U-value calculation gives a target
of 0.457 W/m2K, so with an average U-value of 0.451 W/m2K,
the example building passes.
Boiler Input
The benchmark for boilers for the target U-value calculation is SEDBUK
78%. Where boilers of a lesser or higher performance than the “benchmark”
are employed, then the target is eased or tightened according to the following
example calculation:
| Proposed boiler SEDBUK | 90% = 1.15% x 0.457 (Target) = 0.525 W/m2K |
| Benchmark SEDBUK | 78% |
The new adjusted target then becomes 0.525 W/m2K. This now presents the possibility of……..
The Bescrete ‘Star Performer’ block & the target
U-value method
Even under the new Part L this remains a truly universal block for residential
construction. Generally speaking, the following U-values will provide
a pass with SEDBUK 78% boilers.
| Element Exposed ground floor Exposed roof Exposed walls Windows Rooflights Doors |
U-value (W/m2K) 0.35 (usually 50 mm polystyrene or similar) 0.16 (250 mm of insulation) 0.41 (see note below) 2.0 2.0 1.5 |
Separating wall house / unheated space (e.g. integral garage)
Under the old regulations this was referred to as a semi-exposed wall
and, depending on the degree of exposure of the unheated space, a protection
factor was added. This protection factor, however, did not change the
U-value of the wall’s construction. Further, such a wall was required
to have a minimum U-value of 0.60 W/m2K.
The new Part L now adds into the actual U-value calculation for the wall
a thermal resistance figure (Ru) for the unheated space.
This means that a 100 mm Cellular Bescrete block applied with 27 mm of
Gyproc Thermal board plus, or similar, will give a U-value of 0.564 W/m2K.
This is still sufficient to give a comfortable “pass” by means
of the target method when adopting the U-values for the exposed components
of the building detailed in Table 3, providing the boiler rating is no
worse than SEDBUK 78%.
What this means to the house builder.
Most house-builders wish to continue their normal method of build with
minimal change. Further, simplicity of ordering and administering materials
on site is paramount.
Many have been advised that the only solution to Part L is Aerated Concrete
blocks for the exposed walls. This is not the case, and where Aerated
blocks are employed this will inevitably require the site to have two
different types of block (Aerated for exposed walls, and dense for sound-resisting
walls). This increases site supervision and increases the risk of misplacing
blocks, leading to possible infringements of Part E and Part L.
In addition, in contrast to the old Part L, exposed beam-and-block ground
floors will now require added insulation in most cases, even when using
low-density beam infill and foundation blocks.
An example of this is a beam-and-block suspended floor incorporating 40
mm of expanded polystyrene insulation, and having an exposed perimeter
of 40 LM.
Such a floor would have a U-value of 0.32 W/m2K when using
100 mm Cellular Bescrete block as the foundation and beam infill block.
This is more than sufficient to provide a “pass” using the
target U-value method, providing the other criteria in Tables 2 and 3
are adopted.
The same floor incorporating aerated infill and trench foundation blocks
would provide a U-value of 0.28 W/m2K.
Even with the advent of Part L 2002, the 100 mm Cellular ‘Star
Performer’ remains THE truly universal house-building block:
- Inner leaf of external wall with partial or total fill (U-value should achieve 0.41W/m2K)
- Wall adjacent to unheated space (apply 27 mm Gyproc thermal board
plus or similar)
- Infill block for suspended beam-and-block floor
- Foundation block in soils……..
- 275 mm sound-resisting separating walls (complies with current AD E and proposed forthcoming amendment)
3. Carbon Index method: This provides flexibility in
design of new dwellings, whilst at the same time achieving similar overall
performance to that obtained by following the Elemental Method. The Carbon
Index adopted in this method is defined in the SAP, and the requirements
would be met if the Carbon Index for the dwelling (or each dwelling in
a block of flats or a converted building) is not less than 8.0
As a general rule, the SAP rating would have to be at least 100 with a
SEDBUK 78% boiler to obtain a pass by this method.
Energy advisory service
We are members of National Home Energy Rating and employ specially
trained and certified energy assessors operating the universally accepted
NHER energy and SAP assessment programmes. We are thus able to offer a
full energy advisory service, including advice on meeting the Part L regulations.
All assessments are backed with full SAP worksheet prints, along with
Regulation compliance sheets and serial numbered N H E R SAP certificates.
All assessments are to BS EN ISO 9002 and are subject to regular spot
scrutiny by National Energy Services.
Furthermore, all U-values quoted are calculated by method BS EN ISO 6946
using the latest BRE software.
Please telephone, e-mail or fax for details of this service.