4.3.3.1.4 Carbon Index Method
The Carbon Index Method – where the building’s
performance is related to the CO2 release associated with heating
and using the building. This new calculation method can be used as
a way of showing compliance and the requirements will be
met where the Carbon Index (CI) for the dwelling is not less than
8.0. Values range between 1 – 10.
Calculating the CI requires the same data about the dwelling as the
SAP calculation and is an extension of it
CI = 17.7 – 9.0 log10(CF)
where CF = C02 emissions (kg/year) / (Total Floor area (m2 ) + 45.0)
4.3.3.1.5 Energy ratings
(dwellings)
Calculations include the thermal insulation of the building fabric;
the efficiency and size of the heating system; ventilation and optionally
solar gain.
There is no obligation to achieve a particular SAP Rating but the
dwelling client must be provided with one displayed in the dwelling.
Under SAP 2001 the maximum SAP rating is 120 compared with SAP 1998
when it was 100.
4.3.3.2 Buildings other
than dwellings
For buildings other than dwellings the three methods of showing
compliance with the regulations are:
a) the Elemental Method
b) the Whole-Building Method
and
c) the Carbon Emissions Method
The requirements and three methods are similar in concept to those
for dwellings so the same advantages apply when using Aircrete in
the building fabric.
The integral thermal insulation properties of Aircrete enable simple
design details which provide continuous insulation around the building
avoiding excessive thermal bridging. In cavity wall construction
the Aircrete inner leaf can be built
first facilitating inspection of the integrity of the insulation
before constructing the outer leaf.
The low air permeability properties of Aircrete enable design details
which minimise air infiltration through the building fabric. The
accuracy of Aircrete blocks used in thin joint construction helps
minimise air leakage.
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4.3.4 Thermal bridging, air leakage
and condensation
The 2002 changes to Approved Document ’L’ have addressed these issues in greater detail. To supplement the BRE document BR262 “Thermal Insulation: avoiding risks” (2002) additional material was published under the title “Robust
Details - Limiting thermal bridging and air leakage: Robust construction
details for dwellings and similar buildings”. This information
was to help meet the new energy efficiency aims without introducing
any inherent or potential risks associated with such aspects as interstitial
and surface condensation risks and with undue leakage, both inward
and outward.
These “Robust Details” apply to dwellings and buildings
with a ‘domestic internal environment’ although special
situations such as drying rooms and swimming pools are not covered
and will need special consideration. Based on existing
industry practice these details can be achieved with the skill available
on site.
For walls, the details cover U-values in the range 0.35 to 0.47 W/m K
although where lower values are required these details can be assumed
not to increase the risk of greater air leakage or thermal bridging.
Where thicker insulation levels are required it should be ensured
that the wall construction can be built without compromise in technical
risk.
4.3.4.1 Applying the robust
details to the use of Aircrete
Four types of wall construction are covered in this document:
• SOLID WALLS with external insulation
• CAVITY WALLS with
a fully filled cavity
a partially filled cavity
an internal surface insulation
For each wall type, details are shown for a wall junction with:
• ROOFS - pitched and flat
• FLOORS - timber and concrete - solid and suspended ground
- intermediate
• SEPARATING/PARTY WALLS
On each detail, because the thickness of insulation material required
will depend on the thermal properties of the material to be used,
a thickness is not given.
Instead, wherever it is critical a minimum R value (R = t / λ where
t = the thickness and λ = the thermal conductivity of the insulant)
is quoted.
Note that any type of masonry material or insulant may be used if
it is suitable for the intended purpose.
Figures 6 to 10 show examples from the Robust Details document.
Generally, where Aircrete blocks are used, a lesser thickness of
added insulation will be needed.
For simplicity, throughout the Robust Details document, all cavity
constructions have been shown with a brick outer leaf. However,
an external finish of render, tile hanging or weather boarding on
other types of masonry, including Aircrete
blockwork, are suitable for the purpose and may be used.
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