Acceptable Construction Details, Thermal Bridging and Air

Acceptable Construction Details, Thermal Bridging and Air
Acceptable Construction Details,
Thermal Bridging and Air
Permeability
Sean Armstrong,
Technical Adviser,
Building Standards,
DEHLG
26/03/2009
Outline
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Overview of TGD L
Overview of Guidance wrt
Airtightness and Thermal Bridging
Overview of Acceptable
Construction Details
Building Standards - Strategy
Strategy 2008-2010
Promote high quality, safe and sustainable design and
construction, notably, through ongoing review of the
Building Regulations and prioritisation of energy
efficiency and eco-design.
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Part L
2008
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What principle underpins Part L?
Reduce Demand
for Energy
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Increase supply from
renewable and
efficient sources
What are the main provisions of the
Regulations for new dwellings?
a)
Primary energy consumption and associated CO2 emissions
Energy consumption and emissions should be 40% better than 2005 Reference House.
Ie. MPEPC=.6, MPCPC=.69
b)
Renewable energy sources
10kWH/M2/Annum Thermal, 4kwh/m2/annum Electrical or a combination or CHP
c)
Building fabric
Fabric Insulation, Thermal Bridging, Air Infiltration
d)
Space and water heating
Oil or gas fired boilers should have a seasonal efficiency should be not less than 86% as specified
in HARP Condensing boiler
MVHR as per GPG 268
e)
Owner information
Operation and maintenance of the:
Building
Fixed Services
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3) Building Fabric
a)
b)
Fabric insulation
a)
Elemental U Values
b)
Area weighted average elemental uvalue of doors, windows, rooflights
reduced to 2.0
Air infiltration
a)
b)
c)
Thermal bridging
a)
b)
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On site testing
Use of Acceptable Construction
Details
Use of Acceptable Construction
Details
Y value
a) Building Fabric –U values
Fabric insulation
Area weighted average elemental u-values
Area weighted average elemental u-value of doors,
windows, rooflights reduced to 2.0 (opening area 25% of
floor area)
0.27
2
0.20
0.16
2
Unheated
0.22
Attic
Av e
r a
g e U v- a lu
e 2 .0
1
0.27
Average
U-value
1
2.0
0.25
0.25
2
Unheated space
0.27
0.25
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2
b)Building Fabric -Thermal Bridging
Demonstrate by calculation that the all thermal bridges meet a
table of acceptable values in TGD L, Table D1
Use acceptable details that have been assessed and limit
thermal bridges to acceptable values as per Table D1 in TGD L
Use alternative details that limit risk of mould growth and
condensation using a calculation method for the temperature
factor in TGD L
APPROPRIATE ON SITE INSPECTION & QUALITY CONTROL
Value of Y = 0.08
Alternatively, Value of Y = 0.15
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c) Building Fabric –Air Permeability
1.3.4.1 To avoid excessive heat losses, reasonable care
should be taken to limit the air permeability of the
envelope of each dwelling. In this context, envelope is the
total area of all floors, walls (including windows and doors), and
ceilings bordering the dwelling, including elements adjoining
other heated or unheated spaces.
1.3.4.3 Achievement of reasonable levels of air permeability can be
facilitated by adopting the standard details referred to in
Paragraph 1.3.3.2 (Acceptable Construction Details) above,
together with an appropriate performance specification and the on-site
inspection regime and related quality control procedures, referred to in
that paragraph.
1.3.4.4 Air pressure testing should be carried out on a proportion of
dwellings on all development sites. See Sub-section 1.5.4 for
details of the test procedure, extent of testing, use of test results in
DEAP calculations and appropriate measures to be undertaken where
the limit set is not achieved. When tested in accordance with the
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procedure referred to in Sub-section 1.5.4, a performance level of
Air Leakage
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Source: Leeds Metropolitan University
Low Carbon Housing Learning Zone
Guidelines to improve air tightness
Design Stage
Keep it simple! Simple designs are more likely to get built right.
Decide which layer of the construction provides the air barrier. Stick
with this. Use the pen-on-section test to check continuity and to
identify key details
Pay careful attention to the design of junctions between elements to
ensure continuity of the air barrier.
Minimise penetrations of the thermal envelope, whether by services or
structure or construction.
Construction Stage
Ensure that details of all design changes involving elements of the
external envelope are distributed throughout the design, procurement
and construction teams
It is important that the project programme reflects the required
sequence for effective formation of the air barrier and insulation
installation
Communication and Education – Personnel involved in procurement
and constructing the building fabric should understand the need for
insulation continuity and airtightness.
Quality Control -Quality Assurance (QA) should be extended to check
for insulation continuity and airtightness
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Thermal Bridging
Thermal Bridge:Part of the structure of lower thermal resistance that bridges
adjacent parts of higher thermal resistance and which can result in localised
cold surfaces on which condensation, mould growth and/or pattern staining
can occur.
Thermal bridges fall into two categories:
(a) Repeating thermal bridges (such as timber joists, mortar joints, and
mullions in curtain walling). The additional heat flow due to the presence
of this type of thermal bridge is included in the determination of the Uvalue of the particular building element which contains these bridges.
(b) Non-repeating thermal bridges (such as junctions of floor and roof
with the external wall, and details around window and door openings)
where the additional heat flow due to the presence of this type of thermal
bridge is determined separately
Acceptable Construction Details address Thermal
Bridge Type B
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Main thermal bridges using
traditional cavity construction
details
Junction of Gable
wall with ceiling 7%
Ground floor
perimeter 15%
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Lintels 23%
Sills
23%
Jambs 6%
Sect. 1.3.3 TGD L Thermal Bridging
1.3.3.2 The following represents alternative approaches to making
reasonable provision with regard to limitation of thermal bridging:
(a) Demonstrate by calculation in accordance with the methodology
outlined in Appendix D that all key thermal bridges meet the
performance levels set out in Table D1 of Appendix D.
(b) Adopt details that are similar to, or demonstrated as equivalent to,
generic details that have been assessed as limiting thermal bridging
to an equivalent level to that set out in Table D1 of Appendix D. A
set of such details for typical constructions will be developed in
consultation with relevant construction industry organisations and
will be made available in a document “Limiting Thermal Bridging
and Air Infiltration – Acceptable Construction Details”.
(c) Use alternative details which limit the risk of mould growth and
surface condensation to an acceptable level as set out in Paragraph
D.2 of Appendix D.
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Deap Calculations
1.3.3.3 DEAP allows for thermal bridges by including an allowance
for additional heat loss due to thermal bridging, expressed as a
multiplier (y) applied to the total exposed surface area.
Where provision for thermal bridging is made in accordance
with options (a) or (b) of Paragraph 1.3.3.2, this multiplier
should be taken as 0.08.
Where option (c) of Paragraph 1.3.3.2 is used, it will be
necessary to allow for each thermal bridge separately in the
calculation.
Alternatively a multiplier of 0.15 may be used.
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Significance of Thermal Bridging in
DEAP
27%
16%
9%
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Methodology outlined in Appendix
D
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The procedure to establish linear thermal
transmittance () is outlined in BRE IP 1/06.
Modelling Software should perform to IS EN ISO 10211 Parts
1 and 2. Several packages are available that meet this
requirement. –Therm (free), HEAT, Physibel
The guidance in BRE Report BR 497 Conventions for
calculating linear thermal transmittance and temperature
factors on inputting parameters should be used for modelling.
This allows different users of the same software package and
users of different software packages can obtain correct and
consistent results.
Thermal Bridge and Linear
Thermal Transmittance 
D.3 Linear Thermal Transmittance and
Additional Heat Loss
The linear thermal transmittance () describes
the
heat loss associated with a thermal bridge. This
is a
A
property of a thermal bridge and is the rate of
heat
B
flow per degree per unit length of bridge that is
not
accounted for in the U-values of the plane
building
elements containing the thermal bridge. The
linear
transmission heat loss coefficient associated
with
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non-repeating thermal bridges is calculated as:


Example of models
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Option A
(a) Demonstrate by
calculation in
accordance with the
methodology outlined
in Appendix D (BRE
IP 1/06, Software to
ISO 10211, Inputs to
BR497) that all key
thermal bridges meet
the performance
levels set out in Table
D1 of Appendix D.
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Option C
.
Use alternative details which limit the risk of mould growth and surface
condensation to an acceptable level as set out in Paragraph D.2 of
Appendix D
D.2 Mould Growth and Surface Condensation
The temperature factor (fRsi) is defined as follows:
fRsi = (Tsi – Te) / (Ti – Te)
where:
Tsi = minimum internal surface temperature,
Te = external temperature, and
Ti = internal temperature.
For dwellings, the value of fRsi should be greater than or equal to 0.75 so as
to avoid the risk of mould growth and surface condensation.
For three dimensional corners of ground floors this value maybe reduced to
0.70, for all points within 10 mm of the point of lowest fRsi
Where option (c) of Paragraph 1.3.3.2 is used, it will be necessary to allow
for
each thermal bridge separately in the calculation of a value for y.
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Option B
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Adopt details that are similar to, or demonstrated as
equivalent to, generic details that have been assessed as
limiting thermal bridging to an equivalent level to that set
out in Table D1 of Appendix D. A set of such details for
typical constructions will be developed in consultation
with relevant construction industry organisations and will
be made available in a document “Limiting Thermal
Bridging and Air Infiltration – Acceptable Construction
Details”.
Acceptable Construction Details
Format
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Details have been developed by DEHLG, HomeBond
and SEI.
They were developed in Consultation with an Industry
Working Group made up of represenatives from
different Sectors of the Construction Industry.
The guide is presented in 2 sections.
Section 1 discusses the general theory of insulation
continuity and airtightness in construction.
Section 2, in seven separate parts, provides
indicative detail drawings of thermal insulation and
airtightness provisions for specific construction
interfaces.
Acceptable Construction Details –Section
1
Explains how to achieve minimise thermal bridges at design stage
and construction stage
Provides an Index to drawings
Explains how thermal bridging multiplier (y) can be used in DEAP
Provides pictures and guidelines of best practice with regards to
achieving airtightness in Buildings
Provides examples of how to calculate value for y for TGD L
example
Provides an appendix 2 of Psi (ψ) values for commonly used
details
which can be used when value for y is obtained by calculation.
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Acceptable Construction Details –
Section 2
Consists of drawings for each construction type.
21-25 Drawings for each construction type and 4
common drawings
Type
Type
Type
Type
Type
Type
Type
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1 Cavity wall insulation
2 External insulation
3 Internal insulation
4 Timber Frame
5 Steel Frame
6 Hollow Block Internal Insulation
G General Details(common to all constructions)
21-25 Drawings for each construction type and 4
common drawings
Details -Introduction Page
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Example Detail-Gable Wall
Blue line on
Drawing indicates
air barrier
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Example Detail-Foundation
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Example Acceptable Construction
Detail
Lintel
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Deap calculations
Heat loss through thermal bridging is not accounted for
in the u-value calculation for the plane building
elements containing the thermal bridge and therefore
must be evaluated separately. It is usually expressed
in terms of a fraction known as y. In order to
determine the value of y to be used in an energy rating
calculation, an assessor has three choices:
a)Use 0.15 where no calculations have been performed and
where Acceptable Construction Details have not been
used;
b) Use 0.08 where the Acceptable Construction Details have
been used in all details;
c) Or use a value for y which can be determined
through calculation, this procedure must be
followed where a value for y other than those
outlined above is used by the assessor; sample
calculations are provided later in this section
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Example Calculation
Roof: Pitched tiled roof, insulation laid on attic floor,
part between joists and part over joists.
Walls: Cavity wall (dense concrete blocks) rendered externally,
with partial fill insulation in the cavity and 50mm cavity retained.
Floor: Concrete slab-on-ground floor with insulation under slab
All dims.
are internal
Example using Table D1 Psi values and Appendix 2, Diagram 1 Concrete lintel
Junction detail
ACD Concrete Lintel
ACD Sill
ACD Jamb
ACD Ground Floor
ACD Intermediate Floor within a dwelling
ACD Eaves
ACD Gable (insulation at ceiling level)
ACD Corner(normal)
ACD Party wall between dwellings
Appendix 2 Party wall with floor
ACD Party wall with ceiling
Appendix 2 Rising wall
2
y factor ( exposed surface area 243.3 m )
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Lm
Psi
25
23.2
43
23
23
14
9
10.2
10.2
9
9
9
0
0.04
0.05
0.16
0.07
0.06
0.24
0.09
0.03
0.11
0.22
0.22
L x Psi
0.00
0.93
2.15
3.68
1.61
0.84
2.16
0.92
0.31
0.99
1.98
1.98
17.54
0.07
Psi value source
Diagram 1, Appendix 2
Table D1/IP1/06
Table D1/IP1/06
Table D1/IP1/06
Table D1/IP1/06
Table D1/IP1/06
Table D1/IP1/06
Table D1/IP1/06
Table D1/IP1/06
Appendix 2
Appendix 2
Appendix 2
Acceptable Construction Details
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Available on DEHLG website;
http://www.environ.ie/en/TGD/
Draft document available on
website
26/03/2009
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