8/20/2019 Thermal Bridging

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   These can be either 2-

dimensional (where 2 planesintersect) or 3-dimensional

(at the junction of 3 or more

planes).

Geometric thermal bridgesare due to the shape of the

building or, more specifically, due to the shape of the thermal

envelope. An increase in the complexity of the buildinggeometry is liable to increase the occurrence of junctions that

display a degree of geometric thermal bridging.

3-D junctions offer a level of complexity that is often difficultto visualise from 2-D drawings alone and frequently require

changes in material as well as in direction.

Designs must ensure that such issues are not left to site

operatives to solve but areaddressed fully at the design

stage, considering suchconcerns as continuity of

insulation, buildability, buildsequence and procedural

issues.

When calculating heat loss from a dwelling, variationsbetween as-designed and as-built details can have asignificant on the overall result. The example illustrated hereshows the difference made by

omission of the intermediatefloor perimeter insulation in atimber framed dwelling.Designs need to ensure thatthe likelihood of such errors occurring are minimised.

LowCarb4Real; Design collection:Thermal bridging 

•  A thermal bridge is created when materials that are poorer insulatorsthan surrounding materials come in contact, allowing heat to flowthrough the path created.

• Insulation adjacent to a bridge is of limited help in preventing heat loss(or gain) due to thermal bridging; the bridging has to be eliminated, re-built with a reduced cross-section or with materials that have better in-sulating properties, or with an additional insulating component (a ther-mal break).

UrbanBuzz Project—Developing Low Carbon Housing: Lessons from The Field—LowCarb4RealLead Organisations: Leeds Metropolitan University and University College LondonProject Partners: Good Homes Alliance, National Trust, Taylor Wimpey, Redrow, University of Leeds.Contacts:Prof. Malcolm Bell, Leeds Metropolitan University (m.bell@leedsmet.ac.uk), Prof. Bob Lowe, Bartlett School of Graduate Studies, UCL (robert.lowe@ucl.ac.uk)

• Repeating:  where bridges occur following a regular pattern, such as

that made by wall ties penetrating a cavity wall or timber studs in atimber frame external wall..

•Non repeating:  where bridges occur that are detail specific, such asthe bridging of a cavity wall by a combined lintel or at a door thresh-old.

•Geometric:  at the junction of two or more planes, such as at thecorner of an external wall or at the eaves.

Types of thermal bridgeWhat is a thermal bridge?

Rules to assist in the avoidance of thermal bridging Design Rule Description

Prevention Rule Where possible, do not interrupt the thermal envelope.

Penetration Rule Where an interrupted insulating layer is unavoidable, thermalresistance in the insulation plane should be as high as possible.

Junction Rule At building element junctions, insulating layers should meet without anygaps. Insulating layers should join without interruption or misalignment.

Geometry Rule Design edges to have as obtuse angles as possible.

 Adapted from CEPHEUS (Cost Efficient Passive Houses as European Standards) - Projectinformation No. 36 (Feist, Peper & Gorg, 2001).

Repeating thermal bridges  Geometric thermal bridges 

Non-repeating thermal bridges 

With a higher level of insulation, the relative importance of thermal bridges in-

creases in the energy balance. With this in mind, repeating thermal bridges whichmay have been acceptable in the past need to be designed out to achieve the en-

ergy performance standards required in today’s lower carbon emission dwellings.

 Timber studwork in standard timber frame construction, and in pre-manufactured

insulated panels (left ), may create repeating thermal bridges that will need to be de-signed out; most commonly by the addition of a supplementary layer of insulation.

 At Stamford Brook (right ), traditional stainless

steel wall-ties would have significantly in-creased the heat loss through the masonry-cavity external walls, effectively bypassing

much of the 142mm of retro-filled blown-

fibre insulation. The use of Kristiansen RefusI 

250mm glass-filled thermoplastic polyesterwall-ties (with a thermal transmittance well

below that of steel) went some way to re-ducing the repeating thermal bridging.

Non-repeating thermal bridges are intermittent and often caused by disconti-nuities in the designed thermal envelope. These may be constructive or ma-

terial thermal bridges, frequently observed around openings and other in-stances where materials of different thermal conductivities form the external

envelope. Thermal bridges can be quantified by calculating the linear thermal transmit-

tance (Ψ-value), measured in W/mK. This is the additional heat loss (or gain)

through the building envelope per metre length of that detail, and can be cal-culate by using software such as the freely available THERM package from

LBNL (http://windows.lbl.gov/software/therm/therm.html ) and the resultant values fedback into SAP to establish the DER rather than use the default values for ac-

credited construction details (see left ).

Design changes, whether in the design or construction phases need to be re-

viewed and thermal bridging re-calculate to avoid potential problems (below ).

In more extreme cases thermal bridging can significantly increase the risk of

surface or interstitial condensation and promote mould growth.

 Thermal bridge at the Stamford Brook bay window head, the design included an insu-

lated head liner which was omitted in the construction phase for aesthetic reasons.Worst case, 20mm gap:

Ψ = 0.18 W/mK

Compromise 42mm gap:Ψ = 0.068 W/mK

Steel toe added to inner leaflintel

SAP Table K1 value forcombined lintel:Ψ = 0.30 W/mK

Original separate linteldesign at Stamford Brook

Ψ = 0.016 W/mK