Thermal Bridging

Xtratherm Advanced Detailing delivers…

A major factor in the performance of the building fabric is not simply the amount of insulation you install, but how it interconnects with other components and the other insulated elements within the design. It has been estimated that up to 30% of the heat loss in a well-insulated building is through these ‘Non Repeating Thermal Bridges’. It is critical when calculating the performance of low energy buildings that this heat loss is measured and minimized.

With Xtratherm’s Advanced Detailing and engineered jointing of boards the insulation continuity of your project is assured.

• Thermal Bridging - The principals

  What is a thermal bridge?

  Why should my detailing improve with low energy design?

  A major factor in the performance of the building fabric is not simply the amount of insulation you install, but how it interconnects with other components and the other insulated elements within the design. Building junctions, where building elements meet such as at corners or reveals are less well insulated than the main element.

  It has been estimated that up to 30% of the heat loss in a well-insulated building is through these ‘Non Repeating Thermal Bridges’ at wall/floor junctions, corners, reveals, ceiling heads and sills etc. It is critical that when calculating the performance of low energy buildings that this heat loss is measured and minimized.

  With Xtratherm’s Advanced Detailing and engineered jointing of boards the insulation continuity of your project is assured.

• Thermal Bridging - Measurement

  What is a Y-value?

  How big a factor is it in my SAP rating?

  As with every element/component within the energy strategy of a building design, U-values, air tightness, boiler efficiency etc, the ‘Continuity’ of the insulation layer has a numerical value within SAP and is called the ‘Y’ value.

  From the changes to Part L 2010 regulations, the Y value must be actually calculated by the building designer rather than being assumed. The details used to calculate the Y value should be witnessed on site and included within the ‘As built’ SAP.

• Thermal Bridging and FEEs

  'The Fabric First' approach is scored in the Code for Sustainable homes?

  Is Part L going the same way?

  Part L Building Regulations has followed the Code for Sustainable Homes and set a separate performance target for the building fabric to ensure that the building is inherently energy efficient.

  The fact that targets have been set for each of these design elements have been illustrated within FEEs1, and includes a Thermal Bridging Target of around U=0.05, provides clarity as to what a designer needs to achieve in a specification to achieve the Zero Carbon goal – so how do we achieve that?

• Where do you get the junction performances from?

  If my junction detail is better than average where do I get the resultant PSI value 'score' from?

  The list of junctions that need to be accounted for in any design is given within Table K of SAP 2009 2.

  For each of these junctions, appropriate detailing to ensure the ‘Continuity of insulation’ can be satisfied by following details printed in the publication Accredited Details for Construction3, published by the DCLG. The ACDs4 show each of the details with its components and gives a sequence on how the detail should be constructed, they also allow that detail to be signed off on site to the satisfaction of the energy assessor/designer.

  Using these ACD details will allow the assessor to ascribe a PSI (Ψ) value to each junction within the design (Column A below); for example the wall/floor junction has a default value of 0.16 taken from Table K of the SAP workbook. If no detailing is specified and witnessed on site the default values apply. (Column B below)

  The designer/assessor should measure the length of each of the junctions in the designed building and multiply the length by the respective PSI value. The total of all heat losses through the junctions are then totalled and divided by the total heat loss area of the building to determine the Y-value for input into SAP. It has been estimated that every 0.01 reduction in the Y-value will result in a 1% energy reduction in the design.

  ² SAP 2009 – www.planningportal.gov.uk/buildingregulations/approveddocuments/partl/bcassociateddocuments9/acd
  ³ ACD – www.planningportal.gov.uk
  ⁴ Here ‘Accredited Construction Details’ means:
  – For England & Wales and for Northern Ireland: Accredited Construction Details (As above)
  – For Scotland: Accredited Construction Details (Scotland)

• Xtratherm PSI Values

  How can simply using Xtratherm products give me a thermal bridging advantage?

  Xtratherm have taken the ACDs and replaced generic square edged insulation materials with high performance Xtratherm PIR that have engineered jointing and ancillary products such as pre-formed corner boards to improve the continuity of the insulation layer. We then recalculated the PSI values (Our team were the first to be qualified by the BRE on their Thermal Modelling Course.) to achieve performances that give significant improvements over the Accredited scores from Table K.

  Good U-values in these elements coupled with good detailing and decent air tightness ensures that, no matter where that energy comes from, conventional or low or zero carbon technologies, the heat loss from the building is minimized.

  By improving Xtratherm jointing detailing and providing higher performance materials we simply improve the performance of the tried and tested ACDs.The industry has been using the Accredited Construction Details for many years now. They have been developed by the Government to deliver the continuity asked for and clearly identify the build sequences that are required retaining traditional methods and skills – but with the inclusion of Xtratherm Partial or Full Fill the thermal performance of those details are greatly improved.

• Full fill or Partial fill? Aerated or Dense Block?

  There are a lot of different specifications - can I get a score for each?

  On this site, we have provided a range of PSI values depending on whether you are partially filling your cavity wall with Xtratherm Products or using our built in full fill solution CavityTherm. They have been used in wall specifications using dense, medium, or aerated block work. Some specific details for Northern Ireland building techniques are also given.

  You can see the advantages that Xtratherm insulation with engineered detailing actually provides.

• Wall U-values? 0.21 - 0.12 w/m2K

  Thermal bridging actually gets worse the better the U-values in the walls and floors - what do Xtratherm performances fall within?

  The list of junctions that need to be accounted for in any design is given within Table K of SAP 20092.

  We have also given resultant PSI values for each junction that has been accredited within a spread of target wall U-values, from a current regulation target of 0.21W/m2K to a Passive level of 0.12W/m2K. Individual certificates illustrating the resultant PSI values can be downloaded on ACDs.

• Semi Detached House - Sample Calculation

  We show you how to calculate and take advantage of the Y-value in SAP

  Example house Y Value Calculation
  Semi Detached House – TER 18.24
  Using 125mm CavityTherm built-in Full Fill
  Total Envelope Area = 190.580 (m²)

  					PSI (Ψ) Value Choices
  			Table K		Xtatherm Values
  Key Junctions (Reference Table K SAP 2009)			Default	ACD’s	Dense	Medium	Light
  			A	B	C	D	E
  	L (m)		Ψ	Ψ	Ψ	Ψ	Ψ
  E2 – Lintels (Keystone Hi-Therm Lintel)	13.970	x	1.00	0.30	0.01	0.01	0.01
  E3 – Sill	12.170	x	0.08	0.04	0.03	0.03	0.03
  E4 – Jamb	27.150	x	0.10	0.05	0.00	0.00	0.00
  0E5 – Ground Floor	19.600	x	0.32	0.16	0.16*	0.06	0.03
  E6 – Intermediate Floor within a dwelling	19.600	x	0.14	0.07	0.00	0.00	0.00
  E10 – Ceiling (Insulation at eaves)	11.000	x	0.12	0.06	0.05	0.05	0.05
  E12 – Ceiling (Insulation at gables)	8.900	x	0.48	0.24	0.22	0.10	0.05
  E16 – Corner (Normal)	20.400	x	0.18	0.09	0.05	0.05	0.04
  E17 – Corner (Inverted)	10.200	x	0.00	-0.09	-0.06	-0.06	-0.06
  E18 – Party wall between dwellings	10.200	x	0.12	0.06	0.05	0.05	0.05
  P1 – Party Ground floor	8.900	x	0.16	0.08	0.08*	0.07	0.04
  P2 – Party wall Intermediate Floor within a dwelling	8.900	x	0.04	0.00	0.00*	0.00	0.00
  P4 – Party Ceiling (Insulation at ceiling level)	8.900	x	0.24	0.12	0.12*	0.10	0.05
  Total L x Y			41.04	16.65	8.85	5.55	3.60
  Y-Value (L x Y/Total Area)			0.220	0.09	0.05	0.03	0.02

  (* Value taken from Table K Accredited column)

Request Xtratherm Accredited Details