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Ashgrid U-Value Calculations

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Introduction
Building Regulation Part L Conservation of fuel and power which came into force in April 2002, introduced the need to take into account thermal bridging of a bar/bracket system in the calculation of U values for twin skin metal roofing and cladding.
Previously the calculation of U values for these systems had been carried out in accordance with the requirements of BRE paper IP5/98 Metal cladding: assessing thermal performance. This paper stated that for a bar/bracket system with brackets at least 1m centre the thermal bridging was not significant.
One of the document referred to in the new Part L, BRE 443 Conventions for U-value calculations states that although the advice given in IP5/98 was appropriate at the time of its publication, that these bridging effects should now be taken into account. MCRMA Technical Paper No. 14 Guidance for the design of metal roofing and cladding to comply with approved document L2:2001 gives advice on suitable calculation methods.
These methods involve two-dimensional models for the profiles and bar, and three-dimensional models for the bracket, which are analysed using finite element analysis computer programs HEAT2 and HEAT3. These programs are fully compatible with BS EN ISO 10211-1:1996 Thermal bridging in building construction - Heat flows and surface temperatures Part 1. General calculation methods, and so meet the requirements of Approved document L.
Notes
  • The U-value calculations are based on the AG40 bar and bracket.
  • Other systems will require separate calculations.
  • For horizontal cladding, contact Ash & Lacy.
  • AG40 system calculations for Building Regulation approval will be provided by Ash & Lacy on receipt of an order. Alternatively contact David Roy for full calculations.
  • Contact David Roy for 'building specific' calculations.
Ashgrid
The calculations are only valid for the AG40 bar and standard bracket (100mm - 280mm deep). Sway bracket are not included.

Roof Application

roof

Vertical Wall Application

text

Note: The depth is the Bracket Depth and The Insulation Thickness

Insulation
The analysis is based on insulation with a Thermal Conductivity of 0.04 W/mK and  0.038W/mK
Surface Resistances
External
m2K/W
Internal
m2K/W
Roof 0.1 0.04
Wall 0.13 0.04
Bars
The analysis is based on the insulation being cut between the bar as shown below.

bar

Profiles
Liner

liner

Valid for Depth varying from 15mm to 20mm

Valid for Pitch varying from 150mm to 350mm

Outer Sheet

outer

Valid for Depth varying from 30mm to 35mm

Valid for Pitch varying from 150mm to 250mm

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