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Mystery of the 'U' Value; Building Fabric Efficiency

Updated: Dec 8, 2019


Before studying architecture, I had heard of the U Value and knew it had something to do with understanding the energy saving characteristics of building materials. We were reminded us of the important difference between energy and power in a recent lecture by Toby Cambray of Greengauge MEP.

The definition provided from a rather handy site https://www.firstinarchitecture.co.uk/a-quick-and-easy-guide-to-u-values/ helps to explain.

When the u-value calculation is at its lowest, the more efficient the construction will be at keeping heat loss through the structure to a minimum. Architects are responsible for helping to design energy efficient buildings with expert assistance from those like Toby who detail mechanical and electrical engineering requirements.


U-values are considered in the Building Regulations Approved Document Part L and are easier to calculate for new buildings.

Whilst preparing drawings to make the scale model I need to think about thermal resistance from the outside wall and the interior walls. So resistance in the individual building materials that I choose is very important which also affected by the choice of how thick I make each layer.

R=d/k where

R= Thermal Resistance (m2K/W)

d= Thickness of material (in Metres – very important)

k= thermal conductivity of the material (W/m K)


Whilst looking at the environmental conditions for my Watercress Farmhouse I identify a number of areas where thermal bridges might exist and must draw these in detail to be able to calculate the u-values at the most vulnerable heat loss points of the structure .

Insulation is a key part of reducing u-values. Those that exist in the market today are most commonly know by their trade names such as Celotex, Kingspan and Rockwool. The types of insulation available however are quite broad and each has its own merits. Sustainable options are now important and must meet as a minimum the standards set by Building Regulations. AECB take this one stage further and Passivhaus claim to reduce energy loss 90% more than current UK standards.



Thermal bridges are localized areas of low thermal resistance. The rate of heat flow though a thermal bridge depends on a number of factors:

- The temperature difference across the thermal bridge;

- The thermal conductivity of the materials passing through the insulation layer

- The cross sectional area of the thermal bridge.

- How easily heat can get into and out of the thermal bridge which, in turn, depends on:

- The relative area and surface resistance of the surfaces of the thermal bridge facing the source of heat and those facing the heat sink.

- The lateral heat flow paths in the assembly that can bring heat to and from the thermal bridge


You may think this is something new, but humans have been working on this since the stone age... just keeping warm is basic instinct!



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