How To Calculate U Values with Our Free Online Tool
Article By: Tom Ayre | Last Update: November 2020
Table of Contents
What are U Values?
A U Value is a number that illustrates the rate at which heat is transferred through a structure. The standard unit of measurement for this is W/m²K.
Broken down this means:
The rate of heat flow in watts directed over 1 x 1 metre square of a structure.
The lower the U-Value the more efficient the structure is, for instance, a U-Value of 0.5 will lose heat at double the rate of a 0.25 structure.
What U-Values Should I Adhere To?
In The UK we recommend adhering to the regulations from the “Planning Portal – L1A: Conversation of fuel and power in new dwellings”
The document outlines some U-Value figures, these include:
- Roof 0.20 W/m²K
- Wall 0.30 W/m²K
- Floor 0.25 W/m²K
- Swimming Pool Basin 0.25 W/m²K
- Windows & Doors 2.00 W/m²K
How To Calculate A U Value
Step 1 – Calculating Rsi Value and Rso Values
So what are Rsi and Rso?
Rsi is the surface resistance of the internal face of a roof, wall, floor etc. Values of the Rsi are given from the Architects pocketbook as:
- Roof / Ceiling 0.1
- Wall 0.12
- Floor 0.14
These figures are given in the units of m2k/w
Rso is the surface resistance of the external face, again, this could be a roof, wall, floor etc.
Values of the Rso are given by the Architects pocketbook as:
- Roof / Ceiling 0.04
- Wall 0.06
- Floor 0.04
Step 2 – Calculating R1, R2, R3 Values
Calculating your material r value (thermal resistance) for the material is relatively simple. You need to look at how your building element is constructed and work out the r value for each element, see the diagram.
The first part is to work out your material thickness in millimetres (mm).
Once you have done this we need to look up the materials thermal conductivity which is expressed in m2K/W.
Wikipedia has a list of thermal conductivity values here.
Typical building materials include values of:
- Acoustic Tile 0.06
- Brick 0.7
- Concrete – cast dense 1.4
- Concrete – cast light 0.4
- Fibreglass 0.04
- Glass (window) 0.8
- Hardwoods (oak) 0.16
- Particleboard (high density) 0.17
- Particleboard (low density) 0.08
- Softwoods (pine) 0.12
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Step 3 – Adding Airspace Resistance (Optional)
If you have a cavity wall or air gap in your construction element then this needs to be accounted for in your U Value calculation. For the air gap calculation, for simplicity, we don’t include a thickness element
The thermal resistance of an air gap in a cavity wall can be simplified to 0.18 m2K/W.
For a roof space above an insulated ceiling, the value can be taken as 0.04 m2 K/W