WHAT IS U-VALUE?
U-value is a sum of the thermal resistances of the layers that make up an entire building element – for instance, a roof, wall or floor. It also includes adjustments for any fixings or air gaps.
In other words U-value tells us how well something lets heat move from inside to outside. The harder it is for heat to get through, the lower the U-value. So, we want a lower U-value. For example a wall with a U-value of 0.4 W/m²K losses heat at half the rate of a wall with a U-value of 0.8 W/m²K.
The U-value of an element depends on all materials contained within it. For example, with a cavity wall, this would include the plaster, plasterboard, blockwork, insulation, cavity and brickwork.
Pic.credit:firstinarchitecture
The U-value is usually used to measure the effectiveness of insulation materials that are used in commercial, industrial and domestic buildings. U-values generally range from 0.1 (very little heat loss) to 1.0 (high heat loss).
U-VALUE UNIT
The U Value is calculated in W/m2.K (Watts per square metre Kelvin). To break it down: it shows how much heat (in Watts) passes through 1 square metre of a material when the temperature on one side is 1 degree (K or ˚C) different from the other side.
HOW TO CALCULATE A U-VALUE?
To find the U-Value of insulation, you start by checking its R-value. R-values show how good a material is at stopping heat, and you can always find them in the product details.
Here's the formula for calculating R-value:
The basic U-value calculation is relatively straightforward. U-values are derived from the thermal resistances of the components constituting a specific section of the structure. Once you know the R-values of those materials, you can use them in a formula to get the U-value.
Pic.credit:firstinarchitecture
It's essential to note that, in addition to material resistances, the internal and external faces also contribute resistances, which need to be added. These values are constant.
U-VALUE CALCULATION EXAMPLE
Calculating straightforward U-values involves examining the construction of a building element layer by layer. To illustrate, let's take a look at a cavity wall example:
WHAT IS THE U-VALUE IN THE UK? | U-VALUE CHART
As of June 2022, Approved Document Part L underwent revisions, encompassing updates to specified U-values for both new residential constructions and renovations to existing residences.
The table below provides the mandated U-values in accordance with Approved Document Part L – Dwellings – England:
IS A LOWER U-VALUE BETTER?
U-value gauges how efficiently or inefficiently a component allows heat to move from the inside to the outside. If the component makes it challenging for heat to pass through, the U-value is lower. Essentially, our goal is to achieve a lower U-value.
WHAT IS A GOOD U-VALUE?
A building element's U-value holds significant importance, particularly because specific standards must be met in accordance with Building Regulations. In this context, the critical U-value figures are outlined below:
External Walls U-value requirement: 0.16 W/m²K
Floor U-value requirement: 0.11 W/m²K
Roof U-value requirement: 0.11 W/m²K
These figures represent the desired U-values that need to be achieved for optimal energy efficiency and compliance with regulatory standards. In general, lower U-values are considered good as they indicate better insulation and reduced heat loss.
WHAT IS THE IDEAL U-VALUE?
The ideal U-value refers to those currently required for constructing passive houses, which are homes that either do not require any heating source at all or need very little. To build them, products with very high R-values above 7.85 m²K/W and low thermal conductivity below 0.0016 W/mK are used. Products with such properties can be classified as having an ideal U-value.
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