Ug, Uf, Uw, Uwhat? : An intro to the U-value and those most important to Passive House design

For someone outside the building sector, many of the shorthand symbols frequently used in Passive House related content – including educational videos, press releases, webinars, and even some of our other blog posts – can be quite confusing. What the heck is a U-value? What are all those different subscripts? And what the heck is this symbol -> Ψ? Don’t fret. We will get into all of those, as well as an explanation regarding their relevance to Passive House design.

A brief explanation of the U-Value

The heat losses through a standard building component, i.e. external wall, window, floor, roof, are defined by the U-value also known as the heat transfer coefficient, or thermal transmittance coefficient. This value indicates the rate of heat transfer through a specific component over a given area if the temperature difference is one degree (1 Kelvin). The measurement unit of the U-value is therefore “W/(m²K)”. Thus, the lower the U-value, the lower the rate of heat transfer, and the better the insulating property of the element! In other words, the more slowly heat moves through the material.

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