U-value: what the thermal transmittance coefficient says about your house

Every quote for new windows, roof insulation or a façade contains a number with an awkward unit: 0.24 W/(m²·K). This article is aimed at owners of an existing residential building who want to know what it means.

Six questions: what does the U-value measure, which value is good, what does the law require, how do you find your own, what do Uw, Ug and Uf mean — and what does the energy performance certificate say?

#What is the U-value — and what exactly does it measure?

Therein lies the most common confusion: an insulation material is advertised with its thermal conductivity, the lambda. The U-value only arises when all the layers come together — plaster, masonry, insulation, cladding. § 37 of the Building Modernisation Act (Gebäudemodernisierungsgesetz, GModG) says so explicitly: the calculation is made “taking into account the new and the existing component layers”.3

Why that matters: private households use more than two thirds of their final energy consumption for heating. How much of it escapes through the building components is decided by their U-values — put into context in the overview of the energy performance certificate.

#Which U-value is good? Guide values for wall, roof, windows and basement

“Good” has three yardsticks, all from the law: the typical value in existing buildings, the maximum value on renewal and the new-build reference.

What a better value delivers does not depend on the target value but on the distance from the starting value — times the area. The consumer advice centre (Verbraucherzentrale) works through two cases: an uninsulated wall at 1.5 falls to 0.19 with 16 centimetres of insulation — 87 per cent less loss, paid back after 14 to 15 years. A wall at 0.50 reaches 0.18 with 12 centimetres, but only after more than 70 years. What pays off first is what is poor and large in area.

#Which U-values does the law prescribe on renovation?

There is no duty to improve a component just because its U-value is poor. The requirement is tied to the measure: anyone who renews, replaces or installs an external component for the first time must comply with the maximum values of Annex 7 to the Building Modernisation Act.1 For residential buildings: 0.24 for external walls, roof surfaces and top-floor ceilings, 0.20 when replacing a roof waterproofing, 0.30 against the ground and unheated rooms, 1.8 for a new external door.

Two restrictions belong with this. First, the de minimis rule: alterations affecting “no more than 10 per cent of the total area of the relevant component group of the building” are exempt.2 Two replaced windows trigger nothing, a new façade does. Second, instead of each component value, the calculation may be made for the whole building — primary energy demand and transmission heat loss may then exceed their reference values by no more than 40 per cent.5

Only one requirement applies unconditionally: a top-floor ceiling without minimum thermal insulation must be insulated to 0.24 — also fulfilled if the roof above it is insulated. If the owner of a one- or two-family house was living there himself on 1 February 2002, the duty only falls on the next owner.4

#How can I tell the U-value of my house without a measuring device?

Not by measuring. The usual route is expressly permitted: if the parameters of the components are missing, established empirical values for comparable age classes may be used. The federal government publishes them as flat-rate values by year of construction and type of construction; anyone who uses them complies with the recognised rules of technology.8 A solid top-floor ceiling up to 1968 is listed there at 2.10, a plastic window with insulating glazing up to 1994 at Uw 3.00.

Three things make it more precise. If insulation was added later, the flat-rate value is converted using the thickness and thermal conductivity of the new layers; if the lambda is unknown, 0.04 W/(m·K) applies for mineral fibre and plastic foams, 0.05 for blown-in and natural insulation materials. For radiator recesses the double wall value counts. And anyone who wants it more precise for their region searches the Altbauatlas by postcode, building age and component — the notice itself refers to this database.

#What do Uw, Ug and Uf mean for windows?

A window consists of parts that behave very differently, which is why there are three values. Ug is the glazing, from “glazing”. Uf is the frame, “frame”. Uw is the whole window, “window” — glass, frame and edge seal weighted by area, and therefore always worse than Ug alone.

In addition there is a quantity working in the opposite direction: the g-value, the total solar energy transmittance. It says what share of solar energy passes through the pane into the house — 0.60 in the reference building.7 A pane that strongly reduces heat loss usually lets in less solar heat.

#What does the U-value have to do with the energy performance certificate?

It is the raw material of the demand certificate (Bedarfsausweis): there an issuer records every component with its area and U-value and calculates the energy demand from them. The consumption certificate (Verbrauchsausweis) knows no U-values; it reads bills — the distinction is drawn in consumption certificate or demand certificate.

And there the U-value has a say. For residential buildings with fewer than five flats whose building application was submitted before 1 November 1977, a demand certificate must be issued on sale or letting — unless the building already met the level of the 1977 Thermal Insulation Ordinance (Wärmeschutzverordnung) on completion or was later brought up to it.6 This is proven via the component values: façade of wall and windows at most 1.45 to 1.75 depending on the floor plan shape, roof 0.45, basement ceiling 0.80, plus at least double or insulating glazing.8

The component values themselves do not appear in the certificate, only their result — as can be seen in the energy performance certificate template. How the building figures are derived from them is explained in primary energy demand and final energy demand, which measure pays off in energy-efficient renovation and funding. Anyone who orders a demand certificate online states the year of construction and type of construction — the issuer derives the U-values from them.

#Conclusion: one figure per component, three yardsticks behind it

The U-value is the smallest unit of building energy: one figure per component, small is good. What your house has is stated by the federal government’s flat-rate values; what has to come out of a renewal, by Annex 7; what pays off, by the distance between the two times the area.