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Understanding Windows: U Values, Psi Values & G Values

In this second blog I come to the bit in my talks on high performance windows which I am always most concerned about.  It can be a bit over-technical and maybe dull but it seems to me that without some understanding of basic terms, what I talk about later in my presentations (and in future blogs), makes a lot less sense.

Heat loss through a window can be broken down into four elements, as depicted by the four red arrows in the diagram below.

What are U values?

Most readers will I am sure be passingly acquainted with the U value.  This expresses the amount of heat loss through any building element per square metre of surface, and for one degree difference between the internal and external temperature.  So Watts per square metre per degree difference, or W/m2.K.  So we can express the thermal performance of the glass, Ug,  of the frame, Uf, or indeed the whole window, Uw, as a U value, in W/m2.K.

What are Psi values?

Psi is a Greek letter, given as ψ, and it simply denotes heat loss over a given length rather than area.  So at the edge of the glass there is some extra heat loss because the edge of the glass is not as good an insulator as the rest of the glass.  This heat loss we give as heat loss per metre of glass edge per degree temperature difference between inside and outside, so W/m.K.  This is ψg.  We have the same thing for the window installation and this is given as ψw.

What are Uw values?

When we see a  whole window U value (Uw) quoted, this will be adding together the heat loss through the glass, the glass edge and the frame.  The equation below explains this, but the following picture demonstrates things best.

This formula is given in BS EN ISO 10077, which is the European standard for calculating the thermal performance of windows and doors.

However, I am now going to refer yet again to the Passivhaus methodology, from which we can learn so much about optimising the performance of our building envelope.  Passivhaus is more interested in the performance of the complete window, and includes the ψw in the calculation, which you can see at the end of the formula below.

This installed U value is actually much more helpful in determining the total performance of our window, and really for optimising the performance.  It is effectively ‘lost’ in UK methodology, making us largely unaware of the real importance of good installation detailing. 

The g value of the glass simply tells us how well the glass transmits heat from the sun.  It is expressed either as a percentage, or simple decimal.  A g value of 1.0 (100%) would tell us that all solar heat could enter the building (so without any glass), and of 0 (0%) would be for an opaque material. The more sheets of glass, and the more coatings, the more there is to stop the sun’s heat reaching inside, so the worse the g value will be. 

Typical g values for a Building Regulations compliant double glazed unit will be in the region of 0.7 (70%) and high performance triple glazing around 0.5 (50%).  Solar control glass, which may be need in certain designs, would typically have g values around 0.3 (30%) or even less.

On the face of it this all makes double glazing look better – after all who doesn’t want free heat?  But remember that it is the energy balance which is so important, not the absolute values for heat loss or heat gain, and the energy balance of triple glazing will be better for most of the UK.

That’s a longer story though, and one I shall be looking at in my next blog on glazing. 


Chris Herring 

*Aw = Area of window

Ag = Area of glass

Af = Area of frame

Lg = Perimeter of glass

Lw = Perimeter of window

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