Glazing unit’s dew point

Dew point of glazing or the moment of condensing the glazing.

The words “Why is the glazing sweating?  After all I payed for the best.” are very popular.

In this article I will explain briefly why this is happening.

The dew point of the glazing depends on few factors:

Exterior Temperature

Interior Temperature

Heat Transfer coefficient of the glazing

Air Humidity in the room

The ratio between these four defines the condensation of your glazing.

We hear very often from customers that they are unhappy with the glazing they bought because it is constantly sweating. And every time when a consultant goes on the spot he finds that the humidity of the room is very high. This problem occurs most often when the building is new and the structure is still damp following the wet construction processes. Or the rooms where the problem occurs is usually with higher humidity like kitchens, bathrooms e.t.c.

It goes without saying that the correct choice of the glazing is very important. For low-emission glazing the dew point is way lower but this doesn’t mean that there is no such point.

I’ll explain the dependencies in the diagrams below.

In the first example (diagram 1.1) I’ve used 2 regular float glass 4mm filled with air. The Heat Transfer coefficient of this kind of glazing is usually around U = 2,8-3,0 W/sqm*K. It can be seen from the example that when the Air Humidity is 50% and the Temperature of the room is 20 degrees the dew point comes around -9 degrees. At humidity 60% the dew point is at -3 degrees.

Case 2 (diagram 1.2) shows what is happening when the window is made of 2 float glass 4mm, in which one of them is low-emission and filled with ARGON.  In the right conditions and type of the glasses it can be reached coefficient of Heat Transfer  U = 1,1 W/sqm*K.  The diagram shows at 60% and 70% humidity and 20 degrees room temperature, the dew point hits respectively at -27 and -10 degrees.

In case 3 (diagram 1.3) is used a single glass. The results speak for themselves.

diagram 1.1

diagram 1.2
diagram 1.3

It’s important to understand that the greatest role for the glazing condensation plays the Air Humidity. And of course the type of glazing. It’s been a long time since we used regular glazing in Europe . Having in mind that 85% of a window is glass it is very important not to save money when choosing the glasses.

Here you can download a diagram for defining the dew point of glazing:

Diagrams: Studio CAD

Source: Studio CAD

If you need a professional opinion when choosing glass or glazing don’t hesitate to contact us.

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