After investing in some high-performing double-glazed or triple-glazed windows, you notice a change. You enjoy every morning: grab your coffee, look out at the beautiful view outside your shiny new panes … Only to realise there is fog on the exterior glass. Oh no! You didn’t spend all that money for foggy glass! Your heart hits your gut. Did you buy defective windows? Did your installers mess up the seals? Don’t worry! Before you pick up that phone, be reassured that in most cases, the appearance of exterior window sweat means quite the opposite: It means your windows aren’t failing … they’re succeeding. In fact, they might be working a little harder for you than they actually have to.
The Science of the Morning Dew
Condensation on the exterior of your windows is a fascinating topic that brings together two disciplines: climate science and, well, glass. Let’s dive in.
What Is Exterior Window Sweat? Exterior window sweat is essentially just condensation. Every once in a while, particularly during spring and autumn mornings, the outdoor air temperature drops and humidity levels increase slightly. When damp air meets a cool surface, it reaches its dew point – the temperature where air becomes saturated and turns into liquid water. Grassy yards tend to collect their share of morning dew and anyone who has ever left their car parked outside overnight knows how often the windshield needs a good wiping down. And that can happen even if it doesn’t rain!
Why Didn’t Your Old Windows Do This? This never happened with your 20-year-old windows! And you know what? You’re right! The vast majority of old windows, especially if they were single-pane, simply didn’t have the same kind of insulation performance. Your outdated double-hung or sliding-glass units were probably pretty drafty compared to your shiny new replacements.
In other words, heat from your cosy home could escape easily through the old units and into your yard, warming the outer pane of glass. This meant the glass was always warmer and the dew point remained higher. So even during damp mornings, the outdoors stayed too warm for the air to condense onto the glass. For Double Glazing Cardiff, visit https://choicetradeframes.co.uk/upvc-windows-doors/cardiff/
Energy-efficient windows act more like thermal vaults. Most feature advanced technology like argon gas fills and special Low-E (low emissivity) coatings. Such systems are designed to minimise the transfer of heat from one side of the unit to the other. This means that when cool, damp air gets close to these windows, there’s little opportunity for the heat to escape and keep the glass warm.
If the inner pane does its job and keeps the indoor heat from escaping, then the outer pane is going to stay quite cool. If you combine this cool glass with the humid morning air … you get condensation.