Find the Best Insulating Windows: Why Uf Values are so Important

To save you looking back, here’s the formula to calculate an overall Uw for a particular size and configuration of window:

¹ transmission through the glass spacer bar (used to separate the glass panes in a double or triple glazed unit).

Essentially, this means the Uw output is created by thinking about the frame, the glass and the glass spacer bar proportionally. The larger the window, the less frame area, the more glass area. Since the insulation of a window’s frame is (normally) worse than the glass insulation: the larger the window, the better the overall Uw value.

As a good real life example, ask for both a tiny stone mullion window and a massive 6m long sliding door, using exactly the same type of frame and glass. The stone mullion unit will get a far worse Uw value out of this calculation because such a small amount of glass can’t compensate for the frame.

This makes good sense, so the calculation works…right? A lower Uw value always means you’ll get the best insulating windows…?

The Inherent Problem

Let’s look at a different example:

• A timber window with a Uw value of 1.38
• A heritage aluminium window with a Uw value of 1.3

We’ll pretend we’re a prospective customer aiming to buy one of these two systems, and we’ll pretend they’re about the same price

We’re all busy. Uw values are quick, easy to understand. So we’ve judged that these two systems are incredibly similar in terms of insulation; barely worth considering that difference of 0.08 in the real world.

Except this analysis couldn’t be more wrong. And this has a massive real-world consequence for a homeowner. Choosing purely based on that overall number means you likely aren’t getting the best insulating windows.

These u-values are calculated based on a single opening window. (So, not adding any extra frame sections within the space to skew results.)

Consider the size of your window casements at home: are they actually going to benefit from the excellent triple glazing with sub-optimal metal frame?

Having quoted an untold number of windows we can safely say that most will fall somewhere in the middle of this graph, more-so when you consider the addition of a central frame section for windows with multiple opening sections.

A number isn’t helpful if it’s not clear where that number came from

The aluminium-clad timber frame has a frame insulation value of 1.56 Uf; the aluminium’s frame is all the way up at 3.38 Uf.

So, in this comparison, it’s the glass that’s pulling the aluminium frame back in line with building regs with a triple glazed unit all the way down at 0.8 Ug. The timber product is using double glazing at a 1.19 Ug.

Therefore, our more detailed comparison is:

• An aluminium-clad timber system with Uf=1.56, Ug=1.19, averaging to Uw=1.38
• A heritage aluminium system with Uf=3.38, Ug=0.8, averaging to Uw=1.3

But why does that matter if the resulting average Uw is basically the same? Will they not feel the same when installed?

Think about how that heat is being lost

It’s easier to imagine in an extreme situation: you have an awesome glass unit floating in mid-air. So good, that it counteracts the lack of frame around it such that it has a good average Uw.

Every bit of heat is being lost through the terrible “frame”, regardless of how amazing the glass unit is. The temperature outside will become the temperature inside no matter how hard the glass tries to stop it.

Therefore, it’s important to view huge differences between Uf and Ug with caution. Consider both to make sure you’re getting the best insulating windows.

This is why customers don’t expect, yet do experience, condensation on the inside of brand new aluminium windows+doors: it’s not clear how a Uw value behaves in real life until you look at the Uf vs Ug difference.

Anti-Bias

For full disclosure: the numbers in the example above are perfectly accurate – we sell both of these products!

We consider the Unik Funkis Aluminium-clad timber product one of our most versatile offerings. As a double glazed window it’s a no-brainer for replacement windows and, with their multiple glass and frame offerings in triple glazing, we can tailor the performance for any scenario.

Whilst we’ve just made it sound fairly poor in comparison, we will never remove Smart’s Heritage aluminium from our range either. No other product can sensibly fit into a traditional stone mullion window and achieve such a good level of insulation. This is the only viable option for replacing old slim-framed steel units.

The lesson on these two products beyond their U values is that not every product works in every application; it’s so important to choose the correct product based on what’s required.