By Jon Vanstone, chair, Certass TA.

The British construction industry has a wonderfully predictable reflex. Change the rules and, before long, someone decides the most expensive option has just become compulsory.

So it is with the Home Energy Model. No sooner has SAP been shown the door than a new certainty begins to circulate that triple glazing is no longer a choice, it is destiny.

It is a neat story and it offers a simple upgrade path with the comforting sense that compliance can be solved by specifying a better version of something familiar. It is also, on closer inspection, slightly misleading.

The Home Energy Model marks a genuine shift in how we assess buildings. Where SAP relied on simplifications and averages, HEM models how buildings actually behave. Heat loss, solar gain and ventilation are considered together, interacting over time rather than sitting politely in isolation. In doing so, it removes some of the convenient illusions the industry has grown used to.

One of those is the idea that windows are not especially critical. In reality, they always have been, but compared to a well insulated wall, even a good window is a weak point. Under SAP, that could blur into the background. Under HEM, with more precise modelling of heat loss and solar gain, it becomes much harder to ignore.

So yes, better glazing will matter more, but this is also where the narrative starts to drift from regulation into something a little more commercial.

HEM measures outcomes. It does not ask whether triple glazing has been installed, it asks whether the building performs well enough. That distinction allows multiple routes to compliance rather than a single, predetermined answer.
A designer can improve windows, but they can also improve insulation, airtightness, ventilation or services. The model simply measures the combined effect. In that sense, HEM is less interested in what you install and more interested in whether it works.

This is why claims that triple glazing will become a universal requirement should be treated cautiously. They are not entirely without foundation, but they tend to reflect a commercial position rather than a strict reading of the regulations.

From a Certass perspective, that distinction is fundamental. Compliance has always been about demonstrating that an installation meets Building Regulations, not about mandating a single specification. The move to HEM reinforces that principle by placing greater weight on real performance and dependable product data.

In some situations, triple glazing will be the simplest and most effective route. Highly insulated homes, large glazed areas and heat pump led designs all benefit from reducing heat loss as far as possible.

In others, the answer is more balanced. Glazing also contributes to solar gain, particularly in well oriented buildings, which means performance becomes a trade-off rather than a one-way upgrade. In these cases, improving other elements of the building may achieve the same outcome more efficiently.

What HEM really changes is not the list of acceptable products, but the visibility of the consequences of choosing them. Designs that previously appeared compliant may now be exposed as underperforming.

What this means in practice
For installers and specifiers, HEM shifts the focus from individual products to overall performance. Decisions that were once made in isolation now have clearer and more measurable impacts on compliance.

From a Certass standpoint, this increases the importance of accurate specification, correct installation and reliable product data. As the model becomes more sensitive, there is less room for approximation between what is designed and what is actually built.

Glazing will inevitably come under greater scrutiny. In some cases, this will lead to more triple glazing, not because it is required, but because it offers a straightforward way to improve performance without revisiting the wider design.

However, it should not be treated as the default answer. Many projects will still achieve compliance through a balanced approach across multiple elements, and understanding where glazing sits within that balance becomes more important than simply choosing the highest specification available.

For installers, this creates a more informed conversation with clients, who may arrive with assumptions shaped more by marketing than regulation. Being able to explain how glazing contributes to overall compliance becomes increasingly valuable.

It also reinforces the role of competent person schemes. As modelling becomes more detailed, demonstrating that installations meet regulatory requirements in practice becomes critical, with schemes such as Certass providing that assurance.

The Home Energy Model does not mandate triple glazing, but it does make its performance harder to ignore.

That does not make it inevitable. It simply means, more often than before, it will be the option that makes the most sense.