Section J is the energy-efficiency part of the National Construction Code (NCC Volume One, Part J). It sets how much a building is allowed to gain and lose through its envelope - walls, roof, and above all the glazing - so the building can be conditioned without using more energy than the Code permits. It applies to the commercial classes (roughly Class 2 to Class 9), and for a glassy modern facade, the windows carry most of the load.
What Section J actually asks of the facade
Two numbers do most of the work: the U-value (how fast heat flows through the window - lower is better) and the SHGC (how much solar heat the glass lets in - which you want low on hot orientations, and sometimes higher where winter sun helps). Section J also now pays attention to thermal bridging - the metal paths, like an aluminium frame or a slab edge, that quietly short-circuit the insulation. Get the glazing mix wrong and the whole building can fail its energy target on paper.
The two ways to comply: DtS and JV3
You meet a set of fixed elemental limits - each part of the envelope hits its required U-value and SHGC. Simple and certain, but conservative: it can force more expensive glass than the building really needs, because it can't trade a strong wall off against a weaker window.
You model the whole building's energy use and show it performs as well as a reference building that just scrapes DtS. More work, but it lets you trade off - spend where it counts and claw back the over-spec DtS would have demanded.
Where Section J trips people up
Almost always at the glazing - and almost always because the number on paper isn't the window that gets installed. A Section J report leans on a glazing performance value; if that value came from a generic catalogue product and a different system is fabricated and fitted, the building's real performance drifts from the one that was certified. The frame, the sub-frame, the actual size, the actual glass - they all move the U-value. That's the gap worth closing before procurement, not after a certifier queries it.
Section J is proven on a number. The question is whether that number is your actual window - or a generic stand-in.
How we help prove it
We model the actual installed window to ISO 15099 - real size, real system, real glass and sub-frames - so the U-value and SHGC in your Section J case reflect what's really built, and hold up at shop-drawing stage. On a whole project, our FHK+ed pre-procurement check proves the specified systems for Section J (energy) and Section F/H (condensation) before they're locked in, then spot-checks what's actually supplied on site. You can also model your own windows in BTW. There's more on energy & Section J and on thermal bridging.
Not sure your glazing will clear Section J?
Send us one window you've specified, or the project's glazing schedule. We'll model the actual systems and tell you plainly whether the Section J numbers hold - before they're priced and procured.
Ask the lab →About Section J. Section J is Part J of NCC Volume One (energy efficiency). Deemed-to-Satisfy sets elemental limits; JV3 is a verification (performance) method; NABERS Energy is an alternative verification pathway. AFRC Lab models fenestration performance (U-value, SHGC, condensation) to ISO 15099 as evidence to support a Section J case; final NCC compliance is determined by the project's assessor or certifier. Results are indicative to the systems and sizes provided. AFRC Lab (Advanced Fenestration Research & Certification) is an independent consultancy; it is not the AFRC. BTW and FHK+ed are AFRC Lab offerings.