Low Carbon Box

20/06/2026

Windows in Part L1 2026 - The good news: the g-value fix in SAP 10.3

Here's some genuinely good news from the new building regulations — particularly for anyone dealing with overheating concerns on a south-facing home or a property with large glazed doors.

Under the previous rules, specifying solar control glass — the type that reduces how much heat comes through the window in summer — could actually penalise your home's energy assessment. The calculation assumed a certain amount of solar gain, and if your glass let less through, your energy score suffered.

The new SAP 10.3 (used for Part L 2026 energy calculations) has fixed this. The calculation now takes your actual glazing specification into account, so if you need solar control glass to keep your home comfortable in summer and comply with the overheating regulations, it won't count against your energy performance.

For self-builders and developers with south-facing designs, this removes a frustrating trade-off.

👉 Full guide to what's changing with windows under Part L 2026: https://www.lowcarbonbox.co.uk/why-your-window-spec-just-got-harder-the-part-l-2026-u-value-changes-explained/

18/06/2026

Windows in Part L1 2026 - The supplier checklist (4 questions to ask before you fix the spec)

If you're working with a window supplier on a new build or self-build project, here's something worth knowing before you sign off the specification.

Under the new building regulations (Part L 2026), window U-values have to be calculated differently — using each window's actual size and design, including all the frame components. But the tools to do this properly are still being developed, and not every supplier is across the change yet.

Before your window schedule is fixed, ask your supplier to confirm four things in writing:

1. The U-value for each different opening size
2. The calculation method used
3. That it meets Future Homes Standard / Part L 2026 requirements
4. That glazing bars, frame type, door panels and spacers are all included

Catching this early keeps your options open. Full guide here:

👉 https://www.lowcarbonbox.co.uk/why-your-window-spec-just-got-harder-the-part-l-2026-u-value-changes-explained/

13/06/2026

Part O Overheating - When you need TM59 instead.

If your design fails the Simplified Method, that isn't the end of the road. TM59 dynamic simulation is the second compliance route under Part O, and it opens up far more design freedom.

It's worth considering when the building fails the Simplified Method's glazing or ventilation limits, when bedroom windows have to stay closed at night for noise, when the scheme is Passivhaus or very airtight, or when you simply want more flexibility than the fixed allowances allow.

We run TM59 using DesignBuilder with EnergyPlus and a location-specific summer weather file. It's more involved, but it can credit external shading and solar control glazing — the things the simple method can't.

👉 When TM59 is the right route: https://www.lowcarbonbox.co.uk/part-o-overheating-a-plain-english-guide-to-the-simplified-method/

Part O requires all new homes in England to pass an overheating check. Here's how the Simplified Method works — glazing limits, ventilation requirements and window sill heights explained.

11/06/2026

Part O Overheating - What the Simplified Method can't credit

If your plan to beat overheating relies on external shading, MVHR, air conditioning or blinds, it's worth knowing the Simplified Method can't credit any of them.

External shading (brise soleil, awnings, shutters, overhangs), mechanical ventilation with heat recovery, and comfort cooling all count for nothing under this route. Internal blinds and curtains can't be credited under either the Simplified Method or the full TM59 assessment. Trees and planting are out too.

If one of these is genuinely the only way to make a scheme work, TM59 dynamic simulation is the route to explore — external shading in particular can make a real difference in a full model.

👉 Read the full guide: https://www.lowcarbonbox.co.uk/part-o-overheating-a-plain-english-guide-to-the-simplified-method/

Part O requires all new homes in England to pass an overheating check. Here's how the Simplified Method works — glazing limits, ventilation requirements and window sill heights explained.

08/06/2026

Part O Overheating - What counts as glazing (only the glass)

One of the most common Part O misunderstandings: people measure the whole window opening. Only the clear glass area counts towards your glazing total — frames, glazing bars and solid panels are excluded.

On a glazed screen we recently checked, the structural opening was 2.26 x 2.1m (4.74m²). The actual glass came to 3.27m² — about 69% of the opening. The 3.27m² is what goes into the calculation.

This matters most with bifold doors and large glazed screens, where the frame takes up a real chunk of the unit. Measure the sum of each glass pane, not the overall width times height.

👉 More on what counts: https://www.lowcarbonbox.co.uk/part-o-overheating-a-plain-english-guide-to-the-simplified-method/

06/06/2026

Part O Overheating - The west-facing glazing trap

Here's a number that catches a lot of designs out. A west-facing house of 100m² with cross-ventilation is allowed just 11m² of total glazing under Part O's Simplified Method.

That's a tight allowance once you add up living room windows, glazing in the front door, and any rooflights. The orientation of your most heavily glazed wall sets the limit for the whole house.

Interestingly, the same west-facing house without cross-ventilation is allowed 18m². It shows why glazing design and ventilation strategy have to be worked out together, not separately.

👉 Full breakdown of the orientation tables: https://www.lowcarbonbox.co.uk/part-o-overheating-a-plain-english-guide-to-the-simplified-method/

Part O requires all new homes in England to pass an overheating check. Here's how the Simplified Method works — glazing limits, ventilation requirements and window sill heights explained.

05/06/2026

Every new home now has to pass an overheating check

Building regulations changed in 2022, and a lot of self-builders and homeowners still don't know about it. Every new home in England now has to pass an overheating check before it's signed off.

For most standard houses with straightforward glazing, that's the Simplified Method under Part O. It looks at three things: how much glazing you have, whether there's enough ventilation to offset it, and whether your windows are safe to open at night.

We've written a plain-English guide that explains what the numbers mean for your project and when you might need a different route.

👉 Read it here: https://www.lowcarbonbox.co.uk/part-o-overheating-a-plain-english-guide-to-the-simplified-method/

03/06/2026

this is worth a read for sustainable houses - HPH399: The Good Building Book – with Jon Broome and Nick Grant... - House Planning Help https://www.houseplanninghelp.com/hph399-the-good-building-book-with-jon-broome-and-nick-grant/

10/05/2026

If you are thinking of a battery system with or without PV on your roof to help manage your home energy, then this podcast by Everything Electric podcast is well worth a listen.

Duracell Energy – Home Energy Independence! - Everything Electric https://fullycharged.show/podcasts/podcast-297-capture-store-and-save-energy-on-your-terms-duracell-energy/

Robert speaks with Mark Millar of Duracell Energy to discuss the importance of home energy independence & the whole home energy ecosystem.