Low Carbon Box

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.

09/05/2026

Planning in South Oxfordshire? You'll need a Planning Sustainability Statement.
Since April 2021, South Oxfordshire District Council has required all new planning applications to demonstrate a 40% reduction in carbon emissions under Planning Policy DES10. The calculation is done using SAP (for homes) or SBEM (for non-domestic buildings), and it needs to be carried out by an accredited energy assessor.

We recently completed an assessment for a 3-bedroom detached house in Cholsey, designed by DP Architects. The approach was straightforward: better-than-required insulation levels, energy-efficient building services (including waste water heat recovery from the shower), and a NIBE air source heat pump running at 358% winter efficiency.

The result: a 42% carbon reduction — over the 40% target, without overcomplicating the design.

If you're working on a project in South Oxfordshire and need a Planning Energy Statement, we can help. Details at the link below.

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Low Carbon Box have completed several Planning Sustainability Statements to demonstrate how proposed developments meet this requirement.

08/05/2026

There's a common assumption in housebuilding that better-insulated homes are more prone to overheating. Our latest technical study suggests that's not the full picture.
We ran three construction options through a TM59 overheating assessment — cavity wall, timber frame, and an enhanced cavity spec — using a typical 100m² three-bedroom semi and the Birmingham DSY1 2020 High 50 weather file.
A few things stood out:
Cavity wall construction showed 29% fewer overheating hours in bedrooms compared to timber frame. Thermal mass is doing real work here — it slows temperature swings in a way lightweight construction can't match.
The enhanced cavity spec (better wall and roof insulation using PIR) actually outperformed the standard cavity. More insulation slows heat transfer from outside in, not just from inside out. On a hot day with the sun on an exposed wall, that matters.
All three options initially failed TM59 Criterion A in the living room. The fix was straightforward: reducing the glazing g-value from 0.66 to 0.40 — solar control glass — brought all three into compliance.
The full study is on the website if you want the detail.
https://www.lowcarbonbox.co.uk/reducing-overheating-risk-cavity-vs-timber-frame/

Technical study comparing cavity vs timber frame construction for Part O compliance shows cavity construction passes easier.

04/05/2026

Post 3 of 3 We have a 100% pass rate on overglazed extension SAP assessments — no redesigns required.

Plain English guide to how it works, what your options are, and how to get started:

https://www.lowcarbonbox.co.uk/your-extension-has-too-much-glazing-heres-how-to-fix-it/

Published by Low Carbon Box | Sustainable Building Consultants You’ve spent months working on your extension design. The architect has drawn it exactly how you imagined — big bifold doors, a full-width roof lantern, floor-to-ceiling glazing on the south wall. Then someone mentions that it might ...

04/05/2026

Post 2 of 3 Under Part L, there's a route called the whole-dwelling SAP calculation.

It models the full house + extension and compares carbon emissions against a standard compliant design.

https://www.lowcarbonbox.co.uk/your-extension-has-too-much-glazing-heres-how-to-fix-it/

Solar gains from south-facing glazing often tip the balance — we've passed projects at 73% glazing.

Published by Low Carbon Box | Sustainable Building Consultants You’ve spent months working on your extension design. The architect has drawn it exactly how you imagined — big bifold doors, a full-width roof lantern, floor-to-ceiling glazing on the south wall. Then someone mentions that it might ...

04/05/2026

Post 1 of 3 Your architect says your extension has too much glazing.

Before assuming you need to redesign — read this:

https://www.lowcarbonbox.co.uk/your-extension-has-too-much-glazing-heres-how-to-fix-it/

The 25% glazing rule for extensions isn't the end of the road. 🧵