H1 6th Edition: A Mangawhai Heads Case Study

Under the H1 6th Edition, adding underfloor heating to bathrooms no longer means piling insulation under the slab. Using the thermal modelling method (H1/VM1), we recently added bathroom underfloor heating to a Mangawhai Heads home in Climate Zone 1 and kept standard 90 mm timber framing with R2.2 wall insulation, without changing a single wall.

Why was underfloor heating such a headache under the H1 5th Edition?

Under the 5th Edition, heated floors often triggered very high insulation requirements under the heated wet areas, or forced designers to over-insulate the entire floor slab to compensate. That meant extra cost, awkward slab edge details and an unhappy builder.

The 5th Edition is now on its way out. Previous editions of H1 can only be used until 26 November 2026. From 27 November 2026, all H1 compliance must use the 6th Edition documents (see MBIE's H1 acceptable solutions and verification methods). For more on what changed, see our post on the H1/VM1 changes.

What changes for heated floors under the H1 6th Edition?

Under the 6th Edition, you often need minimal or zero extra insulation under bathroom floors compared with the 5th Edition's demands. But there's a trade-off: the 6th Edition requires us to assume a much higher proportion of timber framing in the walls, up to 38%.

Many designers assume that adding 38% framing to your H1 calculation will wreck the building's thermal performance, forcing them to move from 90 mm to 140 mm walls or specify expensive high-performance insulation. With H1VM1 thermal modelling method, that isn't necessarily what happens.

Case study: underfloor heating in Mangawhai Heads (Climate Zone 1)

When the client for our Mangawhai Heads project asked for bathroom underfloor heating, we modelled the house using the 6th Edition parameters, including the higher framing percentage.

The result: we changed nothing in the walls. The design comfortably complied with standard 90 mm timber framing and standard R2.2 wall insulation.

For the client: underfloor heating without a budget blowout.

For the architect: no need to redraw plans for thicker walls.

For the builder: no oversized framing or complex slab edge insulation details.

Why didn't the 38% framing penalty force 140 mm walls?

Because dynamic thermal modelling assesses the home's heating and cooling across the whole year, not just the R-value of one wall.

Timber is a thermal bridge with a lower R-value than insulation. When the 6th Edition makes us allow for 38% timber framing, the wall's overall R-value drops, which makes the house slightly harder to heat in winter.

But a lower R-value also lets the house shed excess heat more easily in summer, which reduces the cooling load.

Mangawhai Heads is in Climate Zone 1, a relatively warm climate, and the design already complied with a healthy margin. When we modelled the higher framing percentage, the heating penalty was effectively offset by the cooling benefit. Balancing the two, standard 90 mm framing passed comfortably.

	Calculation method	Thermal modelling (H1/VM1)
How it assesses the building	Compares R-values against a reference building	Simulates heating and cooling energy over a full year
Effect of 38% framing	Lower wall R-value counts against you	Heating penalty can be offset by cooling benefit
Underfloor heating	Can push insulation upgrades elsewhere	Assessed as part of the whole home's performance
Typical outcome	Conservative, often over-specified	Finds the practical

Will the H1 6th Edition force big design changes?

In most cases, no, especially if you use H1VM1 thermal modelling. Colder climate zones such as Zones 5 and 6 can be more demanding. But it's unlikely the move to the 6th Edition will force major design changes when the building is modelled properly.

The calculation method is rigid and conservative. Thermal modelling is dynamic, so we can find the practical balance between client comfort, such as underfloor heating, and construction efficiency, such as keeping 90 mm framing. Read more in Stop over-specifying: the engineering reality of H1 reports and our practical guide to H1 climate zones.

Frequently asked questions

Do I need extra insulation under underfloor heating for H1 6th Edition?

It depends. Under the H1 6th Edition, bathroom underfloor heating typically needs minimal or zero extra under-slab insulation compared with the 5th Edition. Using the thermal modelling method (H1/VM1), the heated floor is assessed as part of the whole home's annual heating and cooling performance rather than against a fixed rule.

What is the 38% timber framing rule in H1 6th Edition?

The H1 6th Edition requires a higher assumed proportion of timber framing in walls, up to 38%, when calculating wall thermal performance. Because timber conducts heat more than insulation, this lowers the wall's effective R-value. With thermal modelling, the impact can be offset by other factors, such as reduced summer cooling loads.

Can I still use 90 mm framing under H1 6th Edition?

In many cases, yes. In our Mangawhai Heads case study (Climate Zone 1), the home complied using standard 90 mm timber framing and R2.2 wall insulation under the 6th Edition, using the thermal modelling method. Colder climate zones may need more, so each design should be modelled.

When does the H1 5th Edition end?

The H1 5th Edition can be used until 26 November 2026. From 27 November 2026, H1 compliance must be demonstrated using the 6th Edition Acceptable Solutions and Verification Methods, or an alternative solution.

Key takeaways

Under the H1 6th Edition, bathroom underfloor heating usually needs little or no extra slab insulation.

With thermal modelling, that framing (38%) penalty can be offset. Our Mangawhai Heads project kept 90 mm framing and R2.2 insulation.

The 5th Edition ends on 26 November 2026, so new projects should be designed to the 6th Edition now.

Need to prove H1 compliance without over-specifying?

If you're an architectural designer working through the 6th Edition, or a client has thrown underfloor heating into the mix, Reshape Consulting can help. Find out more about our H1 compliance reports and thermal modelling, or contact us for a quote on an H1 modelling report.