Even a well-insulated house can overheat on a scorching day. The HES system that H0ME brings to Italy, in the 320mm build specified in its datasheet, has a thermal time lag of 18 hours: the heat peak reaches the inside face of the wall 18 hours after it hits the outside. That's not the same as saying every bit of heat takes 18 hours to cross the wall.
Insulating and slowing down are two different jobs
Thermal conductivity — the Greek letter λ (lambda) — measures how readily a material conducts heat under set conditions. For a given thickness, a lower λ means more resistance to heat flow. The HES mix is rated at λ = 0.048 W/(m·K) in its datasheet.
That number matters, especially for insulation. But on its own it can't tell you how a wall behaves on a summer afternoon. Outside, temperature shifts hour by hour, and so does the sun hitting the roof, the walls and the windows.
So you have to look at how the whole assembly responds as conditions change: how much the heat wave gets damped, and how late it arrives inside. That delay is the time lag. Damping measures something separate: how much the swing in temperature shrinks as it crosses the wall.
The methods for calculating these dynamic thermal properties come from ISO 13786, adopted in Italy by the national standards body, UNI. And the result depends on how the materials are actually put together — time lag can't be predicted by comparing two λ values, or two insulation names.
Hemp-lime and EPS: what can fairly be compared
EPS (expanded polystyrene) is light and insulates well. But next to HES hemp-lime it generally has lower volumetric heat capacity — with so little mass, it stores less heat per cubic metre.
Insulating means making it hard for heat to pass through. Storing means absorbing some of that heat and giving it back later. Two different jobs, and in summer both matter.
None of this means every hemp-lime wall beats every EPS system outright. Time lag doesn't come with a material's name — it comes from the full build: layer thicknesses, densities, specific heat, finishes, the order everything sits in.
A fair numeric comparison needs two fully specified walls, calculated with the same method. Skip that step, and any claim of an outright winner isn't worth taking seriously.
What we do have for HES is a precise, bounded figure: 18 hours of time lag for the 320mm build in the HES datasheet. That figure holds for that build. It's not a rule for every hemp-lime wall, nor a head-to-head against an EPS system.
We go into how the mix is built in our piece on the HES mix. The particle size of the hemp shiv also shapes the material's behaviour, though on its own it can't predict how the whole wall performs.
What it means for your house
A longer time lag pushes the moment you feel the heat peak indoors later into the day. That's useful to a designer: it leaves room to square the wall with the rest of the summer strategy.
It's no guarantee of a cool house, though. Orientation, glazing, shading, thermal bridges and damping all play their part. Once the air outside cools, well-managed ventilation helps flush out the heat that built up during the day. External shading stops the sun before it reaches the glass.
Time lag is one piece of the summer design, not the whole of it. We've covered humidity separately in The Breathing Wall: Comfort and Humidity.
FAQ
What is thermal time lag?
It's the delay between the outdoor temperature cycle and when that same pattern shows up on the inside face of a wall. It isn't how long a single unit of heat takes to travel through.
Why does EPS store less heat than HES hemp-lime?
Because it's extremely lightweight and has little mass available to hold heat. The actual outcome still depends on the whole wall build.
Is 18 hours enough to stay cool in summer?
No. You still need shading, well-managed ventilation, and a design that suits the site's orientation and climate. The 18 hours describes this specific HES build, not the comfort of the building.
We explain how the wall works on our Technology page. Previous instalment: "How to Choose the Right Plot". Coming up next: "Toward the First Build Site: Getting Ready to Break Ground."
Article produced with the support of artificial intelligence and reviewed by the H.0.ME. editorial team.