6pm in August: The Physics That Moves the Heat

Six o'clock, August. You open the door and the heat hits you. The house spent all day soaking up sun and handed it back the minute you walked in.

Several things cause this. Windows with no shade. Appliances that have been running since morning. How and when the place gets aired out. The walls matter too. The HES system that H.0.ME. brings to Italy is rated at 18 hours of thermal lag on a 320 mm wall, a figure from the HES technical data sheet and from testing carried out at Vilnius Gediminas University and the BRE Centre at the University of Bath.

Heat here behaves like a wave.

The Delay That Matters

Across a day, the outside face of a wall warms up, then cools back down. The inside face does the same thing later. That gap between the two peaks is thermal lag.

Worth clearing up: this is not how long heat takes to cross the wall. It is the distance in time between the moment the sun hits the outside hardest and the moment you feel that hit indoors.

Say the outside surface peaked at 2pm. With an 18-hour lag, the indoor peak lands at 8am the next morning. Outdoor air at that hour is often cooler than it was the afternoon before, so if conditions allow, you can open the windows and push some of that stored heat back out.

The peak doesn't vanish. It shows up later, when you have better tools to deal with it.

The Peak Arrives Weaker, Too

Thermal lag tells you when the peak lands. Decrement, also called attenuation, tells you how much weaker it is once it gets there.

Think of the wave again. It loses energy on the way in. The temperature swing that reaches the inside face is gentler than the one beating on the outside.

Timing and intensity are two different measures, and together they describe how a wall behaves over the course of a day. ISO 13786:2017 (adopted in Italy as UNI EN ISO 13786) defines these dynamic thermal properties and how to calculate them. Performance is always assessed for the whole component: every layer working together, never one material on its own.

We don't have a certified decrement value for the HES system. We won't invent one, and we won't work it back from a single material spec.

The HES data sheet also gives a thermal conductivity (λ) of 0.048 W/(m·K). That number tells you how well the material conducts heat. On its own it doesn't set the wall's thermal lag; thickness and the makeup of the whole wall count just as much.

We cover how the material itself works in our piece on the HES mix. The size and spread of the hemp shiv particles shape the structure of the composite, and that's on the hemp shiv particle size page.

What This Means for Your Home, in Practice

A wall with good summer performance keeps the worst of the outdoor heat from landing during the hours when the house is already struggling. Thermal lag on its own won't keep a house cool.

Sun coming straight through the glass puts heat inside no matter what the walls do. External shading is what stops it: shutters, blinds, awnings. Ventilation works the same way, and timing is everything. Open up when the air outside is cooler and you clear stored heat. Open up at midday and you bring more of it in. Shading and natural ventilation both show up among the passive cooling strategies recommended by ENEA, Italy's national energy efficiency agency.

The design as a whole decides the outcome: openings, roof, orientation, systems, and what people do day to day. Humidity changes how hot a room feels, though that's a separate question from thermal lag; we get into it in our piece on breathable walls and comfort.

H.0.ME., a company and general contractor, holds the Italian license for the patented technology, in place since 2019. It hasn't built a house with HES technology yet. The first site is in preparation now. That's where we'll find out how walls, openings, and everyday use work together.

DATA_BOX:

> The number: 18 hours of thermal lag with a 320 mm wall. Source: HES technical data sheet; testing by Vilnius Gediminas University and the BRE Centre at the University of Bath.

FAQ

Does thermal lag help in winter too?
Not in the same way. Here we're talking about it as a defense against summer heat. In winter, what counts is the wall's thermal resistance, how well it limits heat loss.

Is thermal lag enough on its own to keep a house cool?
No. You also have to block direct sun and ventilate when the air outside is cooler.

Does wall thickness change the lag time?
Yes. The 18 hours in the HES data sheet refer to a 320 mm wall. Change the thickness or the composition and the component behaves differently.

To dig deeper into the system, visit the Technology page. In this series: previous episode, EP4 «27,653 m² Before Us». Next up: the breathability of the wall.

Article produced with the support of artificial intelligence and reviewed by the H.0.ME. editorial team.