On a summer evening after a day of full sun, the air in your bedroom can feel as heavy as it did in the early afternoon. The wall has soaked up heat all day, and it hands it back just when you want to sleep. A wall with little thermal lag lets the heat wave through in a few hours, and you feel it almost at once.
One number tells a different story: 18 hours. That is the thermal lag declared for a 320 mm wall in the HES/HES-mix system: how long a wave of heat takes to travel through the wall, from outside to inside. It is one of the key figures of the HES system that H0ME is bringing to Italy (source: HES technical data sheet). For the material itself, see how HES-mix works.
Insulation is only half the story: timing matters too
Talk about walls and most people think of insulation: how much heat gets through. But there is a second question: when does it arrive? With similar insulation, different materials and build-ups give the heat back after a few hours or after many. Thermal lag measures exactly that: the delay before the outdoor heat peak makes it through the wall.
The HES system works on both fronts. The declared conductivity of HES-mix is λ = 0.048 W/(m·K) (source: HES technical data sheet), a low value that slows heat moving through the material. The mix is hemp-based, with the right particle size; our dedicated article explains its role in insulation and comfort. The data sheet also declares the lag: 18 hours at 320 mm.
Think of a stone left in the sun. It warms slowly and releases its heat hours later. A wall works in a similar way: thickness turns into time.
Why 18 hours matters
When the sun heats a façade in the hottest hours, the heat peak starts its journey inward. With 18 hours of lag, a peak hitting the outside in the early afternoon would shift to the following morning. Ventilation helps whenever the outside air is cooler than the air indoors. The hottest moment passing through the wall lands far from the evening, when you are home, eating and sleeping.
The peak does not disappear. It moves to a moment that is easier to manage. The design puts this to work together with ventilation and cooling, choosing the right combination case by case. In winter the focus is different: limiting heat loss to the outside, which the home's overall energy design takes care of.
What it means in your home
Combined with shading and natural ventilation, the wall works for your night-time comfort. A later peak leaves more room for cooling during the hottest hours and helps keep indoor temperatures steadier, even after a scorching day.
If you are planning your own home, this figure shapes even simple choices: how much to rely on air conditioning alone, where to put the bedrooms, how to orient the house against the afternoon sun. It doesn't replace good design; it makes good design easier, leaving more room for every other choice, from shutters to ventilation.
Picture an ordinary July day. The house warms in the afternoon sun, you sit down to dinner, then head to bed. A long delay means the outdoor peak doesn't coincide with those moments, so you can open the windows when the air is cooler and close the shutters when the sun is strongest.
For the moisture side, read the wall that breathes: thermal lag and vapor management work together in the same material. All the system's technical data is on the Technology page.
18 hours: the declared thermal lag of a 320 mm HES-mix wall (the delay before the outdoor heat peak travels through the wall to the inside).
Source: HES technical data sheet.
FAQ
Are thermal lag and insulation the same thing?
No. Insulation is about how much heat gets through; thermal lag is about how long it takes. Together they shape your comfort, but they answer different questions.
Do 18 hours cover the whole summer, not just one hot day?
The 18 hours is the value declared in the HES data sheet for the 320 mm wall. Summer comfort also depends on shading, ventilation and the design of the home.
Does this apply to any wall thickness?
No. The 18-hour figure is specific to a 320 mm HES-mix wall; other thicknesses give different lags.
This is the first instalment of the series "18 Hours: Bringing a House That Breathes to Italy", the story of the road to the first H0ME home built with HES technology. Explore all the HES system's technical data on the Technology page, and don't miss the next instalment, on how the design takes shape.
Article produced with the support of artificial intelligence and reviewed by the H.0.ME. editorial team.