It's July, and it hits 38 degrees by afternoon. Evening comes and the air outside cools, but the bedroom can stay warm past midnight. Beyond insulation, what often decides it is timing: when does the heat the wall soaked up during the day reach the inside? A light wall lets it through fast. A wall with more thermal mass holds it back and takes the edge off. If it arrives later, it meets a house you can already cool with night air.
U-value always matters; thermal lag completes the picture
The U-value on a spec sheet tells you how much heat passes through a wall under steady conditions. It counts in winter and in summer. For summer comfort you need two more numbers. Thermal lag is how many hours pass before a temperature swing on the outside face shows up on the inside face. Attenuation is how much weaker that swing is when it gets there.
Two walls with similar U-values can behave differently at night. That depends on the conductivity, density and specific heat of the materials, and on the order of the layers. Porosity matters too: the particle size of the hemp shiv shapes the material's structure and how it handles heat.
Breathability is about vapor, not heat: see the wall that breathes.
Three walls, three ways of using mass
The reference method is UNI EN ISO 13786, which assesses the dynamic thermal behavior of a complete building element. Each build-up is calculated on its own, with its own thicknesses and materials, so wall families can't be ranked in advance. Three construction families help you get your bearings:
- Timber frame with lightweight insulation: light structural elements, little mass in the wall. Thermal mass comes mostly from elsewhere, like floors and interior finishes.
- Hollow clay block with EPS external insulation: the mass is in the clay, the EPS on the outside. Performance depends on thicknesses and layer order.
- HES lime-hemp monolithic block, 320 mm: one element that combines insulation and some mass in the same thickness. At this thickness, the HES technical data sheet declares a thermal lag of 18 hours.
These are a guide to the logic, not a room-by-room schedule or a ranking of systems. The declared figure belongs to the HES technology, which H0ME has brought to Italy as licensee since 2019. It is an HES network figure, not one measured on a finished H0ME build. Every project is checked by calculation on its own build-up during design.
What it means for the person sleeping inside
No wall makes cool air. A wall with good thermal mass delays the temperature swing and tends to flatten it, instead of passing it through in a few hours. That can help cut cooling needs, especially when the wall works with the rest of the house.
Summer comfort comes from three things combined. The first is the wall's mass, which acts as a flywheel. The second is window shading: sun through glass heats a house however good the wall is, and with the openings shaded the benefit of the delay shows much more. The third is night ventilation: you open the windows once the outside air drops below the indoor temperature and flush out the remaining heat.
Planning a new home? Bring these questions to the first meeting: which way the facades face, which windows get the most sun, how air will move through the house at night. The first consultation is free and helps you sort things out before choosing materials.
FAQ
Is a wall with a high thermal lag enough on its own for a cool night's sleep?
It helps a lot, but the best result comes from the whole: shaded windows and airing the house in the cool hours work alongside the wall.
Is the UNI EN ISO 13786 calculation the same as a real on-site measurement?
It's a standard calculation method, useful for comparing complete wall build-ups. It applies to each build-up and doesn't replace an instrumental survey on site.
Does a low U-value automatically mean good summer comfort?
The U-value counts in winter and summer alike. For summer comfort, thermal lag and attenuation matter too: the delay and the reduction with which heat crosses the wall.
To see where this delay comes from, the Technology page explains the HES system. In the "18 hours" series, the previous installment was How much does a lime-hemp house cost? Mostly, the finishes decide; the next one goes back to the physics of vapor in the wall.
Article produced with the support of artificial intelligence and reviewed by the H.0.ME. editorial team.