The Wall That Makes Time Work For You: What "18 Hours" Really Means

Picture a summer evening after a scorching day. You'd expect the house to start cooling once the sun goes down — instead, the air upstairs still feels thick, almost like mid-afternoon. That's the wall talking. It spent the day soaking up heat, and now it's giving it back, right when you're trying to sleep. A wall with little thermal lag lets heat straight through in a matter of hours, so it lands inside almost as fast as it hits the façade.

There's a number that tells a different story: 18 hours. That's how long it takes heat to work its way through a 320mm HES/HES-mix wall, from the outside face to the inside. It's one of the defining figures behind the HES building system that H0ME is bringing to Italy (source: HES technical data sheet). The material itself is covered in how HES-mix works.

Insulation isn't the whole story — timing matters too

Ask most people what makes a wall "good," and they'll talk about insulation: how much heat gets through at all. But there's a second question that matters just as much: how long does it take? Two walls can insulate equally well and still behave completely differently — one hands back its heat within hours, the other holds onto it far longer. That delay has a name — thermal phase shift — and it's simply the time heat needs to cross the wall. It's a well-established idea in building physics; here, we're just explaining what it means in practice, not replacing the technical standards that govern it.

HES works on both fronts at once. The declared thermal conductivity of HES-mix is λ = 0.048 W/(m·K) (source: HES technical data sheet) — a low figure, meaning heat moves sluggishly through the material itself. The mix is built around hemp shiv, milled to a specific particle size that shapes both insulation and comfort, as covered in that companion piece. For HES-mix, the data sheet also states the phase shift directly: 18 hours at 320mm.

Think of the difference between a thin pane of glass and a thick stone wall. Both block some heat — but only the stone forces it on a long detour before it ever reaches the other side. Here, thickness buys time.

Why 18 hours is the number that counts

When the sun hits a façade during the hottest stretch of the day, that heat sets off on its journey inward. If the wall takes 18 hours to let it through, it doesn't surface indoors until well after midnight, or the next morning — a point when the outside air and your ventilation are both better placed to carry it away. The peak shifts clear of the evening hours, especially where the design already leans on night ventilation or passive cooling.

That's the real distinction: a wall that lets everything through immediately, versus one that makes heat wait its turn. It isn't blocking heat forever — it's delaying it just long enough to arrive off-schedule. Winter is a separate question — there, what matters is limiting heat loss outward, weighed as part of the whole energy design.

What this means inside your own home

That delay can take real pressure off cooling during the hottest hours, working alongside good shading and natural airflow — the wall isn't doing the job alone, but it's carrying real weight. In a well-designed home, it can also help keep indoor temperatures steadier overnight, even after a brutal day outside — though exposure, window placement, shading, and ventilation all still play their part.

For anyone designing or renovating, this figure quietly shapes decisions that have nothing to do with walls on paper: how much you can lean on air conditioning, where bedrooms end up, how the house is turned toward the afternoon sun. It doesn't replace good design — it gives good design room to breathe. A wall that buys time makes every other choice, from shutters to airflow, easier to get right.

Curious about the humidity side of things? The wall that breathes covers how phase shift and vapour management work together in the same material. Full technical figures for the system live on the Technology page.

FAQ

Is thermal phase shift the same thing as insulation?
No. Insulation is about how much heat gets through; phase shift is about how long it takes. You need both — they answer different questions.

Does 18 hours hold up across a whole summer, or just one hot day?
That figure comes straight from the HES data sheet. In real conditions, exposure, shading, and how well the house is ventilated all factor in too.

Does this apply to any wall thickness?
No — the 18-hour figure is specific to a 320mm HES-mix wall. Different thicknesses give you different phase shifts.

This is the opening chapter of "18 Hours: Bringing a Breathing House to Italy," the story of H0ME's first home built on HES technology, now taking shape. See the full technical picture on the Technology page, and stay tuned for the next chapter — how the project itself starts to take form.

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