The problem: high bills, houses that can't cool down
Summer: a house that bakes, air conditioning running non-stop. Winter: heating bills that climb. One likely culprit is the building envelope. When a wall bleeds heat easily, your HVAC system has to work harder to keep up. Roofs, windows and air-tightness all play a role too, but here we're focusing on the wall, because it's the one place we have a hard number to show you. The HES system that H.0.ME. brings to Italy uses a material with a thermal conductivity (λ) of 0.048 W/(m·K), according to the HES technical data sheet. One clarification up front: "breathes" in the title is a metaphor. λ measures heat transfer only — vapor management is a separate topic, and we'll get to it below.
What λ actually means, no physics degree required
λ describes how much heat flows through one square metre of a material, one metre thick, when there's a 1-degree (K) temperature difference between the two sides (source: NIST, Special Publication 260-261, p. 8). Lower number, heat struggles to get through. Higher number, heat moves more freely.
The figure 0.048 is what's declared on the HES technical data sheet for the material used in the system. Real-world wall performance also depends on installation quality, moisture, actual density, thickness and any thermal bridging in the specific design. The declared value alone doesn't tell the whole story.
This is where thickness comes in. Thermal resistance of a single, uniform layer is calculated as thickness divided by λ (R = s/λ): it increases the lower λ is and the thicker the layer (NIST, SP 260-261, p. 16). The resistance of the whole wall, though, depends on every layer together — not just this one.
Curious about the material blend behind this number? We've covered it in how the HES mix works and gone deeper on particle size in hemp shiv particle size and insulation.
The 18 hours: a delay, not a clock
There's a second figure that completes the picture: thermal lag. The HES data sheet lists a lag of 18 hours at 320 mm thickness. It's also where our series gets its name.
What this actually means: thermal lag is the delay between the moment outdoor temperature hits its peak and the moment heat flow peaks on the inside face of the wall (ENEA, Report RdS/2013/090). It's not a clock. Midday heat doesn't "arrive" indoors at a precise hour like a scheduled train — it's a declared delay for that specific wall, under those specific conditions.
Something else worth keeping separate: how much that peak is also dampened in intensity is a different metric, called attenuation, and needs to be documented on its own. The declared figure is 18 hours at 320 mm, per the HES data sheet. Actual comfort in a real home still comes down to orientation, glazing, ventilation and overall design.
What this means for a home
We won't invent bill-saving figures — that would be misleading without a real, monitored building to back it up. What we can say, because it's verifiable: according to the HES data sheet, a 35 cm configuration of this wall, in climate zone D, supports reaching energy class A4. That applies within the context of the building's full energy design, not the wall alone. Climate zones are assigned by municipality under DPR 412/1993, so it needs to be checked municipality by municipality.
In plain terms: a wall with these characteristics, all else being equal in terms of HVAC, exposure and windows, can help lower heating and cooling demand compared to a less insulating wall. That's the technical starting point for the first H0ME project built with HES technology. It's not yet a promise of results for that specific house.
Full technical details live on the Technology page.
FAQ
Is 0.048 genuinely a good figure, or just marketing?
It's the value declared on the HES technical data sheet. The number itself is objective; what it means for a specific home also depends on installation, thickness and overall design.
Does a low λ also mean a wall that "breathes" well?
No, those are two different things. λ measures heat transfer only. Vapor and moisture management is a separate subject, covered in the wall that breathes: comfort and humidity.
Does this figure already relate to a house H0ME has built?
No. This is the technical data for the HES system we're bringing to the first H0ME construction project, still in preparation.
For the full technical breakdown of λ, thermal lag and energy class, see the Technology page; for comfort, read the wall that breathes. In this series: go back to EP2 "An American Patent, an Italian Licence", or stay tuned for the next instalment.
Article produced with the support of artificial intelligence and reviewed by the H.0.ME. editorial team.