Windows fogged over at breakfast, dark rings in the corners. Behind both, the same setup: vapor in the air, a surface that's too cold, no air moving through. Building science puts 40-60% relative humidity forward as the target range for occupied rooms. To see what vapor does on a cold surface, take a glass of water.

Fill it with ice water and leave it on the table. Beads form outside. That water didn't come through the glass: it was already in the room as vapor, and turned liquid when the glass fell below the dew point of the surrounding air.

That's where hygroscopy starts, and where hygroscopy and ventilation part ways.

Why the Glass Sweats, and What Happens to a Wall

Glass isn't hygroscopic: cool the surface far enough and vapor condenses on it.

A house behaves the same way. A shower puts vapor into the air, and so do a pot on the stove and daily life. Where that vapor meets a wall, a corner, or a window frame colder than the dew point, it turns to water.

So the wall isn't making the moisture. What matters is how much vapor we produce, how cold the surfaces are, and how much fresh air moves through. The US Environmental Protection Agency's guide "A Brief Guide to Mold, Moisture and Your Home" treats condensation on windows, walls, or pipes as a possible sign of excess humidity.

A hygroscopic material acts differently. When the air turns humid it takes in some of that vapor, then gives it back once the air dries out. It doesn't take water out of the house and it doesn't bring in fresh air: it flattens the spike and spreads it over time.

This has been measured in hemp-lime composites. The 2015 study "Moisture buffer potential of experimental wall assemblies incorporating formulated hemp-lime," published in Building and Environment, also found that finishes and coatings change how well a wall assembly buffers humidity.

What This Means Inside an HES Wall

The HES system that H.0.ME. is bringing to Italy uses a mix of lime and hemp shiv. Like the other hemp-lime composites in the literature, it trades vapor with the room, holding some back when humidity climbs and releasing it once the air dries out.

We've been here before, in the posts on how the HES mix works and on the wall that "breathes".

"Breathing" is a figure of speech. A wall doesn't bring oxygen into a house or carry pollutants out. It can soften a humidity swing, and how well depends on the whole wall, its finishes, and the design.

Surface temperature counts too. The HES technical data sheet lists a thermal conductivity (λ) of 0.048 W/(m·K) for the material. By itself it won't tell you whether condensation forms: the full wall assembly counts, along with thermal bridges and conditions inside and out.

As of December 31, 2014, 27,653 m² had been built with the HES system, according to HES Group. That figure belongs to the HES network, not to H.0.ME.

H.0.ME. has held the Italian license since 2019 and is preparing its first build with this technology. It hasn't finished an HES house of its own yet.

In practice, a hygroscopic wall can soak up part of a shower's vapor and hand it back when the air dries. Windows, ventilation, and a design with no cold spots do the rest.

📌 The 40-60% relative-humidity range comes from E.M. Sterling, A. Arundel, and T.D. Sterling, "Criteria for Human Exposure to Humidity in Occupied Buildings," published in ASHRAE Transactions in 1985, as the optimal band for cutting the risks the authors examined. The World Health Organization's 2009 guidelines, "WHO Guidelines for Indoor Air Quality: Dampness and Mould," don't set that number: they call for preventing or reducing persistent dampness and microbial growth in buildings.

FAQ

Does hygroscopy replace ventilation or mechanical ventilation systems?
No. It can take the edge off vapor spikes, but it doesn't refresh the air or remove indoor pollutants.

Does a hygroscopic wall get rid of mold?
No. It can help with humidity spikes, but condensation and mold can still show up on cold surfaces, at thermal bridges, from construction defects, or where air doesn't circulate enough.

Does H.0.ME. already have active building sites in Italy with this technology?
No. H.0.ME. has held the Italian license for the HES system since 2019 and is preparing its first site. The 27,653 m² reported as of 2014 belongs to the HES network's European experience, not to H.0.ME.

For the system in detail, see the Technology page. Last time, in The Room Where You Sleep Better, we looked at humidity in the bedroom. Next Friday we continue toward the first H0ME house with HES technology.

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