Wake up with a dry throat, or that stuffy heavy-air feeling? Temperature is only half of it; relative humidity does the rest. Italy's Istituto Superiore di Sanità, in its COVID-19 Report No. 11/2021, sets a home target of 20–22°C in winter and 24–26°C in summer, with relative humidity between 40% and 60%. In that range, the report says, the respiratory system's defenses do their job. That isn't a promise about sleep.
The US EPA lands close by. Its guide A Brief Guide to Mold, Moisture and Your Home says to stay under 60% relative humidity, better still between 30% and 50%, so mold gets no foothold.
What happens to bedroom air overnight
Every breath puts carbon dioxide and water vapor into the room, and behind a closed door both go up overnight. They behave differently, though.
Relative humidity is a ratio: how much vapor the air holds against the most it could hold at that temperature. Turn up the heat and the vapor stays put, the temperature climbs, and relative humidity falls even though nothing got drier.
CO2 is different. It tracks how many people are breathing, and no wall dilutes it — only fresh air does. The ISS report says as much, telling people to air out rooms several times a day even with a ventilation system installed.
Temperature, occupants, air exchange, room size, other moisture sources: all of it shapes the outcome. Pinning that on one material doesn't hold up.
What a "breathing" wall really does
Some materials trade moisture with the air: they take some up when humidity spikes, give it back when the air dries out.
People reach for the sponge comparison. Half right. A wall is a buffer, not a humidity thermostat: it softens a spike, it won't hold the room at a set number.
Researchers at the University of Bath measured this on hemp-lime test walls with the Nordtest protocol. The 2015 study in Building and Environment, "Moisture buffer potential of experimental wall assemblies incorporating formulated hemp-lime," puts bare hemp-lime and lime-plastered hemp-lime in the top buffering class, and finds synthetic paint over the top cuts that performance down hard.
Still: lab-built assemblies, one protocol, not the HES wall system — and no promise that a real bedroom sits inside 40–60% around the clock.
Where the HES wall fits in
The HES system that H0ME brings to Italy uses a hemp-lime mix as the structural wall mass. H0ME has held the Italian license for US Patent 10,266,451 B2 since 2019.
HES's technical specs give a thermal conductivity of 0.048 W/(m·K), and a 320mm wall with an 18-hour thermal lag. Heat numbers: how fast warmth crosses the wall, how long its peak takes. They say nothing about humidity.
Hemp-lime is hygroscopic, and that helps flatten humidity peaks. In a finished house the result depends on the whole assembly, the finishes, the ventilation and how the rooms get used.
H0ME is studying this as it builds its first HES home. No H0ME home is finished yet: the track record belongs to the wider HES network.
For more, see the breathing wall and humidity comfort, how the HES mix works, and hemp shiv particle size and insulation.
What this means for your bedroom
A hygroscopic wall takes the edge off the evening-to-night swing. It doesn't get you out of opening a window, or of having ventilation designed right.
A cheap hygrometer tells you what's going on. If humidity stays high, find the cause: weak air exchange, condensation, a leak, too much indoor moisture. The wall is no excuse to skip that check.
So "sleep better" is not a medical promise. It means steadier temperature and humidity, and air that gets refreshed.
FAQ
Does a breathable wall replace mechanical ventilation?
No. It can soften some humidity swings, but air exchange still depends on ventilation, mechanical or natural.
What happens to humidity in winter once the heating kicks in?
With the same vapor in the air, raising the temperature lowers relative humidity. A hygroscopic wall eases that swing; it won't guarantee you stay above 40%.
Which finishes should you avoid so you don't seal the wall shut?
The paint's name alone won't tell you. The University of Bath study found synthetic paint can cut buffering capacity hard, while lime plaster keeps it. Check the whole finish system with your designer and supplier.
Want to know more about the building system? Visit our Technology page. Previous episode: Hemp-Lime vs. EPS Insulation: What Happens in Summer. Next up: airtightness and breathability in winter.
Article produced with the support of artificial intelligence and reviewed by the H.0.ME. editorial team.