You get back from vacation, turn the key, and there it is: that shut-up-house smell. It happens to nearly everyone, in any home.
When it comes to heat, though, the wall can help. For the HES system, which H0ME is bringing to Italy, the HES datasheet states a thermal lag of 18 hours at 320 mm thickness. In plain terms, the sun's heat takes about 18 hours to work its way through the wall.
A note on where we stand. H0ME has been a general contractor in Avellino since 2019 and holds the license to US patent 10,266,451 B2. It is preparing its first HES build but hasn't yet built an HES home of its own. What follows explains the technology so you know what to expect.
Thermal lag, in plain terms
By day the sun heats the outside of the wall. That heat doesn't rush in all at once. It seeps through gradually, like water soaking into a thick sponge.
With a thin, light wall, the peak reaches the inside within a few hours, usually when the house is already hot. A wall that slows heat down a great deal delays it by many hours. In the HES system that is about 18: the afternoon heat arrives muted and late, the next morning.
Thickness and material do the slowing. The HES datasheet gives a thermal conductivity (λ) of 0.048 W/(m·K). That is low, meaning the material lets very little heat through. We explain how the mix is made in how HES Mix works. The size of the hemp shiv fragments matters too, and we cover it in hemp shiv grain size and insulation.
In a house shut up for weeks, nobody opens the windows in the evening to let the heat out. How much heat builds up depends on the walls, roof, windows, shading and how much sun the house gets. The wall is one of the biggest factors.
The stale smell has a different cause
Thermal lag deals with heat. That closed-up smell comes from air that hasn't been changed, moisture that has sat still, and what furniture and materials give off. We covered that last point in the previous episode, Drywall and glass wool: what you actually breathe.
On moisture, the wall can lend a hand. The lime-and-hemp biocomposite lets vapor pass through and smooths out humidity peaks indoors. We look at this in the breathing wall. Fewer peaks means less condensation on surfaces.
Two things not to mix up:
- breathability: water vapor can pass through the wall;
- airtightness: the house has no drafts.
A wall can be breathable in a well-sealed house. Vapor moves through it, but fresh air doesn't come in through the masonry. Fresh air only comes with ventilation.
The WHO makes the point, without giving figures, in its WHO Guidelines for Indoor Air Quality: Dampness and Mould: moisture that lingers in a closed space encourages mould and is linked to health problems. That's why keeping humidity in check and changing the air go hand in hand.
What you'll find when you get back
The wall slows the heat coming in and smooths humidity peaks. Together with the rest of the house, it can help limit the heat that builds up. The air, though, is still, and it stays that way until you open up.
Refreshing the air is up to you.
First thing when you walk in
Open up, before you even unpack.
Choose windows on opposite sides of the house and open them together. Air sweeps through and carries the stale air out with it. In September the best hours are usually early morning and evening, when it's cooler outside. Open the interior doors too, or the air will sit in the closed-off rooms.
Everything else can wait. Even a well-insulated house needs this.
FAQ
Does a breathable wall replace opening the windows?
No. Vapor passes through the wall, but fresh air doesn't. To clear out stale air, you have to open up.
Can lime-hemp eliminate the risk of mould on its own?
No. It smooths humidity peaks and reduces condensation on surfaces, but if you don't ventilate properly, the risk remains.
How should I air the house when I get back?
Open windows on opposite sides of the house. The air sweeps through the rooms and genuinely replaces what's inside.
Want to see how the system works? Read our Technology page. Previous episode: EP15: Drywall and glass wool: what you actually breathe. Next episode: coming soon to this blog.
Article produced with the support of artificial intelligence and reviewed by the H.0.ME. editorial team.