Take a hot shower and the bathroom fogs up. Cooking adds moisture to the air at home. So does breathing. Some of that water vapor can pass through the materials in your walls. How easily? Every comparison starts from μ = 1, which is by definition the value for still air.

This is one installment of "18 Hours: Bringing a House That Breathes to Italy," our series on the first H0ME house built with HES technology. Today's subject is a number few people know. It tells you what "breathable" means when someone says it about a wall.

μ = 1: the baseline

μ is pronounced "mu." It's the water vapor resistance factor. It tells you how much a material slows moisture compared with still air of the same thickness, under the same conditions. It's a ratio, so it has no unit.

Measure air against itself and you get μ = 1. A hypothetical material with μ = 10 slows moisture ten times as much. With everything else equal, a lower μ means more moisture gets through. The reference standard for these values is UNI EN ISO 10456.

Where does a material's number come from? The lab. UNI EN ISO 12572 describes the cup test. A sample separates two spaces at different humidity, and you measure how much moisture crosses it.

On a data sheet, read the number and also the conditions it was measured under. The μ value for HES-mix is still being finalized, so we don't give one in this installment.

Thickness matters: enter s<sub>d</sub>

μ alone won't tell you how much a layer holds moisture back. You need its thickness too. That's what the equivalent air-layer thickness, written s<sub>d</sub>, is for:

s<sub>d</sub> = μ × d

Here d is the material's thickness in meters. The result is the depth of still air that would slow moisture by the same amount.

Here's a made-up example, just to show the math. Take a material with μ = 10 that's 2 centimeters thick. That makes d = 0.02 meters. Multiply and you get s<sub>d</sub> = 0.2 meters, the same resistance as 20 centimeters of still air. These numbers are for illustration. They are not HES-mix data.

Same μ, thicker layer, higher s<sub>d</sub>. So "breathable" by itself doesn't tell you much. You need to know the material and how thick it's going to be.

Plaster and paint work the same way. Each layer adds resistance, and a thin finish with a high μ can still hold moisture back.

What this means at home

When you choose a wall, ask to see the planned finishes too. If you want the wall to stay open to moisture, the lime plasters and paints need documented breathability. A product name isn't enough. Ask for the data sheet.

Checking the whole wall, including the order of its layers, is the designer's job. They do it with a hygrothermal calculation for your specific project.

Mold does not form on the hemp-lime biocomposite. Lime is alkaline, and water doesn't stay trapped.

Breathable walls don't replace fresh air. Moisture moving through materials is one thing. Fresh air coming into your rooms is another. Ventilation is also what carries the moisture we produce at home to the outside.

For more on the mix, read How HES-mix Works. For how it connects to comfort, see The Wall That Breathes.

H0ME builds new homes in hemp and lime, either on an empty lot or by tearing down an existing building and rebuilding. The first H0ME house with HES technology hasn't been built yet. The first site is being prepared. Questions about materials come before design decisions, and that's where this series starts.

📌 The data point μ = 1 for still air: the baseline for every comparison. Reference standard: UNI EN ISO 10456. Test method for materials: UNI EN ISO 12572. The μ value for HES-mix is not yet published on the data sheet.

FAQ

Does μ = 1 mean a hemp-and-lime wall is the same as air?
No. μ = 1 is the value for still air. To assess a wall, you need the data for each material and its thickness.

If the wall is breathable, can I ventilate less?
No. Breathability doesn't replace fresh air. Ventilation should be designed around the house and the people who live in it.

Can I paint the walls any way I like?
Pick the paint with your designer. Check its vapor resistance and whether it works with the other layers.

Discover HES Technology. In this series: previous installment, Coming Back in September to a House That Doesn't Smell Stuffy. The journey continues in installment 18.

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