Indoor comfort: what lambda doesn’t measure in a hemp house.
People living in a hemp-insulated house rarely talk about kWh/m²/year.
They say the feeling of cold walls has gone in winter, that the temperature no longer changes from one room to the next, that the house stays cool when it’s hot outside, that it never feels raw, or cold and damp, any more…
None of these sensations can be read in the insulation’s lambda value. Yet they correspond to precise physical phenomena, which can be measured and therefore taken into account from the design stage.
Three measurable parameters.
Perceived comfort in a retrofitted home depends on the balance between three parameters:
• Air temperature: between 18 and 22 °C depending on how the room is used.
• Relative humidity: between 40 and 70%. Below that, the air is dry and irritating. Above it, the risk of condensation and mould increases.
• Air/wall temperature difference: less than 3 °C. Beyond that, you feel the cold-wall effect, even when the thermostat shows the right temperature.
To which can be added:
- Acoustic quality: low reverberation and good sound insulation between spaces.
- Indoor air quality: a low concentration of volatile organic compounds (VOCs), a particularly sensitive point in retrofitting, where many materials are added to an existing volume.
These parameters affect one another. The air/wall difference is the best example. Felt temperature roughly corresponds to the average of the air temperature and the wall temperature. A room heated to 21 °C with walls at 15 °C feels close to 18 °C. To compensate, occupants turn up the heating, which dries the air and lowers relative humidity.
A good R-value doesn’t make a comfortable room.
This is a gap often found in specifications. An insulation material can offer excellent thermal resistance and still leave occupants in a room that is damp, cold and uncomfortable.
Lambda indicates how much heat passes through the wall. It says nothing about what actually happens, and what is actually felt, in the home.
Yet energy regulations are all based almost exclusively on this single criterion.
What the hempcrete block changes in the room.
The hempcrete block acts on several of these parameters at once, not just on heat loss.
- On humidity, it is a capillary-active material: it absorbs excess moisture and releases it when the air dries out. The room’s relative humidity therefore varies less, with no additional mechanical system.
- On felt temperature, its high specific heat slows down variations in the wall’s temperature. The wall stays closer to the air temperature, and the cold-wall effect is reduced.
- On acoustics, the hempcrete block has an absorption coefficient of 0.85. This value depends on the finish: a plaster partly closes the surface porosity, which accounts for most of the absorption. Worth bearing in mind if acoustics is one of the project’s criteria.
- On air quality, the block is rated A+ for VOC emissions. It contains no petrochemical additives or synthetic binders.
Designing for how it feels, not just for compliance.
Taking these five parameters into account from the preliminary design stage makes it possible to anticipate what occupants will really experience in the home, beyond what regulations require. The wall’s moisture regulation plays a central role here. It ties in with the questions covered in our articles on breathable materials and on internal insulation in historic buildings.
A compliant home isn’t necessarily a comfortable one. What makes the difference can’t be read in the lambda value.
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