The simplified calculation in the CTE (Spanish Building Code) only looks at the static U-value and penalises heavy walls. Here you see what your rammed earth wall does under real conditions: how long it delays the wave, how much it damps it, and through which routes of the building code it can — or cannot — comply.
Calculating…
Each layer of the wall stores and releases heat with a certain delay. Move the time control to see how the thermal wave travels from the outside (left) to the inside (right) over a typical summer day.
The static thermal transmittance (the familiar U-value) measures how much heat gets through a wall when the indoor and outdoor temperatures are constant. It is a convenient abstraction: you set 10 °C outside, 20 °C inside, and measure the flow in steady state. It is what the CTE simplified calculation asks for.
The problem is that this constant temperature does not exist. In reality the outdoor temperature oscillates: it is cold at night and hot at midday, and it repeats every day. The static U-value does not see that oscillation, so it does not see the wall's inertia. For lightweight construction that is fine; for a heavy rammed earth wall it completely undervalues its behaviour.
The dynamic thermal transmittance — technically the periodic thermal transmittance |Y₁₂| defined by EN ISO 13786 — measures how much heat gets through the wall when the outdoor temperature oscillates like a real day, with a 24-hour wave. It has the same units as the static U-value (W/m²·K), so the two figures are directly comparable.
In a lightweight wall with no inertia, dynamic U ≈ static U: the outdoor wave passes to the inside untransformed. In a 50 cm rammed earth wall, the dynamic U-value is of the order of ten times lower than the static one: the mass absorbs the heat as it enters, holds it for hours and releases it once it no longer matters. The wave reaches the inside heavily attenuated and shifted in time — which is exactly time lag and damping.
It is the metric the CTE itself recognises in section 3.1.7 of DB-HE 1 for "products of high thermal inertia", and the basis on which the general verification procedure is built. If the static U-value is a photo of the wall in a cold chamber, the dynamic U-value is the film.
The DB-HE has a simplified route (tabulated U-values) and a general procedure via dynamic simulation. The simplified route penalises inertia; the simulation captures it. This is what your configuration yields, in both scenarios:
The 25% is measured over the whole building envelope (roof + façades + floor): if you exceed it, the building-wide K limit is added. The U_lim applies per element, and only to what you replace or substantially modify (DB-HE1 3.1.1, point 2).
If you renovate more than 25% of the envelope, the building must meet the overall coefficient K. Enter the areas and U-values of your elements — openings (windows and exterior doors) also count, and are usually the biggest penalty.
Simplified estimate: area-weighted average U-value (no thermal bridges or adjustment factors for ground/unheated spaces, which the official calculation does include). The façade U-value is taken from your rammed earth wall; adjust areas and the other U-values to your building.
A PDF (in Spanish) with the wall configuration, the HE-1 verification (new build and renovation) and the overall K coefficient, ready to attach to your project report. Supporting guidance document — it does not replace the designer's own calculation.
Professional plan feature. Guidance document only: the regulatory verification remains the responsibility of the qualified designer under the current CTE DB-HE.
Same climate, same orientation, same external finish. Only the construction system changes. The CTE reference wall is ½ pie perforated brick + 6 cm XPS + LH7 + gypsum plaster (zone D).
| Indicator | Your rammed earth | CTE ref. wall |
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