## Thermal lag

In this article we’ll discuss thermal lag, an important value to take under consideration when analyzing walls and, more in general, building structures. We’ll use our thermal bridge FEM software, Mold Simulator Dynamic, to compute thermal lag in accordance with EN ISO 13786.

In case of simple structures (i.e. a set of homogeneous layers) thermal lag can be analytically evaluated using EN ISO 13786 formulae, but in more general situations a FEM simulation is required.
Thermal lag represents a structure’s thermal mass in terms of time; to make it simple, it’s how long it takes to the heat wave to pass through a building structure.

 T0: time when temperature is at its maximum on external surface. A sinusoidal heat wave is applied to external surface. T1: time when temperature is at its maximum on internal surface. The heat wave has been delayed and faded.

Thermal lag = T1 – T0 in hours

You need Mold Simulator Viewer (free) to open it.

## Thermal Bridge Computation

In this brief tutorial we’ll show you how to computer linear thermal transmittance of a typical thermal bridge: a junction between a wall and a floor. We’ll use Mold Simulator as FEM software for its analysis.

## Thermal bridge

The structure is in contact with external environment (on the left) and internal environment (on the right).
In this case, the thermal bridge is caused by floor; a way to evaluate it is to compute its linear thermal transmittance, ψ (psi) in W/mK.

Linear thermal transmittance
This value represents the difference between the theoretical configuration (just a wall) and the real one (a wall with an intersecting floor).

 Theoretical configuration Real configuration Wall length (l): 1.1 m Wall transmittance (U): 1.0980 W/m²K (computed simply considering wall’s material properties) Thermal conductance (L2D): U x l = 1.2078 W/mK Wall length (l): 1.1 m Wall transmittance (U): 1.2548 W/m²K (FEM simulation with Mold Simulator) Thermal conductance (L2D): U x l = 1.3803 W/mK (FEM simulation with Mold Simulator)

Thermal bridge ψ = 1.3803 – 1.2078 = 0.1725 W/mK

You need Mold Simulator Viewer (free) in order to view it.

## Mold Simulator Mac OS X

We’re extremely pleased to announce that we’ve released Mold Simulator for Mac OS X! Finally a new Mac FEM software for thermal bridge analysis. On Mold Simulator page you’ll also find the new 2.0.8 version.