How to calculate the thermal transmittance of doors and windows using the numerical method

The thermal transmittance of a window is calculated using the standards of the series EN ISO 10077. The numerical method for calculating the transmittance has significant benefits compared to the simplified method, especially now that the legislation for the energy certification is very strict.

Let's see why.

This in simple terms, the thermal transmittance of a window is a physical quantity that represents the amount of heat that it must not pass. The higher the transmittance value, the more there will be heat transfer between the building and the outside. Since the heat goes from "hot" to "cold", this will disperse outward during cold days, and vice versa, will come during the hot days.

"Low transmittance window ↔ best"

It 's interesting to note that the 10077 series of standards are not only Italian but also European and international standards.

The first of these standards (UNI EN 10077-1) tells us that the transmittance of a window frame is the sum of the contribution of the glass, the frame and the spacer. Tells us, also, that the transmittance of the spacer - joint between glass and frame - is dependent on the combination of these.

Having said that we come to our topic.

This first standard proposes three methods to determine the transmittance of the frame:

 the numerical method, which uses the second standard series (UNI EN 10077-2);

 measurement in hot room (or Hot Box in English);

 the simplified method with the tables annexed to the first rule.

 

The hot chamber method involves the use of a laboratory test and complex equipment, it is therefore expensive.

The remaining two methods are the most used.

Pros and cons:

 numerical method - best transmittance values ​​(lower) and the cost of more computing;

 Simple Method - U-values ​​worse (higher) and less computation cost.

In fact the cost of finite element numerical method for the not so high (ask for information or to Silipigni CSI spa), the benefits in terms of transmittance are remarkable.

We make the comparison with real-world examples:

 lateral node wood thickness 68 mm → numerical method 1,459 W/m2K 1.8 W/m2K simplified method

 central node wood thickness 68 mm → numerical method 1,343 W/m2K 1.8 W/m2K simplified method

 

As you can see, the difference is remarkable. For frames in aluminum or PVC can be said the same thing.

The reason is simple: the simplified method must necessarily return data conservative, ie it must give the values ​​"abundant" to account for all types of construction that we are marketing. In fact, looking at the data above, we note that this method does not distinguish even between lateral node and the central node.

This brings us to another consideration: the numerical method analyzes the profile taking into account all its characteristics, the simplified method not - for example, for wood profiles only takes into account the thickness and type of wood. The numerical method is, as you might say for a dress, "made to measure". For chassis complex is not essential.

 

How does the numerical method?

The profile section is broken down into smaller elements - usually triangular in shape - called finite elements.

Even the internal cavities are broken down and takes account of their insulating properties. It also takes into account the reduction of convective heat transfer / radiant.

 

Once this phase is called "pre-procession", the profile is fed to the software that deals simulate the physics of heat transfer to each finite element. In short, we could say that at this stage the computer does the math.

Finally the most useful part: the phase is called "post-procession". This is the most interesting phase for a technician who is analyzing the profile, because not only presents the final result - the value of transmittance - but also a whole series of diagrams that are valuable to identify the following strategies to improve the profile.

 

20/09/2011

----------------------------------------

Translated via software

----------------------------------------

Source:

Italian version of CercaGeometra.it

Seguici su Facebook