Thermal Envelope


Thermal envelope is one of the key elements of a passive or a low energy house. Most important parts of thermal envelope include outer walls, roof, foundation, windows and doors.The purpose of the thermal envelope is to prevent heat transfer form interior of a house to its exterior in winter and vice versa in summer. Theoretically, acheiving the passive house standard is not too expensive as many relatively inexpensive insulation materials are avilable on the market. However, it is important to use proper materials for each part of the building and also to install them consistemtly, avoiding "heatbridges".

Although heat transfer is very important it is not the only important feature of a thermal envelope that affects the envelope efficiency. Three characteristics of a thermal envelope that affect its thermal efficiency most are:
You can find more information about all three characteristics by following upper links. In practice however, most often only heat transfer values are compared to get a general impression about energy efficiency of a certain house or building material. The effectivenes of an insulator is indicated by its overal heat transfer coeficient, so called U-value or its inverse thermal resistence, so called R-value. Better insulators have higher R values or lower U values. 
The table below lists typical U and R values for comparison. These are values for typical houses built in central Europe (e.g. Germany).  Although real values might vary you should get a general impression of range of the U or R value.
Typical U values in W/(m2K)
  Outside walls of a house Roof over heated room or attic Windows
House built before 70'   0.9 - 1.4  0.8 - 1.4  2.2 - 4.5
House built in 70'  0.6 - 1.0   0.5 - 1.4  2.3
House built in 80'  0.5 - 0.9  0.5 - 0.7  1.7 - 2.3
Modern house  0.3 - 0.6  0.2 - 0.4  1.3 - 1.8
Better insulated modern house  0.25 - 0.35  0.15 - 0.2  1.1 - 1.5
Low energy house  0.15 - 0.2  0.12 - 0.15  0.9 - 1.1
Passive house  0.1 - 0.15  0.07 - 0.1  0.6 - 0.8
Typical R values in h ft2 F/Btu
  Outside walls of a house Roof over heated room or attic Windows
House built before 70'   4.1 - 6.3  4.1 - 7.1  1.3 - 2.6
House built in 70'  5.7 - 9.5  4.1 - 11.35  2.5
House built in 80'  6.3 - 11.35  8.1 - 11.35  2.5 - 3.3
Modern house   11.4 - 16.2  14.2 - 28.4  3.2 - 4.4
Better insulated modern house  16.2 - 22.7  28.4 - 37.8  3.8 - 5.2
Low energy house  28.4 - 37.8  37.8 - 47.3  5.2 - 6.3
Passive house  37.8 - 56.8  56.8 - 81.1  7.1 - 9.5
It is important to understand, that insulation is only effective when other rules for low energy house are applied as well. These include suitable architecture, airtightness etc. that are described in other parts of the site. For insulation to be effective it is also important to avoid heatbridges and to consistently apply insulation to all parts of the building. 

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