Innovative Energy Engineering



Thermal Control

A thermal control layer should be continuous around the entire enclosure. Thermal bridges, or areas of higher U-value significantly de-rate overall performance and lead to condensation.

Thermal conductivity is a material property and describes Energy Transfer Rate (Watt) per area (m²) and dT (Kelvin) per thickness (m):

Conductivity

In IP units the values may be given in ft or in. Care must be taken and the units properly converted when used in calculations:

Conductivity_in-ft

Thermal Transmittance "U-Value" is defined as rate of heat transfer through specific assembly of given thickness:

U_value

Thermal Resistance "R-Value" is the inverse to U-Value:

R-Value

Overall Assembly R-Value

Obtaining correct assembly R- and U-values allows correct heating load, energy and comfort calculations. A massive (homogeneous) wall can be easily calculated once we know the specific material properties . Once the R, or U-values of wall and openings are known, the overall U-value can be calculated using the "Parallel Path Method" as described in "ASHRAE Fundamentals 2009 27.3.". For assemblies that differ material in only one dimension (i.e. massive concrete wall with continuous external insulation) the R-values of all layers (i.e. drywall etc.) can be added. This is a simplification as it does not take into account thermal bridges. Most assemblies contain some studs, fasteners or other components that conduct heat better than insulation in between them. For those the 2-D heat flow can be approximated by using isothermal-planes method as described in "ASHRAE Fundamentals 2009 27.3.". For example, steel stud walls with R19 insulation may only have an R-value of 8 due to thermal bridging. This has lead to discomfort, building damage and expensive retrofits.

The above describes simplifications while most building assemblies are more complex. Imagine a typical metal building with fiberglass insulation pinched in between the frame and siding and the insulation in the frame cavity is concave. Add some attachment screws penetrating the insulation, some air pockets and the above methods will fail. A 2-D software like THERM can model complex assemblies (modelled as 2-D detail in Revit and imported into THERM as DXF file). Some HVAC load and energy software (TRANE TRACE!) doesn't provide accurate U-values for the default assemblies. The user should verify the correct U-values and correct as needed.

Corners can be treated per the "ASHRAE Fundamentals 2009 4.4." 2-dimensional conduction rules and add half the adjacent wall thickness per corner to the length of the perimeter wall in question. Overall, this does not account for large loads (~1 %). But Including them is easy and neat for the sake of consistency and being conservative.

Minor thermal resistances of air films can be taken into account:

Position of Surface Air Movement Direction of Heat Flow R [SI] R [IP]
Horizontal Still Up 3.46 0.61
45° Still Up 3.52 0.62
Vertical Still Horizontal 3.86 0.68
45° Still Down 4.32 0.76
Horizontal Still Down 5.22 0.92
Any 4.6km/h (7.5 mph) Any 1.42 0.25
Any 9.3 km/h (15 mph) Any 0.97 0.17

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