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. While reviewing heating load calculations of failed designs, it became apparent that not many designers know how to calculate overall assembly U-values. 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. A massive (homogeneous) concrete wall can be easily calculated once we know the specific concrete properties (medium, heavy etc.). however, most enclosures consist of different layers and also have windows and doors. 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.".

The above describes simplifications that can be employed with hand or calculator. But 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 that can model complex assemblies is THERM. Complex structures are easily modelled as 2-D detail in Revit and imported into THERM as DXF file.

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

Application in Load and Energy Simulations

To accurately calculate heating and cooling load the actual enclosure assembly needs to be modelled as accurately as possible. As software for such load calculations does not take away the need to be precise. Heat transfer at corners often is not modelled accurately since the corner gets neglected. The easiest way to take into account corners is to Treat the corner 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.

Some HVAC load and energy software (I'm looking at you, TRANE TRACE!) doesn't provide accurate U-values for the default assemblies. The user should verify the correct U-values and correct as needed.

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