Thermally improved fenestration products offer the benefit of separating the interior portion of the window or door frame from the portion exposed to the exterior. The benefits of such a configuration include decreased energy loss and enhanced resistance to the formation of condensation. For optimal performance, the portion of the frame containing the low conductive material that separates the interior and exterior portions of the frame must be placed in line with, or perhaps behind, the wall insulation or; better still, the full edge of the window should be insulated. Oftentimes neither of these measures is executed.
Thermal modeling of fenestration frames generally assumes an adiabatic condition (no heat transfer) at the boundary conditions of the frame. Such an assumption simplifies the analysis by forcing thermal energy to flow perpendicular to the interior and exterior surfaces without regard for the thermal gains or losses that may pass through the outer frame edge. Whether empirically derived or tested, the overall conductive factor for a fenestration, U-Factor, is typically reported, assuming no heat transfer through the frame edge. U-Factor is a measurement of the energy transferred for a given time over a specified area per degree temperature change. English units are Btu's per hour per degree Fahrenheit per square foot. Note, this is the inverse of R-Value.
Three major zones in the construct of any fenestration contribute to the overall conductive factor - the center of the glass (COG), edge of glass (EOG), and the frame. The adiabatic assumption at frame edges renders two of these, EOG and frame, which is grossly inaccurate, unless insulation is used around the fenestration perimeter. The contribution that each zone has on the overall conductive factor is determined by a weighted average of the areas for each zone. For smaller fenestrations there is less glass than frame, so the weighted area of the frame and EOG plays a greater roll in the overall conductive factor than the COG contribution would play if the fenestration were larger.
The thermal contour images of a generic, thermally broken, aluminum framed window below demonstrate how easy it is to dismantle the engineering prowess of a well designed thermal break, simply by mounting the window forward of the insulating plane. Specifically, the image on the left simulates a window mounted nearly flush with a brick masonry veneer and seated on a steel angle. No insulation is modeled in the channel formed between the bottom "legs"; furthermore, the channel is considered to be exposed to the exterior temperatures.
The image on the right is modeled with an adiabatic boundary condition along the bottom of the window between the vertical "legs". Such a state simulates mounting of the window with the thermal break in line with the wall insulation and having the cavity between the vertical "legs" filled with insulation.
Considering the temperature differences of the interior surfaces of the two models, an astounding 20 °F temperature disparity exists. From an energy standpoint, the conductance of the frame for the brick cavity mounted window is approximately 0.97 Btu/(hr °F ft2), contrasted with the U-Factor of 0.73 Btu/(hr °F ft2 ) for the well insulated frame edge model. In layman's terms, the brick cavity mounted window will conduct 33% more energy through the frame edge. These numbers, albeit extremes, provide a basic understanding of the need for proper mounting and insulating.
There will be further discussion on this topic in blogs to come. For example, how does this affect the condensation resistance factor (CRF)? In the meantime, if you so desire, you may download the free software used to create the models, "Therm", or other great energy software applications from the Lawrence Berkeley National Laboratory's website at http://windows.lbl.gov/software/default.htm.
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