Bridging at wall-to-roof transitions A lot of structural framing combined with the angle of the roof framing can present thermal bridging issues and issues with thickness of insulation.
Adequate insulation from the ground The thermal control layer should extend to protect the building enclosure from the ground. Two lines of faulty reasoning often result in inadequate thermal control from the ground:
“Heat rises” Hot fluids moving via convection do rise, but conductive and radiant heat flows move equally (and significantly!) in any direction. A floor or wall in contact with the ground and lacking adequate thermal control will allow very significant amounts of heat to pass into the ground. The earth is a very large thermal mass; it is able to continuously absorb heat from the building unless the thermal control layer prevents this flow.
“The ground is warm” Surface ground temperatures vary daily and annually, based on solar exposure and air temperature, but subsoil temperatures are relatively constant and range from about 40°F (4.5°C) in the North to 75°F (24°C) in the South. While these temperatures may be closer to the comfort range than air temperatures, if the desired temperature inside the building is higher or lower than the subsoil temperature, a significant amount of energy will be required to maintain the desired indoor temperature due to losses or gains from the ground.
At the construction phase, quality of installation is critical to the performance of a thermal control layer. Even if the layer is well designed, if the installation is incomplete or of poor quality, the results can dramatically underperform. During installation, ensure:
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No gaps or voids Be sure that insulation fills all cavities and makes complete, uninterrupted contact with interior and exterior enclosure elements. This can be challenging, especially when installing batt or board-style insulation around obstacles like wiring, plumbing, and blocking.
Proper density R-value of insulation materials is rated at a particular density. If the insulation is too dense (over packed), the R-value may be lower than anticipated because the air pockets in the material are compressed or eliminated. If the insulation is not dense enough (under packed or incompletely installed), the performance may be lower than anticipated because the greater volume of air spaces will allow for high losses due to convection.
Continuity A thermal control layer often combines different materials throughout the building enclosure, and it is crucial that intersections between elements of the thermal control layer are seamless, especially at crucial junctions like windows, sill plates, and wall-to-roof transitions.
Your thermal control layer is a fundamentally important element in your project. By wisely incorporating all of the above considerations to meet your specific design goals, you can closely determine the comfort and efficiency of your building.