September 2012: Thermal Bridging
Thermal bridging is an inherent consequence from having thermal mass act negatively upon a building’s heating and cooling load often by short circuiting the envelope. The material can be: steel, iron, aluminum, concrete, brick, wood and sometimes glass. We commonly see these kinds of design flaws in winged walls and cantilevered floors, but they can easily take the form of old cast iron windows and framed areas with exposed surfaces that are experiencing huge temperature extremes. Infrared pictures do a great job in documenting how these items impact overall conduction losses, contribute to comfort issues and in some extreme cases cause moisture problems and resultant building component failure.
Many clients don’t realize that a normally framed ceiling will contain about one fifth of the area that is wood and therefore not very well insulated. This is why additional insulation should be run perpendicular to the joists providing what is known as a “monolithic covering.” A similar situation exists where the joists are left exposed either in open floor areas or the backside of knee wall walls. Sometimes these surfaces get so cold that the indoor humidity is attracted to the framing and or fasteners and display ghosting marks. If the case is extreme there will not only be huge conductive penalties, but the surfaces will reach dew point and transport moisture in and out of the building. Often masonry walls crack because this precise reason where they are being subjected to thermal shock and a delta T across the block of some 60 degrees.
Noticing and addressing these unique issues in some buildings can be difficult and costly at the onset but deliver huge benefits over the intended lifespan of the dwelling.