31 January 2017
Posted by: Evolution
Time to read: 4 min
Keeping a home warm and dry is important to the health of the homeowners as well as the home itself. Mould is toxic to humans and can deteriorate the structure of a building.
Understanding thermal breaks and bridges is integral to understanding how temperature and moisture can be controlled.
Every industry uses idiosyncratic jargon that potentially alienates outsiders, and the correct terminology for building science can sometimes have that counterintuitive result. Where the word “break” usually has a negative connotation, in this context it’s a positive.
A thermal break occurs when an insulator interrupts the thermal conductivity in a material, like a polymer strip separating the internal and external sides of an aluminium window frame.
Aluminium is highly conductive of temperature, which is why most metal window frames are so cold in winter. Consequently, aluminium is not an optimal material for external joinery, though it has been used ubiquitously in New Zealand because of its pricing, availability and ability to withstand the unique UV conditions. The solution is to create a thermal break in the frame with a strip of a less conductive material, like polymer.
Not only does a cold window frame indicate a loss of heat, it also creates a “thermal bridge.”
Warm air holds more moisture than cold so where the internal temperature meets the colder external, condensation can form. When an internal surface temperature falls below 12.6 oC under normal indoor relative humidity and temperature conditions the air space near the cold surface will develop moisture.
In a window, this difference of temperature accounts for the condensation in winter that many readers are likely to be familiar with.
Humans need to live in a certain level of humidity. It would be unhealthy to remove all moisture from a home. Therefore, controlling moisture is key to maintaining a healthy environment and preserving the structure of the residence.
According to Paula Hugens, Structural Engineer at eZed, Ltd., “A thermal bridge free construction is obviously important for any energy efficient project; however, it is also very important if you want to avoid problems with structural decay and mould.”
So, thermal breaks are good, a key component of avoiding thermal bridges. Again, the word “bridge” generally has a positive connotation, but not in building science.
Green Building Advisor.com defines “thermal bridging” as the easy pathway for heat flow across a thermal barrier; in other words, a weak point in the thermal envelope. A good example of this concept is the concrete subfloor in many homes in New Zealand.
The warmth absorbed in a concrete floor when the sun shines on it can help heat a home at the end of the day, unless that heat escapes through the sides and bottom due to a lack of insulation.
Creating a thermal break by fully insulating the concrete subfloor, including the perimeter, like the MaxRaft system, can retain up to 80% of heat.
Otherwise, that heat is transferred to the exterior and, worse, the temperature difference between internal and external occurs at the joint between wall and floor, creating an area where condensation can form and be absorbed, resulting in mould.
Increasing insulation in the external walls is important, but placing those walls on a potentially cold floor will compromise the benefits.
Converse to popular belief, a partially insulated floor can be more problematic than one that is not insulated at all and can cause more thermal bridging. This seems counterintuitive, but remember that condensation, and subsequently mould, occurs where warm air meets cold.
Any weaknesses in a thermal envelope allow for this differential to occur within the building. This explanation emphasizes the importance of using a holistic approach to building a better home.
An added concern of thermal bridging is uneven temperatures, which cause air movement and internal drafts, compromising comfort. The promise of a “warm home” should refer to the entire home, the entire time.
According to an article “A Changing Climate Requires a Change in Building Practices” published on the Canada Green Building Council, “Increased durability and performance measures will add upfront costs to a building project, but they can help save on operational and maintenance costs, both of which could see dramatic increases in the future. And it only makes sense to protect your investment by building something that will last.”
Building better now is a wise investment by reducing health problems and decreasing maintenance, repair and retrofitting costs as building codes become more demanding.
Build well and stay dry while avoiding troubled waters, or condensation, ahead.