Passive House design takes into consideration the 3 ways that heat travels – Convection (via a gas or fluid), Conduction (via direct contact of molecules), or Radiation (temperature equalization of surfaces via radiant energy). In this thermal image of 2 buildings, the red and yellow areas show the least thermal resistance, the green and blue areas show more thermal resistance. It is easy to see that the building on the left is losing more heat through the walls and windows than the Passive House building on the right. In this article we will focus on heat loss through conduction via thermal bridges, what that is, and what can be done to control it.
Since conduction depends on molecules being in contact with each other, it would make sense that if you could separate them in some fashion you could control the amount of heat transfer. Fortunately building materials do have different molecular structures as well as heat transmission tendencies that can be converted into a common number for comparison called an R value (thermal resistance value). The higher the R value, the more thermally resistant or insulating it is. Steel studs allow heat to more readily transfer through them than wood studs and therefor have a lower R value than the wood studs. Even the way a material is made or installed can affect its R value. Aerated concrete has a better R value than traditional concrete; wood placed so that its fibers are parallel to the heat flow allows more heat transfer than with the fibers perpendicular to the heat flow.
So now we know heat transfers at different rates through different materials, how do we use that knowledge? As the saying goes, “we follow the money”. Your hard-earned cash is paying for that heat and you want to keep it inside where it will benefit you. Additionally, the environment pays a price for that energy to be created so conserving it makes good environmental sense too. If we follow where heat wants to go, we find it wants to go to a cooler area (the 2nd law of thermodynamics) and will travel the fastest through the least resistive route possible.
The connections where heat transfers the most easily are called thermal bridges. It’s like a bridge giving a car an easy way to get across a ravine – without the bridge the car wouldn’t make it across. A collection of low R-value materials touching each other from the inside of a building through to the outside of a building (or to the soil the building sits on) makes a direct path for heat to flow out of your house. With careful planning, a Passive House eliminates these thermal bridges by changing how the building is put together. You can substitute building materials of higher thermal resistance, design connections in such a way that they do not make a direct path for the heat to follow from the inside of the building to the outside, and test these scenarios with computer modeling programs that will show you how effective the design is before you build it so that you have the opportunity to optimize the design when it is the cheapest to change – in the design phase. But don’t toss the thermal underwear just yet – you might need it if you go camping!
As we follow the Seattle Passive House project I will be showing you some of the computer models that Dan used to design the structure to eliminate thermal bridges. One final note on thermal bridging, the differences in temperature at the point of thermal bridges can cause those sections of the building to be cooler than the adjacent surfaces and this can lead to mold growth by causing condensation from the air to form on that surface which gives us another important reason to eliminate thermal bridges. A future article will deal with humidity levels, proper ventilation, and air quality being another key area of Passive House construction.