Passive House stresses a comfortable environment with a steady temperature year round for its buildings. A very worthy goal, but how is that achieved when it can be below zero part of the year and above 100deg another part of the year. Doesn’t that get expensive?
First, let us examine the 3 ways that heat travels – Convection (air current), Conduction (one thing touching another), and Radiation (using the 2nd law of Thermodynamics where the temperature of items in an isolated environment equilibrate over time – i.e. heat seeks to warm up cold and in the process becomes less hot as cold warms up and becomes less cold.) I am sure all the cringing PHD’s reading these definitions have more to add, but hey – I’m not acting in a vacuum!
So, how does this relate to Passive House? If you notice the first way that heat travels, by convection, it is easy to understand that if we make the building as air tight as possible then heat cannot escape or gain entrance by air currents when we don’t want it to. We measure the air tightness of every Passive House building no matter how big or small, no matter what its function is. Every Passive House building must achieve .06ACH @ 50 Pascals.
The other two ways that heat transfers, conduction and radiation, can allow heat to exit through the structure in what is called a Thermal Bridge. It works just like a bridge you use to drive across a canyon or river – if it is not there, you don’t get across. Think about the warm air inside touching the sheetrock, which touches the two by four behind it, which touches the sheathing on the outside of your house. That can be a thermal bridge. A wall touching a concrete foundation, touching the bare ground can be another thermal bridge. There are many others, but you get the idea.
The final way that heat can be a transferred by thermal bridge is through radiation. This is an invisible thermal bridge, but one that is just as important. You can’t feel the radiation like a convective wind current, you can’t see it touching one thing and then another in a conductive way, but it is transferring heat through frequency waves of electromagnetic radiation in the infrared and visible spectrum.
An easy way to understand these waves is to think about radio waves. If you tune in a radio station and it comes in loud and clear, there is nothing blocking the radio waves that are being sent out. If you can’t tune in the radio station and hear static or other interference then those same radio waves are being blocked. Heat radiation works like radio wave radiation – it transmits easily when nothing blocks it. If you have a big picture window facing south with no overhang or shade, the room that it is in will warm up just through radiation. If you put some kind of shading mechanism on the outside so that those radiation waves are disrupted, you won’t get as much heat transfer. If you build the window with an idea that you want the films, gasses, glasses, and frame components to not transfer radiation easily, you can do that too and now the heat waves within the house are being disrupted from transmitting out through radiation as well.
There is a lot more to designing a house than one might originally think! Fortunately there are wonderful people who work hard to take all that science and put it in an easy to use tool. For 2D heat transfer analysis that tool is called THERM and is available as a free download from the fine folks at Lawrence Berkeley National Labs. You can get your copy here: http://windows.lbl.gov/software/therm/therm.html Be sure to get the User’s manual as well: http://windows.lbl.gov/software/therm/21a/Therm2.pdf
THERM allows you to enter the window or construction components of your project and model them to see how they will respond to the different temperature conditions that your project will face. By testing the areas most likely to be thermal bridges you can make changes to your construction BEFORE it is built, lived in, and develops a problem because of thermal bridging. You can design in thermal breaks where heat transfer is disrupted, add insulation, or use materials with different heat transfer properties. Heat loss is not the only issue to be concerned with – where heat is escaping, the internal portion of the structure that heat is escaping through can develop cool spots that will allow moisture to accumulate and then you have a potential for mold, rot, and rust.
THERM is a great program for detailing the thermal properties of your project. You can enter in different materials, change the thicknesses, run any number of calculations to determine how to best design your building before you start digging in the dirt. THERM is one of the great tools available to PH consultants and any designer that wants to optimize their project. By keeping the heat you want and blocking the heat you don’t Passive Houses can be designed to be comfortable all year around – even if the temperature swings from zero to 100deg during the year.