How thermal mass impacts energy use in buildings.
Slides are here.
There are 3 thermal properties to know: thermal conductance, specific heat, density. The ASHRAE Handbook of Fundamentals has pages and pages listing these for all kinds of materials.
Specific heat: heat required to increase temperature by one degree. e.g. it takes a lot of energy to heat water. In buildings we usually rely on Volumetric Heat Capacity (density * specific heat).
We heat and cool buildings by moving air, but it has a very limited capacity due to the above. Water would have a lot of advantages. So why do we use air? Air is practical to move, we need to move it around anyways so we don't suffocate.
Thermal mass: TM = Volume * VHC. The ability to store heat while experiencing a temperature change.
Note that its much easier to store energy as heat in e.g. a wall than it is to store electric energy in e.g. a battery.
Thermal mass lowers the amplitude of temperature change. When we simulate a 1.6m thick concrete wall we can see the lag time of thermal mass. It takes a while to heat the wall before the flow reaches the inside of the building.
Fever curve: how many hours in the year am I above a certain temperature. Can subtract to find out how many hours are in the comfortable range.
WIPO Conference Hall: a very cool building, all in wood. From a thermal mass standpoint, its a "low mass building". We only need to turn on the HVAC system when there is a meeting (rarely), because it doesn't take very long to heat or cool.
Heat will "flow" from hot to cold. We are constantly in heat flow with everything else. Three forms: conduction, convection, radiation (basic but important to know how this applies to buildings).
Conduction is heat flow through molecular movement. Convection is heat flow through a fluid (gas or liquid). Radiation is transfer of electromagnetic energy from one body to another (e.g. when there's a vaccuum we can still see heat flow).
Thermal conductivity is ability of material to conduct heat. We typically consider it to be constant across a material. For our building envelope we generally want low thermal conductivity (product us from the outside conditions).
Note that standing air is a pretty good insulator, but once it starts moving there is convection, so we try to avoid air gaps in the envelope.
Conductance is conductivity over thickness.
The important thing to remember of all this is the R value and the U value. R is the resistance, and is the inverse of conductance, C.
When you buy insulation at home depot you might by an "R15" insulation, then if you have two of them together you create an R30 wall.
The U value summarizes the entire building envelope.
When designing a building you are given an R-value requirement.
Slides are here.
Phase Change Material: artifically introduce thermal mass into a building. Just as the building is getting too warm, it changes from solid to liquid. Small enough to go into e.g. drywall or paint. Can make drywall behave like 13cm concrete. Currently in early days. Popular where there are naturally ventilated buildings.
Thermal bridging: need to consider insulation and wood framing to calculate R-value for a real wall.
Some states have no energy code (90.1). Other states adapt various versions of the ASHRAE standard. California has its own that it maintains, that is above what ASHRAE mandates.
Canada has the Model Energy Code for Buildings.
It's possible to use straw bale as insulation.
Vaccuum insulation panels have an incredibly high r-value. The vaccuum suppresses all conduction.
Windows. Two qualities:
Visual light transmittance. A percentage. We want it high. The fraction of indicent radiation that reaches the interior.
Solar heat gain coefficient. Sounds similar but is different. The amount of solar radiation (not just visible anymore) that is transmitted by the window. In this case need to consider re-radiated infared radiation from the glazings.
Solar heat gain coefficient should be high or low? It depends. In e.g. Alaska you want all the solar gains, in the southern US you want to avoid them probably. When it comes to windows you can buy products that look the same but differ widely in these kinds of properties. Windows are expensive so you should know what you are buying.
Windows have a label from the National Fenestration Council (I've seen these). At this point, should be able to interpret most of this information.
Solar heat gain is always positive.
Skylights. Nanogel. Cool. Fill it between two glazings and it has great thermal properties.
20% of a window is the frame. This is important to consider.
In the 70s we had good music and bad windows. In the mid-90s we got superwindows.