Embodied Energy

Thermostats in our homes measure air temperature, and so we generally think of the temperature of the house in terms of its air temperature. But there’s more to it than that. We rarely think of the temperature of all the other stuff in your house. It makes a bigger difference than most people give it credit for.

Ever notice that when it’s been cold outside for a few days, and suddenly it warms up to what you’d consider a really hot day? But you don’t really need your air conditioner?

I still need to crunch the numbers a bit – but over the summer I noticed that the factors I thought would be important for predicting how hot my house would get or how much electricity I’d need to cool my house.

I was using these factors:
Outdoor High Temperature
Outdoor Low Temperature
Indoor Morning Temperature
Amount of Cloud Cover

What I found was that there was an additional factor – the average temperature my house had been for the past few days.

It takes a few days for all the materials of my house to settle on a temperature. The air temperature, in contrast, can fluctuate within hours.

So I got to thinking why that might be.

I’ll give two scenarios here:

1) Your house’s air temp has been 65 degrees all afternoon. In the evening you turn on the heater and your air temp goes up to 70 after an hour.

2) Your house’s air temp has been 70 degrees all day, with the heat on the whole time.

Do you think that if you lay down in bed – will you be cooler, warmer or equally temperate in the 2nd example? I’d guess you’d be warmer in the second example. After all, the bed itself is 5 degrees warmer to start with in the 2nd example. The bed doesn’t warm up right away when you turn the heat on – because the thermostat turns off the heater as soon as the air gets to 70 degrees.

Another thing to consider is radiation heat. If the ceiling of your house is 85 degrees (which can happen in many houses on a sunny day), then the energy of the ceiling’s radiation will warm you up more so than if it had been cloudy, and the ceiling was 77 degrees, even if the air temperature was 77 in both cases.

So, two houses at the same temperature reading may not feel the same, depending on the embodied heat of all the stuff in the house.

Like anything – embodied energy can help and hurt you. It’ll probably help you if you’re aware of how to take advantage of it. If you ignore it, it’ll work against you.

One design where embodied heat helps you is with a Tromble Wall http://en.wikipedia.org/wiki/Trombe_wall , which is essentially a material with a lot of mass that is placed in the sun’s rays. In cool weather will absorb lots of heat during the day and radiate that into the structure it’s attached to during the night.

The Spanish have used this concept widely in their architecture. Big thick walls lined with white material act as a temperature moderator. In the hot daytime, they gradually absorb energy, at night when it gets cooler, they give off most of that energy. The next morning, they’re cool again, and get warmed up by the sun. This lets you have a comfortable temperature all day inside because the thick walls are always the temperature that you want them to be, as opposed to the air temperature outside, which is the opposite of the ideal temp.

It works best in an area with wider temperature swings. In the Southern US, where summer temperatures are high day and night – different solutions may be more applicable – but the concept of embodied energy is still worth having in the back of your mind the next time you go over to the thermostat to change the setting.