Homeowner’s Energy Workbook Part 5

Radiant Heat Movement

Radiant Heat Movement

So conductive heat movement is heat moving molecule by molecule for objects that are touching.  Convective heat movement is heat moving by air or water flows.  Radiant heat movement is heat moving by electromagnetic waves.

The sun radiates its energy down to us on the surface of the earth.  When you stand in the sun on a calm, cold winter day, you can feel the radiant warmth from the sun.  Back into the shade and it feels like the temperature just dropped ten degrees!  Stand facing an open fire and feel the radiant warmth.  Turn around and feel that warmth on your back.  If the radiant wave path is blocked, it won’t reach the surface.

Radiant Barrier

Radiant Barrier

I have an odd arrangement in my house with a fairly powerful wood-stove about three feet from the door of an upright freezer.  It was the only way I could get the freezer into the kitchen.  Definitely not an ideal arrangement.  Anyway I have a shade of aluminized Mylar that I pull down when we have a fire going in the wood-stove.  The radiant heat is reflected away from the freezer and the surface of the door stays cools.  Sometimes builders lay reflective material on the top of the insulation in the attic.  This will reflect the radiant heat from the roof getting into the top of the insulation – for a while, until the shiny surface gets covered in dirt and doesn’t reflect any more.

But this is about the meaning of terms so we want to reserve more insulation discussion for later on.  We do need to look at R value and U value, however.  R value is a measurement of the ability of a material to Resist the conductive flow of heat.  The higher the R value the better the insulator.  Since different materials are . . . different in their ability to resist the flow of heat.  Since we know that to reduce the conductive flow of heat to cool depends on the thickness, the number of molecular layers the heat has to move through, the R value of a material is commonly referenced to the thickness of the material, like the R value per inch.

Well if a material can Resist the flow of heat, how well does it conduct the flow of heat?  That is the opposite of the R value and is known as the ‘U’ value.  Why not the ‘C’ value, you might ask?  Because the ‘C’ has been reserved for something else entirely like Centigrade or Celsius, and we wouldn’t want to get confused.  So if a higher R value is a better insulator, a lower U value is also a better insulator.  In fact, if you want to change an R value to a U value, you take the inverse of it or put 1 over it.  An R of 5 is the same as a U of 0.2.  So if someone tries to sell you new windows with twice the U value, you would be getting a very bad deal indeed!

An R value is nice to work with if you are buying insulation in a lumber yard, but what we really want to know is how much heat the house loses on a cold winter’s day or gains in the heat of the summer.  And that, my friend, is the total U value of your house, and that is one of the key things we’re going to figure out.  Every surface of your house that protects you from the outside weather has a U value; every window, every door, every wall, every ceiling, every floor.  We can also squeeze a U value out of the convective losses and add that to the total.  So keep this R value/U value thing in your mind.  It’s going to be important.

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