Insulation does not simply hold hot or cold air in place
For a house to function efficiently, we need three components to work well together: insulation; air movement; and moisture control. I have touched upon these topics recently, but quite by chance, I saw an answer to the insulation question in the title answered by an expert on a website who boiled his response down to one line. When speaking to a client, I discovered the same idea, so I thought a post about how insulation works may be a good idea.
First, a clarification is in order; something you may recall from science class. Do you remember learning about absolute zero? The concept seems to have nothing to do with our daily lives, so there would be no reason to engrave that lesson in your brain. At absolute zero, there is no energy for movement. Everything above that temperature has some degree of energy. You can think of that energy as being equal to heat. On a hot day, we have too much heat, and on a cold day, we have too little heat. You could imagine greater amounts of heat causing ping pong balls to move more quickly. These faster moving balls will force slower or stationary balls to move faster when they collide. This starts to spread the heat around an area.
If we wish to stop this transfer of heat, we need insulation. Insulation has a resistance to transferring heat, which we describe in terms of it having an R-value. The insulating material achieves this by two means: having trapped air pockets, which slow down the heat; and being made of a material which does not transmit/conduct the heat well. Insulation might not stop air flow, so that is why you may also have an air barrier on your home. If there are gaps in your insulation, you could significantly reduce the R-value. The open space will allow for the heat transfer. You also loose insulation effectiveness by thermal bridging. Consider the structure of your attic. You have wood framing members holding up the roof, which go down to wood framing running above your ceiling. As the wood below the roof heats up, the heat is transferred to the other attached pieces, eventually making its way into your home. In older homes with little insulation, you will see the joists (wood framing) above the ceiling when you go into the attic. Newer construction covers this framing to prevent some thermal bridging.
As a reminder, if you are considering insulation for your home, you also have to consider air flow (you need fresh air for quality) and moisture control (to prevent mold). I had the post about why over insulation could be bad, but I wanted this insulation post to be separate, after my experiences.