Attics. Advanced class in thermodynamics

Ya right, I don't even know what thermodynamics means. What I do know is heat transfer is what all this green home energy conservation stuff is all about. Trying to keep cool and warm where it belongs.

Thinking about it, your gas, oil, electricity use is all increased by various losses or waste of hotness or coolness. So what's important is how.

Heat transfers three ways, conductive, ie finger touches hot pan, convection, air above the pan rises and cooler air replaces it to be warmed etc..., and radiative...you feel the heat from the pan at a distance but the air in between is a normal temperature.

It's that last one that is really tricky. Hot surfaces "radiate" heat via invisible waves traveling at the speed of light. If you have ever sat under a restaurant heater you may have noticed it warmed you although the air was still cool. That's radiative heating. So why do we care?

Well all of those radiant barrier ads on the radio and in your mailbox are about stopping the radiative effect of the sun on your roof. They basically stop the radiating of the heat into the attic. They promise huge energy savings for us Texans. But before you run to the phone to get some...lets make sure we understand the challenge.

Your attic is full of stuff, not only your camping gear you never use but also the insulation, rafters, duct work etc. The radiative heat, heats all of this stuff, just like you at the restaurant.

The insulation has a couple of jobs. First it keeps convection from taking all the cool from your ceiling and sending it out the vents. It also slows the heat in the attic from soaking through your ceiling into your living space. (Conduction).

So the air is hot, and the stuff is hot, and as long as the sun is shining on the roof the more radiative and conductive heat will be transferred into your attic. A simple shade tree drastically reduces these two heating effects. But, for those of us in the sun, where we make a mistake is thinking that some air vents and insulation are all we need.

Radiant barrier basically reflects the invisible heat energy back. Then your attic is only heated by conduction and convection. Air moves in, comes in contact with the "under roof" surface that has been warmed by the sun, heats up and rises. This goes on all day and into the night. If you have enough insulation, that heat never really gets to your cool ceiling before it cools off for the night.

So, we could keep piling insulation up in the attic. The air above it and all the stuff would be super hot. That heat will slowly seep further and further into the insulation where it will reach the topside of your ceiling and begin to warm the air in the room. The more insulation I have the longer it will take to become saturated with heat and the slower the heat transfer will be. Or, we can vent the attic and take advantage of convection to keep removing the hot air and replacing it with cooler outdoor air. If we move enough air we can keep the attic reasonable in air temp.

Combine those two and you have a pretty good system for keeping a handle on conductive and convection heat, but we are still at the mercy of the radiant heat. Add the radiant barrier and you have really cooled your attic and given your insulation a chance to do its job.

Taking this a logical step further, there are new homes being constructed with the radiant barriers and insulation under the roof with a sealed up attic and no insulation between the attic and the living area. The idea is that the attic is kept cool by the transfer of heat through the ceiling and conditioning the air. For all of us, it doesn't make sense to do that on an existing home.

Bottom Line - Radiant heat needs to be addressed, especially if your duct work runs through that space. Insulation and venting are good, but not good enough to really make a difference in your cooling bill. But you also can't believe those claims of 30% savings. The pros will tell you its 8 -12%.

It's time to call some contractors and find out what this stuff costs...it's 95 degrees today. Using a simple ROI calculation of...what's the cost of the barrier vs saving 10% on my electricity bill in the summer months should be a good starting point. If that looks reasonable then we will do the harder calculation of 10% of just the air conditioning portion of the electric bill per month.