Insulation & Understanding Heat Transfer in the Home
This entry was posted on June 8, 2011.
First, it is important to remember these two points. Heat transfer occurs in three ways; conduction, convection, and radiation. And, in all three methods of heat transfer, warmer objects always lose heat to cooler objects. Now here is some science and practical applications.
Common insulation types work to reduce the amount of heat transfer by trapping air. These include fiberglass, foam, and cellulose insulation. Their primary method of reducing heat transfer is through stopping convection.
The practical side works like this. As you heat your home in the winter, you are constantly losing heat to the colder outside air. Your thermostat notices a drop in temperature and your heating system runs to keep up with the heat loss. As you cool your home in the summer, the outdoor heat constantly transfers to inside your home. Your thermostat turns on the air conditioner whenever the heat transfer or heat gain in this case has surpassed your desired indoor temperature.
Common insulation types as mentioned above work by slowing the transfer of heat between the inside and outside of your home. These heat differences also create air movements that work inside your walls and attics to further accelerate heat transfer. If you look at the construction of these insulation types, you will notice they are designed to trap or stop the flow of air.
These insulation types are not very effective in reducing the transfer of radiant heat however. In fact, like most home construction materials, they have a very high rate of radiant heat transfer. Because radiant heat is often the primary mode of heat transfer in buildings, this calls for a product that addresses this heat exchange.
Reflective insulation or radiant barriers use layers of aluminum, paper, or plastic to trap air, reducing convective heat transfer. The aluminum component however is extremely effective in reducing radiant heat transfer. The metalized foil materials used in reflective insulation will reduce this transfer by up to 97%.
In addressing all of this, here's an example of heat transfer in a home. Let's use a brick home. Bricks used for building materials have a high emittance rate, around 0.93. Emittance is the ability of a material's surface to emit radiant energy.
As the sun rises in the summer our brick home's exterior starts to warm up. The heat as described before will always want to transfer to cold. This means the home's interior. As the bricks warm up above the inside temperature, heat transfer will start. The bricks will warm the sheathing through radiant and conductive (they are touching) heat. This heat will transfer into the wall cavity. Now, the common insulation types will slow this absorption of heat (warm air) and its movement in the wall. The radiant heat however, without a radiant barrier will continue to pass through unimpeded into the drywall and interior of your home.
Aluminum, the key component in radiant barriers has a very low emittance, or emissivity at 0.03. This means the addition of a radiant barrier will reflect or radiate 97% of radiant heat, thereby keeping it outside the home.
Either way, once temperatures rise and radiant transfer rises, your home will warm up. But in using correct materials or combination of materials you can control the rate at which this will happen. This means less heat in the winter, less air conditioning and more comfortability in the summer, and lower utility bills.
Hopefully this brief explanation is helpful and you can better understand your home and the elements at work.