Insulation and Heat Transfer: Understanding Your Home

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 are some scientific and practical applications.

Common types of insulation work to reduce the amount of heat transfer by trapping air. These include fiberglass, foam, and cellulose insulation. Your primary method of reducing heat transfer is to stop convection.

The practical side works like this. As you heat your home in the winter, you are constantly losing heat to the cooler outside air. Your thermostat notices a drop in temperature and your heating system works to keep up with the heat loss. As you cool your home in the summer, heat from outside is constantly being transferred inside your home. Your thermostat turns on the air conditioner whenever the heat transfer or heat gain in this case has exceeded the desired indoor temperature.

Common types of insulation, as mentioned above, work by slowing heat transfer between the inside and outside of your home. These heat differences also create air movements that work within your walls and attics to further speed heat transfer. If you look at the construction of these types of insulation, you’ll notice that they are designed to trap or stop airflow.

However, these types of insulation are not very effective in reducing radiant heat transfer. In fact, like most home building 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 requires a product that addresses this heat exchange.

Reflective insulation or radiant barriers use layers of foil, paper, or plastic to trap air, reducing convective heat transfer. However, the aluminum component is extremely effective in reducing radiant heat transfer. Metallized foil materials used in reflective insulation will reduce this transfer by up to 97%.

Addressing all of this, here is an example of heat transfer in a home. Let’s use a brick house. Bricks used as 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, the exterior of our brick home begins to heat up. Heat as described above will always want to transfer to cold. This means inside the house. As the bricks are heated above the interior temperature, heat transfer will begin. The bricks will heat the cladding through radiant and conductive heat (they touch). This heat will be transferred to the wall cavity. Now common types of insulation will retard this absorption of heat (hot air) and its movement in the wall. However, radiant heat, without a radiant barrier, will continue to pass unimpeded into the drywall and interior of your home.

Aluminum, the key component of radiant barriers, has a very low emittance, or emissivity, of 0.03. This means that the addition of a radiant barrier will reflect or radiate 97% of the radiant heat, thus keeping it out of the house.

Either way, once temperatures rise and radiant transfer increases, your home will get hotter. But by using the 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 comfort in the summer, and lower utility bills.

We hope that this brief explanation is useful and that you can better understand your home and the elements at work.