Adding insulation reduces heat flow. That’s what insulation does. But how much does wall insulation impact the heat flowing into your house during the summer? And is adding wall insulation the most effective way to reduce your cooling bill?
Adding wall insulation to save on cooling costs seems to make sense, since slowing heat movement and reducing the need for cooling is our goal. In northern climates, if you suggested that insulation wasn’t the best way to retain heat and reduce your heating bill, we would laugh right in your face and maybe throw a coffee at you. And we don’t sacrifice our coffees lightly. So why does what works in the North not work quite the same in the South?
The differences stem from the way that heat moves and the relative importance of solar radiation. In the North, heat loss through solid surfaces is responsible for the largest component of your heating bill. And in the South, when most homes need air conditioning, a well-insulated house is definitely easier to cool than an uninsulated house. But the heat radiating from wall surfaces is responsible for a smaller portion of the air conditioner’s cooling load than the heat flowing from your hot attic and the heat radiating through your windows.
A brief explanation of how heat moves
Heat moves by radiation, conduction, and convection. The most common example of radiation is the energy that comes from that big ball of thermonuclear plasma in the sky. The sun’s warmth on your skin is solar radiation.
Conduction is heat movement between any two adjacent materials, moving from hot to cold. Remember that time you grabbed a hot frying pan handle? The heat transfer mechanism (and the cause of the burn) was conduction.
The third way that heat moves is by convection, which is heat that is transported via air or a liquid. If you open a window in the dead of winter, the main heat transfer mechanism is convection.
The climates are also different
In Georgia, where air conditioners attempt to maintain the indoor temperature at around 75°F, the hottest temperature of the year may be 110°F. In Maine, our heating systems maintain an indoor temperature of 70°F in the face of -10°F outdoor temperatures on the coldest nights. The rate of conductive heat loss (heat energy moving across a wall) is driven by the difference in temperature between the indoors and the outdoors (the delta T), and this delta T is much higher on average in heating climates than in cooling climates. Meaning: if you live in the North, wall insulation is more important to retain heat than it is to prevent heat intrusion if you live in the South.
One exception to this rule concerns attic insulation in the South. The summer sun traverses the sky at a very high angle, beating down on house roofs. Attic spaces can get dramatically hotter at these times, approaching 140°F or 150°F. This can be controlled to an extent with attic venting, but the delta T will remain substantial.
So in the South, high levels of attic insulation are important; wall and slab insulation … not so much, at least when it comes to cooling.
Heat gain: Where does it come from?
So where are you gaining all this AC-straining heat from? Well, your external heat loads include solar heat gain through windows, air infiltration, and heat transmission across surfaces.
Internal heat loads come from the heat emitted by equipment (computer monitors, stoves, heating appliances, televisions), heat from light bulbs, and heat from people and pets. (Whew.) And we’re not even touching the topics of sensible vs. latent heat, time delays and thermal storage, solar angles, surface opacity, etc. There are a lot of factors.
That’s complicated — let’s keep it simple
The biggest factor explaining the differences between winter heating and summer cooling is the sun.
In northern heating climates (at least mine), we may have 5 to 6 months of heating compared to a few weeks of cooling. Adding radiant solar heat streaming through the windows helps lighten the heating system’s BTU load.
In southern cooling climates, the inverse happens. The sun’s radiation pours through the windows but fights against the air conditioner, adding BTUs and increasing the cooling load.
What are the solutions? Minimize solar heat gain through windows whenever possible. This can be done by eliminating west-facing windows, adding shades or awnings, planting shade trees, or buying or building a house with wide roof overhangs. (I’m sure there will be a ton of other great suggestions in the comments).
Adding insulation definitely lowers air-conditioning bills. However, walls are not generally as problematic as attics or windows, both of which allow in more heat.