Adding extra insulation to your house walls is a waste of money, if you live where air conditioning bills are more than your winter heating bills.
Adding insulation in the walls of your home is not a very effective way to lower cooling energy use. Field tests have found that walls are only 5% to 10% of all the cooling loads in the average home.
About twice as much solar heat as gets into your house through the attic than through the walls. Investing money to lower attic temperatures makes much more sense for folks with high air conditioning bills.
Walls are not exposed to large amounts of solar radiation because so many things shade them (roof overhangs, landscaping, buildings, and awnings). Also, the summertime sun is higher in the sky, so most of the solar heat hits your roof not your walls.
The outdoor air temperature is the heat exposure for walls. During the summer, there is usually little temperature difference between the indoors and the outdoors. Typically, the difference is only 15 to 20 degrees.
Stick with the building code-recommended level of wall insulation (R-5 [block] to R-13 [frame]). Don’t spend money on more wall insulation. Use your resources to plant shrubs, trees, and other kinds of shading devices. Blocking the sunshine is more effective than more insulation.
If you live, where winter heating bills are as big as or bigger than your summer air conditioning bills, wall insulation is a priority. If the winter outdoor temperature averages 20 and you keep it 70 degrees in the house, there is a 50-degree difference.
Than much temperature difference, makes heat flow through the walls more expensive. More wall insulation is worth the cost to protect from very cold winter air.