View Full Version : These days, shade is our friend

07-21-2008, 10:14 PM
Put this in the "Helpful hints" file.

This afternoon, when the temp outside peaked at 99.9 degrees, I checked the thermometer tucked into the deep shade of the mulberry trees on the west side of my house. It read 88 degrees. That's nearly a 12 degree difference. Hoorah for vegetation!

If your house isn't nicely shaded in the summer, now's the time to plan for the fall planting of trees and shrubs. (Plant mulberries. They grow fast and yield the earliest fruit. Mulberry jam and cobbler are your friends too, or they want to be anyway.)

Pay special attention to windows. Even if you have the best double pane, argon filled, low e coated windows, they still only amount to about R-4 in insulating value (divide the U-value into 1 to get a rough approximation of the R-value).

Not counting our sun porch, I have 291 sq ft of windows. If those windows were unshaded, we would be pulling in nearly 4,000 BTUs per hour of heat into my house from the hot afternoon outside -- JUST from the windows! Shading knocks a thousand BTUs/hour off the heat gain. Shading plus R-20 insulated shutters for the window brings our heat gain down to 1700 BTUs/hr. The heat gain for our R-33 insulated walls is about 1000 BTUs hour, and about the same from our well-ventilated attic.

So even though windows are a much smaller area than the sq footage of walls and ceilings, as much heat gets in through them in the summer (and gets OUT in the winter) as the walls or the ceiling.

That suggests a need to pay more attention to windows.

It's important that windows be actually shaded. You still get heat coming through them, but you get less heat if they are shaded from the outside.

One value-priced window treatment that we used until our vegetation got sufficiently shady is to duct tape 2 or 3 auto sunshades together and hang them on the outside of the window. If that's too redneck, then hang a white roll-up shade over them to hide them from the neighbors.

While it takes a few years to grow a tree, climbing vines like morning glories can cover a window quickly. To make an easy trellis:

+ put eye hooks in the eaves,
+ thread string through them,
+ loop the string through a piece of lumber or pipe anchored to the ground below them.
+ Grow the morning glories up the strings (or other fast growing vines with lots of leaves).

To calculate heat loss or gain, use this formula:

Summer heat gain from the outside to the inside
Heat gain = (Ceiling area or wall or window area in sq feet) TIMES (Temp on the outside MINUS Temp inside) DIVIDED BY R value of ceiling or walls or windows

Reading over this, the formula looks a bit confusing to me. So let's try this:

For interior summer heat gain in the summer:

-- subtract the interior temperature from the exterior temperature,
-- divide this by the R value of the ceiling, wall, or window,
-- multiply the resulting number by the sq ft of the ceiling, wall, or window.

Heat gain = A X ((T1-T2)/R)

where A = area in sq feet
T1 = exterior temp
T2 = interior temp
R = R value of the wall, ceiling, or window

You can use this same formula to calculate heat loss in the winter, just reverse the outside/inside temps so you are subtracting the outside temp from the inside temp (temp inside MINUS temp outside).

You need a separate calculation for your windows, your walls, and your ceiling in a spread sheet, and substitute different R-values to determine how adding insulation to walls or ceilings, shade to the outside, and/or insulated shutters for your windows, ventilating your attic, etc., will impact the movement of heat -- to the interior in the summer, to the exterior in the winter. Remember: heat always moves towards cold.

Bob Waldrop, Oklahoma City

You can read more energy conservation tips at my website, which only offers free info, it doesn't sell anything -- Energy Conservation News and Resources (