Heat Reflective Windows & Damage to Landscaping

An aspect of home improvment that may affect landscape gardens

by Mario Vaden

 

This advice is about heat reflective energy efficient window damage to landscaping. Damage to landscaping from energy efficient windows is neither common nor rare, and is worth keeping in mind. Even Portland landscaping in a mild climate can face this problem, especially with the warm dry summers.

This is not a difficult problem to identify. If anything is challenging about this kind of landscape damage, its that the problem is not well known, so it can be sneaky, so to speak - reminds me of the phrase in the movie Mr. Deeds - "you underestimate the sneakiness"

Step One: recognizing the origin of the damage. One improvement for house construction and remodeling that affects landscaping, are the windows with heat or light reflective characteristics.

These energy efficient windows present a challenge for landscape design and landscape maintenance, by causing lawn, shrubs and trees to receive much more sun energy than needed - enough energy to cause damage. The damage is fairly common, but can be very subtle, and many times, the cause is not recognized. Even professional landscape and tree professionals may not recognize this kind of damage, because many decades of horticulture happened without the presence of new style windows.

Newer house windows are designed with up to 2, maybe 3 panes of glass. A coating, or glazing, is placed on at least one of the pane surfaces. In addition to reflecting "radiant" heat, new windows are designed to reflect ultraviolet light rays back outdoors. With most of thes newer windows on a building, plants outside can receive much more radiant heat and ultraviolet light.

Which surface the glazing is on, depends on the manufacturer, or the climate where the building is located. If the glazing is on one surface, a lot of "heating" heat is salvaged and reflected back into the house in winter. If the glazing is on another surface, a lot of heat from the sun is reflected back outside, to keep the building cooler. In either case, heat-reflective glazing on any window pane surfaces will reflect some of the sun's heat back to the outdoors. Depending on the window and glazing, from 11% to almost 75% of the sun's radiant heat will be reflected back outdoors - to the landscaping area.

Here are some some practices that can reduce or prevent damage.

1. Increased duration of irrigation.
2. Increasing frequency of irrigation.
3. Placing trees between the path of the sun and windows.
4. Choosing plants that can tolerate a lot of warmth.
5. Locating "hardscaping" in yard areas with the highest reflected heat.
6. Changing the spacing and overlap of irrigation sprinkler heads.
7. Using awnings or exterior window shades.
8. A combination of suggestions #1 to #7.

Expounding on #5 - Placing a patio or deck in a hot area may not be desirable. Even if you chose this alternative, it would probably be more practical for construction of a new house. On the other hand, hardscaping need not be a patio, if you consider decorative gravel as hardscaping. There are styles of fine decorative gravel or sand paths or beds, that are raked for decorative effect. These areas can contain decorative boulders, step stones, garden art, sculpture, ponds, water bowls, and even tough plants like pines or junipers. These don't need to have an "oriental" look - it's versatile.

If you don't care for hardscape, and want plants or lawn in the zones with reflected heat, then options of modifying irrigation, using trees or wisely choosing plants, are still available.

The size of the heat reflection challenge will be determined by:

1. Whether or not the yard is already landscaped.
2. What plants already exist on the site.
3. How hot the climate is.
4. The type of windows.
5. How large the windows are.
6. How many windows there are.
7. The direction the windows face.
8. How many hours the sun directly "strikes" the windows.
9. Whether or not the zone with reflected heat has plants.

The amount of glass surface is a large factor. If a house has only one 2 foot by 2 foot bathroom window facing the west (sunset direction), it's doubtful that heat reflection damage will be noticeable. But if a house has three 6 foot by 8 foot windows facing the west, heat reflection will be significant at the least. If a house with large windows like that has large bedrooms above on the second story, heat-reflection is a bigger concern.

The zones of reflected heat for each home need to be identified, and that must be handled on an individual basis for each house. Buildings are positioned on properties differently from one another, and the size, shape and number of windows differ from house to house. Another factor to consider is the angle of the sun. The sun is "lower" in the winter and then higher in the sky in summer (north hemisphere). Due to the angle of heat and light reflecting from the sun, the zone of reflected heat on the ground will be closer to a building on the south side in summer, and farther from that south side in the winter.

The zone of reflected heat on the south side, will also move across the yard from west to east on any day of the year as the sun arches across the sky on the south, aiming the light at a different angle each hour of the day. So, on the south - any day of year, there is an elongated hot-zone. The whole zone is not hot at the same hour of the day. The heat moves from one side to the other during the day.

The west side is a bit different. The zone of reflected heat on the landscaping, or ground, will move during the day, but not exactly like the south side of the house. On the west side, the sun's rays won't even hit the window until 1 pm to 2 pm.

When the sun first strikes the west windows, when the sun is high, the heat will reflect more downward, at a slight angle, closer to the building. As the sun sets, the heat and light will come in from a different angle, and the zone of reflected heat on the ground will move out and away from the building. In other words, the "hot-spot" on the ground will start next to the building and creep westward away from the building as the sun sets, as long as nothing blocks the sun's energy.

All we need to do, is look out a window and visualize, or experience where the sun will be, or is. It's as easy as pointing with a finger at the sky to where the sun is, or can be. And whatever angle the sun's energy strikes the window from, it's energy will reflect back out in the opposite direction, at about the same angle. If you drew an imaginary line from the sun, to your window, and then from the window to where the reflected energy hits the ground - it's a V shape. And the imaginary V shape will change during the day as the sun moves. The width of the V shape will change as the sun moves.

This kind of heat damage has been one of the most noticeable increases of landscape problems, so that's why the warning and suggestions are here. Maybe you live in a private forest. If you do, then there is no heat-reflection challenge for you. If your house is exposed to the sun, and heat damage is a problem, this advice may prove useful.