Designing for Thermal Comfort and Cooling
Contributor
Fall 2024
It’s been another sweltering summer in cities across California.
From San Diego and Los Angeles to Santa Clara and Sacramento, gardeners found themselves drenched with sweat. Again.
Even without the impacts of global warming, cities can be up to 22°F (12°C) warmer than outlying rural and natural areas. This is due to the abundance of heat-absorbing surfaces, such as asphalt and concrete; the lack of cooling vegetation, such as big trees and vigorous groundcovers; and the amount of heat generating sources, such as air conditioners and cars (Labs and Watson 1983; Environmental Protection Agency 2014).
Combine these heating attributes with global warming and you get a recipe for public and ecological health calamities.
Urban warming is linked to the formation of harmful air pollutants, like low-level ozone, a known respiratory reducer. It increases the chances of heat-related illnesses, such as heat exhaustion. It increases the temperature of water runoff, contributing to the degradation of waterbodies. It also alters growing seasons, affecting urban, rural, and natural landscapes and ecologies.
Gardeners, designers, and landscape designers can provide an antidote.
When shading and evapotranspiration are combined, an urban landscape will drop summertime temperatures by up to 9°F (5°C). When an entire community is shrouded in trees, it will be, on average, 6°F (3°C) cooler than tree-less communities. When those trees are combined with vigorous groundcovers, the temperature difference between the vegetation and asphalt can be as much as 25°F (14°C) (McPherson and Muchnick 2005; South Carolina Energy Office n.d.).
Cooling Strategies
This diagram illustrates six cooling strategies available to gardeners and landscape designers. These recommendations are ranked according to effectiveness per dollar spent. However, effectiveness is site-specific. Humid areas, for example, do not benefit from evaporative processes.
A. Decrease Solar Exposure
Decreasing the amount of solar radiation a landscape receives and stores involves intercepting the sun. Since the hottest time of day is when the sun is high in the sky, intercepting devices must be placed close to a structure or outdoor living areas to be effective. Shading devices—structures and trees—are mostly used on the south and west sides of a building.
Structures
Shading structures include arbors, awnings, canopies, gazebos, pavilions, pergolas, sails, solar panels, trellises, and umbrellas. Fabrics and wood are the favored materials for cooling.
Trees
Some trees do a better job of shading than others. Some species may cast a larger or a darker shadow. Below are the trees that typically have a large and dark shadow.
Tables:
Unfortunately, there has been little quantifiable research on the amount of shading each tree produces. The reason is not because of a lack of interest, but because there is too much climatic and environmental variation, both of which affect a tree’s growth and their amount of shade (Urban Forest Ecosystem Institute 2024). California bay (Umbellularia californica) is a great example. Along the coast it casts a huge shadow, but inland its shadow is sparse and porous.
Pro Tip: Pruning for Shade
Pruning plays an important role in influencing the amount and degree of shade that a tree casts. For example, limbing up and removing the interior branches encourages a tree to grow with great reach, but it becomes airy and its shadow light. On the other hand, tip pruning and removing the outer gangly branches encourages more compact growth, which reduces its reach, but increases its density and the darkness of its shadow. Pruning for shade is typically best done in late fall so a tree has five months to recover.
B. Increase Air Flow
Wind has a fantastic cooling effect and can drop the perceived summertime temperatures by as much as 8°F (4°C). Wind wisps warm air away from objects. Naturally, the greater the temperature difference between the wind and object, the greater the cooling. Wind also speeds evaporative processes, which is why people feel cooler in a breeze (our skin generally has a higher moisture content than the air).
Increasing air circulation is also great for landscape health. Pests, such as diseases and insects, are less of a problem in a well-ventilated landscape. Having an afternoon breeze run through a garden is also good for public health if near industrial areas, roads, or freeways.
If the landscape is large, trees and walls can guide wind to living areas. But more likely than not, increasing airflow often involves removing obstacles, not increasing them. Two techniques encourage airflow.
Guiding
Whether with appendages on buildings, overhead fabric, well-positioned hedges, or walls near structures, wind can be guided into and out of areas and buildings. Wind behaves much like water—but it’s more fickle.
Funneling/Propelling
Squeezing wind through a constricted opening will increase its velocity, which increases cooling. Compression can be a product of architecture, vegetation, or walls. While wind velocity increases at the compression point, it dramatically decreases on the leeside of the feature.
Pro Tip: When to Block
There are three situations where wind should no longer be guided into a landscape, but actively blocked.
Too Hot
The cooling effect of wind stops working above 95°F (35°C). After that, wind starts adding to the heat index and feelings of discomfort. Wind should be blocked if temperatures regularly exceed 95°F.
Too Cold
The wind chill effect becomes more pronounced as temperatures drop. The cooler it gets, the more wind contributes to heat loss and discomfort. Wind should be blocked if temperatures are regularly under 76°F (24°C).
Too Intense
The faster the airflow, the greater its ability to move lung-damaging heavy particulates. Perennial or persistent winds over 15 mph (24 kph) should be blocked and dulled. Good examples include the easterly Santa Ana and Diablo winds from late fall through early spring.
C. Increase Evaporation
Evaporation can create a large cooling effect. Naturally, the drier the environment, the better evaporative cooling works. Humid environments do not benefit from evaporative processes.
There are three sources for evaporative moisture: plants, open sources of water, and aerosolized water.
Plants
Evaporative processes favor the fast growing and big-leafed trees. Some of the best are avocado (Persea), banana (Musa), birch (Betula), catalpa (Catalpa), cottonwood (Populus), deciduous oaks (Quercus spp.), empress tree (Paulownia tomentosa), fig (Ficus), magnolia (Magnolia), maple (Acer), hoja santa (Piper auritum), mulberry (Morus), redwood (Sequoioideae), sapote (Casimiroa edulis), tuliptree (Liriodendron tulipifera), and weeping willow (Salix babylonica and hybrids).
Open Water
Open sources of water include fountains, ponds, and shallow pools.
Aerosolizing Water
Fountains that splash a lot of water and ponds with water jets—even if small—will increase evaporative cooling. Another effective device is a mister attached to a garden hose.
Pro Tip: Within Reach
Putting water within reach of four of our five senses is essential. Having water within reach will not only help cool an area, but it will also have a beneficial impact on our feelings of well-being. Water is visual and dynamic, it has a distinct smell, it feels familiar and safe, and its sound is predictable yet a bit erratic—all of which makes us feel better about the space we occupy.
D. Increase Heat-Shedding Materials
Choosing materials for cooling means managing reflectance and density.
Reflectance
Reflecting solar radiation back into the atmosphere is an effective method for reducing surface temperatures. Light colors cool down surfaces like driveways, walkways, and walls.
However, caution is needed. Walking surfaces that are too bright causes excessive reflection, which can cause discomfort and anxiety. Earth-toned materials diffuse sunlight and are ideal.
Some of the best solar reflecting materials
Density
Low density means less weight, mass, and heat storage. The heat-absorbing surfaces in a landscape are the driveways, patios, retaining walls, roads, structures, and walkways. Some of the materials that can be used instead of asphalt and concrete include bricks, decomposed granite (DG), fly-ash concrete, papercrete, pavers, porous asphalt, porous concrete, turf blocks, and any wood product.
Pro Tip: Wood Rules
Of all the materials one can use in a landscape, wood is an outlier. It has two qualities that make it distinct from all other materials—both of which make wood the perfect material for managing thermal comfort.
Low reflectance
Wood products have low reflectance, which means they do not disperse a large amount of the solar radiation they receive back into the environment. Radiance ranges from just 5 percent to 48 percent. Wood greatly dampens light and radiation.
Poor conductivity
Wood is a terrible conductor of radiation, which is a good thing, as it will not store radiation as heat. Wood-wrapped buildings and wooden benches will reduce discomfort on a hot day.
E. Increase Earth Cooling
The temperature four to five feet (1.2 to 1.5 meters) below the surface is usually much cooler than the air temperatures during summer. Temperatures vary across the nation, ranging from 55°F (13°C) in the north to as high as 74°F (23°C) in the south. When it is 90°F (32°C) outside, even a floor at 74°F feels fantastic. Below are three common methods to increase earth cooling.
Subgrade
Subgrade construction means putting a structure underground. The structure may be partially or wholly underground. Despite being the most effective method for passively regulating the temperatures of a building, it is not widely used. Building down into the earth is far more expensive than building on top.
Berm
A berm is a compacted pile of earth that is usually vegetated. Berms are built up against structures to increase thermal protection. They are much less expensive than sub-grade construction. Mounded soil can also be used to create outdoor rooms.
Slab On Grade
The cooling of concrete and other hard surfaces is accelerated by placing the dense surface directly on grade (highly compacted soil). Typical construction calls for concrete to be poured on a bed of sand or gravelwhich adds a layer of insulation from the cooler temperatures of the soil below. Concrete slab on grade, however, is only used for low impact surfaces, like driveways, walkways and light-duty parking lots. Putting bricks and pavers directly on grade and in contact with the soil underneath them is also much less resource- and time-consuming than placing them on sand.
Pro Tip: Partnership With Stormwater
Cooling with earth dovetails well with stormwater management. Both demand changes in contour. Creating depressions increases opportunities for infiltration, and building up soil increases opportunities for cooling by buffering wind and increasing the amount of surface area available for planting.
F. Insulate Surfaces
Berms, green roofs, living walls, and vines are all excellent at protecting structures from heat gain and loss. These techniques work because of shading, thermal mass, and evapotranspiration.
Pro Tip: Expect Expense
Living surfaces can feed pollinators, grow food, clean and slow runoff, provide recreation, and increase people’s feelings of wellbeing, but they come with two caveats. First, living surfaces are expensive. They increase constructing and maintenance costs (Formeller 2010). Second, irrigating a feature attached to a building can moisten the building, reducing its longevity and the health of its occupants. A dry building is a healthy building.
This article was sponsored by:
Resources
American Concrete Pavement Association. 2002. “Albedo: A Measure of Pavement Surface Reflectance.” R&T Update 3 (05). [pdf]
Brenzel, Kathleen Norris. 2001. Sunset Western Garden Book. 7th ed. Menlo Park, California: Oxmoor House.
Environmental Protection Agency. 2014. Keeping Your Cool: How Communities Can Reduce to Heat Island Effect. Publication No. 430F14041.
Formeller, Ieszic. 2010. “A Healing Home: The Application of Therapeutic Landscape Design Theory to the Residential Setting.” Master’s thesis, California State Polytechnic University, Pomona.
Kourik, Robert. 1986. Designing and Maintaining Your Edible Landscape Naturally. Santa Rosa: Metamorphic Press.
Labs, Kenneth and Donald Watson. 1983. Climatic Building Design: Energy-Efficient Building Principles and Practice. New York: McGraw-Hill Book Company.
McPherson, E.G. and J. Muchnick. 2005. “Effects of street tree shade on asphalt concrete pavement performance.” Journal of Arboriculture 31 (6): 303–310.
Roth, Fred, Leon Boroditsky, Emina Darakjy, David Roger, and Nancy Sappington, Nancy. 2018. Street Trees Recommended for Southern California, 2nd. ed. Anaheim: Street Tree Seminar, Inc.
South Carolina Energy Office. “Landscaping for Energy Efficiency.” Energy Briefs. [pdf]
Spalding, George H. 1971. “Some Outstanding Shade Trees for Southern California.” Lasca Leaves 21: 64–70.
Urban Forest Ecosystems Institute. 2024. “SelecTree: A Tree Selection Guide.” California Polytechnic State University, San Luis Obispo.
Share:
Facebook
Twitter
LinkedIn
Responses