Impact of Low-Emissivity Glass on Site Design 

by Natasha Andjelic, PLA

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As landscape architects we bring our expertise to select appropriate planting and hardscape materials for the health and wellness of plants and people alike. Sometimes, however, even the most well thought out planting plan can be adversely impacted by unforeseen environmental factors, whether natural or man-made. One such factor is the solar reflection hotspots created by energy efficient low-emissivity glass (low-E). In some cases, the reflected light from low-E glass creates extreme temperature micro-climates.

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Low-Emissivity Glass Effects

The popularity of glass facades has exploded in recent years and spread globally as an expression of a new and transparent Green Architecture. Low-E glass is now mandated by new building and energy codes for new construction. The glass is coated with a thin layer of metal or metallic oxide that lets visible light pass in, hence providing thermal benefits to interior spaces, but it also reflects the ultraviolet light out. Thus efficiently keeping buildings and homes warmer in the winter and cooler in the summer.

 

Low-E glass, which typically has a green tint, reflects 30-50% of the sunlight’s energy compared to clear glass, which reflects only 10%. It is this characteristic of Low-E glass, that can at times reflect light beams measuring upwards of 200˚F. At those temperatures, vinyl siding on houses can melt, cars parked in the beam’s light can be damaged, and there have even been reports of people feeling a sensation of ‘burn’ from such reflections. At 140˚F and above, skin burn can occur. A Hotel in Las Vegas and an office building in London are two of the more high-profile examples of this issue.

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When the glass façade surrounds a planted courtyard, the reflected solar heat can raise the ambient temperature of the space creating a very specific microclimate. As designers, this can work to our benefit by extending the season of outdoor use in cold weather climates, but conversely it can also be a concern during high summer temperatures.

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Identifying Potential Hotspots

There are several methods for evaluating whether or not installed glass in new construction (or building renovation) may have an issue with reflected solar hot spots in the landscape. A number of modeling software including AutoCAD, Rhino and Energy Modeling Software/ Energy Plus, can assist in evaluating the areas of potential solar hot spots as well as the potential temperature the concentrated reflections might be emitting. This modeling is particularly important to review in curved or sloping building facades, as the curve can amplify and/or concentrate the collective solar reflections.

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If it is identified that the project might have an issue with solar hot spots, solutions for mitigating the

problem during design should be implemented. These include architectural interventions such as brise soleil or any vertical or horizontal feature that can break the continuous plane of a sloping or concave façade. Solar orientation of the building should also be assessed to minimize the potential for solar hot spots. These solar hot spots are typically most intense when the sun angle is low, (fall, winter and early spring).

 

On site, materials proposed that are in the direct path of a hot spot should be reviewed for temperature tolerance / melting points as specified by the manufacturer. This includes items such as bollards and light posts as well as irrigation heads and other plastic covers. Permanent or temporary shade structures such as trellises or umbrellas are also highly effective in providing user comfort.

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Sun Scalding

Plants, even cactuses in the desert, have a range of temperature extremes they can tolerate. Here in the Northeast, the maximum ambient temperature tolerated by trees is approximately 120˚F. If a tree is located within a reflected beam hot spot, it can develop sun scalding and damage the trunk, thus leaving the tree susceptible to infections and potential long-term structural issues. Sun scalding happens in nature as well and is usually the result of the freezing of bark cells when cells are awakened by high solar glare from snow and are unable to go dormant before the dropping temperatures of nightfall. Europeans have a long history of painting the young trunks white to mitigate the issue. Such an approach can be evaluated with consideration to aesthetic goals.
 

In addition, concentrated heat spots warm the earth and can impact root activity, spinning the trees into a continuous freeze/thaw cycle. This can result in cracked tree bark. The heat can alter the cycle of dormancy needed by most temperate plant species having a deleterious effect on the plant’s health.

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Tree Selection
The best option for the trees is to avoid planting them in solar hot spots. However, sometimes the need for shade and the benefits afforded by trees outweigh the risk of planting them in close proximity to the hot spots. At such time, tree selection can help sustain the long-term health of the trees. Compound leaf arrangements and small leaves create less transpiration and exposed surface that might be susceptible to scalding than large simple leaf trees such as oaks and maples. In addition,  specifying tree species able to tolerate extremes of temperature and water such as those often listed on recommended urban tree lists are suggested and include:

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• Metasequoia glyptostorboides / Dawn Redwood
• Taxodium distichum / Bald Cypress
• Styphnolobium japonicum / Japanese Pagoda Tree
• Gingko biloba / Gingko
• Ulmus parvifolia / Chinese Elm

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Other considerations for tree specification including specifying small caliper trees (2′′ typ.) rather than larger ones as they have a better chance of adapting to extreme growing conditions. In the long run, the young trees will typically grow quicker, bigger and healthier than their larger caliper counterparts. Soil mixtures and depths should also be carefully considered, with the recommendation to install up to 3’ of topsoil in urban courtyard areas where subsoil compaction and existing utilities compete with healthy root growth. Irrigation is recommended, as the added solar spots can cause the trees to transpire at an augmented rate. Irrigation should be concentrated on the roots to promote the natural cycle of transpiration and be activated with soil moisture sensors. Spraying the leaves with water in an effort to cool the trees is not recommended as water droplets on leaves can create light prisms that may burn the leaf surfaces.

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Other Plant Material
All proposed planting materials should be assessed for heat tolerance. Planting deep rooted grasses such as tall fescues or warm season grasses that go dormant when the solar hot spots might be most intense will perform better than a Kentucky blue grass lawn for instance. From a maintenance stand point, replacing a few ornamental grasses if required is also easier than replacing a patch of burned lawn.

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Conclusion
The increase in use of Low-E windows creates wonderful opportunities for large interior spaces that visually connect to the outside and nature. Understanding both the opportunities and constraints this material embodies has become part of the analysis  landscape architects perform to design for successful sites. Architects in collaboration with landscape architects can study the siting of both new buildings and exterior improvements with an understanding of how large expanses of southern-oriented Low-E glass can potentially impact adjacent open spaces and design for buildings and site that minimize the potential adverse solar radiation impact on the health and wellness of its occupants, as well as potential adverse impacts on neighboring flora and fauna.

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