Design for Extreme Heat

Description

This guide is intended to provide basic concepts and strategies for designing homes to be safer during extreme heat events (Figure 1).

 

Buildings can be designed to provide protection during extreme heat events
Figure 1. Buildings can be designed to provide protection during extreme heat events (Source: University of Arizona 2021).

 

Design Considerations

When designing homes for extreme heat, the overarching goal is simply to keep the building occupants cool enough to avoid health risks. Consider the following when designing a house to be resistant to extreme heat:

  • A primary approach to keeping occupants cool is to keep the interior environment of the building cool. The first step is to minimize heat gain to the house (for example shading).
  • Consider strategies which help keep a person’s body cool whether or not the building is kept cool (for example ceiling fans). These are particularly useful for houses without central A/C, or for times when a home’s air-conditioning has failed, lost power, or simply cannot keep up with extreme conditions.
  • Consider the possibility of power loss through blackouts or brownouts during heat waves. Backup power systems, passive cooling techniques, and simply designing a home to minimize heat gain can help address loss of air-conditioning due to a power outage.
  • Rather than designing the entire house for extreme heat, consider designing a designated “cool room” or cool zone within the house. This approach can save cost, may be the most appropriate approach for a retrofit situation or for cooler climates, and may be the most effective way to ride out a power outage.
  • Consider the specific needs of the most at-risk populations during extreme heat such as people who live alone, are elderly, are very young, have existing respiratory issues, have mobility limitations, or have other physical or cognitive impairments. Ensuring that residents with limitations have access to reliable cooling, good air quality, and dependable communication is one of the most important considerations in preventing hospitalizations or fatalities during a heat wave.

Design Approach

An overall three-prong approach to designing or retrofitting a home for extreme heat is suggested. This approach can apply to all homes, whether new or existing, air-conditioned or not, and regardless of climate. The suggested approach is as follows:

  1. Minimize heat gain to the home.
  2. Provide emergency cooling.
  3. Provide a backup power source with enough capacity to supply emergency cooling.

Minimizing Heat Gain

The first step in designing for extreme heat is to minimize heat gain in the building or a designated cool zone. The primary sources of heat gain to a house are solar radiation, hot outside air, thermal radiation from nearby surfaces, internal equipment, and body heat from the occupants themselves.

Solar radiation is usually the most significant heat source affecting a house. Its primary entry point is directly through windows and skylights. It will also heat up roofs and walls, driving heat into the house. During summer, the sun shines strongest on the roof and on the east and west sides of a house. Shading or reflecting sunlight from these areas is one of the most effective strategies for reducing heat gain. This can be done via landscaping, roof overhangs, window overhangs, awnings, shutters, blinds, screens, porches and other architectural features, low-SHGC windows or storm windows, and cool or light-colored roof and wall finishes (Figure 2).

Hot outside air can enter a house directly by infiltration through cracks and gaps. These gaps may be around doors, windows, wall and floor penetrations, and vulnerable points in the structure such as where the rim joist meets the foundation sill plate or where the bottom plate of an exterior wall meets the subfloor. Infiltration also brings in humidity, which can reduce the body’s ability to cool itself and adds load to the air-conditioning system. Ventilation air (fresh air) which is brought in through the HVAC system can cause more heat and humidity gain than infiltration. Air sealing, energy recovery ventilation (ERV), and ensuring the home does not get more ventilation air than it needs are the most effective ways to reduce heat gain from outside air.

Thermal radiation from surfaces and objects near the house can be a source of heat gain if those surfaces are exposed to direct sunlight. Nearby buildings and pavement are generally the biggest sources of external thermal radiation. Trees, bushes, and other vegetation are not large contributors to this effect because the surfaces of plants stay cooler due to evaporation. Inside a vented attic, thermal radiation from the underside of the roof deck can cause significant heat gain to HVAC ductwork or to the attic floor. Green landscaping, water-permeable surfaces, light-colored pavements, and attic radiant barriers are effective ways to reduce heat gain from thermal radiation.

Internal heat sources such as kitchen equipment, lights, appliances, and the occupants themselves can be a very significant heat source. Heat and humidity from showers, baths, and cooking can be major contributors. If cooking is avoided, and bathroom heat and humidity are exhausted properly, heat gain from internal sources can be greatly reduced.

Note that some strategies for minimizing heat gain in the summer (e.g., light wall and roof colors; low-SHGC windows) will also increase the need for heat in the winter. In cooler climates, such strategies should be carefully weighed against wintertime effects.

 

Shade trees planted on the east or west sides of a house are one of the most effective measures that can be taken to reduce heat gains
Figure 2. Shade trees planted on the east or west sides of a house are one of the most effective measures that can be taken to reduce heat gains (Source: Purdue University 2018).

 

Strategies for Minimizing Heat Gain

      Windows and Skylights

      Attics and Roofs

      Walls

      Foundations

      Site

      Systems and Appliances

Emergency Cooling

Emergency cooling can come in many forms. The requirements for emergency cooling are simply that it can keep occupants cool enough to keep them safe during extreme heat, that it is reliable, and that it has a small enough power draw that it can be comfortably powered by the planned backup power system. Emergency cooling can come in the form of air-conditioning, evaporative cooling, refrigeration, or even passive cooling strategies.

The emergency cooling system could be very small or very large. Emergency cooling does not necessarily mean an additional system or unit to the basic HVAC that is planned for the house. As long as the base HVAC system for the house is reliable, well-maintained, and has sufficient backup power, it can be considered the emergency cooling system.

Mechanical air-conditioning is generally the most practical defense against extreme heat. Typical air-conditioning systems are designed for standard local conditions and may not have the full capacity needed during an extreme heat event. A designer may be tempted to oversize the system based on possible heat wave temperatures. Oversizing is not recommended, however, due to side-effects such as poor humidity control, short-cycling, reduced equipment life, a noisier system, larger space requirements, reduced efficiency, higher operating costs, and higher upfront costs.

Instead, right-size the equipment according to published local design temperatures. Although it may not have the capacity to meet the increased demand during an extreme heat event, a right-sized or even undersized air-conditioning system will still most likely provide enough cooling to keep inside temperatures well below dangerous levels. Using shades, choosing not to cook, turning on fans, and moving activities to the coolest areas of the house can make up for shortcomings in air-conditioner capacity.

Beyond selection of air-conditioning equipment, much can be done to improve system performance. Ensure new ductwork is well sealed. For existing ductwork, hire a home energy rater or other professional to test the air tightness of duct work and identify areas that can be sealed. Hire an HVAC contractor to do the sealing and perform equipment service tasks such as checking the refrigerant charge, clearing condensate lines, and cleaning coils.

Many existing houses and some new houses do not have air-conditioning – especially in cooler regions. If installing a whole-house A/C system is too costly, mini-splits, window units, or portable air-conditioning units can provide cooling to a single room or area of a house. These types of smaller systems can also be planned as lower-energy backup cooling for houses with backup power as a way to reduce load on the generator or battery bank during a power outage (Figure 3).

 

A mini-split air-conditioning system can be a highly effective low-energy approach to provide cooling to one designated zone in the house
Figure 3. A mini-split air-conditioning system can be a highly effective low-energy approach to provide cooling to one designated zone in the house (Source: MN Department of Labor and Industry 2020).

 

Ground-source heat pumps (GSHPs) are less vulnerable to extreme heat events than standard air-conditioners because they rely on stable ground temperatures instead of fluctuating air temperatures. These types of systems can be expected to provide more cooling at a higher efficiency than comparable air-source systems under heat wave conditions. These systems require a site evaluation during the design process.

Evaporative cooling systems can be very effective in dry climates. Other low-energy and passive systems such as wind- or fan-driven comfort ventilation, exhaust cooling, or night flush, as well as earth-coupling strategies, can provide viable cooling in many climates or can supplement other strategies (Figure 4). Some of these strategies are particularly useful because they require no power at all.

 

ENERGY STAR-rated ceiling fans save energy when operating and provide cooling movement to reduce the need for coolant-based air conditioning.
Figure 4. Ceiling fans are a simple and effective way to provide occupant cooling in any climate (Source: Courtesy of Tim O’Brien Homes).

 

Refrigeration can also be a part of an emergency cooling plan. Ice and cold water are highly effective at helping to lower a person’s body temperature.

Thermal mass within the house can moderate temperature fluctuations by absorbing heat during the day and releasing it at night. This evens out the load on the HVAC system, reducing peak cooling needs but increasing the load at night. The overall result is generally beneficial for both energy consumption and peak demand.

Strategies for Emergency Cooling

Mechanical Air-Conditioning

Evaporative Cooling

Refrigeration

 Earth Coupling

 Heat Pump Water Heaters

Airflow and Night Flushing

Backup Power Systems

Extreme heat events tax the power grid and can cause blackouts or brownouts throughout the heat wave area. Incorporating a backup power system such as a generator or batteries can help guard against this risk (Figure 5). Reducing energy consumption and demand when on backup power will allow a smaller system to be installed and will lengthen the amount of time the system can provide power. Even if a home is not intended to have a backup power system when first constructed, steps can be taken to facilitate the easy addition of a generator or renewable energy backup power at a later date.

For addressing extreme heat, the requirements for a backup power system are that it has enough capacity (kW) and storage (kWh, Wh, or Amp-hr) to supply the emergency cooling system for at least a few days.

Design measures that can reduce the load on a backup power system include minimizing heat gain through the measures listed above, using high-efficiency A/C and appliances, and only powering key electrical circuits or a designated cool room. Key electrical circuits may include a refrigerator, freezer, mini-fridge, icemaker, emergency portable A/C, single-room cooling, charging outlets, medical equipment outlets, a well pump, and/or other vital equipment. The electrical panel breakers for non-essential circuits can be labeled and turned off during an emergency.

Behavioral and HVAC control measures that can lower the load on a backup power system include setting the thermostat cooling set point a few degrees higher, flushing the house with night air if it is cooler than inside air, using ceiling fans instead of air-conditioning, using blinds and other shading, and keeping cooking equipment and other appliances turned off.

Passive cooling design and thermal mass construction can help homes to weather an extreme heat power outage. Passive cooling techniques can be fully functional without power. Incorporating thermal mass in the home’s construction can delay a home’s temperature increase by several hours. For short-term outages, thermal mass can help span the periods when A/C is not available.

From a broader community perspective, homes that reduce electricity demand during peak periods (generally late afternoon and early evening) can reduce the chance of grid interruptions. The combined effect of multiple homes taking such steps will have real impact on the reliability of the power system.

Most of the strategies to reduce load on a backup power system can also be used to reduce load on the grid. Additionally, HVAC controls and grid-enabled equipment can be used to automatically implement load-shifting strategies such as only cooling during non-peak times. Installing a ground-source heat pump system can also mitigate demand on the grid, as these systems are less affected by outdoor air temperatures than air source heat pumps.

 

A battery storage system can provide reliable back-up power during a grid power outage
Figure 5. A battery storage system can provide reliable back-up power during a grid power outage (Source: NREL 2021).

 

Strategies for Backup Power

Communications, Accessibility, and Indoor Air Quality

Addressing communications and accessibility as a design goal means considering the reality of how people with physical and/or cognitive limitations might manage an extreme heat event, particularly if there is a power outage. Strategies to address communications and accessibility can be varied and case specific. The strategies presented below are very general and primarily meant to establish basic awareness.

Indoor air quality becomes particularly important during an extreme heat event. High heat and humidity tend to aggravate existing respiratory issues. Extreme heat can also cause increased air pollution, making the issue worse. Extreme heat also often coincides with regional wildfires causing particularly low air-quality. Any measures taken by a designer to reduce stress on the respiratory system can mitigate these issues (Figure 6).

 

Installing high-MERV filtration can mitigate air quality issues associated with extreme heat
Figure 6. Installing high-MERV filters can mitigate air quality issues associated with extreme heat (Source: EPA 2022).

 

Strategies for Maintaining Communications, Accessibility, and Indoor Air Quality

  • Provide a small backup power source that can keep cell phones charged for several days.
  • Ensure telephone or internet is reliable.
  • Ensure house-bound or immobile occupants have access to means of cooling and communication.
  • Use high-MERV filters to mitigate heat-related respiratory aggravation. (See High-MERV Filter.)

Cool Rooms

A cool room is an area of the home designed and equipped to be a shelter-in-place zone during extreme heat. This approach can save money and increase the likelihood of surviving a power outage. Designing for a single room can simplify approaches which may be impractical, not climate-appropriate, or out of budget for the whole house. Even in homes which have had no intentional design toward resisting extreme heat, simply identifying ways to isolate the naturally coolest room in the house can be a life saver.

Strategies for Designing a Cool Room

More Info

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References and Resources*
Author(s)
U.S. Environmental Protection Agency
Organization(s)
EPA
Publication Date
Description
Report about opportunities for ventilation when traditional methods are limited by weather conditions or by contaminants in the outdoor air.
Author(s)
Forestry and Natural Resources
Organization(s)
Purdue University
Publication Date
Description
Blog article describing considerations for how to use trees for shading to reduce cooling energy demand.
Author(s)
Kyle Mittan
Organization(s)
University of Arizona
Publication Date
Description
Article from University of Arizona on keeping homes cool during extreme heat, both personal and system changes.
*For non-dated media, such as websites, the date listed is the date accessed.
Contributors to this Guide

The following authors and organizations contributed to the content in this Guide.

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