Mixed-humid climate construction

Building in a Mixed-Humid Climate Zone

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One of the main reasons we build houses is to protect ourselves from the environment. Setting aside extreme environmental events, climate (which is part of the environment) is the overarching system that surrounds our homes. When designing or retrofitting a home, one of the first steps you should take is understanding the climate system as well as the microclimate around your home. 

Two key merits of designing a home for the respective climate zone it’s in:

  1. Maintain thermal comfort for house occupants at all times with minimal active heating and cooling, resulting in great energy savings as well as resilience since the dependency on the power grid is low.
  2. Keeping the house resilient and durable for many years with minimal impact from mold, decay, and pests, which ensures a healthy and safe environment for occupants. The key here is keeping water, in all its forms, away from the house structure and using designs and materials that allow wet elements to dry quickly.

 

Adapting to Climate Zones

About 50% of household energy in the U.S is used for heating and cooling (space and water).  This rate could be dramatically cut to almost zero in new constructed homes, and can be significantly reduced in existing homes depending on the scope (and budget) of retrofitting projects.

Data from recent years shows the acceleration of extreme climate events, and that we will be experiencing significant changes in the climate, some subtle and growing slow (rising temperatures, droughts, sea-level rise) and some harsh and unexpected  (extreme storms, precipitation, wildfires).

Designing a home based on historical data or for today’s climate is a good starting point and that’s what building codes help us achieve. However, assuming houses are built to last two to three generations (say 80 years) the goal should be to ensure those designs can meet the future challenges and demonstrate the same efficiency and durability throughout those years. To reach true sustainability, it is important to adopt best practices that adhere to local climate zones, micro-climates, and predictable extreme events. 

Setting aside extreme events, this blog focuses on the basic need of adhering to your local climate zone. You cannot design the same home in Arizona and Indiana, Texas, or Oregon. In each region, the trajectory of the sun, the direction of wind and rain, the ranges and fluctuations of temperatures and humidity, the amount of precipitation, snow, and ice, the risks from wildfires, floods, and earthquakes differ. Therefore, we want to convey two key takeaways: 

  • First, all the mentioned climate factors command a thoughtful approach to the design and alignment of house elements, such as the location of rooms, windows, and solar panels, the slope of your roof and the size of the overhangs, the elevation of the house and location of key energy and water systems. A design that considers the climate zone will keep the house cool in the summer and warm in the winter. It will also help your house dry quickly after exposure to water which will improve resistance to mold, decay, and pests. These are just a few examples.
  • The second takeaway is the use of materials, assembly, and detailing. For example, a humid climate poses different challenges compared to a dry climate. Different materials and how you assemble them can make a difference between a house that quickly accumulates moisture and develops mold and decay versus a house that dries quickly and serves you for many years and in a healthy fashion. 

What about the cost? The difference between a true resilient house and standard construction isn’t necessarily the cost. Many times the difference is being aware, asking the right questions, choosing the right professionals and then properly designing and assembling the house, using the right materials. 


U.S Climate Zones

Building America”, a program funded by the Department of Energy (DOE), has divided the U.S into 8 climate zones and provides ample information and data on design and building practices based on those climate zones. Their main aim is to help homeowners achieve the most energy-efficient homes, but they also provide key strategies for durability and adaptation to the relevant climate zone, providing further safety, comfort, and cost savings.

 

Mixed-humid climate construction

 

To determine the climate zone relevant to your property, check the Building America Best Practices Volume 7.3 Guide to Determining Climate Regions by County (DOE 2015) for a list of counties and climate zones.

All content below is credited to Building America and the DOE, although we have refined and added some nuggets! Still, Building America is a great professional, reliable and motivating source to follow.

 

MIXED-HUMID

A mixed-humid climate is generally defined as a region that receives more than 20 in. (50 cm) of annual precipitation, has approximately 5,400 heating degree days* (65°F basis) or fewer, and where the average monthly outdoor temperature drops below 45°F (7°C) during the winter months.

States that are partially or entirely within the Hot-Humid climate zone:

  • North Texas
  • Oklahoma
  • Kansas
  • Missouri
  • Arkansas
  • North Mississippi
  • North Alabama
  • North Georgia
  • South Carolina
  • North Carolina
  • Virginia
  • South of West Virginia
  • Kentucky
  • Tennessee
  • South of Illinois
  • South of Indiana


Parts of the mixed-humid climate zones are subject to frequent and intense rainstorms, severe thunderstorms, and hail. Some areas are at high risk for tornadoes and high winds. Large parts of the Midwest and the South have been subject to flooding. The mixed-humid climate is subject to hurricanes along the Eastern seaboard. The region is at low risk of earthquakes except for localized areas around Arkansas and South Carolina (USGS 2010). Precipitation in the mixed-humid climate varies widely, from 20 inches in the drier parts of Kansas, Oklahoma, and Texas, to more than 50 inches in Tennessee and North Carolina. 

 

In this climate region you should mainly focus on: 

  • Moisture and precipitation management
  • Solar radiation
  • Hurricanes & Tornados
  • Flooding
  • Hail
  • Lightning
  • Wildfires in risk zones
  • Earthquakes in risk zones
  • Pests

 

Moisture and Extreme Precipitation

Probably the biggest challenge for maintaining a durable home is keeping its structure dry. Water in its various forms - liquid, solid (ice), vapor (moisture) - finds its way onto the exterior (rain, snow, ice), interior (floods, showering, cooking, breathing), and within the structure (leaks). Here are some key strategies to explore with your architect and contractors when designing or retrofitting a waterproof house:

  • Most importantly: properly design and build your walls, roofs, and the foundation floor (crawl space/basement). A solid design will help drain water quickly and allow materials to dry. The design defines the various layers you should use such as sidings, vapor, air, and water barriers, insulation, drywall, etc.), the order by which you will lay them from the outside in, and the materials you use. The build is how well you attach them and run the detail so there are no cavities/leaks and thermal bridging. Check our blog on “water” for further details.
  • Design the right size and angle of overhangs (eaves and gables) with a proper gutter and drain system that is capable of routing water from heavy storms away from the house.
  • Landscaping is key. By digging a ditch or creating a small barrier/slope, and planting the right plants, you can help stop excess rainwater from running off and ending up damaging your property or overwhelming the local sewer system or water reservoirs. 
  • Use door jambs that are designed for water and rot resistance.
  • When installing windows use sill wrap, corner shields, and adhesive flashing tape to protect against water intrusion.
  • Use cement backer board behind tubs, showers, and kitchens
  • Install a dehumidifier - these systems suck wet air, cool it down, and condense the water back into a container or a pipe system and back into the world, preferably for good use and/or away from the house
  • Install a thermostat with humidity controls
  • The EPA still allows the use of paints containing mildewcide which potentially can repel bacteria but is also deemed toxic. Instead, look for VOC-free, or Low-VOC (some of the low-VOC become free after drying for a couple of weeks). 

 

Solar radiation

Simply put, solar radiation heats up the roof, walls, windows, and doors, and that energy then heats up the interior of the home. In order to have an energy-efficient home (use less energy) and resilient (reduce the dependency on the energy grid and cooling systems, even during extreme heat waves), you should:

  • Install a reflective roof and use light or reflective exterior wall colors.
  • Install a radiant barrier in the attic.
  • Install overhangs, covered porches, awnings, pergolas, or shade trees to minimize solar heat gain (avoid shading the roof due to moss).
  • Place the air handler and ducts in conditioned space or go ductless with mini-split heat pumps.
  • Install high-performance, low-emissivity windows with low solar heat gain coefficient.
  • Locate windows on the right sides of the house to catch coastal breezes.
  • Create a tight thermal envelope and install a positive pressure ventilation system.
  • Use non-heat-producing Compact Fluorescent Light (CFL).
  • Install ceiling fans and look for solar-powered fans (as backup).

 

Hurricanes, Tornadoes and Strong Winds

Hurricanes and Tornadoes are not the same. While there are hurricane proof building codes and solutions that can save lives, the same house hit directly by a tornado may endure more damage. Therefore, tornadoes command better choice of materials such as concrete blocks with reinforced steel or Insulated concrete forms (ICFs) walls, scrutinized design of the structural load and assembly, and a shelter room.

Damage from storms varies depending on the category of the storm and the location of the house. The main risks from storms are falling trees, poles, and flying debris, power outages from days to months, major flooding and excess rain, and loss of water supply. In all categories, there is a risk of damage or full removal of the roof, sidings, and other exterior elements, structural damage (walls, roof) to complete displacement or destruction of the house. In storm risk areas:

  • Structural load design and assembly
  • Designing walls to resist uplift using hurricane strapping and other metal fasteners that provide a continuous load path from foundation to roof.
  • Consider steel-reinforced concrete walls.
  • Anchoring walls properly to foundations.
  • Designing roof geometries that are less prone to wind damage than gable roofs and installing continuous roof underlayment. 
  • If you decide to go with a gable roof, properly planning the length and width of the gable overhang, strengthening gable ends, and outlooker attachments at gable ends. 
  • Eave designs with extended fascia providing drip edge, and recessed soffit vents (Zoeller 2006).
  • Adequately securing chimneys to the structure.
  • Ensuring windows and doors meet appropriate design pressures (“impact windows and doors”) in addition to being protected from windborne debris. 
  • As with windows and doors, there are hurricane-proof garage doors and tracks. You may also reinforce an existing garage door using a garage door bracing kit.
  • Safe-room / Shelter - according to the Federal Emergency Management Agency (FEMA), a safe room is a: “hardened structure specifically designed to meet the FEMA criteria and provide near-absolute protection in extreme weather events, including tornadoes and hurricanes. Near-absolute protection means that based on current knowledge of tornadoes and hurricanes, the occupants of a safe room built in accordance with FEMA guidance will have a very high probability of being protected from injury or death.”
  • Outswing doors.
  • Hurricane shutters.

 

Flooding

Floods are the most common natural disaster in the United States. We tend to think that floods happen mainly around coastlines and during hurricanes, but America is experiencing more frequent and devastating floods along creeks and rivers (“riverine floods”), lakes and ponds, and areas with inadequate drainage systems. In some cases, extreme precipitation events (“atmospheric rivers”), in-land tornados, and melting snow/ice can also cause floods in unexpected locations. In flood risk zones consider:

  • Elevating your home above BFE (Base Flood Elevation level).
  • Build slab-on-grade foundations and grade lots to drain away from the structure. In areas of likely coastal flooding build on concrete or pressure-treated piers.
  • Adoption of Water Resistive Materials.
  • Elevating Essential Infrastructure.
  • Backing up Critical Systems.
  • Install a generator-ready electrical service panel to run generator-powered shop vacs, fans, and heaters to dry out the house and reduce water damage during post-storm recovery when electric power outages are common.
  • Building Rain Gardens and Barrier Systems.
  • Buy flood insurance.

 

Hail

Hail is a form of precipitation consisting of solid ice that forms inside thunderstorm updrafts, sometimes building rapidly and without advanced warnings. Hail can damage homes and landscaping and can be deadly to people, livestock, and pets. In Hail risk zones consider:

  • Harden your roof using impact-resistant shingles (Go for Class 3 or 4 UL2218 tested shingles)
  • Protect windows and skylights with either permanent or temporary shutters.
  • Cut trees around the house that may fall on the house during hail storms.
  • Stowaway (indoors) all backyard and front yard furniture during the storm.


Lightning

Lightning is a giant spark of electricity in the atmosphere between clouds, the air, or the ground.. Lightning is one of the oldest observed natural phenomena on earth. It can be seen in heavy snowstorms, in large hurricanes, and obviously, thunderstorms (with or without rain). 

Lightning protection systems do not prevent lightning from striking the structure, but rather intercept the lightning strike and provide a conductive path for the harmful electrical discharge to disperse safely into the ground. 

The installation of a complete lightning protection system (see components below) includes several components. These components must be properly connected to each other in order to minimize the chances for any sparks or side flashes. In addition to the conductive and grounding elements, you need to further protect the house from electrical surge which might flow through the house piping and wiring networks putting these elements at risk as well as the appliances connected to them.

  • Air / strike terminals - these are basically lightning rods made of conductive materials (mainly copper) mounted on a structure (usually the roof) and intended to intercept the lightning strike
  • Cable conductors - route lightning current from the air / strike terminations at the top and to the grounding system
  • Grounding - there are various grounding solutions, but common to all is that they effectively route the electrical charge into the ground
  • Bonds - help interconnect the above lightning protection components and other electrical systems in the house to the grounding system. Their main job is to  eliminate the opportunity for lightning to sideflash internally.
  • Surge protectors - A surge protector or arrester is a device that protects electrical equipment from surge current events caused by lightning or other electrical events.
  • Landscaping - lightning strikes may cause severe damage to trees. In addition, trees close to your property may either damage the structure after being struck, or the lightning may move from the tree to a conductive material in the structure. You can either avoid large trees in proximity to the structure, or install lightning protection systems on the tree which may reduce such risks.

 

Wildfires in risk zones

Wildfires pose a risk for the lives of people who live near those ecosystems and their homes. Moisture is one of the main factors that determine wildfires frequency and since the changing climate in recent years brings dryer winters, the consequences of wildfires are becoming more devastating, and the fire season becomes longer. In fire risk zones consider:

  • Avoid new construction in WUI zones and choose a location that is not at high risk of wildfire. 
  • Use non-combustible or fire-resistant materials for exterior components such as roof, sidings, windows, doors, vents, and gutters. For example, use Class A-rated roof shingles and  borate pressure-treated lumber in framed homes.
  • Create defensible space by surrounding your property with noncombustible materials and remove vegetation away from the house.
  • There is no “fire-resistant” vegetation. Design the landscaping around the house with high-moisture plants that grow close to the ground and have a low sap or resin content. Choose plants that resist ignition such as rockrose, ice plant, and aloe. Plant hardwood, maple, poplar, and cherry trees that are less flammable than pine, fir, and other conifers.
  • Install interior and exterior fire sprinklers.



 

Earthquakes in risk zones

If you are planning a new construction, you need to design and build the house with enough strength and stiffness to properly perform during an earthquake. According to FEMA, “actual earthquakes can generate forces considerably higher than those used for code-prescribed design.”

As a minimum, build your home according to local and state codes. But based on the specific risk zone you are in, consider above-code techniques that can be applied to further enhance a structure to obtain minimal damage and best protect the occupants and belongings. These would be:

 

  1. Adopting I-Codes from the International Code Council (ICC) where local codes have not yet adopted them, or no code is currently required.
  2. Codes define the measures or factors for strength and stiffness. Discuss with your experienced architect, engineer, or contractor if you need to go above these metrics.
  3. Designing strength and stiffness to both vertical and lateral movements. Many times focus is on vertical, although lateral loads pose the same if not higher risks, therefore, adding strength and stiffness to lateral movement can be beneficial. 

If you are retrofitting, there are many measures you can take to make your house more susceptible to earthquake forces. Here are some options you can explore:

  • Replace unreinforced masonry or deteriorating concrete foundations with reinforced concrete.
  • Add steel frame and structural sheathing to a soft-story wood-frame.
  • Secure frame to the foundation with anchor bolts.
  • Inspect exterior masonry walls periodically for cracks and reinforce them; In case of chimneys, brace them to the roof structure.
  • Inspect for loose roof tiles and properly anchor roofing material to a braced roof frame.
  • Strap water heaters to the building frame.
  • Secure bookshelves to walls with screws or straps.
  • Secure light fixtures and fans to ceiling joists. 
  • Secure bookcases to wall studs.
  • Strap computer monitors and televisions to walls or desks.
  • If your home is heated by natural gas, use flexible pipe connections for gas appliances. 
  • Install a seismic actuated gas valve, which shuts off the gas during severe earthquakes.
  • Manufactured homes should be tied-down and anchored



 

Pest control

Pests do not only risk your property, they are also a threat to your family’s health. As with other hazards, prevention and being on the offense is a better strategy than being on the defence after pests have gained access or control over parts of your property.

The following methods are layers of protection that perform well together to reduce the threats from pests.

  • Start with studying the threats in your location. You can learn about the main or common threats from local building codes and other local public resources. 
  • Once you gain knowledge, either hire a professional or try to monitor and identify if you have any ongoing infestation.
  • Whether you have an ongoing infestation or not, the main objective is to create an environment that rejects or eliminates pests;
    • Use pest-proof building materials;
    • Eliminate food sources, hiding areas (cracks), and other pest attractants;
    • Use traps and other physical elimination devices; and when necessary, selecting appropriate poisons for identified pests.
  • Termites and carpenter ants are a threat to your home structure. Main strategies here are keeping your structure and the soil around it (18 inches or so) dry, and creating barriers to block easy access: 
    • Use the Termite Infestation Probability (TIP) maps to determine environmentally appropriate termite treatments, bait systems, and treated building materials for assemblies that are near soil or have ground contact.
    • Keep all wood (including siding, decking, and fencing that attaches to the house) from soil contact to minimize the presence of wet wood, which attracts carpenter ants. 
    • Use termite flashing and insulation products with termiticides or use fiberglass rigid insulation when insulating slab edge or exterior foundation walls
    • Provide roof drainage to carry water at least 3 feet beyond the building.
    • Apply decorative ground cover no more than 2 inches deep within 18 inches of the foundation.
    • Keep plantings at least 18 inches from the foundation with supporting irrigation directed away from the finished structure.
    • Specify and install an environmentally appropriate soil treatment and a material treatment (treated wood, termite blocks) for wood materials near grade.
  • The CDC provides a good starting guide to help protect your home and the health of your family from various pests such as rats, flies, roaches, mosquitos, and fleas.

 

Summary:

Climate (which is part of the environment) is the overarching system that surrounds our homes and one of the main reasons we build houses - to protect ourselves from weather and natural phenomena. 

Setting aside extreme events, homes should be designed in a way that adheres to the local climate zone characteristics and the microclimate around the home and meet the following objectives:

  1. A resilient and durable home with minimal impact from mold, decay, and pests - ensuring a healthy and safe environment for occupants
  2. Provide thermal comfort for house occupants at all times with minimal active heating and cooling, resulting in great energy savings and resilience (less dependency on the grid)
  3. Provide ongoing supply of quality Air and Water

Achieving these objectives require the right design, choice of materials and proper construction. Building codes are a good starting point however, they set the minimum requirements and don’t always cover all the best practices. Assuming houses are built to last 50 - 100 years, the goal should be to ensure their design can meet future challenges and demonstrate the same efficiency and durability over such time. 

To reach true future sustainability:

  • Research and understand your climate zone
  • Study how to mitigate the risks it poses
  • Tap into information from local governments, communities and neighbors
  • Hire certified professionals that will help you achieve these goals in the most cost-effective way. 

Remember, working with the environment and adhering to the local climate zone is the necessary first step. The next step is being ready for future extreme events, those - “one in a century events” - which now occur more often. These require additional measures and planning.

 

KEEP COOL. BUILD RESILIENCE. EAMPACT.

 


References:

https://www.eia.gov/energyexplained/use-of-energy/homes.php

https://www.ncdc.noaa.gov/billions/

https://basc.pnnl.gov/

https://www.energy.gov/sites/default/files/2015/10/f27/ba_climate_region_guide_7.3.pdf

https://www.energy.gov/sites/prod/files/2013/11/f5/40percent_hot_humid.pdf

https://www.energy.gov/sites/default/files/2013/11/f5/40percent_mixed_humid.pdf

https://www.energy.gov/sites/default/files/2013/11/f5/18899.pdf

https://www.energy.gov/sites/default/files/2013/11/f5/cold_climate_guide_40percent.pdf

https://www.energy.gov/sites/default/files/2013/11/f5/marine_40_guide.pdf

 

Footnotes:

* Heating and Cooling Degree Days

NOAA defines: “Degree days are based on the assumption that when the outside temperature is 65°F, we don't need heating or cooling to be comfortable. Degree days are the difference between the daily temperature mean, (high temperature plus low temperature divided by two) and 65°F. If the mean temperature is above 65°F, we subtract 65 from the mean and the result is Cooling Degree Days. If the mean temperature is below 65°F, we subtract the mean from 65 and the result is Heating Degree Days.”

A degree day is a solid gauge to calculate if your home improvements have merit. After you take measures to improve your energy efficiency with the proper home insulation and airtightness, energy-efficient HVAC systems (or a “passive house” ventilation system), you will be able to observe how the new energy bills fare against the past. While extreme events of high or low temperatures at any given year might skew the results, you should still be better off after installing proper insulation and taking advantage of energy-efficient HVAC systems or “passive house” strategies.

 

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