What Makes Up a Net Zero Home? How a Vapor Barrier Plays a Role
The Secret Below-Slab Ingredient for Building an Energy-Efficient Home
2020 used to seem so far away.
Even back in 2008, when the state of California adopted its Long-Term Energy Efficiency Strategic Plan (CEESP), its first core residential vision – 100 percent of new homes achieving Zero Net Energy (ZNE) by 2020 – seemed like a distant (and not urgent) target date.
And yet, here we are.
Come January 2020, every new home constructed in the Golden State will meet a new standard:
“A Zero Net Energy (ZNE) Code building is one where the net of the amount of energy produced by on-site renewable energy resources is equal to the value of the energy consumed annually by the building..”
The U.S. Department of Energy uses the term Zero Energy Building (ZEB) in a helpful guide to illustrate the systems and design in place to meet the goal.
In California, the strategic plan set in motion an array of standards, initiatives and incentives to prepare for 2020:
- Requiring solar panels on all new residential construction
- Driving the lighting industry to more energy efficiency
- Offering technical training and education to the building industry
Many industry experts believe mandatory installation of solar panels will help most builders reach the zero net energy goal.
But while the focus of the ZNE goal may be the solar panels on top of the building, there’s a powerful way energy-efficient homes can work from the bottom-up, rather than just channeling solar energy from the top-down: a high-performance below-slab vapor barrier.
Watch Case Study Video: ABC Green Home Uses a High-Performance Vapor Barrier to Meet Net Zero Energy Needs
Energy Efficiency Begins from the Ground Up
With all the focus on solar on top of a building, it’s easy to forget there are six sides of a building – the bottom side of the building is often disregarded, or perhaps, misunderstood.
Protecting the building envelope from below with a high-performance vapor barrier below the slab can provide any building with a variety of benefits:
- Protecting the building from water vapor, which can destroy moisture-sensitive flooring and provide a food source for mold
- Preventing intrusion of soil gases like radon and methane
- Improving the indoor air quality for a building’s inhabitants
These are important attributes for any building project. They also tie into long-term sustainability goals of durability and human health.
But when it comes to energy efficiency and achieving zero net energy, a low-permeance vapor barrier performs one critical task: reducing the relative humidity in the building envelope.
Lower Humidity Leads to Lower Energy Load
No matter the climate you’re building in – even a relatively arid region like California – water vapor will always diffuse upward from the groundwater supply back into the atmosphere. This process, diffusion, is the natural outcome of a moisture gradient: the ground is warmer and relatively more humid than the air, resulting in a vapor drive – even through a concrete slab.
This diffusion of water vapor provides a food source for mold and a danger to moisture sensitive flooring, as outlined above. As more vapor permeates into the building envelope, the relative humidity of the built environment increases.
Most people understand the role of insulation in their building, which keeps the conditioned air – whether warmed or cooled – in place, thereby reducing the amount of times an HVAC system kicks on to regulate the air temperature.
But consider how an HVAC system works. Its main function in heating and cooling your home is by removing humidity from the air. When you use a high-performance vapor barrier with an extremely low permeance rating, you eliminate a potential source of humidity from entering the building envelope.
Less humidity in the air means less work for your HVAC system. Less work for your HVAC system means a lower energy load for your zero net energy home.
Two Sides to the Zero Net Energy Equation
Going solar is the initial instinct when trying to solve the zero net energy mandate in California's CEESP residential action plan. Produce enough solar energy and you can offset all kinds of energy-producing activities or shortcomings in a home’s construction.
While solar panels are critical to a cleaner energy future, this mentality discounts the other half of the zero net energy equation: you can also balance your energy equation by demanding less energy load in the home.
To its credit, CEESP does not fall into this imbalanced equation: their focus on the lighting industry, insulation and more efficient appliances, to name a few, will drive all kinds of energy savings.
A high-performance vapor barrier with extremely low permeance is often the very first component to be installed on a job site that contributes to the energy efficiency side of the equation.
Start with a High-Performance Vapor Barrier
Much of the concern around meeting CEESP’s bold adoption target dates centered around costs to builders. Making better choices around building components and installing new systems like solar stoked concern about the bottom line.
This is another place where a below-slab vapor barrier is what builders would call “short money.” In most buildings, designers are already calling for below-slab protection in the form of a vapor barrier or retarder. This historical cost is already “baked in” to most cost estimates.
Selecting a vapor barrier with the right characteristics to contribute to a zero net energy home may only contribute a tiny marginal increase to the cost of the building’s construction.
Here are two key traits to look for:
Water Vapor Permeance: a true vapor barrier will have a permeance of 0.01 perms or lower and will meet this specification under ASTM F1249’s test method. This test method measures the water vapor transmission rate through plastic film using a modulated infrared sensor. This is a much more reliable, accurate and consistent test method than ASTM E96, which some manufacturers may employ to “cherry pick” a more favorable but widely variable result.
Permeance After Conditioning: how a vapor barrier performs in a laboratory environment is one thing. The construction environment is far less forgiving. ASTM E154 provides four different tests for a vapor barrier’s permeance after conditioning to elements like soaking, heating, low temperatures and exposure to soil organisms. How a vapor barrier performs after conditioning to protect against water vapor is far more indicative of the performance you can expect in the rugged below-slab environment. A high-performing vapor barrier should be able to retain its permeance of 0.01 perms or lower after conditioning.
So why does permeance matter when it comes to below-slab vapor barriers? We explain that in this Stego IQ episode.
2020 is Upon Us
It is full speed ahead for California and its bold ZNE standard. If history is any guide, California’s standards when it comes to cleaner air, cars – and now homes -- will soon expand to other regions and states.
The zero net energy future is here. Are you ready?
Written by Tom Marks
Tom Marks has over 10 years of experience as the Rocky Mountain Regional Manager for Stego Industries, LLC. His main role is as an educational and technical resource to help project teams implement appropriate and effective sub-slab barrier systems. In addition, Tom leads Stego’s Green Team and Footprint Project, helping position Stego as an industry-leader in material ingredient transparency, healthy buildings, and corporate responsibility.
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