Confused by “Net Zero,” “Near Zero” and TRUE “Zero Energy” housing?
There's a lot of confusion around these terms. We asked building science expert Paul Duffy to sort it all out for us.
May 11, 2018 by Paul Duffy
Have you ever wondered why there is so much talk about “Near Zero” or “Net Zero” energy housing versus Zero Energy Houses? After all, we have zero energy land vehicles (they are called bicycles), zero energy water vehicles (they are called sailboats) and zero energy aircraft (they are called gliders). When you really think about it though, even if you have “zero energy” applications like these, some energy input is needed—either human power or some renewable source like wind and solar.
Now, I could go into a long-winded explanation of the laws of thermodynamics in an explanation of why this is so, but that is not really what I wanted to focus this article on. Let it simply be said that we will need to consume at least some energy to make buildings comfortable. The only other option is that you put up with some amount of discomfort—a house that is somewhat too hot in summer or too cold in winter—not necessarily unlivable but outside the normal comfort ranges we expect when mechanical systems temper the indoor air.
If you reframe the question, however, it may be possible to heat and cool a house with the renewable energy that is available on site. Imagine, if you can, a house that could be heated and or cooled with only the solar energy that is generated from the roof of the building. Such a building is not a “zero energy consuming building,” but it might be thought of as “net zero”, or at the very least, “near zero” energy consumption because the energy captured on the roof offsets the need for heating and cooling.
Now let’s consider the host of factors that affect the captured solar energy that is available for any given building. The amount of energy available will be affected by:
- Latitude of the site
- Shading of the roof
- Slope of the roof
- Orientation of the building
- Time of Day
- Season of the year
To have enough energy available consistently, it is likely necessary to oversize the energy collection system such that it over-produces power when conditions are optimal, and so some power is stored to be available to fill in the gaps when available power is sub-optimal (e.g. overnight.)
Storage of energy, via batteries, is expensive so builders usually look for other options. Where allowed, the most economical option is to back-feed power into the electrical utility grid selling the power to the electrical utility. This illustrates the “Net Zero” concept… if energy sold to the utility equals the energy purchased from the utility, the net of those transactions is zero. In future, utilities will increasingly become the vehicle that facilitates such transactions, rather than being energy suppliers only, as they were originally set up.
The Path to Net Zero
The prospect of energy independence holds a lot of appeal. So how does this affect the way you build houses?
Firstly, the amount of energy available on the roof is much smaller than we are used to having as our “peak” energy used in our buildings so it is necessary to take aggressive steps to conserve energy.
Secondly, certain measures are going to be much more effective at minimizing energy use versus others. So it is necessary to clearly understand what to prioritize and what to minimize, and in what order, to get the desired result.
Finally, you have to realize that net zero energy means different things to different people. Some use net zero energy use (e.g. 0 kWh net energy use) as the target. Others use net zero energy cost as the target (e.g. $0 energy use—usually utilities buy back energy at rates that are much lower than the rates at which they sell them so net zero cost might actually mean you sell a lot more power than you buy to get to net zero cost.)
In most cases, people are promoting concepts of “net zero ready” or “near net zero”, providing a very efficient building but not adding the cost of solar collectors on the roof because solar panels are not cost-effective at present. Whether you realize it or not, the market is very confusing partly because people are promoting near “net zero” concepts where the finished house still consumes a significant amount of energy.
Steps on the Path
Assuming you want to move along the path to net zero housing, where do you begin?
- Start with a focus on air tightness. The debate over how tight is too tight is over… the world has moved on! Air leakage is undesirable because it leads to increased energy consumption, it compromises the performance of air permeable insulation, and it can lead to concealed condensation which leads to rot, corrosion, mold, and other health and durability problems.
- Increase insulation levels. However, be smart about it. Focus on poorly insulated areas: thermal bridges, poorly insulated basement walls, uninsulated slabs on grade. It also makes sense to use continuous insulation—an exterior layer of spray foam or insulating sheathing versus heavier framing with larger cavities for insulation.
- Use high quality windows. Today’s wall systems are insulated to RSI 3.6 or more. Windows typically one tenth of that value. It makes most sense to improve the thermal performance of the weakest links… and typically that means improving the windows and doors.
- Avoid running mechanical systems and ductwork through unheated spaces. Even if you bury ducts in a mountain of insulation, the pressure of fan power can cause even minor defects in ductwork to leak a lot of air!
- Use heat recovery on ventilation air. If you go to so much trouble to keep the heat in, it makes no sense to exhaust heated air to the exterior without recovering at least some heat.
You will notice that I largely avoided the whole topic of mechanical systems, not because they are not important but because system choices are highly dependent on some very basic assumptions—assumptions that vary greatly across the country. Huge regional variations exist.
In some parts of the country, fossil fuels like natural gas are plentiful and cheap so a low energy house naturally includes the lowest cost fuel. But fossil fuels are typically non-renewable so it becomes difficult to convince some customers we should be using them as our primary heating fuel.
In other parts of the country there have been conscious policy decisions to only use electricity in the specifications for net zero housing because it is readily available and because it is the only fuel that can be generated locally. Electricity as a heating fuel is costly however—costly enough that it usually makes sense to incorporate air source or even ground source heat pumps into the design to minimize peak load requirements.
Regardless of fuel type, provincial codes are tending to require high efficiency equipment because this has been a relatively simple change to mandate and it results in significant greenhouse gas reductions. The only downside is that Code requirements requiring high levels of energy efficiency leave relatively small gains available in terms of moving to higher efficiency heating and cooling equipment. Current industry practices already tend to be at the high efficiency end of the spectrum.
Trends and Innovations
When you talk to builders of net zero or near net zero housing, they inevitably want to talk about their wall assemblies and similar innovations, so it is always topical to discuss trends.
Thirty years ago, in the aftermath of the “energy crisis”, builders of super energy efficient houses were experimenting with double walls, fat walls and similar concepts – and that trend has returned. Radically thicker walls, with multiple layers of framing and lots of insulation, are one way to improve performance. The trouble is, this is almost a “brute force” approach—if you are prepared to live with radical changes to your construction process, sequencing problems, detailing issues and a total re-engineering of the houses you build, this might work for you. But for many or perhaps most builders, this is not the preferred approach. It is simply too radical a departure from the way they are used to building to be their preferred method.
A growing trend is the so-called hybrid wall. This approach uses layers of different insulation types to increase R-value, air seal, control vapour flows, and fulfil other functions in the wall. Consider a wall comprised of conventional 2×6 framing, with a 3” layer of medium density, closed cell spray foam on the interior of the sheathing and a lower cost insulation filling the rest of the cavity. You can add insulating sheathing or even more closed cell spray foam for a wall assembly with a thermal performance of R-40 or more. There are some cautions with this approach. Do not skimp on the amount of medium density closed cell spray foam you use in the cavity. Medium density foam will effectively become a vapour barrier at thicknesses over 1 ½ “ so if you are not careful you could end up with condensation on the interior of the foam… using more foam (at least 3” thick) tends to avoid this problem in most parts of the country because the interior surface of the foam is closer to the interior, and therefore warmer so it avoids the cold temperatures that cause condensation. There is an added hidden benefit to this approach—the spray foam in the cavity adds structural rigidity to the wall, resisting high winds with added racking strength. Looking to promote a more durable, highly efficient home? This might be the approach that works for you!
Finally, if you follow the technical magazines, there has been a lot of talk about the so-called “perfect wall.” This is a wall with all of the insulation on the exterior of the framing running “continuously” over the studs. Proponents argue that if you have enough insulation on the exterior, you don’t need insulation in the cavity, and that whole concern about concealed condensation goes away. The Structural Insulated Panel (or SIP) manufacturers have been arguing in favour of this approach for years. Without framing to interrupt the insulating layer, thermal bridges are eliminated and a higher effective insulation level can be achieved in the same space. More recently, foam sheathing manufacturers and spray foam manufacturers have gotten into the act with wall assemblies predicated on high levels of continuous insulation applied outboard of the stud cavity space.
This trend is being helped along by ever more stringent codes requiring not only high insulation levels but also:
- At least some continuous insulation on the exterior
- Continuous air barriers
- Water resistive barriers, and
- More sophisticated vapour/condensation control schemes.
Foam sheathing and spray foam manufacturers are actively pursuing schemes to combine these features into their wall system solutions. In commercial buildings, particularly, the trend is to have one material perform several features. If you have to add more insulation, best to do it in a way that combines multiple features into the one product solution.
Manufacturers of insulating sheathing are promoting use of their materials with sealed joints, auxiliary flashings, transition membranes and other accessories that air seal, limit water ingress, and redirect rain back to the exterior as part of their insulation solutions.
Spray foam manufacturers are pursuing similar schemes using medium density closed cell spray foam in combination with transition membranes and flashings. The key difference is that, because foam adheres to the exterior sheathing bridging gaps and sealing around brick ties, penetrations and other elements, the amount of additional work required to provide a continuous air barrier, vapour barrier, water resistive barrier, as well as continuous insulation is greatly reduced. It is not uncommon to see savings of 10% to 20% in installed cost versus other approaches.
Clearly, the path to net zero is very challenging, but savvy builders are re-engineering their homes and using innovative approaches to build better, more durable, as well as, highly energy efficient homes. Even houses built as recently as five to ten years ago did not have access to the array of innovations coming on the market today.
Paul Duffy is an independent Building Science, Engineering and Management Consultant. He may be reached at firstname.lastname@example.org