A CO2 meter outdoors, next to a graph of indoor air metrics

TEDI and Ventilation, the tail that wags the dog

I will keep this post as short as possible because it is complex, and because it is complex I’ve made a calculator. Without discussing the actual interplay of 8 key variables pertaining to TEDI and ventilation, it would be impossible to separate facts from opinions, and pet theories from hard analysis. In a nutshell however, you know how irritating it can be to see someone idling their car or truck when nobody is even inside? This is exactly how I feel about ventilation systems that are exhausting perfectly good conditioned air when nobody is home. I conclude that we need demand-based ventilation controls in ALL residential projects, something we only really see on new ICI projects, but to come to this conclusion, we need to first understand the impact that ventilation rates, recovery efficiency and operation schedules have on high-performance buildings. Once you have electrified everything, made an airtight enclosure, and gotten your aggregate building-level U-value as low as possible, ventilation becomes a significant part of total building energy loss, and this post is about how to do better.

This ventilation calculator (scroll way down) can help architects and designers separate the ‘wheat’ from the ‘chaff’ on the issue of what the overall impact of ventilation rates are on TEDI, and why we may need to break apart TEDI. Lastly, we should consider whether Ventilation losses might be better represented alongside but independent of TEDI, as it muddies an understanding of the building envelope performance on its own terms.

TEDI, for the uninitiated, is Thermal Energy Demand Intensity (in kWh/m2/yr), but it is equally a definition of the amount of energy wasted, as it is the amount of energy demanded, when you think about it from the tailpipe point of view. Much of what the calculator does is reverse the building physics equations from TEDI, to get a required U-value. Just as the physics govern the arc of a thrown ball on the surface of a planet with a known gravity both forwards and backwards in time (since the equations permit reversibility even if the arrow of time we experience does not), then so too can building physics equations be reversed to work-out data from the tailpipe view.

We just went through a global pandemic. At first, public health authority ‘experts’ flatly denied the airborne transmission of SARS-COV2. What we witnessed was the unscientific expression of over 100 years of disease transmission dogma, and if the whole crazy story interests you I wrote a longer post about that over here:

Architects, Air and SarsCov2

All that said, we know better now right? Covid transmission can happen through the air, and so buildings play a major role in mitigating transmission. Even 60 minutes reminded us of this recently with Dr. Joseph Allen of Harvard’s Chan School of Public Health.


Good. We’re all set, and we know what to do for the next pandemic right? Monitor Air Quality, Increase Ventilate Rates and Filter the delivered Air. But wait a second; Dr. Allen tells us we need to increase ventilation. In fact, we may need to increase ventilation to when we had it right, back in 1900, to a level of about 15 litres per second per person which is 54 m3/hr/person.  Ok so what?


Well for starters, at the heart of the Passivhaus standard is a core assumption, based on an obscure EuroNorm (EN) called EN 1946 Part 6, that assumes each occupant needs 30m3/hr of fresh air delivered to their nostrils (sometimes converted to 0.3ACH, when you know building volume). That is the same as 8.33litres/second, which is about HALF of the rate suggested by Dr. Allen. This 8.33litres/second also forms part of the equation that establishes the climate-independent Heat Energy Threshold of 10W/m2, and in turn the TEDI target or Qh of  15kWh/m2/yr. (see page 126 of the PHPP v9.0 guide). The central tenet of Passivhaus is that there is no heating system required, independent of the climate or location the project is in, however occupants must breath, and so since fresh air must be provided and stale air exhausted, this exhaust forms a major component of heatloss. For this reason the greatest percentage of heat must be recovered from this exhaust air through controlled ventilation with heat-recovery, and then the only source of heating in a Passivhaus would be the small difference that can temper this fresh air by consideration of this equation:

Climate-independent Passive House requirement
The minimum fresh air flow rate for one person is 30 m3/h (according to the DIN 1946 – health criterion). At 21°C and standard pressure, air has a heat capacity of 0.33 Wh/(m3K). Fresh air can only be heated by a maximum of 30 K (to 51°C) in order to avoid dust carbonisation or the burning of small dust particles in the air.

This results in the following capacity needed per person:
Ppers = 30 m3/h/pers * 0.33 Wh/(m3K) * 30 K = 300 W/pers

This shows that the heating of the supply air can provide 300 Watt per person. Assuming 30 m² of living area per person, this would result in 10 W per m² of living area, regardless of the climate. This is an output unit, i.e. the values are based on the day with the highest heat output required (heating load). In order to meet this criterion, a Passive House will require different levels of insulation depending on the climate zone: more in Stockholm, less in Rome.

from: https://passipedia.org/basics/building_physics_-_basics/heating_load

So, what happens if we take a perfectly compliant PHI certified project, and ratchet up the ventilation to almost double what we might have formerly assumed? Would the project still be certifiable? Or would it fail?

If you are a regular user of PHPP you can try this out for yourself, but if you would like to try a more dynamic calculator where you can try tweaking the different variables and see immediately the impact of ventilation of TEDI and the necessary adjustment to overall building U-value, you can try the calculator here (or in the bottom of this post, there is a more graphically squished version). The secondary goal here is to determine just HOW MUCH the rate of ventilation contributes to the percentage of energy loss captured by the  conventional TEDI metric.

Maybe increasing ventilation is no big deal and we can recover all of that energy required for additional ventilation with DOAS with heat recovery aka. ERVs aka HRVs, but what if we can’t? What if we can only reasonably recover about  80% of it at best (in a climate where we have -10ºC during a good part of the heating season)  – that last 10% can be a standards compliance killer. Here’s how:

Eight key variables that govern the impacts of increasing ventilation rates in any heating climate as below:

  1. Treated or Conditioned Floor Area
  2. Occupant Load (how many people per unit area – this gives a total number of occupants you you can over-ride that)
  3. Ventilation Rate (in litres/second/person – the calculator gives you equivalent units in m3/hr also)
  4. Occupancy Schedule (how many hours are people using the space in a  year? Schedules have a huge influence on meeting targets – or not!)
  5. Ventilation Recovery Efficiency
  6. Climate: which for the heating season is governed by Heating Degree Days (HDD) for your location
  7. Total Gains (Internal/Solar/Occupant, etc).
  8. Envelope Air Leakage

From these inputs alone you can derive the aggregate Building Envelope U-value you will need to meet compliance targets. However, certain energy efficiency standards are predicated on a set level of ventilation. Passive House is just one of these. The BC Step Code is another. The Toronto Green Standard is yet another – all of these are adopting TEDI targets that frustratingly incorporate system ventilation losses.  This is why I am calling ventilation the tail that wags the dog. If you increase ventilation rates for Passive House – why in turn would you not increase the combined TEDI targets? But I digress, let’s play with some sliders and see the dynamic results of changing variables live!

Maybe 0.3 ACH is less critical in a home where there is a lower likelihood of spreading disease as in say, a movie theatre or hospital or shopping mall or church or restaurant where superspreader events have happened, but humans still need to breathe. We inhale 17,280 times a day on average, and that is about 0.84kg of O2, and we exhale about 1kg/day of CO2 – crazy eh? So with revised guidance for adequate ventilation of 15l/s/person or 54m3/hr/person, what we need to clearly understand is the impact of increased fresh air delivery with efficient heat recovery, on the overall energy use and loss targets of standards.

TEDI Components

A simple increase of ventilation rates can blow compliance with a TEDI target. But by how much? This is why we wanted to make a calculator – so you can use sliders to see the effects dynamically. If ventilation goes from 8.33l/s to 12.5l/s, what is the effect on TEDI? How much of TEDI is a function of heatloss from unrecovered ventilation air, versus envelope transmission and air-leakage losses? When you have a way to calculate that dynamically, you can see how much energy your ventilation system contributes to TEDI, and then work it backwards to determine what kind of aggregate whole-building U-value you would need to arrive at a TEDI of 15. Obviously if the calculator shows you need a negative U-value – you have a problem. In case that is not clear, a negative U-value would mean in effect the envelope would have to GAIN more energy from the environment than it loses.

The trouble here is this, when you increase ventilation rates, this can constrain the envelope requirements to the point of the absurdly impossible, especially in the case of high-occupancy environments, such as conference facilities, churches, or other high-congregate uses. What’s the solution? Well, since energy performance is annualized, designers will often mess around with the operation schedule, or the building occupancy count, or the efficiency rate of the HRV/ERV/DOAS w. HR, to get U-value results closer to something that is ‘economical’ and buildable. This is the gaming that often happens behind the scenes in an energy model when a pass/fail TEDI of 15 MUST be achieved, but which may not be practical (ie. the top tier of the Toronto Green Standard v4). Even CaGBC ZCB standard tacitly allows you to mess around with schedules to reflect occupancy more accurately. And hey, we agree! Why ventilate when nobody is home?

Smart Ventilation

This is where CO2-based-ventilation controls come into play. CO2 sensors are commonly available for a low price now. Based on the principle that every dollar not spent is a dollar saved, every minute an ERV/HRV does not need to run is energy and dollars saved.

CO2 sensors immediately react to even subtle increases in this trace gas, and so can ‘know’ when a people (or pets) are in a room, and ventilate until a level closer to that of outdoor air is reached. Just as a thermostat has a setpoint, so can a ventilation have a CO2-based setpoint, but most ‘smart’ thermostats can’t respond to air-quality sensors, not yet. We aim to change that, more on that development later. With CO2-based controls, we can design a building to ventilate for whatever worst case a standard requires us to (ie. CSA F-326), but then dial that back to both optimize ventilation system runtime and thus equipment life, but also fine tune the systems to ventilate only when it is necessary and not when it is not, limiting heatloss, and thus, automatically adjusting the ‘Occupancy Schedule’ parameter to optimize runtimes, reduce equipment wear and tear, and further reduce ventilation-associated heatloss.

TEDI as defined

TEUI is pretty universally defined as Total Energy Use Intensity, as a measure of total building end-use energy or the energy delivered to and used in the building (not primary energy, ie. from the power plant to the building), divided by conditioned building area over a period of one year.

That said, TEDI is not so clearly defined. Generally, and depending on who you are talking to, Thermal Energy Demand Intensity is comprised of:

  1. Passive: Envelope Transmission losses, a function of aggregate U-value
  2. Passive: Envelope Air Leakage Losses (can be tested with a blower door)
  3. Active: Service Hot Water Use with or without Drain Water Heat Recovery (Toronto is afaik unique to include DHW, we don’t agree with it).
  4. Active: Ventilation system losses (unrecovered by ERV/HRV), including any losses from direct vent appliances like range hoods with no energy recovery.

It is clear that 1 & 2 are connected to the building envelope. But 3 & 4 are connected to engineered systems. The former relates to Passive systems, designed by Architects, the latter relates to Active systems, designed by engineers. So why should one TEDI metric combine all of these things? Why not just focus on TEUI instead to gauge a pass/fail for any building energy ratings system or standard?

TEDI related to Enclosure and TEDI related to Ventilation should be treated separately. Here’s why:

  • TEDI Enclosure: A measure of Envelope efficiency, U-Value and Airtightness, and passive building resilience related to the Architect’s  professional scope of services
  • TEDI Ventilation: A measure of Ventilation System Efficiency, no relation to passive resilience (think of a power outage during an ice-storm or heatwave) related to the Engineer’s professional scope of services

How the Calculator Works

The calculator functions should be self-evident, but what you may not expect is that, if you set a combined TEDI Enclosure + TEDI Ventilation, something we will call ‘TEDI Combined‘ for clarity, such as 15kWh/m2/hr, the calculator will work backwards to tell you what Aggregate Building Envelope  U-Value you would need to achieve to meet that TEDI Combined. If the U-value is lower than 0.15, you will need to achieve an even higher level of envelope efficiency than Passivhaus. If you arrive at a negative U-value, try increasing the Ventilation Recovery Efficiency, reducing the Ventilation Rate or decreasing Occupied Hours or Occupant Load  to bring the U-value into alignment with 0.15 or greater. Higher than 0.15 you can build easily enough in Canada. Less than 0.15 can be challenging, and expensive.

The section below called ‘TEDI Enclosure Component’ are additional features under further development. Ignore it for now but eventually we’re building out a complete dynamic TEUI solution with more general parameters.


What implications do increased ventilation rates have for architects and how we design?

  1. To understand early on what challenges we will face as a function of the 8 key variables to determine whether our energy targets and envelope U-value targets can be met.
  2. To understand the impacts of increasing ventilation rates on TEDI, and whether this will impact our ability to achieve certifications that previously relied on outdated or lower ventilation rate scenarios.
  3. That we may need to apply pressure on standards and certifying bodies to separate TEDI Enclosure from TEDI Ventilation, since in many instances, the TEDI Ventilation component alone can exceed the total targeted TEDI Combined. In some instances it may not be possible to wish away these impacts with adjusting or reducing other parameters such as scehdules, occupant loads or ventilation efficiency.
  4. TEDI Combined needs to be clearly defined by standards and certifying bodies, and based on defined standards for occupant loads, occupancy schedules and a specific measure of ventilation recovery efficiency.
  5. We need to understand that a net-zero operational TEUI is possible even when TEDIcombined exceeds the 15kWh/m2/yr threshold, and that TEDI should not form part of a pass-fail metric unless references is made to TEDIenclosure and/or TEDIventilation separately.

Just as the pandemic changed everything and notably increased ventilation rate guidance, perhaps it is also time energy ratings systems and standards be similarly changed to align with both health-oriented AND planetary climate-oriented outcomes, since both are possible.


Many thanks to Dr. Ted Kesik and Evelyne Bouchard for originating this discussion, helping to clarify concepts, find mistakes, debug code, and further the ideas that led to the development of this calculator. If you have comments or corrections to suggest, please leave them below!


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