Sorry grandma, out with the Lüften, in with the ERV

There are some traditions we learn from our grandparents that are cherished and passed to the next generation like making tourtière, singing old traditional songs together and repairing things. Then there are the not so good things like smoking cigarettes, going to mid-night mass, cheering for the Leafs and opening windows routinely and indiscriminately.

First off, for this article; be it resolved that having a balanced, dedicated ventilation system will deliver and distribute consistently cleaner, more uniformly oxygen-rich, and less CO2 air, than open windows for a period each day.*

Apparently it’s all the rage for North Americans experiencing this German tradition of Lüften that often show a lithe, yoga-pant wearing woman standing in front of an open breezy window with a facial expression of complete serenity festooned with billowy curtains.  I mean yes, there’s got to be some 80 odd days a year – notably spring and fall – when you can do this without an energy/pollution penalty and I do love those days/nights, don’t get me wrong.† But unless your home is short, air tight, over occupied and windows are growing mold, Lüften-away. Otherwise, the heart of a Canadian winter or those languid, soupy, city summers when you bring in a new batch of humidity-laden air back into the house – especially at nighttime when the temperature drops (seduction) but the relative humidity (RH) outdoors increases (sucker punch) – it’s no reliable strategy for fresh air and energy savings – especially if you have to bump the temperature up or down after windows are closed. 

What do I mean when I say there is an “energy/pollution” penalty? Let me break it down. The two most common measures of comfort and the optimal ranges suggested are 21oC indoors at 50%RH. The first number is easy to understand, it’s the temperature of the air, but the second one, the RH is a bit more involved. For example, air at 21oC can hold varying levels of humidity, and above 70%RH, the air can start to feel uncomfortably sticky. Air conditioners were built to change the first number (Sensible Heat) but struggle a bit more in affecting the second number, RH (Latent Heat), especially if the AC cycles on and off too quickly. In fact, an AC system has to work long and hard to pull out or “precipitate” the excess humidity held by air, and it should be noted, the expensive part of cooling is in dehumidifying. 

The ‘pollution’ part from the outside includes temperatures that are too hot/cold, pollen, noise, humidity/dryness, dust and smoke. Think Manitoba forest fires and odours. Think the guy smoking pot on his porch every night, fireplace chimneys a-belching, the contractor cutting concrete without water and noises for motorbikes to drunken revelers at 2AM.

A properly installed, balanced ventilation machine can stickhandle all these contaminants significantly better than an open window. Meanwhile, indoors, carbon dioxide rises inexorably while oxygen levels drop. If you haven’t measured CO2 in an occupied room, you should, because you’ll know right away that CO2 builds inexorably with each occupant; think a couple in a bedroom with a dog and boom, you’re over 1000ppm‡ by midnight. Having guests for dinner or have kids? The peak looks like K2 by 10PM.

And therein lies the rub; unless you’re in breezy, temperate Victoria BC, most Canadian homes need a proper balanced ventilation system preferably a dedicated system with good controls. And please, if you’re a Passive House enthusiast, resist drawing big red arrows suggesting that “cool” summer air might magically flow in through a main floor front window out the back top floor skylight window.§ Also, try not to shoot yourself in the foot by suggesting they open windows while operating a ventilation system. Make the house as airtight as possible so that the investment in a balanced ventilation system earns its keep. NB: resist talking about “natural ventilation.” 

Windows are a home dweller’s eyes and connection to the outside world and that connection is pivotal to feeling happy, secure and in sync with the rhythm of the day.  So instead of opening up windows in extra humid or cold times of year because those cool Germans do it in a mild climate, install that dedicated balanced ventilation system and then get outside as often and as long as possible to not contaminate# the indoor air, say hello to neighbours and take in the great outdoors with every breath.


* I could find no standard on the Lüften method: how long, how often, opposing windows only etc. Let’s assume here for argument’s sake that the pattern is each morning for 2 hours, windows are opened.

† As a Windsor area youth and adult life in Toronto up until age 53, I’ve never lived in a house with air conditioning. So, I know all about opening windows for “ventilation” even in the dog days of summer.

‡PBC’s Matt Adams shared with me some technical perspective on where this "threshold" came from and if it matters.

 §For the air to move through the house, it needs a force; either a steady wind or ‘stack effect’ which requires a difference in temperature between inside and out.

#CMHC’s Don Fuglar always said that humans are like Schultz’s caricature of Pig-Pen who is always surrounded by a cloud of dust.


Photo by Lera Platova on Unsplash


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