Do Trickle Vents Really Work?

In our post “How does indoor air pressure affect Ventilation and Air Quality?” we discussed the pros and cons of negative, positive, balanced and balanced with HRV/ERV ventilation. We really do think that balanced ventilation is the way to go, but not everyone is convinced.  In the UK and Europe, negative pressure ventilation is common through the combined use of trickle vents and kitchen and bath exhaust fans.  Trickle vents are offered on new window frames and there are options for retrofitting them into existing frames.  

It may sound like a drippy register, but “trickle vents” are designed to provide “background ventilation” and remove condensation from a house. They supposedly have several advantages:

• You can ventilate your home during most types of weather, no matter if it’s rainy or not.  The design of most trickle vents does not allow water to be forced in even during windy rainstorms.

• You can ventilate your home more safely than opening the window.  Homes and apartments with first-floor access have considerably more risk of break-ins than higher stories, so opening windows, even with nightlatches, is often a safety risk.  Trickle vents allow safe ventilation, whether you’re at home or away.

• You can control the ventilation with adjustable trickle vents.  Too much draft on a cold day is not nice, so you can open the vent partially or close it altogether on some windows.

• Trickle vents supposedly provide minimal ventilation where homes are built tightly and windows are rarely opened.  This allows toxic chemicals that build up from off-gassing and normal daily activities like cooking and cleaning, to be diluted and vented.

• Trickle vent options include ones that automatically open at preset humidity or temperature levels.

Of course, there are cons to trickle vents.  Complaints most often center on noise and wind.  If the home is located on a busy street which has a lot of car and truck traffic, or is in a windy area, trickle vents break the insulative qualities of the window to let in traffic and wind noise.  Some designs are better than others at attenuating noise, so design of the vent is important to consider when installing new windows. 

Now, do they work?  Since they were mandated in the UK for some time (2006-2021 from what I’ve read), it seems to me that they were proven to do the job, but in reality, they probably were not.  According to a research project funded by the National Renewable Energy Laboratory as part of the Building America program (here is a summary by one of the authors), fresh-air ventilation like trickle vents usually does not work the way it is intended.  In order for it to do so, the home must be sealed very tightly except for the vent, and rooms must be in constant negative pressure.  In most cases (which are normal or poorly sealed homes) cracks around the main entrance door overwhelmed the amount of ventilation coming through dedicated vents like trickle vents.  Ventilation in multi-family homes in particular is unpredictable, because pressure in the corridors and apartments fluctuate depending on weather, wind, and occupation of the buildings.  The end result is that the theory of exhaust-only ventilation was not working in real life.  According to the theory, “consistent negative pressures of up to 20 Pa are needed to draw air from the vents at the rates desired”.  No wonder there are so many moisture, mold and air quality problems in modern homes!  The air exchange is not working as exhaust-only ventilation plans have been designed.

In the end, the author of the aforementioned research project still advocated for tighter buildings and controlled ventilation via balanced ventilation and energy recovery units and whole-home dehumidifiers (if necessary).  These do have higher up-front costs, but they deliver consistently healthier air, and the paybacks are short (under 5 years).  

Obviously, if I lived in the UK and was required to have trickle vents in windows, I might not be inclined to “do the research” because they seem to be a reasonable source of ventilation.  Living in 2022, even better ways of getting fresh-air ventilation are constantly being designed and with many sources of information at our fingertips, we can choose more wisely.