Vector image of person coughing.
October 31, 2020

Do you feel overwhelmed by all the information that’s out there with regards to COVID-19 and ways to prevent/protect against it as we move into the winter?

There are 3 ways that COVID can be transmitted (fomite or surface contact, ballistic droplets, and aerosols). We’re going to focus on what we can do from a building science perspective, and that falls squarely on ventilation and air filtration, to combat aerosol transmission.

There’s a lot of information - good and bad - out there. It’s easy to be confused about what to do and how to apply guidelines.

The Blue House Energy team is dedicated to improving people’s knowledge of building science and the need for good indoor air quality ( Healthy Indoor Environments has been in our online training catalogue for years). We are always learning about best practices and diving deep into the technical end of things. Every couple of weeks while we’re all working our way through the COVID prevention maze, we’ll continue to summarize the work of top level building scientists and air quality specialists and post it here and in social media. 


Here’s our top 8 items for the end of October 2020:

1. This week, based on requests, we’ve produced a one-page infographic on DIY box fan filters. We did not come up with these designs. We just pulled some simple diagrams together in one place. Our sources are noted. 


2. John Semmelhack, (Twitter: @JohnSemmelhack) offers home performance consulting + contracting. The self-professed heat pump + airflow whiz and his team at did some in-field air flow testing on the Comparetto Cube and 4 variations on the simple single filter-taped-to-the-fan version. Steve Rogers (@TEC_Minneapolis) advised on the test setup. 

The results (below) show that the Comparetto Cube (second set of columns) performs better than the simple version, in terms of air flow. 


Here’s a summary of some comments from John’s Twitter feed (7 Oct 2020) about the box fan filter set ups: 

These are a low cost approach to room air cleaning. If you can afford a right-sized HEPA air cleaner, it will provide better filtration, but it probably won’t be any more quiet or energy efficient at similar airflows to the better box fans filter setups. Simmelhack did a noise level comparison with a HEPA unit at high speed to verify this (both about 55db(A)).

There’s a way to roughly calculate the effectiveness of the box fan filter set up that is similar to a CADR (Clean Air Delivery Rate) calculated for a HEPA air cleaner, but we’ll leave that exercise for another article. 

3. This animated graphic article from El Pais does a fantastic job of showing how aerosol transmission occurs and accumulates over time.

4. & 5. Shelly Miller, professor at Colorado University Engineering @CUEngineering teaches classes on air pollution, indoor air quality and environmentally toxic chemicals. She also studies urban air pollution problems and health. Dr. Miller was interviewed for an article published on 18 October  in USA Today: Ventilation and air filtration play a key role in preventing the spread of COVID-19 indoors. She and several of her colleagues have assembled a fantastic FAQ document on protecting against aerosol transmission. You can read more of Dr. Miller’s articles on ventilation and air infiltration at The Conversation, and follow her on Twitter @ShellyMBoulder.

6. Geek out on the American Society of Heating, Refrigerating and Air-Conditioning Engineers  (ASHRAE) COVID-19 resources. ASHRAE’s Epidemic Task Force (ETC), chaired by Bill Bahnfleth, Professor of Architectural Engineering at Penn State University, published a position document on infectious aerosols way back in April. The document outlines ASHRAE's position that facilities of all types should follow, as a minimum, the latest published standards and guidelines and good engineering practice. . There are lots of downloads, including a one-page guidance document on re-opening buildings. There is also one for schools as well as one for residential buildings. 

7. The Harvard School of Public Health is another organization that has an excellent set of resources. Their Schools for Health page has a 5-step guide to checking ventilation rates in classrooms that can be downloaded for free.

8. Good news! This new WHO video that recommends a layered intervention approach that includes increasing ventilation rates (natural or mechanical) and limiting recirculation. Ventilation that will provide 6 ACH is recommended.

As always, we don’t work in a vacuum, and there are many people doing great work. We’ll have another round up that will highlight more excellent work. Shout out to you all. Thank you for your passionate sciencing!!!


Do you have questions about ventilation and air filtration? Give us a shout!


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