Managing Indoor Air Quality

November 19, 2018

We learned during our first winter in our passive house that some smoke particles enter our house through the ventilation system. The smoke particles are so small that the filter doesn't catch all of it. For the last five years, our indoor air quality management routine had been unplugging our Zehnder ComfoAir 350 heat recovery ventilator (HRV) when we smelled smoke inside the house. In the last couple of weeks, we had to refine our seasonal practice because the outdoor air quality had been poor due to large fires in California.

Here's what we are doing at Midori Haus to manage our indoor air quality:

1. External air quality check. We look at the real time air quality index to see what our air quality index number is. If it’s good (green 1-50) then we leave on our heat recovery ventilator (HRV) and operate it as normal. Our normal operation is HRV fan set on medium speed and boosting up to high when we’re cooking in the kitchen. If the air quality is unhealthy (151 -200) or worse, we unplug our HRV. If the air quality is moderate (51-100) we stick our nose outdoor and sniff the air to decide whether to unplug our HRV or not. Just like temperature, there are micro variations of air quality within a given city. For example, last night we noticed and felt the air quality on the east side of out city to be much worse than on the west side. 

2. Check for major fires and wind direction. We look at the California statewide fire map to see what fires are burning nearby. Depending on the size and number of the fires, we may be affected by fires burning over 100 miles away. The wind pattern makes a difference, so we look at the weather data to find out the wind direction. The 10-day forecast section is useful because it will show the change in forecasted wind pattern. With may fires burning north and east of us, winds blowing from south and west bring us relief, especially since those directions are over the ocean.

3. Check air quality levels inside the house. We look at the data from our FooBot. This is a relatively inexpensive air quality monitor for homes. We have it placed in the kitchen near the HRV supply vent. The device glows orange when air quality is bad and glows blue when it’s good. I can see the data and history on the FooBot app on my iPhone. This screen shot shows the state of our indoor air quality after leaving the HRV turned off overnight. The composite score is 53 (a score based on some algorithm) and the breakdown of the air quality components are: Fine particles (4 is great), Volatile compounds (592 is poor), and Carbon dioxide (2000 is poor). We've noticed that Volatile compounds level spikes up when we cook. We have a recirculating venthood in our kitchen, so we rely on the HRV to evacuate the cooking smells. The only way to rapidly clear the air in the kitchen is to open the windows, but we don't want to do that when the outdoor air quality is bad. When we turn off the HRV for an extended period of time, Carbon Dioxide level builds up, like on this screenshot. At these times we turn on the HRV for a short while to flush out the VOC and CO2. During this time the particle count goes up, but we let the air purifier handle that.

4. Run an air purifier. Couple of years ago, Kurt did some research and bought a Plasmacluster Ion technology air purifier from Sharp. Most of the time it sits in his office at work, but recently he’s been hauling this back home in the evening and weekends. This cleans the air and we can feel it. We have recent experience where this device changed our panic to calm. On the morning of Saturday November 10 our house stank of skunk in addition to the smoke. Our neighbor’s dog got sprayed in the middle of the night and HRV sucked in the skunk smell in addition to the smoke. The smoke and skunk particles are so small that they get through despite the layers of filters we have. We were about to host an open house and tour for the International Passive House Days event and we were worried about the indoor air quality. We ran the air purifier for 10 - 30 minutes in each room and it cleared up nicely.

5. House plants in place to clean the air. We have house plants in every room. Plants provide oxygen and absorb carbon dioxide. Several years ago, Kurt bought a lot of peace lily plants after learning that the plant removes chemicals such as benzene and formaldehyde. These plants also help with the humidity. 

We enjoy living in Midori Haus built to the Passive House (Passivhaus) standard. The tightly sealed enclosure, about 10 times tighter than conventionally built houses, keeps random air from coming in from random places. The heat recovery ventilator provides us with continuous filtered fresh air. Only during these extended bad air quality days do we need to pay special attention to our ventilation system to keep our indoor air clean. Our next experiment will be to install the Zehnder ComfoWell F9 filter (yellow filter that has MERV 15 rating) at the air intake, next to the charcoal filter. If it arrives before the outdoor air clears up, we’ll be able to measure the result of this new filter.

Walls and Ceiling

November 6, 2012

We’ve removed asbestos and lead based paint in the original house.  So we should have safe and healthy indoor air in the house, right?  Not if you don’t pay attention to the material you introduce into the house.  Earlier this year we read a book by Bill Bryson titled, At Home: A Short History of Private Life.  A curious and well-researched book containing facts about homes in England and US, there were many astonishing examples of the rooms, materials used for homes and how people lived.  In Chapter 14 he mentions ways in which how our houses can hurt us.  One example is wallpaper.  After 1775 a popular shade of green was made by soaking the wallpaper in a compound containing copper arsenite invented by a Swedish chemist, Karl Scheele, and the color was known as Scheele’s green.  Use of wallpaper increased after the wallpaper tax was lifted in 1830 in England and by the late 19th century 80% of English wallpapers contained arsenic.  Today most of us know that arsenic is toxic.  But back then they didn’t and the rich green color containing arsenic was used in candles, clothing and even food coloring.

A more recent example of walls in your home causing harm is the reactive sulfur gasses coming out of certain drywalls manufactured in China.  According to CDC report the people who lived in US homes that were built between 2001 and 2008 containing imported drywalls experienced headaches, eye irritation, difficulty breathing and other health problems.  The Chinesedrywall website appear to have collection of information related to this issue.  By the way, the term drywall, sheetrock and gypsum board appear to be used interchangeably.  For a quick overview of the history and manufacture of drywall see this video.

We know that people made decisions based on information available to them at that time.  Some of these, like asbestos, has beneficial qualities like fire-resistance and sound absorption that made it attractive to use in various building materials at one time.  It’s only later when the side effects of these materials that caused serious illness, such as lung cancer from inhaling asbestos fibers, that made these materials fall out of favor.  So, in 2012, we’ve chosen materials based on what we know today to be benign and promote good air quality.

Our house was built in 1922 so the original walls and ceiling were made of lath and plaster.  Here are some photos of the original wall taken during deconstruction.

Plaster above the door chipped away to reveal the lath underneath

“Lath” is the narrow wood strips nailed horizontally across  wall studs.

Plaster oozing through the lath holds it in place

Below are the “after” pictures of the new walls.


We chose to have fiberglass batts installed in the interior wall cavities between rooms.  The purpose for this is not for thermal insulation but for sound attenuation.  We noticed that the hardwood floor over a crawlspace seem to carry the sound throughout the house, perhaps similar to how a sound reverberates within a guitar. We were told that another way to dampen the sound is to use different thicknesses in the drywall, for example using 5/8″ and 1/2″ on either side of the wall studs.  We didn’t do that.  Hopefully the fiberglass batts will dampen the sound of me playing my flute to prevent annoyance to others in the house.


AirRenew drywall from CertainTeed was selected.  This drywall product promotes indoor air quality in two ways — 1) Traps volatile organic compounds (VOCs) such as formaldehyde in the air and makes it inert and traps it inside the drywall; 2) Resistent to mold and mildew.  You can watch a short video of this here.

Ceiling and walls installed using AirRenew.

Lydia Corser from GreenSpace advised us to be selective of the plaster used to texture the drywall.  Her store, located next to Habitat’s ReStore on the west side of Santa Cruz, is a great place to get get paint, flooring, countertops and various interior materials for your home.

We selected M-100 hypo-allergenic powder compound from Murco to be used to texture the wall.  This product is formulated with no VOC’s, preservatives, mildewcides or fungicides.

The compound is mixed with water.  I was told that this product is a little bit harder to mix than the usual texturing material.

Though harder to mix application is the same effort.

Indoor Air Quality – Testing For Radon

Radon was one of the topics covered in the Indoor Air Quality class we took recently.  We understand that the concentration of radon gasses that naturally come out from the soil various from region to region and in some cases from house to house.  Our instructor, Rich Prill, Washington State University’s Extension Energy Program, mentioned that one of his clients found higher level of radon compared to the neighbor’s houses because there is a subterranean stream running under the client’s house some 10-20 feet below surface that affects the gas composition of the soil under the house.  So, even if the radon zone map of California show that we are in a moderate area (predicted average indoor radon screening level between 2 and 4 pCi/L), we should test for it.

What is radon?  
Radon is a noble gas that doesn’t react with anything else.  It’s present in our soils, rocks and sometimes water.  The problem with radon is that it has a 3.8 day half life during which it goes through radio active decay, releasing alpha particles that can cause lung damage.  So, if you breathe in radon, which is a colorless and odorless gas, these large alpha particles can damage and mutate cells in lining of your lungs.  Radon is the second leading cause of lung cancer after smoking.

So where does it come from?  You’ve heard of uranium, right?  Uranium 238 is the most prevalent naturally occurring radioactive material in the ground.  It has a half life of 4,500,000,000 years.  Uranium 238 eventually turns into radium 226, which has a half life of 1590 years.  Then radium 226 turns into radon, which has a half life of 3.8 days. At this  point you may ask, “What is half life?”  Half life means the period of time after which only half the mass of the original radioactive element remains.  So, suppose a radioactive material starts with 100% of a radioactive material, after a single span of its half-life only 50% of that radioactive material would remain. The other 50% of the material would have converted either into energy or another element which may or may not be radioactive.

Why do you want to test for it?
Radon is a colorless, odorless gas and it occurs naturally and it’s in our soil at different concentration.  The gas comes out of the ground and it can get into your house if the floor is not airtight.  Most houses with crawl space like ours is not airtight.  We know it’s not airtight because we can smell the dampness and mold from the crawl space when it rains.  So, if there is any radon in the ground, it would certainly get into the house.  In fact, when it rains the water in the ground acts like a air and vapor barrier so any radon gasses in the ground would look for path of least resistance, or dry soil to release.  The dry area happens to be under the house so when it rains the crawl space may have extra concentration of radon.

The testing result could be different from day to day, depending on the season, rain and stack effect.

EPA gathers data from the various testing conducted by labs and publishes results (aggregated by zip codes) on a map.  California Department of Health publishes a report that lists the number of tests conducted in a given zipcode as well as the number of tests that resulted in level above the EPA threshold of 4pCi/L.  You can find the PDF here.

Next Steps
So, I just bought some short term radon test kits at the neighborhood hardware store and will test the house.  If the results show a level higher than 4pCi/L then we would take measures to mitigate it by putting in a pipe under the plastic sheet used for sealing off the crawl space and venting it to the outside.  You can find a PDF of radon resistant construction here.

UPDATE:  Our result for the short term (96-hour) Radon test was .2pCi/L, which is quite low!

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