Editor’s Note: This is one of a series of blogs by David Goodyear describing the construction of his new home in Flatrock, Newfoundland, the first in the province built to the Passive House standard. You’ll find his complete blog here.
In a previous post I talked a little about food security. I tried to focus some of the landscape in our yard on producing food — and it was a huge success! We had a steady supply of baby root vegetables and greens until the end of September. The garden was quite a bit of work but it was amazingly enjoyable to watch it flourish and be able to pick vegetables throughout the summer. I battled nature and learned a lot from my planting mistakes. I will put those lessons to work next year as I add new crops.
Getting ready for the fall harvest was no easy task. I had never owned a cellar before now, nor had I harvested or stored vegetables. A lot of effort went into researching the old methods of root cellaring vegetables to keep them fresh for a long time. Storage for most root vegetables is basically about creating underground conditions to keep the vegetable “alive.”
A low, constant temperature (less than 5°C, or 41°F) with high humidity (95% plus) is necessary and can be achieved by adjusting dampers to regulate air flow through the ventilation stacks inside the cellar. Luckily, my father and my father in-law have been around a few cellars. They grew up in a time when every family had a garden. It was a necessity. They provided me with much of the knowledge needed to create good storage conditions.
Storage is pretty easy for potatoes. A large bin with slatted sides promotes air movement around the potatoes. I made some bins from leftover flooring, and they also proved useful for storing carrots, beets, and parsnips.
My fall harvest provided our family with a great bounty. Some of the root vegetables were ready for harvest through the summer so we picked away while the garden continued to grow. Nature provided us with turnip greens, lettuce, beet greens, and a variety of small vegetables, including carrots, rutabagas, potatoes, beets and onions.
By October the potato plants had completely died back and I let their skins “harden off” in the ground to toughen them for storage. By mid-October we pulled the potato plants. We left them to dry for a day, and moved them directly to the cellar. We ended up with about 300 pounds.
The cellar at this point was cool (less than 10°C [50°F]) but humidity was low. A perfect combination to promote drying for a couple of weeks to further harden-off the skins before closing up the door for winter. I left most the vegetables (carrots, parsnips, cabbage, beets) in the ground until the beginning of November while I waited for the root cellar to cool. Gently pulling them (on a cold day) to ensure minimal damage, cutting off the greens and leaving the dirt on them is all you really need to do for preparation.
They were then stacked inside of a burlap lined crate with sawdust between each of the layers. I paid careful attention to keep the roots from touching each other as that can promote rot in storage. One month later, the carrots had little green sprouts growing out the top, a good sign that the vegetables are still alive and kicking!
Although the snow-covered garden is a beautiful sight, looking out at an empty garden does lead to some heavy feelings. This being said, I get to enjoy the fruits of our labor (or should I say vegetables) for the whole winter and will start again in the spring. Creating food security does take work. Some of the work is easy. Some of the work is hard. The end game is all pure enjoyment! Going to your personal grocery store on a cold winter day to pick roots for a warm winter stew is priceless. Next year I plan on adding more perishable varieties of vegetables that lend themselves well to canning. Now that the days are short and the nights are long, I guess I’ll have plenty of time to research what comes next.
Airtightness and indoor air quality
It was a fairly cold fall this year with a noticeable drop in temperature around mid-September. At the same time, it was somewhat humid, as would be expected for our climate. Humidity inside the house was high. After a purge with open windows on a dry day, humidity would drop dramatically in a very short period of time. After closing the windows humidity would continue to climb over several days until we would have to purge again.
Although the temperature inside the house was comfortable, the high humidity sometimes made it feel stuffy. Running a dehumidifier wasn’t much of an option because dehumidifiers just add sensible heat to the living space. Our heat-pump water heater helped a little, but our hot water usage was just too low for it to be effective.
I really should have listened to my Passive House designer from the beginning and installed a minisplit. Lesson learned! From what I recall, his primary concern wasn’t actually humidity. Instead, he expected the interior temperature to be an issue during the summer months, although the house has performed fine in that respect.
In any case, we opted to install a minisplit in order to address dehumidification. The minisplit, a Fujitsu 9RLS3H, made a huge difference up until the end of September and was able to keep the humidity around 50% while running in dry mode. As the exterior temperature dropped, I expected the interior humidity to fall as well, albeit more slowly than it would have with an HRV. It was much slower than expected (the opposite was true during the summer). It would take days to drop a couple of percentage points. Depending on the moisture load in the house, on some days relative humidity would increase, reach a new equilibrium humidity, then start falling again. I was concerned that as the temperature decreased, I would see a significant amount of condensation on the windows and doors. In response, we ran a dehumidifier continuously.
Construction contributes to high humidity
Many new airtight homes see high humidity levels. Where does all the moisture come from? Construction materials are filled with moisture. Drying those materials takes a long time. A concrete slab could take years to dry. However, the house had been closed in for almost a year so I suspected that most of that construction moisture would have dissipated. Given that we could purge the house of humidity quickly (it could drop 10-15 percentage points in 10-15 minutes just by opening the windows), I suspected there was some sort of ventilation issue.
I wondered whether it was the ERV, the airtightness of the house, or construction moisture. Was the current moisture load too high for ventilation to eliminate? I think the answer is a combination of things. Most houses are leaky (3-5 ach50). Even the ones that people call energy-efficient (less than 2 ach50) are still leaky when compared to the airtightness of this house — 0.36 ach50. At this leakage rate, the natural air infiltration is about 2% of the volume of the thermal boundary. At 3 ach50 the natural infiltration is about 20%. If 20% of the air in a house (under average weather conditions) is being exchanged from outside to inside I would think that the effect it has on interior humidity must be astoundingly greater than infiltration in an airtight house.
It is well known that gaining energy efficiency through airtightness can lead to indoor air quality issues if ventilation is not addressed properly. I started investigating every aspect of the ventilation system, from materials to installation methods and balancing. Then I started thinking, and here’s where it got complicated.
Tracking down the source of the problem
I lit our Walltherm stove when we moved in back in April. Smoke started leaking from the flue pipe joints. The joints are typically not sealed since the natural draft of the chimney should prevent smoke from entering the living space. When I opened the windows, the smoke infiltration stopped. The next day, I decided to do some testing. I placed my hands around the stove pipe joints and I could feel cold air leaking through them.
My first intuition was that the ERV was unbalanced, leading to depressurization of the house. I checked the airflow balance (using a digital differential manometer) according to the manufacturer’s test method and found that it was fine. After some additional experimenting, it became evident that the house was under negative pressure even though the ERV was balanced.
I increased the amount of fresh air supply to the building until the manometer read slightly positive. I tested this setup by burning a match near the stove pipe and the smoke was immediately sucked into the stove pipe joints. I had to unbalance the machine by almost 30 cubic feet per minute in order to achieve this. I left the ERV in this unbalanced state so I could use the wood stove. I decided at the time that I would revisit the issue in the fall when I had more time.
The depressurization and smoke issue seemed quite complicated and a solution eluded me for many months. I figured it was related to the ducting of the ERV, so when fall rolled around I started an investigation. My initial thoughts were that the depressurization was caused somehow by the duct configuration on the supply and return, so I switched them just to test the hypothesis. The good news was that the ERV basically remained balanced, meaning that the supply and return duct runs had nearly equivalent lengths.
I rebalanced the unit and once again could feel cold air leaking from the stove pipe joints. I decided to change the ducting at the outside hoods. I switched them and made a few other configuration changes and rebalanced the machine — with much disappointment. After rebalancing, the cold air kept seeping out the joints of the stove pipe. Each test led to the same result and further despair!
Creating neutral pressure inside the house
Then I thought that if the results are always the same it obviously has nothing to do with the unit. Instead, there must be some underlying fundamental physics going on. Then it came to me: Even a tight house is still a “leaky” plenum. Both fans in the ERV operate independently. Fresh air is being fed into the house by the supply. The exhaust has no way of differentiating where air is being drawn from. It simply pulls air from the space. It will pull some air from the fresh air sources but air will naturally start infiltrating through holes in the envelope since they offer a path of least resistance — unless they are plugged.
Plugging the holes can be done artificially by unbalancing the ERV so that the difference inside and outside the envelope is neutral. There are obviously more holes in the envelope then just the stove pipe so when pressurizing by using supply air, you have no control over which holes the supply air exits through. As a result, a large amount of air (in my case 30 cfm) could potentially be needed to effectively “plug” the holes and create neutral pressure.
I am guessing that with airtightness at 3-5 ach50, most homes are so leaky that there is little to no depressurization. I can almost bet that anybody in a Passive House who has balanced their ventilator would see depressurization if a manometer was used to measure the difference between interior and exterior pressure.
I am yet to find anybody willing to test this. If you do, please let me know! This effect is simply not unique to Flatrock! Physics is the same everywhere.
It seemed like a good idea to just continue operating the ERV in an unbalanced state to deal with the stove issue. What could possibly go wrong? The problem is that a solution to one problem can often lead to problems elsewhere. Needless to say, unbalancing the ERV had unexpected consequences. After a lot of research I determined that operating in an unbalanced state (supply greater than exhaust) will collect more of the moisture from the exhaust air stream than when it is operating in a balanced state.
When it is dry outside, interior humidity will drop much slower in this unbalanced state. In the summer, this unbalanced state will lower the moisture removing capacity of the unit and the interior humidity will climb much quicker than if the machine were balanced. The assumption during planning is that our ventilation would take care of exhausting some humidity during the winter but leave enough to be comfortable. The assumption is fine as long as the machine is balanced.
My solution for the wood stove was based on a lack of understanding about the mechanism of moisture migration from one air stream to another in the ERV core. I balanced the ERV airflows and interior humidity would drop dramatically quicker than in the unbalanced state. So operating in an unbalanced state fixes the stove issue but leads to humidity problems! This also explained why humidity climbed so quickly during the summer when it was humid outside. So another solution had to be investigated.
Another solution emerges
I had read that wood-burning appliances in tight homes can be problematic… and they can be! Good quality assurance is absolutely necessary. This being said, I feel like this ongoing depressurization issue is beyond the understanding and scope of most ventilation installers. I do think that other people who are building super-airtight houses should measure to see if the house is depressurized even though the airflows are equalized. Equality of the supply and exhaust exchange rates does not imply that the house pressure is neutral and therefore could lead to dangerous backdrafting, flooding the living space with carbon monoxide.
I read some time ago that the Zehnder brand of ventilators have a function that helps prevent depressurization. I believe it’s called “ChimneySweep.” When activated, it unbalances the unit in order to provide 10% more makeup air, and therefore pressurizes the house to prevent back-drafting of smoke into the living space. My experience was that 10% would not be enough, and in fact I required almost a 30 cubic feet per minute differential between the supply and exhaust (on a ventilation rate of 95 cfm) in order to balance to neutral pressure.
By December I had concocted a strategy that would work. I decided that a normally open motorized damper on the supply duct would do the job. I balanced the ERV unit (with the damper open) to pressurize the house, checking pressure between the inside and outside using a differential manometer. Then I adjusted the damper’s closed position so that the supply and exhaust were equal.
When I want to use the wood stove I would just flip a switch on the wall by the ERV control to force the damper to open, which would pressurize the house. It worked like a charm. If the damper motor fails, the spring-loaded damper opens and the wood stove can still be used. I feel this is a much safer solution and has redundancy built in. Unfortunately, it is another mechanical part that adds complication, but the safety factor makes it worth the complication. The added benefit of pressurizing the house is that when I add wood to the fire there are now no issues with smoke spilling into the living space. The stove now works like a charm!
So how about energy use? I am pleased. I currently heat about 3,376 square feet of interior living space, including both the house and the garage. In December 2018, we depended purely on electricity since I wasn’t ready to use the stove until I fixed the ventilation issue. The temperature in the house and the garage (woodworking shop) are both set to 20°C (68°F). We used a total of 1,576 kWh of electricity.
I have found that the heat loads of the house and garage are definitely lower than the lowest heat output of the Fujitsu heat pumps. How do I know this? Simple: The heat pumps short cycle.
This can be easily confirmed by measuring the amperage at the breaker panel and tracking how long the machine is drawing power. At 0°C, the heat load for the garage is below the lowest modulated heat output for the Fujitsu unit (3100 Btu per hour). I calculated the load to be somewhere around 2800 Btu/hour, including air infiltration and heat loss through the envelope.
The heat pump short cycles every 10 minutes or so. Unfortunately, this does affect efficiency. How much I have no idea, but operating in a steady state would be more desirable. The heat pump in the house has short cycled less. It is the same size as the model in the garage and there is more heat loss due to a larger envelope, so that makes sense.
As the nights get even colder I have witnessed longer run times and less short cycling, which is great for efficiency. I think that the usage could have been less if the heat pumps short cycled less. I am still investigating what is going on but it appears to be a common thing with heat pumps operating under low-load conditions in airtight houses. So far, our bills have been about 38% of that used in our previous home which used electricity and propane.
BLOGS BY DAVID GOODYEAR