Fresh air ventilation — exhaust fan only option in a tight envelope?
I have a small detached building that I converted to finished space, including open cell spray foam insulation in the walls (R15-ish), blown in fiberglass in the ceiling (R38), and a ductless mini split for heat/air. Floor is a concrete slab (insulation unknown, but probably none). The building is a single room.
If I am in there all day, the CO2 level slowly rises from about 500 ppm to as high as 1400+ in about 10 hours. Takes a full day or two of me being out of the building for it to drop back down to 500ish or less. I would not have guessed that my building is extremely airtight, given that it has a garage door on it (which I have insulated and sealed as well as I could), but the way the CO2 levels increase when I am in the building makes me think it must be reasonably tight.
I believe I need mechanical ventilation to keep the CO2 levels at sub-1000, preferably sub-700 ppm. ASHRAE 62.2 indicates I need ~20 cfm fresh air.
My question is this: Is an exhaust-only ventilation option likely to be sufficient in this situation? Or does exhaust-only not work well in a tight envelope?
For exhaust only, I’m considering a Panasonic Whisper Wall 70 cfm unit. If exhaust only is a bad idea, the only reasonably priced through-wall options I can locate is the TwinFresh 32 CFM ERV.
Any input appreciated!
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
With exhaust-only you have no control over where the ventilation air is entering the house, or which areas are getting the most ventilation.
The Lunos e2 ductless HRV would hit your ASRHAE 62.2 ventilation rates without pressurizing/depressurizing the house, and would be delivering the air in two locations, rather than one, as it would be with the Twinfresh. But it's more expensive (by more than 2x).
With a single open room an a single point of ventilation it would be best to put it somewhere in the middle of the room, not one location on an exterior wall. But no matter where you put it the CO2 issue would be resolved even with a single-point HRV.
That 70 CFM exhaust fan will only deliver 70 CFM at some specific back pressure, which might not be specified, but I expect is probably pretty small (small backpressure allows maximum airflow, which gives them the biggest CFM number to market their fan with). If your building is really tight, that means the fan will see a lot of resistance to airflow which is what backpressure is. You might get very little actual airflow with this type of exhaust-only setup, surely much less than the fan’s rating. Better would be the fan and some kind of air inlet across the building from the fan. Best would be the HRV option Dana suggested.
Bill
It's hard to believe that a building with a garage door is that tight. But you did the testing and know that you need to ventilate, which is great. Here are a couple of articles that may be helpful.
https://www.greenbuildingadvisor.com/article/breathe-easy-with-balanced-ventilation
https://www.greenbuildingadvisor.com/article/five-ways-to-do-balanced-ventilation
For a single room, an exhaust only system system could work without significant back-pressure by adding several passive air-vents. That would be a lot cheaper than either of the balanced ventilation options, but involves an energy penalty, as the incoming air isn't heated.
From my limited experiments with CO2 measurements in my home, CO2 levels are not a proxy for air sealing. If there is no air circulation within the building (e.g., furnace and AC fans aren't running), CO2 builds up in a closed room surprisingly (to me) fast. I think you're on the right track to install a ventilation system, and a somewhat balanced system seems better than exhaust only.
How big is the building? Is there a bathroom or is it just general space?
Where are you located (your climate might affect the best ventilation choice)?
Thanks for the responses. Generally sounds like the consensus is that an ERV would be preferable to exhaust only.
To respond to a few of the posts/questions:
I do not know for sure how tight the building is. I have just been using the rapid CO2 rise and the fact that it takes a long time for CO2 levels to dissipate after I leave as a proxy for building tightness. That said, with spray foam insulation in the walls, and assuming I did a decent job sealing up the garage door, it seems at least plausible that the envelope is reasonably tight.
There is air circulation in the building. I have a ductless mini split and a ceiling fan. The mini split indoor unit basically runs 24/7 providing at least some internal air circulation. The ceiling fan runs whenever I am in the room, providing additional circulation. But yes, point taken -- CO2 levels are probably not a great proxy for air sealing.
Building is ~300 sq ft. No bathroom. Just a large single room. Located in southeast US with hot summers, high humidity, but mostly mild winters (though we do have at least a handful of nightly lows in the 20s or so. Right now it is actually about 25 degrees outside, though it is supposed to get up into the 50s today).
This is a small space and most standard HRV/ERV and ventilation fans (like the 70 CFM Panasonic) will be overkill. Although not the cheapest options, I'd suggest considering something from the Lunos line. There are options that can meet the 20 CFM, but also go lower:
https://foursevenfive.com/products/ventilation/