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Using an ERV for constant positive pressure

marleyandbowie | Posted in General Questions on

I own a single story mid-century home built partially on a basement and partially on a crawlspace. Due to decades of moisture and rodent issues the air quality in these spaces is pretty nasty. While I have greatly improved the situation in my short time here, there is still a lot of work to be done and it may take me another year to fully clean and renovate the interior foundation spaces. In the meantime I have completely sealed off the basement/crawlspace and have been using exhaust fans to keep these spaces under negative pressure which has been helping a great deal in preventing that air from coming up into the living space. I also did my best to seal all penetrations in the living space ceilings so that the attic is pretty tight.

I’m now realizing that it could make sense to keep the living space under slight positive pressure at all times, and I’m wondering if I should do this with an ERV or HRV system that allows for custom/unbalanced ventilation. We currently don’t have an HVAC system, we are using wood and propane stoves during the winter and window AC + dehumidifiers during the summer, so I think decentralized/ductless would be preferable but if there is a highly recommended centralized unit that would be better for this I could install a ducted unit in a central closet with some short duct runs. Would love any suggestions on equipment, roughly what CFM rating I should be looking at, if it makes more sense to go HRV or ERV, etc.

Relevant details: It’s one of those weird mid-century ranches cobbled together over decades. 1900 sq ft open concept floor plan, 2×4 framing with a mix of fiberglass and polyiso in the walls, partially vented attic partially cathedral, avg tightness for a mid-century home updated in the late 80s – early 2000s, mix of old and new windows. Southeast NH, Zone 5B.


  1. acrobaticnurse_Eli | | #1

    It's unlikely that you could make your house truly positive pressure without substantial equipment and/or a lot of air sealing, and you would have an easier time increasing the negative pressure on your crawl space, but if you just want to lean towards positive I'd recommend bringing in fresh air via a dehumidifier such as Santa Fe. My Santa Fe Advance 90 is not currently ducted, but could be set up with a duct kit to bring in ~65 cfm of fresh air. My Broan 210CFM ERV can be programmed to have a positive balance, but seems to only allow about a 50 cfm difference. If I program it to bring in 170 cfm it will require me to send out at least 121 cfm, just 49 cfm positive which is less than my similarly priced dehumidifier. You could do both an ERV and ventilating dehumidifier, but it's not clear if that's really necessary. If outdoor air is clean, and temperature and humidity are fine you could just put a fan in the window, or a blower door in the doorway ;-). Add a filter if needed. I've seen videos focused on essentially a fan and a filter in the window for getting fresh air when the outdoor weather is mostly good. 

    I liked Corbett's video for going over the math on the difficulty of making a house positive pressure here:  He went over a free calculator for seeing how much cfm would be needed to induce positive pressure.

    When I worked as a nurse in covid ICUs, some units had loud hepa filters that vented air out of the patient room. They were so loud it was hard to hear what anyone was saying and it was difficult to keep the rooms cool in the summer or warm in the winter, but it achieved negative pressure in rooms that hadn't originally been set up for it.  For rooms that were designed to have negative pressure when they were built there was no noise and conditioning wasn't a problem and we had monitoring equipment to verify that negative pressure was achieved vs just holding a tissue to the door to see if was sucked in (something you could do to see if your crawlspace is negative pressure). I later worked in another ICU that used the same HEPA filters but didn't vent anything out of the room, they just focused on filtering it. Still loud but the temperature was more bearable. It can be important to know what you're trying to reduce exposure to, and if it's something you can filter out, if you need to vent it out, or if you've successfully sealed it out. 

    For now I have an encapsulated crawlspace with a dehumidifier that has a merv 13 filter and a continuous radon monitor in the main living space to see if radon ever becomes an issue. In the main living space I have a hepa filter in the main social room, the main bedroom, and the guest room, along with an in-wall dehumidifier. I recently added an ERV since I realized HEPA doesn't filter everything and I've appreciated the drop in CO2. I've thought about adding other monitoring to see if I should be doing anything else, but for now as long as CO2, radon, and humidity seem fine, and the temperature feels comfortable, I am likely ok. 

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