GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter X Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

EVR sizing for a small portion of a normal size house?

mackworth | Posted in Mechanicals on


TLDR: I want to put an EVR on my second floor only but I can’t find once small enough.

I am located in the northern climate zone, in Massachusetts. My house is about 1950 sq/ft and was built in 1915 but has been renovated twice in the last 10 years that have adversely affected indoor air quality, especially on the 2nd floor. I have no AC currently but its a long term goal.

My 2nd floor is only about 750 sq/feet for 4 bedrooms and 2 baths. This floor was entirely renovated down the studs which included new windows, vapor tight boxes, certainties membrane on walls/ceilings. It’s probably about as tight as you can make a 1915 house without spray foam.

Maybe about a 3rd of the 1st floor has spray foam and the rest is Certainteed membrane. The basement is a very leaky stone foundation that will probably stay leaky forever.

My second floor can get very stale and I would love to add some fresh air to it. I was hoping to install an EVR in a conditioned space in my attic to add fresh air to my 4 bedrooms and put an intake in my hallway. The problem I am running into is that I am not sure how to size the EVR. If I only wanted to provide fresh air to the 4 bedrooms and a return in the hallway, do I only take into account the square footage of those 4 rooms and ignore the 2 bathrooms and the rest of the house? If so, that’s only 450sq roughly which with an air change of .35 is only 20CFM which the EVRs I can find seem not to go down that low. Should I be taking into account every even if I don’t plan on ducting into the floor?

It’s possible that I could maybe give up some closet space to duct down either fresh air or pull stale are from my 1st floor is really necessary, but I am hoping to avoid that. I would also like to avoid a Lunos system.

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. matthew25 | | #1

    How do you know the air is stale? Are you measuring CO2 to determine that? I would propose maybe improved air circulation or conditioning might be all you need unless you have a reason to think outdoor air will help.

  2. Expert Member
    Michael Maines | | #2

    There's not a big downside to slightly over-ventilating with an ERV. In fact, some building science experts recommend going beyond ASHRAE 62.2 requirements.

    The reality is that all of the air in your home is connected, even the basement or crawlspace air. I would size the system to ventilate the whole house, even if you're only ducting to a few rooms.

    The units I'm familiar with can be set to run on a timer, so over a 24-hour period you can ventilate to pretty much any level you want.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |