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Unvented crawl space mechanical ventilation question

dsh1109 | Posted in General Questions on


I am building a diy ICF house on a crawlspace (ICF from footer to roofline) in Louisville, KY Climate Zone 4A. I planned on having an unvented crawl space and have a question in regards to the energy efficiency of what I am thinking. I was hoping to save on the HVAC installation cost by installing a 36k BTU 4 zone mini split diy mini split and using supplemental gas or electric heat when it gets to cold outside for the heat pumps. The house will be roughly 1750 sq ft. 

Code book reads like so on unvented crawl spaces
One of the following is provided for the under-floor space:

  1. 2.1 Continuously operated mechanical exhaust ventilation at a rate equal to 1 cubic foot per minute (0.47 L/s) for each 50 square feet (4.7 m2) of crawl space floor area, including an air pathway to the common area (such as a duct or transfer grille), and perimeter walls insulated in accordance with Section N1102.2.11 of this code.
  2. 2.2 Conditioned air supply sized to deliver at a rate equal to 1 cubic foot per minute (0.47 L/s) for each 50 square feet (4.7 m2) of under-floor area, including a return air pathway to the common area (such as a duct or transfer grille), and perimeter walls insulated in accordance with Section N1102.2.11 of this code.
  3. 2.3 Plenum in existing structures complying with Section M1601.5, if under-floor space is used as a plenum.

I was planning on option 2.1 installing a bathroom vent fan in the sealed crawlspace venting to outside and a one transfer grille in the floor to allow air flow from the living space.  I see per code I would only need 35CFM (1750/50=35CFM) on this vent fan but was planning on a 50CFM unit or so. My question is this option for an unvented crawl seems kind of wasteful and would negate the effects of a air tight well insulated house if I am constantly blowing out conditioned air compared to the other two options. However If I use the other two options I will need to either buy another mini split for the crawlspace or hire an HVAC contractor to run duct work and use a normal ducted system. What do you all think is the cheapest / most energy efficient option?

Last question, am I reading the code book wrong all together and it is not saying to vent air outdoors from the crawl space, but option 2.1 is actually talking about having an exhaust fan pull air from the crawl space and venting to the main floor with a transfer grille to provide circulation or vice versa?

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  1. walta100 | | #1

    In my opinion you are not looking at what the code is trying to tell you.

    The crawlspace needs to be vent to the outdoors or conditioned so that the crawlspace is more or less the same temp and humidity as the rest of the house.

    2.1 You must blow the more or less conditioned air out of the crawlspace into the outdoors install vent so that air will be replaced with air from the living space. Note all of the air exhausted will be replaced with unconditioned air that will find a way to leak into the house.

    2.2 You install supply registers in the crawlspace and return registers to the living space.

    2.3 You use the crawlspace as your return air ductwork.

    When people can’t bring themselves to call it a “conditioned crawlspace” and use words like encapsulated or unvented it makes me think they are looking for the free lunch of not having to spend money to condition the air in the crawlspace.

    If you do not condition the air in the crawlspace, from time to time the walls of the crawlspace will get cooler than the dew point of the air in the crawlspace. When that happens the walls will get wet. This is a game of Russian roulette will the wall get wet enough for long enough for mold to grow and wood to rot.


  2. Expert Member


    Part of this depends on how comfortable you will be sharing the air from your crawlspace with the living spaces above. Even newly built crawlspaces aren't usually kept very clean, and are more likely to be damp or harbour radon. The advantage of venting to the outside is that it helps to offset this.

    If you are fine with sharing air, a cheaper option that an extra mini-split or more ducting might be to just add a fan moving air from the main floor down to the crawlspace.

  3. Expert Member
    Akos | | #3

    ICF houses tend to be pretty air tight, which means you'll need mechanical ventilation.

    The simplest way to condition and vent your crawlspace is to install a stale air pickup for your HRV/ERV there. Size the pickup to provide the code required ventilation airflow. This is much better use of the ERV stale air pickup than trying to vent a rarely used powder room or bathroom.

    Besides being much more efficient than exhaust only, venting your crawlspace this way has the benefit of not mixing the air with the rest of the house and removing any radon.

    1. dsh1109 | | #4

      Thank you for the very helpful reply. You have opened up a whole new can of worms for me with the HRV/ERV thing that I have never heard about. I am slightly confused on how to proceed. Can you tell me where you would place all of the stale air pickups and have the distributions go to? Just pick up in the crawl and back to the crawl or other areas too? My assumption from what I looked at is an HRV/ERV picks up air from the house and outside mixes/cleans it and then redistributes to the house in certain areas.

      I've included a floor plan if that would be helpful its a 4 bedroom. Mini splits will be in the 3 bedrooms not including the guest bedroom and in the main living area.

    2. exeric | | #8

      Akos, this is a really interesting "out of the box" idea. It seems to make a lot of sense. It solves several problems at once as you noted. I hope people here do not just dismiss the idea because it's so unconventional and start considering using an ERV's return air duct to help condition the crawl space air with the already conditioned house air.

      I just couldn't let such a good idea go unremarked upon. The OP could do far worse than taking you up on that idea.

  4. Jon_R | | #5

    The latest model code also provides an alternative of sticking a dehumidifier in the crawlspace. But a crawlspace ERV return will provide better odor and radon control.

  5. dsh1109 | | #6

    Looking into costs and radon mitigation I believe I will go with a option 2.1 with a exhaust to outdoors and a transfer grille from main floor to crawl space for now and comfort, time, and money calls for it do an ERV system later. Thanks for the help guys.

  6. Expert Member
    Akos | | #7

    I would run your place through one of the on-line load calculators (coolcalc or loadcalc.ent).

    Based on the design, my guess that smallest four zone multi split would be about 2x oversized at design conditions. This means it would be ridiculously oversized most of the time which will cause it to cycle. This tends to increase energy use by a fair bit and will cause comfort issues on the smaller zones as they tend to get overheated or overcooled as refrigerant is bypassed through them.

    If your budget is tight, the best bet is a single wallmount in the living space and resistance heat for the bedrooms. During the day, as long as the doors are open, most of the heat will be from the mini split so the resistance heat will not need to run much.

    There is no need for propane or other backup. A decent cold climate mini split will easily heat your place. Add in the resistance heat in the bedrooms, you'll have enough to make it through even the worst polar vortex.

    If this option doesn't work, the next best thing is a single ducted heat pump for the entire house.

    This can be mounted in the crawlspace with simple runs to each room. You can even make a nice access hatch for it from the main floor by framing a large opening in the floor of closet of the guest bedroom and putting the floor there on a hinge.

    You can look at MrCool's Universal or Midea DLCSRBH24AAK/DLFSAAH24XAK which would cost less than your four zone multi split.

    As for ventilation, something like a Panasonic Intellibalance 100 is not much more than the cost of two decent bathroom exhaust fans. Besides the energy efficiency improvement, this will give you much better indoor air quality. You really have to experience it, it is hard to describe how nice it is to walk into your home and to have the air always feel fresh.

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