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Unvented Crawlspace Air mixing

I am currently building a house with an unvented crawlspace, SE Tennessee, hot humid 3 seasons. The construction is as follows: Engineered wall system (concrete with embedded metal studs, insulated) with a 2" concrete slab through the crawlspace, with barrier installed under slab. No openings to the outside, access to the crawlspace located in the partial basement. 4' tall walls. In other words, the crawlspace is built like a 4' tall basement connected to an actual basement.

I am currently weighing my ERV/HRV/air mixing options. I need to decide what the best approach is to dealing with the crawlspace air. Since it is constructed as a mini basement, do I treat this as a good source of air and encourage mixing this with the house in order to dehumidify the crawlspace air? Or is it best to separate this air as its own thing?

One option I have considered is supplying fresh air to the crawlspace through an ERV and exhausting air from the master bathroom. Then providing open registers from the crawl to several locations in the house above. As the house is ventilated (hopefully) it will draw the fresh air from the crawlspace into the house and condition. I am thinking this will take care of humidity issues in the crawl through conditioning with the house, create an easy way to distribute fresh air throughout the house, and hopefully provide a known source of makeup air as negative pressure is created from the exhaust.

The flipside is, would drawing this air from the crawlspace have a negative effect and actually draw moisture and humidity from the ground and create a constant battle with this in the house. In other words, I can't decide if this crawlspace is a "good" or "bad" source of air. Thoughts?

Asked by Bryan C.B.O.
Posted Feb 8, 2018 4:55 PM ET


3 Answers

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You may want to read this article (http://www.greenbuildingadvisor.com/blogs/dept/musings/building-unvented...) on unvented crawl spaces. As the article notes, you can install a floor vent to allow air movement between the conditioned and unconditioned areas.

Does your ventilation plan include ducts that are separate from your HVAC system?

Do you have a small opening between the crawl space and basement?

Answered by Steve Knapp CZ 3A Georgia
Posted Feb 8, 2018 6:56 PM ET
Edited Feb 9, 2018 1:28 PM ET.


Steve steered you in the right direction. Code requirements are explained in one of my articles, Building an Unvented Crawl Space. Whether or not the crawl space has "good air" or "bad air" is a matter of some debate; if you are worried, install a small exhaust fan in the crawl space rim joist, blowing out, and provide makeup air through a grille in the crawl space ceiling (the floor above).

While the article explains requirements in the International Residential Code, your local code may have different requirements -- so check with your local building department.

Answered by Martin Holladay
Posted Feb 9, 2018 5:40 AM ET


If there are radon or mold smell issues, then crawlspace air is bad and should be exhausted.

Summer exterior air that has passed through an ERV is not as dry as interior air (ie, it's less effective at keeping it condensation free).

Answered by Jon R
Posted Feb 9, 2018 7:42 PM ET

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