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drk 01-19-2011 03:25 PM

conditioned crawl space,
i have two contractors at odds on how to condition a crawl space after some mold removal....they agree on materials, cleaning, etc. but they disagree on venting. one says to just put a positive air vent at one end of the house and the other says to put a return air vent in the crawl space...any opinions as to which is better....or any pro and cons

AndyGump 01-19-2011 03:47 PM

So you actually mean to condition a crawl space?

Or are you looking to vent the space for moisture removal and/or to keep it dry?

I have never heard of putting a return air in a crawl space, what is the purpose of that?

Thank you .


drk 01-19-2011 04:10 PM

conditioned crawl space
andy....the 'modern' theory of crawl space conditioning say to seal off all vents, put down wall and covering for the dirt floor and pipe your heat and ac into the crawl space ...the opposing views are one say just put a positive vent to the outside of the house and the other says to put a return air vent into the return air duct...this position maintains that by doing so the system is closed and air temperature and moisture are close throughout the home....essentially making your crawl space like a basement in terms of air (temp and moisture).....the purpose of the project is to eliminate excessive moisture and hencemold spores. I am wondering if there are any benefits to the vent vs the air return....the air return guy says why pump conditioned air and $ to the outside. any opinions appreciated

COLDIRON 01-19-2011 04:27 PM

I would install 4 or 5 vents in the crawlspace and just naturally let it ventilate. Maybe 2 in front and 2 in back, depends. Make sure you don't have anything causing moisture down there, IE: Dryer vents, water leaks, etc.

I would not hook up a return air from there.

1acman 01-19-2011 05:20 PM

Have you thought of sealing the crawl and then installing a dehumidifier to maintain a mold free level of humidity? You can also add fresh air and ventilate it on demand or timed. Aprilaire makes a good dehumidifier, so does thermastor and honeywell. I have not seen anyone add supply or return air to a crawl and do not think I would recommend it.

Gary in WA 01-19-2011 06:31 PM

I’m not HVAC but I pulled these from my library, maybe you can gleen something from them….
Here are the code choices;

AC and the crawl;

Cold climates;

Plan for radon if needed;

Cover the foam board on the walls;

I found this very interesting;


1acman 01-19-2011 07:08 PM

Good info, thanks Gary.

timchi29 01-19-2011 09:07 PM

I bought a house and it had a dirt crawlspace the entire size of the house. I had 4" of concrete pumped in for the floor (have 46" of clearance as of today), installed insulation on the walls and have a dehumidifier. I live in Chicgao and it maintains 60-65 degrees all year with a humidity level of 45%. it is a great storage dry storage area.

Earnie 01-20-2011 06:42 AM

Sealing the crawlspace is the next project on my list.

Due to the unseen termite/bug possibility behind the block wall vapor barrier, I plan to coat the block walls with Zinsser WaterTite vs. installing 16 mil vapor barrier up the walls. If the Zinsser product does not adequately control air and moisture, I can still add the 16 mil to the walls.

beenthere 01-20-2011 03:28 PM

A supply at one end, and a return at the other.

or, as above, in some cases, a dehumidifier will work as well. but can cost more to operate.

drk 01-20-2011 04:44 PM

crawl space
To been there or anyone else:

i have a 2500 sq ft irregular shaped crawl space....i have been told to put two 6"supply vents roughly 12ft from outside walls for heat and air and to put a 12' return vent in the middle of the crawl space on the return trunk this is to be done after remediation of mold (not much but some in various spots) and the placement of a vapor barrier and the sealing of all vents...what do you think? also would you add a dehumider? thanks dan

COLDIRON 01-20-2011 04:56 PM

Am I understanding this right? When return and supply are mentioned does that mean vents with it's own fan independent of the house HVAC system ? If so I would definitely recommend that to keep fresh air moving through the crawl.

beenthere 01-20-2011 05:06 PM

Don't really need to condition the crawlspace to human comfort standards. And it doesn't take much to warm a crawl space up.

I would probably only put in 2 supplies(one at each end) because of your description of shape, and a 8" return.

Good chance you'll need the dehumidifier in the summer to keep the humidity down in the mild times of the year, when the A/C hardly runs.

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