How to condition basement/crawl space?

[maui19]

I am trying to condition the space below my house--about 2,100 SF. It is a cinder block "basement" that is on grade, so about 1/2 the perimeter is below grade while the rest of it is above grade. The "above grade" sections had been open, but I closed them off and installed smart vents to satisfy flood vent requirements.

My goal is to somewhat condition this space so the I can store items there without worrying about mold and rust (I am located in the South, where humidity is a big issue). One of my air handlers is in the space, along with the associated duct work that supplies the first floor.

My biggest concern is humidity in the summer. The space is never going to be really tight, but it's not going to be really leaky either. (I don't care about heat, because it never gets below 58º in the winter due to half the foundation being below grade.)

I was thinking about just punching a vent or two in the first floor ductwork to get some conditioned air in the space during the summer. the other option I'm considering is to put a mini-split down there. I feel like the mini-split might be overkill, but I'm not sure. A dehumidifier is another option, but my experience is that they tend to really heat up an enclosed space.

Thoughts?

 

[Reach4]

Start by planning a moisture barrier for the crawl space. You want to stop moisture from coming up from below.

 

[Sponsor]

 

[Fitter30]

Crawlspace dehumidifier. There are two types one is the big box store type 90% @ 80° the other has a certified rating A.H.A.M. 60% @ 80°. Box store all plastic except except for the refrigeration. $200-$400. Aham rated $1000+. Two maybe three problems with your crawlspace one is the vents the other is cement block if it isn't waterproof and 10 mil plastic not on the floor. Humidity flows from high to low. They will pull RH from the ground, through the block , vents are not air tight and will pull from the 1 st floor. I've got 1200 sq ft have honeywell 70 pint put it on a remote RH sensor and a outdoor thermostat to shut it off below 40. Also purchased a remote RH and thermometer under $20 from amazon to keep track of the condition space. Need < 40% RH to keep mold away.

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[maui19]

I should clarify that this is more like a basement than a crawl space. 9' ceilings, poured concrete floor, two spaces for cars (with garage doors.

 

[Fitter30]

I should clarify that this is more like a basement than a crawl space. 9' ceilings, poured concrete floor, two spaces for cars (with garage doors.

Is there a separate room for the garage with ceiling? Air conditioning removes .1-.3 gallon of moisture an hour per ton. Basements normally don't have much of a load so run time limited.

 

[maui19]

Is there a separate room for the garage with ceiling? Air conditioning removes .1-.3 gallon of moisture an hour per ton. Basements normally don't have much of a load so run time limited.

No it is one large room, with two garage doors at one end. The doors are insulated if that makes a difference. The room is not 100% tight from outside air, but it's fairly tight.

 

[Fitter30]

RH flows from high to low just like temperature. It will even flow from the house to the basement through the floor.

 

[maui19]

I am concerned that a dehumidifier will raise the temp in the basement. Is that a problem?

 

[wwhitney]

Because the basement is open to your garage, I believe the whole space needs to be treated as a garage. IRC M1601.6 would therefore prohibit using your existing HVAC system to condition the space, you don't want to mix garage air and living space air. Also, the whole space should be separated from the living space in accordance with IRC R302.5 and R302.6.

You could limit the applicability of these garage prohibitions by building a compliant wall to separate storage area from the garage area, with the appropriate type of door as required by R320.5.1

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https://up.codes/viewer/south_carolina/irc-2021/chapter/3/building-planning#R302.5

With that out of the way, a good way to approach dealing with the garage or storage area humidity would be:

Step 1: Air/vapor seal the space, to minimize water vapor ingress from exterior air and from the ground. [If there is a slab on grade throughout the area, do you know if there's a vapor barrier underneath it? You can do a "plastic sheet" test on the slab to get a general idea of the vapor drive from below.]
Step 2: Install a logging temperature/humidity meter, e.g. something from Ambient Weather, including an outdoor unit.
Step 3: After you have a year's worth of data (although if are getting RH numbers above 60% or 65% you may want to take some more immediate steps), you can analyze what your needs are and coming up with an appropriate plan.

If the space remains a garage, your options for humidity control are (a) dehumidifier (b) separate HVAC system (e.g. a mini split) or (c) if most of the times of excess relative humidity are times when the outdoor absolute humidity is below the absolute indoor humidity, you could use a smart fan that provides outdoor air only during those times. I would expect (c) is unlikely.

If part of the space becomes a storage area properly separated from the garage, you also have the options there of (d) add a register and a return to you existing HVAC system, or just (e) install a passive air transfer vent between the conditioned space and the storage area. The effectiveness of (e) will hard to model but it could be a reasonable approach if the magnitude of the problem is not large. Whether (d) is reasonable would require knowing the capacity of your existing HVAC system, as well as the demand your living space places on the system (e.g. from a manual J calculation or better from logging run time data on the A/C system to correlate with the temperature/humidity data. The latter could be done by putting a temperature/humidity sensor in/above of the HVAC supply registers).

Cheers, Wayne

 

[Fitter30]

Surprised that the garage has two doors and not ceiling and walls not at least 2 hour drywall.

 

[wwhitney]

Surprised that the garage has two doors and not ceiling and walls not at least 2 hour drywall.

The separation required by R302.6 is much less than a 2 hour rating. It's basically 1/2" gypsum board, bumped up to 5/8" on the ceiling if there is habitable space above. See https://up.codes/viewer/south_carolina/irc-2021/chapter/3/building-planning#R302.6

Cheers, Wayne

 

[Fitter30]

The separation required by R302.6 is much less than a 2 hour rating. It's basically 1/2" gypsum board, bumped up to 5/8" on the ceiling if there is habitable space above. See https://up.codes/viewer/south_carolina/irc-2021/chapter/3/building-planning#R302.6

Cheers, Wayne

Can see that in a attach garage not in basement. And its not separated.

 

[wwhitney]

Can see that in a attach garage not in basement. And its not separated.

I'm saying that since this "basement" has space for two cars, it's an attached garage, and as the rest of the space is not separated from the area for the cars, the entire "basement" requires R302.6 separation from the rest of the house. So hopefully this "basement" has 5/8" gypsum on the ceiling and 1/2" gypsum on the walls.

Cheers, Wayne

 

[Fitter30]

I'm saying that since this "basement" has space for two cars, it's an attached garage, and as the rest of the space is not separated from the area for the cars, the entire "basement" requires R302.6 separation from the rest of the house. So hopefully this "basement" has 5/8" gypsum on the ceiling and 1/2" gypsum on the walls.

Cheers, Wayne

Understand what u wrote. They must be in a area where there no codes. I'm in a area that i don't need a building permit. Very rural in the ozarks, Missouri. It's common for multiple families living on the same property. One of the top 5 poorest counties in the state. I'm surrounded by cattle farms 1/4 mile driveway surrounded by woods.