Using ICF makes it easy and logical to go with an unvented conditioned crawl space. Having indoor air instead of outdoor air circulating through the crawl space keeps it dryer and minimizes, or eliminates, mold problems. That is, after the house is complete and being lived in!

But, and it is a big but, the unvented part can bite you in the rear during construction. I just had a sickening revelation a couple of days ago.

After I put down the subfloor over the crawl space the only decent sized opening into it was the access hole. However, there are thousands of small holes, as in space between subfloor sheets, gaps between the ledger board and wall, nail holes, etc. Well, guess what? Rain water can easily move down into the crawl space but lots of air can't. We had several inches of rain in the weeks between the time the subfloor was put down and the roof sheating got put on. I sensed the crawl space was fairly humid, and in fact there was standing water for several weeks on the rat slab I have down there. I finally vacuumed the water out and installed a fan over the access hole to force ventilating air down there. Too late. Mold had already started to form on the underside of the subfloor and on the I joists. A couple of days ago a contractor I had come over to give me a bid to thoroughly clean the crawl space pointed out the mold.

So now I have, fortunately not very serious, mold to clean out in addition to all the sawdust, dirt, foam particles, etc. that has accumulated after the floor was installed.

Next time I build an ICF house, and I do plan to build another one, I will cut holes in the subfloor for the heating/cooling registers as soon as the floor is down. I will then place a blower over the access hole and make sure the crawl space gets ventilated periodically as needed, especially after a rain. I also won't leave standing water down there either.

I recommend you consider doing the same.

That sounds like 'very' good advice!!!

Another way to minimize construction water intrusion into the unvented crawl space is to install a 4" drainage pipe to daylight underneath the ratslab. This will take care of the gross amounts of water. It does not eliminate having to dry out the area with a blower or dehumidifier to get it below 65% humidity. 65% humidity is the magic number at what stage the mold spores proliferate.

I have that with an opening, with a strainer, at the surface of the rat slab. I'll put a plug in it when we move in. I got rid of the gross amount of water that way. The standing water stayed behind in a couple of low areas of the slab.

I've wondered about the humidity level at which mold proliferates. That's a good number to know. Thanks.

Mac, I guess it wasn't possible to wait until the roof was on to put the subfloor in over the crawl space?

We have done insulated crawlspaces using a 2" "bull floated" slab over 6mil visqueen, and sump drain in middle  -just in case

Possible, maybe. Practical, no. We built the walls from off the subfloor and used it for bracing. The lot has a 4' slope from one corner of the house to the other. One part of the crawl space is 5' high!

I see. I knew there must be a good reason why you did it in that order, because you seem very good at planning and problem solving.

Humid or soaking crawl spaces are the main cause of mold contamination, insects, and structural decompose & also the reason for bacterial infections. If you keep crawl spaces always dry and hygienic then you can protect yourself from unwanted disease. For that you can use dehumidifier, but before that consider these safety points :

• Look for standing water in the crawl area? If it is there there are chances of electrical shock .

 

• Look for excessive debris in the crawl area

 

• Look for chemical odors in the crawl space

 

• Look for evidence of rodents or snakes or insect pests in the crawl space?

After ensuring these points you are ready to use your crawl space dehumidifier and making you crawl space dry and ready to use .

I saw the results of someone who closed their crawlspace vents and had with all kinds of black mold weakening the joists.

I'd either 1) completely seal the crawlspace with a dehumidifier or 2) make sure there is no exposed concrete* and use a humdistat controlled fan to the interior. Both solutions with an alarm in case the mechanicals fail.

* - consider what happens when the concrete is still cool from earlier, the house has the windows open, is 80F and 95% humidity and some of this air goes into the crawlspace. 

Building code requires ventillation of crawl spaces. About 1 sq. in/ sq. ft.

Posted By Jerry D. Coombs, PE on 18 Mar 2010 12:01 AM
Building code requires ventillation of crawl spaces. About 1 sq. in/ sq. ft.

Not if it is conditioned space!

Posted By robsam on 13 Mar 2010 06:24 AM

Humid or soaking crawl spaces are the main cause of mold contamination, insects, and structural decompose & also the reason for bacterial infections. If you keep crawl spaces always dry and hygienic then you can protect yourself from unwanted disease. For that you can use dehumidifier, but before that consider these safety points :

• Look for standing water in the crawl area? If it is there there are chances of electrical shock .

 

• Look for excessive debris in the crawl area

 

• Look for chemical odors in the crawl space

 

• Look for evidence of rodents or snakes or insect pests in the crawl space?

After ensuring these points you are ready to use your crawl space dehumidifier and making you crawl space dry and ready to use .

If  the crawl space is sealed, insulated and conditioned you will not experience any of these problems