Moisture in Crawl Soace
gregxyz123456
| Posted in General Questions on
I have a crawl space under an exterior concrete patio (only exterior above patio). The patio is supported by a steel pan and wood structure bc I live on a steep hill. Moisture from humid air condenses on the steel pan (the ceiling of the crawl space). I am in the process of sealing water and air infiltration and the dirt floor of the crawl space.
Should I also insulate the steel pan ceiling to reduce condensation? If so, how should I insulate it (spray foam, open or closed cell?). I’ve read about only insulating the walls of a crawl space but I don’t indoor living space above the crawl space.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Search and download construction details
Replies
+1 on (depending on climate) blocking the moisture from the floor and walls and then venting away what little moisture makes it past that.
You can air seal it well, install vapor barriers and then put in a low temp capable dehumidifier. More complex, but it will be very effective and it won't run enough to use much energy.
That’s the plan. Thanks
Interesting question. What climate are you in? You might want to put a vapor barrier on the dirt floor but keep the space vented.
I’m in upstate SC. It’s very humid here frequently, even during the winter. I have a vapor barrier which I am upgrading and tried operating it with vents in the walls. The vents bring in humid air which condenses on the metal pan ceiling. It’s almost raining inside because of the condensation. It must be operated as a closed system to avoid continuously bringing in moisture. I may even need to run a dehumidifier. I was just wondering if taking away the cold condensation surface would be a further enhancement.
Given that climate and the experience you report, I think I'd focus on corrosion protection rather than trying to keep it dry. If you sealed the steel from the bottom with spray foam, you would still have moisture coming down from above, so I think the steel is going to be damp most of the time. I assume it's galvanized? You could simply inspect every five years and plan to replace when it looks too bad. Maybe add a sacrificial anode, or spray with car underbody protection stuff--perhaps linseed oil based if you want something non-toxic. https://www.solventfreepaint.com/vehicle-rust-proofing.htm
Traditionally, a space like that would have been left as-is. Having no communication with the living space, it doesn't really matter if it rains in the crawl. Water raining from the steel decking would just soak into the ground, re-evaporate, and recycle. Over the very long term (50-100 years), the steel pan will eventually rust out completely. Depending on how the concrete patio was reinforced (or not), it might eventually crack and settle, and then the whole thing gets torn down and rebuilt.
Stopping this process will be somewhat expensive and may or may not improve matters much. Spray foam on the metal deck would limit condensation when the outside air is cooler than your ground temperature. It won't help a bit when the condensation is forming from ingress of outdoor air with a dewpoint higher than your ground temperature. The condensation will just happen on the surface of the foam instead. Adding a vapor barrier to the floor and closing the vents will also probably not help much. The small amount of moisture that soaks through will still accumulate in the space. This is why we refer to closed crawl spaces as "sealed and conditioned" crawl spaces. You need to do both to stop the condensation. In your case, that probably means running a dehumidifier in the crawl space year-round. If you do a very good job of sealing the space, the energy cost of dehumidification should be low. Spray foam on the metal deck will help in this case to keep the space warmer in winter, with heat being produced by the dehumidifier.
I had a similar assembly in a previous house. Make sure the front of the porch has a drip edge (essentially a groove cut into the cement). Otherwise, capillary action can move a lot of water into the crawlspace whenever it rains.
Spray foam would be your only real option to insulate the steel because you need the insulation to adhere to the steel (no gaps). If you were to use something like rigid foam, you'd end up with bulk water accumulating between the foam and the steel and that would lead to more problems.
What I would do is just put a rust inhibitor on the steel and call it good. Go over the steel with an abrasive pad and stiff wire brush to get rid of the worst of the rust (you're mostly just trying get the loose rust off), then put a coat of Rustoleum's "rusty metal primer" on the entire surface of the steel -- everything you can access. That primer can be left as is (it's brown), or painted with one of their enamels. I like to use the "gloss smoke gray" over the primer on exposed steel beams for example.
Once you've painted the steel like this, it will stop rusting (more accurately, rusting will be greatly slowed down). After that, I'd just seal the space off from the living area and leave it alone as others have mentioned. You're really just trying to protect the structure there from degrading, similar to what you'd do with an exposed outdoor structure. There is no need to seal and insulate such a space if it's not tied into the living spaces.
Bill
Thanks for all the comments. They are very helpful. The reason I’m trying to reduce/eliminate water is the structure inside the crawl space is untreated lumber and the OSB wall is getting damp. I don’t want it to fall apart from all the moisture, but mainly I don’t want some to fret about it when I sell the place. Despite all the moisture over 10 years, the metal pan and 2x6 studs are still in good shape but the OSB wall is clearly degrading (the 3/4” cementitious stucco on the outside is fine).
Oh, the untreated wood and OSB are parts that I hadn't been thinking about. I'm not sure I understand the overall structure--is there a foundation, and then a very short 2x6 wall on top of the foundation, and then the metal pan on top of that? With wood joists too? How high is the 2x6 wall? Or is it just 2x6 joists and the OSB is just covering the ends of them?
The 2x6 wall that is covered with OSB ranges from 4’ to 14’ high ( I’m on a hill that slopes away to to the right). The metal pan (sitting on concrete foundation uphill and the 2x6 wall downhill) with concrete on the pan form a patio above the crawl space. So I really need to maintain the integrity of the 2x6 wall or support the pan in another fashion.
There are no joists. The metal pan is supported by the above noted walls and a mid-pan beam. The metal pan deadheads into the OSB wall (which is covered by stucco).
That's a whole 'nuther issue. With untreated structural wood supports, yes, you really do have to keep it dry. Sealed and conditioned is really your best bet.
You can paint the untreated wood with Coppercoat, which is a preservative paint. It's a little like the stuff they infuse into treater lumber, but since it's only painted on, it's not as good -- it's only on the outermost part of the wood. It is better than nothing though, and does help to keep things from starting to rot or mold.
Bill