Hello all,
Hope this is the correct forum and I am only a DIYer, not a professional –
We have a basement crawlspace with gravel & dirt at bottom, wood framing in middle and plywood on top. No idea if there is a plastic barrier under all the gravel. However I found mouse droppings and excessive relative humidity @ ~70% here. I checked under the edge of the plywood and can see wood framing and then gravel dirt.
Taking out the wood framing is a large job and am not sure if it is connected to other parts of the basement as well. In addition, I would rather not disturb the radon pipe installed in the middle.
To remediate moisture issues and prevent pests, I started covering the floor with DMX 1 step Underlayment. This reduced humidity to ~55-60% and seems to be working. I plan to put in a 10 mil plastic vapor barrier to seal it all up.
However, would this end up trapping moisture and causing mold with the plywood & wood framing under the plastics?
I left a small part open just in case this is a bad idea. Pictures attached.
Thanks!
Replies
There is a good chance your subfloor and joist will collect mold. There are two types of crawlspaces vented and non vented. Both require that there is a barrier from moisture rising up from the earth on the earth.
It does take a bit of experience to know how to install it properly, so would encourage you to consult with a professional in your area unless you are feeling confident. In general I prefer a non vented type as I’ve found it to control moisture much better, but has to be installed correctly. If you live in a flood plain or very high water table area vented is your best option as there is no barrier that can hold back large amounts of water. It’s very important that a vented crawlspace vents properly. Sometimes supplemental power venting is required to get the crawlspace to continually dry out. It is quite a bit of work for a homeowner to tackle alone would be best to hire some help.
The magazine has had a number of articles on this topic over the years. For example: https://www.finehomebuilding.com/2015/05/13/creating-a-sealed-crawlspace
Personally, I'd take the wood out. It has no business being in that environment. Then I'd install a vapor barrier on the floor, taping to the masonry walls with Siga's Fentrim. Then I'd look into how to properly seal a crawlspace. Vents almost never actually dry crawlspaces. In humid climates, they actually tend to make them damper.
Thanks!
I did consult with a couple of water proofing companies and there were a lot more items above the plywood at that time. Random metal sheets and heavy shelving put in by previous owner.
Both gave good tips - they essentially said area is dry and in good shape. BUT will gather mold as time goes on so they suggested removing all organic material. We also agreed it was easy DIY (in our minds).
After removing the steel sheets and shelving, I discovered the wood framing (joists?) under the plywood and the radon pipe cemented to the plywood. That is why I am now reluctant to remove the plywood and joists.
I just had another look at your pics. Are your joists directly on top of earth with no accessible crawlspace and air space? If that is true, it’s not really ever going to function like a crawlspace and needs to be treated like wood on grade which needs to be sealed very well and non-vented.
Correct. It is wood joist over gravel dirt. Then plywood on top of the joists.
So I can try to remove most of the plywood and seal it all up. That should not cause mold to the wood joists?
Thanks.
Okay, so it’s not really a code compliant crawlspace and can’t really ventilate it very well. It should be treated more like wood foundation type of construction which is rarely used. I’m not as familiar with wood foundation construction as it is not allowed in my jurisdiction. I know it is allowed in some regions of the US, so if you are in one of those areas best to refer to your local code for installing. I have a business in the UK along with my US business. Most of the homes I work on in the UK are pre 1800’s and all of the ground floors are built like yours, but rest on dirt, no gravel. Re doing ground floors is big business here, but not for the rot, for the lack of insulation and mice control. What we do here is first, seal off all the vents in the masonry walls. Then install rigid foam to the foundation walls and ground. After that we install EPDM over the foam and seal it completely. They do allow a heavy mill plastic, but the EPDM is usually not that much more expensive here and we find it seals a lot better on the seams. We seam it just like a flat roof. It also involves removing and reinstalling the floor deck which is 1 1/2” thick T&G heart pine floor boards (no subfloor), very time consuming. This method works well for the UK and it rains constantly here.
If you can't ventilate it right, just install a crawlspace dehumidifier in it. Insulate as well as you can, and the dehumidifier will take care of the rest. These are heavy-duty systems that are specialized to handle the high moisture levels in spaces like basements and crawlways, so you can't go wrong. As for the vapor barrier, go for 20-mil pool liner, should do the trick best, there are other posts here on the forum that provide more insight into the topic.
https://www.optimainstitute.com/dehumidifiers/for-crawlspace.html
https://www.familyhandyman.com/project/how-to-install-a-vapor-barrier-in-the-crawlspace/
https://www.dummies.com/home-garden/home-improvement/basement/installing-a-vapor-barrier-in-your-crawlspace/
FWIW, I edit the Ask the Experts department in Fine Homebuilding as a freelancer. The next issue has a question and answer on a very similar topic. Ben Bogie, a builder in Maine I know and trust implicitly, recommends some specific products for running plastic in a crawlspace. Siga makes a line of tapes that are the key to his system, most notably Fentrim, which sticks well to masonry.