[toddly]

Background: Ever since we bought this fixer, it has had a musty smell that seems to come from a converted 'bedroom' that had formerly been a garage. I have removed the 2X4 and plywood floor that had apparently been installed over top of the garage floor, (back in 1966 according to an old newspaper they considerately left behind under the floor). They apparently 'sealed' the corners and edges with tar, then put a sheet of plastic over it, built a subfloor, and then called it a bedroom.

I want to seal the floor and insulate under it; and I want to do everything correctly. I am already digging a french drain outside to reduce groundwater from soaking into the slab.

I want to clean the tar off the slab, fill the cracks, seal the slab, then rebuild an insulated floor. What I need is advice on the following:

1. What type of cleaner/tar removal will not interfere with a good slab sealer? I've noticed that many sealers tell you not to pre-wash the cement with soap, as it may inhibit the sealer from adhering properly.

2. I need to fill deep cracks in the floor slab and also cracks in the sides of the concrete wall foundations. Any advice on which type of crack filler product would be best? Something thick or something watery?

3. I need a good slab and foundation sealer. I've read reviews of dozens of products and still need some advice on which would likely be best for my project. I want the sealer to be absorbed as much as possible and adhere well.

4. Is there any brand of rigid foam insulation that would be good for putting under the rebuilt flooring?

5. Lastly, I've been told that I'll have to have some way to vent moisture that will inevitably condense on the colder concrete slab. Can I just leave the plywood floor unsealed along the edges, so that moisture can vent along the walls and into the room? I haven't a clue.

Any advice would be greatly appreciated. Thanks!

 

[sands]

Seems like the problem you have is that water is getting into that space and the moisture is inevitably creating that smell. Not only that, the moisture can cause a whole other set of problems for you. Your best bet is to first figure out where the water is coming from and stop it from coming in completely. Slowing it down with the french drain will help but you have to get to the root of the problem. How many exterior walls does the space have? Perhaps the water is coming in through the foundation wall? Can you provide any pictures? After resolving the water issue, you want to install two vents for cross ventilation of the space. Which I'm assuming is a crawl space.

 

[toddly]

Sands,
Thanks for the input. As I mentioned, the room used to be a garage, meaning that there is no crawlspace beneath it, only a poured concrete slab. Some of the water is coming up through the slab, and that is why I'm putting in the french drain. But because the concrete is on the ground, it stays cold and will always get condensation, which is why I would like to understand how to vent the subfloor.
Sands, can you explain that process a bit for me? If I'm putting slabs of rigid foam insulation underneath the plywood subfloor (built atop the slab), how do I allow for air flow and ventilation? Besides, isn't any moisture likely to remain trapped under the moisture barrier of 6mil plastic?

Thanks for ANY advice you can offer.

 

[sands]

Hi toddly,

Coming up with a solution for the ventilation is a tough one but let's revisit that in a little bit. Let's focus on what needs to be done regardless of the ventilation.

I would fill in the cracks with mortar mixed with some concrete bonding adhesive. For the really small cracks you might want to open them up a bit to get a good amount of cement into the cracks. Small cracks are hard to fill and even when you fill them in you're only getting the surface and not all the way into the crack. Prior to filling in the cracks however, you want to clean the concrete of any dirt and apply some concrete bonding adhesive to the cracks so that you ensure a good bond. Once cracks are filled, I'd use maybe a product like drylok(concrete waterproofer) to seal the slab. And as you mentioned, make sure all the cracks on the outside of the slab are filled in as well.

Once that's done, you can proceed with the plastic, insulation and so on.
If you're noticing smells even after all that you could try this- Drill SMALL holes into the rim joist and cover it with a screen or vent cover. But make sure bugs and pests can't get in. The holes, I wouldn't make more than 3/4" big. You said it was a 2x4 floor, so most likely a 2x4 rim joist. I wouldn't place the vent, venting towards living space. Ideally, you want it venting outside.

 

[toddly]

Sands,
Thanks so much for your input. I like your idea, but want to ask a few clarifying questions before I proceed with the sealing and vents.

First, on other DIY websites, I've been discouraged from sealing the slab because they say that any water that gets in, or condensation, will then be trapped. Whereas if the slab is unsealed, the water can always be reabsorbed back into the slab. But that's only true if I don't ventilate, right? Theoretically, moisture build-up near the edge of the plastic sheeting could go through the holes along the rim and vent outside. Right?

And what about the issue of moisture trapped under the plastic moisture barrier? It shouldn't be a problem, because it will be trapped, right? And any substantial accumulation will eventually make its way to the edge, where it will mix with air and vent out, right?

With regards to the venting, how many holes would you drill along the rim, and how far apart? Do the vents go directly under the vents, or are they to simply improve airflow? And how many vents per wall? The room is 14'X21'.

Finally, my moisture barrier is a 10'X25' sheet; so I'll be cutting off four feet of one end and attaching it to the side. I realize overlapping the two plastic edges would be ideal, but if I attach them edge to edge, is there a certain waterproof, mildew proof tape that you would recommend that I use?

I hope that about covers my concerns. I can't think of anything else at the moment. Thanks again for your help!

 

[sands]

Toddly,

No problem, glad to help.

It's important to know where the water is coming from. If water was getting into the space and sitting on top of the concrete, then sealing the concrete would be counterproductive. If moisture is coming up through the concrete, then I believe sealing it would be productive. When concrete slabs are poured for applications like this, a plastic vapor barrier is usually placed before the concrete is poured to prevent moisture from coming through. I believe the sealer would emulate that effect. You mentioned you had cracks in the concrete. The moisture could've been coming through from the cracks so once those are patched, it should ameliorate the moisture issue.

As far as the vents are concerned, that's more of a last resort option for me. There is some debate whether crawlspaces should be vented or not. Depending on where you live, new construction homes require crawlspaces to be sealed rather than vented.

So I would go ahead and patch the cracks and keep an eye on the concrete. See if condensation forms, and then wait until you get some good rain to see if any outside water is getting into the space. From there you can better assess your next step. If condensation is forming and no water is coming in from the outside, I would seal the concrete.

I would leave off the venting for now. Once everything is done, I would put a humidity reader in the room to get an idea of the humidity level and if the levels are high, go ahead and try the venting option. I would first get a vent cover, and the size of the cover will determine the hole sizes and how many holes to drill. Small holes maybe 1/2" apart. The less holes the better. I would put two vents total for cross ventilation but keep in mind that cold air in the wintertime could be an issue if not properly insulated. You don't want to compromise the integrity of the rim joist, so minimal amount of holes is ideal.

 

[toddly]

Sands,
Okay. So I'll patch the cracks and see if any water gets in. If it's just condensation then I'll go ahead and seal, and I'll hold off on the vents until I see the humidity readings later this fall.

And I'll look into getting that Drylock sealer.

I guess I've got a clear plan now. Thanks so much for your help Sands.

-toddly

 

[sands]

No problem toddly,

best of luck!

 

[geffy]

subscribed, this seems like a very interesting thread

 

[Gary in WA]

Welcome to the forums- to all of you!

Slabs are pretty easy to get wrong... important to stop the rising moisture sitting on the slab (won't see any due to convective air currents as it encourages evaporation). Plastic is a poor choice as it doesn't lay flat and ANY air space will condense water there and give a "dirty diaper" -same as a basement concrete wall- no plastic; http://buildingscience.com/sites/default/files/migrate/pdf/BA-0202_Basement_Insulation.pdf

Tar is a good impermeable barrier, with builders paper to walk on while framing floor, Fig.2; http://buildingscience.com/documents/insights/bsi-003-concrete-floor-problems?full_view=1

A lot more info; http://buildingscience.com/document...g-the-plank?searchterm=slab%20vapor%20barrier

Clean the slab, fill the cracks with a quality filler, tar/paper it (doubled), add the foam board- thickness per location, add the sleepers with anchors, add joists to level the deck fastened to sleepers; no air vents as it is not a crawlspace, and against Code. No Dryloc; http://neutocrete.com/drylok-an-honest-review/

Gary

 

[toddly]

Gary,
Thanks for jumping into the conversation. I just bought 5 gallons of Drylok Extreme today, but can easily return it. It says that it's supposed to inhibit mold growth, but I'd rather not chance it.

So, are you saying that I do not need to seal the slab? Just put tar paper over it?

But tar paper will eventually deteriorate, right? If I were to seal with plastic, then put foam insulation on top, wouldn't that adequately press the plastic to discourage excess sitting water?

Or would you suggest I use a product like neutocrete?

I'd be happy to go with the tar paper, but I don't see it as sealing the concrete. The old tar that was put on the slab in 1966 is all dried out and comes right up. Is there a sealer you would recommend, that doesn't facilitate mold growth?

Or would you go with no sealer at all?

Thanks for helping me clarify these issues. I want to get it right.

Todd