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vpr80 04-08-2012 12:29 PM

What to do about a Damp Crawl Space under front porch?
I have a bi-level with a narrow front port that runs half the width of the house. It's an L-shape along the front and then extends in right under the front door and stairs up/down. It is fully enclosed from the lower level which is obviously fully finished with a door under the upper stair case for access.

The crawl space is concrete and cinder block on all sides. There is no water getting in, but the area is always cold and musty. When you open the door, you can smell the must and cold. We don't really have anything there except some of the house networking and other low-voltage wiring. The area under the stairs right behind the door is basically our network room with all the panels and such. I've recently put the printer there and a small file, but now you can see the papers picking up the dampness if you leave them for a few hours.

Basically I want to solve this once and for all.

First, I was going to use Super Thoroseal to cover all of the concrete to help reduce the dampness and then see how that works. I know adding ventilation will help, but (1) I don't want the dampness being pulled into the basement and making it more humid and (2) that area is cold so it would cool the basement and require more heating. Also don't really have much space anywhere to add vents, closet on one side and small area under stair case on the other.

Lastly, the other option is to make an insulated half-wall right under the front door and cut off that "raw concrete" area under the porch from the "interior" unfinished area under the stairs. I actually like this idea the best, but I've been told that it will make the problem under the porch even worse.

Ok that's long enough, any help is greatly appreciated. Thank you.

TarheelTerp 04-08-2012 02:27 PM

1 Attachment(s)

Originally Posted by vpr80 (Post 894472)
It is fully enclosed...
I know adding ventilation will help, but...

As my grandmother would say: "There's no but's about it"
Cycle air through the space to the outside.
Something like this will do:

vpr80 04-08-2012 02:40 PM

I can't cycle to the outside. The area is fully under grade level on that side of the house. Only option is to vent to the inside.

So if I try to vent to the inside, what can I do to keep the dampness and cold down so as not to affect the interior?

cortell 04-08-2012 03:05 PM

6-mil polyethylene sheeting over the rat slab should drastically reduce the humidity if moisture is coming up through the floor. However you really need to find a way to vent that area. The sheeting reduces venting requirements to 1/10th. I.e., without plastic, vent opening must equal 1/150th of the crawlspace footprint; with sheeting, 1/1500th.

Jaansu 04-10-2012 06:59 PM

what if it were a deck???
I have a related question. Any good way to reduce humidity under a deck? I will be replacing a pressure treated deck that I believe failed since the area underneath doesn't have enough ventilation, keeping the board wet. The deck is surrounded on three sides by the house and most of the remaining side by a massive board that supports that edge of the deck. How to get in more air movement underneath?

I thought one solution might be to cover most of the ground with waste vinyl flooring and 5 mil plastic sheeting that is tilted to run rain water to the edge. Reducing the water absorbed into the ground may reduce it coming back up again? Or perhaps leave a couple of boards out and cover with a lattice to provide some chimneys?

cortell 04-10-2012 07:29 PM

I think plastic might make things worse. Chances are you'll end up with pools here and there; they'll take forever to dry up and they'll invite mosquitos. Venting that area is the key. Without pics of that exposed side, it's hard to suggest something specific. I'd also dig out a few inches of dirt and replace it with 3/4" gravel.

Jaansu 04-10-2012 08:04 PM

Ventilation under deck
Thanks Cortell. There isn't much to picture about the front side. There is only a 2" space running below the ledger board and above a trim board. I will remove the trim to allow a little more air but I can't do anything else to the support.

You are probably right about the pools. It would be hard to keep it all flat and tilted toward the edge.

Are there any sort of exterior fans that could be mounted to bring in air?

cortell 04-10-2012 08:17 PM

I'm not sure what you mean "above the trim board". Also, by ledger board, I assume you mean a rim joist. A picture (or two) is worth a thousand words. We're not mind readers and we need details to help you out.

Fans are an option, but you need to be able to vent that area sufficiently for the fans to be effective. If you can provide more pics, I can tell you if I see a potential solution.

Jaansu 04-11-2012 07:53 PM

improving ventilation under deck
4 Attachment(s)
Here are pictures of the sides of the existing deck and the surface. It took a while to resize them so I apologize for any weirdness you see. On the side, the top plank is a support beam resting on 4X4s. The bottom plank is a 1X6 that could be removed. Most of the perimeter is flush to the house. What do you think?

cortell 04-11-2012 08:32 PM

Jaansu, I think the solution is crazy simple here. You have plenty of ventilation potential there (much more than I suspected, which is why pics are so important). Get rid of those 1x6 planks! Every single one of them--on all sides. Then trim back some of that vegetation on the third pic. Two feet at least. Problem solved. I'd bet on it.

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