Timer for Crawl Space Fan?

Discussion in 'HVAC Heating & Cooling' started by vpr80, May 9, 2012.

  1. vpr80

    vpr80 New Member

    Messages:
    29
    Location:
    NJ
    Does anyone know of a timer/switch that can be set to run for a certain amount of time per hour? For example, if I want to run the fan for 5 minutes every hour or so?

    Thank you
  2. Dana

    Dana In the trades

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    Is this fan exchanging outdoor air to the crawl space or indoor air? (In a NJ summer outdoor air would be adding to the moisture under the house, not removing it.)

    There are interval timers of all types, but knowing the purpose in better detail may change how you really want to control it compared to what you're thinking might make sense.
  3. vpr80

    vpr80 New Member

    Messages:
    29
    Location:
    NJ
    I have a small 4'x25' sealed crawl space under my front porch. All closed in with no vents, just a door to the inside that's kept closed. It's cinder block all around on a concrete slab. It's always really damp and cool in there. I painted the whole area with Thoroseal to help with the water, but the front wall is still a bit wet to the touch and sometimes there is a very small puddle when it rains outside so I'm getting a bit of water in there. It's usually high-60's temp and also right around 60 percent humidity.

    I was reading online that it would help to vent the area with some conditioned air from the inside so I wanted to install a small bathroom fan ~50cfm to run on interval to keep the area better vented.
  4. Dana

    Dana In the trades

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    Dealing with the drainage issues will be more important than ventilation and waterproofing. An exterior perimeter drain to take the water away, grading the surface to slope away, and adjusting any gutters/down spouts to keep it well away are all-important first steps.

    Since the crawlspace is below a porch, not conditioned space, ventilating with outdoor air would be more appropriate.
  5. vpr80

    vpr80 New Member

    Messages:
    29
    Location:
    NJ
    All of the items that you mentioned have already been done. It greatly reduced the water seeping in, but a very small amount still does get in and also condensation forms. I am talking about a few ounces of water, nothing more.

    Venting to the outside is hard because the side walls are fully below grade so it would have to be a vertical vent to the porch and I am not thrilled with that idea.
  6. Dana

    Dana In the trades

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    60% RH @ 60F isn't much of a mold or rot hazard for the floor joists of the porch the way 60% RH @ 80F is. The "relative" part of "relative humidity" is "relative to TEMPERATURE". Mold and rot hazard doesn't begin in earnest until it's 70% RH @ 70F. Cool the same air down to 60F and the RH of that air becomes 99%- on the hairy edge of condensing. Heat 70F/70% RH air up to 80F and it's RH is 50%.

    If you're going to vent it to the interior, use rigid foam board to insulate and the whole thing, including the floor and the underside of the porch joists, sealing the edges with 1-part spray foam. Then just leave the door open to the rest of the space. Using 2" of EPS or XPS would be best (R8-R10), but even 1" of either is probably "good enough." The rigid foam is vapor-retardent, but at least somewhat vapor permeable. It'll slow the rate of moisture migration from the walls/slab into the space, and from the space to the underside of the porch joists, and it'll be warmer.

    If that's too pricey a proposition, use 6-mil poly stapled to the foundation sill to vapor-seal it from the ground moisture. Use duct mastic to seal the edges and seams (overlap sheets by a foot or so). Then leave it unvented, with the access door closed, and monitor the the RH in that space. Use housewrap to air-seal the porch floor, which is fairly vapor permeable even air tight, which lets any moisture finding it's way in to be able to dry upward. Moisture getting into that space would then be primarily whatever leaks or diffuses through the porch floor, but as long as it's air-tight there won't be much air-leakage to cause summertime condensation.

    Running a fan to ventilate it with conditioned space air doesn't fix the problem, it only moves it indoors, adding unhealthy moisture into your house.
  7. vpr80

    vpr80 New Member

    Messages:
    29
    Location:
    NJ
    I was actually thinking the other way around...to vent INTO the crawl space the conditioned air from the interior of the house. Is this not good?

    Also keeping the door open immediately helps to reduce the RH in the area, but that's not really an option since the door is in the middle of the floor and next to the stairs so it has to stay closed at all times.
  8. Dana

    Dana In the trades

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    If you are basically exhausting basement air into that space and letting it go up through the floor boards in the porch it depressurized the house, and air comes into the house via myriad air leaks.

    In the summer that's sucking humidity/latent-heat into the house.

    In the winter it's putting the more humid conditioned space air into the much colder crawlspace, where it will condense on the underside of the porch.

    The BEST you can do with a ventilation scheme is somehow either ventilate the crawlspace with outdoor air pushing the crawlspace air back out the the great outdoors. In summer that may add humidity to the space during the stickest day, when the outdoor dew point is above the temperature of the slab, but on average it will have a drying effect. In fall/winter/spring it'll have a much stronger drying effect.

    The second-best is to air-seal it to the outdoors, and ventilate it with basement air, with the return air going back to the basement, which at least localizes the problem.

    Or, you can go with a drying scheme that doesn't rely on ventilation: Vapor & air seal against the below-grade ground moisture & bulk leakage moisture with polyethylene, air-seal it from the porch with a vapor-open membrane like housewrap, and let the humidity in the crawlspace track that of the great outdoors (which was my last suggestion.)

    If you air seal it and insulate it with rigid foam (my other suggestion), you can replace the door with a screen door (or remove it completely), and it'll track the indoor humidity without excessive wintertime heat loss or introducing lots of moisture into the house.

    Without seeing the place I couldn't say for sure, but if a decent sized floor grille (like a furnace duct return grille or similar) were installed on each end of the porch, it may naturally vent to the outdoors just fine, and you could just weather strip (and even insulate) the access door in the basement stairwell and keep it closed. But if there isn't sufficient roof overhang to prevent rain and snow from getting in that could create other issues. They wouldn't have to be dead-on the ends to work.
  9. vpr80

    vpr80 New Member

    Messages:
    29
    Location:
    NJ
    How about a few pictures to help. First picture is the entrance through the door to the area under the stairs. You can see the small opening to the crawl space in the back. Second picture is the crawl space itself. Approx 4'x'4'x25' with concrete floor and top and cinder block walls. The right wall in the picture is the exterior wall that gets condensation some seepage. The white pain is two coats of Thoroseal.

    So what I wanted to do was to ventilate the area with conditioned interior space. At one end would be a small 50cfm bathroom fan on a humidistat and the other would be a small vent back to the inside. Do you think I should still insulate with rigid foam?

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  10. CrawlSpaceMoisture

    CrawlSpaceMoisture New Member

    Messages:
    1
    Location:
    Charlotte, NC
    Okay, to answer your first question - just grab a plug in timer like you'd use for a lamp. Set it to come on for maybe 5 minutes every 2 hours or something like that.
    Next, a couple of thoughts. RH vs Temp is important if you're trying to determine if a moisture issue is from condensation. You don't appear to have any wood joists, so it's academic that the important number for fungi is the moisture content of wood. Anything above 16% or so is bad.
    Also, if you had a wood floor substructure, it is a tremendous mistake to staple any poly on the walls to the sill plate.
    Here's what I would do- you could insulate the walls, that would help with the humidity level, but probably unnecessary from what I'm seeing there. IF you do, be sure to check with code enforcement as your building code probably specifies flame spread requirements which limits what insulation you can put on the walls.
    But really, what I'd do? Buy a dehum and put it in there.
    If you were getting more water than you're describing, I'd say your real answer is to dig it out and waterproof.
  11. vpr80

    vpr80 New Member

    Messages:
    29
    Location:
    NJ
    I'd love to but I have not been able to find one small enough. I need one that can be routed to a drain line and also has a pump since it would need to drain up and out. All I've been able to find have been either small household ones that will just die in no time or massive and super expensive ones that are meant for much, much larger areas under a whole house.
  12. Dana

    Dana In the trades

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    Location:
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    In winter it's likely that ventilating it with conditioned space are would cause condensation on the walls, since it's probably averaging below 40F in there in January.

    Even with a wooden floor structure to the porch, with a CMU/cinder-block wall there aren't significant issues with putting poly on the interior the way there can be with poured concrete with limited above-grade exterior. The foundation sill would still dry into the floor-joist cavity of the porch floor, and the capillary draw up the CMU is a fraction of what it would be with poured concrete.

    But putting rigid EPS or XPS (but not polyiso) on crawlspace floor, exterior walls, and ceiling would work, but it's probably not worth the expense due to the difficulty of having to cut it all up to get it in there only to re-assemble and re-seal. A closed cell spray foam solution (walls & ceiling only) might work though. With no ignition sources and not used for storage the requirement for ignition barriers for foam have been largely relaxed (or even eliminated) where there isn't active air communication with the rest of conditioned space (which your ventilation system would arguably become) The CMU and concrete is the ignition barrier to the conditioned space, and the toxic smoke issue is contained if all you have is that tiny access hatch. There are spray-on ignition barrier products out there as well.

    Still, this is an expensive solution to what should be pretty straightforward if you can vent it on both ends to the exterior, even if you have to dig in L-shaped PVC vent stacks that go out through the sidewalls and come above grade at the ends, then weatherstripping the access hatch to limit stack-effect draw into the house. Even a 3" or 4" vent fashioned from drain pipe with a screened vent-cap on each end to keep the rain out coming up in the garden next to the porch would care of it. It's a bit of digging, and a bit of work with a hammer-drill/roto-hammer but the result would be reliable, cheap and nearly maintenance-free.
  13. vpr80

    vpr80 New Member

    Messages:
    29
    Location:
    NJ
    How about this, what about completely sealing that space by closing up the entry way under the stairs? Nothing is really happening under there so why not just close it up permanently. Would that create a petri dish of sorts of all kinds of stuff to start growing?
  14. Dana

    Dana In the trades

    Messages:
    2,841
    Location:
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    Unless you put organic materials like wood/paper/cotton in there as a medium on which the fungi & molds can grow, it's pretty much just a cave. But since the foundation waterproofing is on the outer perimeter and known to be leaking, passive venting to the exterior is probably a good idea even if you brick up the access.
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