Ventilating sealed crawl space?


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Old 10-11-15, 09:48 PM
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Ventilating sealed crawl space?

We recently had our crawlspace walls spray foamed with closed-cell foam, from the floor up to the rim joists (I think that's what its called). The floor is poured concrete and presumably had a plastic sheet installed in the middle of it (I did that test where I taped a 2' x 2' piece of plastic to the floor for ~48 hours - no moisture). I have a dehumidifier in there that clicks on rarely. So, its very dry in there (no water in sump pump hole when I checked yesterday) and conditioned. There is a slight hole (maybe 3" x 6") to the utility room which is right in the middle of the house. The crawlspace is under about 70% of the house, and I'm sure there are other cracks that might allow some air to flow from the crawlspace to the rest of the house (although the ceiling of the crawlspace still has the pink batt insulation from before we spray foamed).

So, my questions is if we need to somehow have more air exchange between the crawlspace and house. Its very dusty/dirty in the crawlspace, so I'd rather not exchange more air but if I need to, then I need to. (Our HVAC is a central A/C unit and a hot water baseboard system.) Also, would it matter one way or the other if I drop a loop from the hot water baseboard system down into the crawlspace to make it warmer (which would make the air rise and presumably go up through the cracks into the house, causing more exchange)?

Any advice would be greatly appreciated. Thanks!
 
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Old 10-11-15, 11:14 PM
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Reading some information from Alaska recently and I was surprised they actually require a ventilation system for a sealed crawl space. Here in the lower 48 I haven't heard it being required. However, once sealed and insulated that space can become part of the total conditioned space. As for the dust and dirt, having a concrete floor, I would get down there with a shop vac and make it spotless. Reason, a lot of the air you breathe passes through that crawl space.

The one opening you mentioned plus the typical holes and leaks will not provide a lot of air exchange. Remember, you need to be able to handle the summer humidity as well so sharing the ac can be necessary. Plus, a moisture test only says that right now there is no moisture, but the presence of a sump pump implies that there may occasionally be a water issue.

Bud
 
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Old 10-12-15, 02:38 AM
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However, once sealed and insulated that space can become part of the total conditioned space.
The Washington state energy code has required that sealed crawl spaces become part of the conditioned space for at least 15 years. I don't remember the ratio but it is something like 5% or perhaps 10% of the total circulating volume of air in a forced air system (both heating and cooling) must circulate through the sealed crawl space. That means discharge from the supply duct into the space as well as a return duct opening.
 
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Old 10-12-15, 06:21 AM
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Furd, the op here has baseboard heat and only ac using any ducts, so I would assume that is going to necessitate some form of mechanical ventilation, like a bath fan on a timer down there exhausting to either outside or to the living space above. I think the information I read said they vented to the outside.

BTW, thanks for the correction on venting as I don't recall ever reading about required crawlspace venting and it certainly was never discussed in any of my many classes.

Bud
 
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Old 10-12-15, 10:43 AM
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If you've insulated the walls and not the ceiling of the crawl space, you do not want to open it to the outside air, you would want to open it to the house air. If it's dusty down there, clean it - I did that at my in-laws' lake place when they built it. Not fun but part of owning a home.
 
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Old 10-18-15, 08:41 PM
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Its absolutely filthy down there, and probably something something like 40' by 60' or so, and truly a CRAWL space on hands-and-knees the whole time. It would take me forever + overtime to clean it. There is no way I'm going to find time to clean it, so I guess it stays dirty until I retire in several decades.

Also, could you clarify how a lot of the air I breathe in my house comes from the crawlspace? Its spray foamed walls and concrete floor - where is the air coming from that goes into my house? Unless you mean it just sort of drifts into my house from the crawlspace, to be replaced by air from the house drifting down into the crawlspace. But if so, then I can't imagine what's driving the air to move. Its cooler in the crawlspace, so the air is not rising via heat. Any thoughts would be greatly appreciated!

Thanks again.
 
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Old 10-18-15, 09:57 PM
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What type of floors do you have above the crawlspace?

Is your home more than 1 story tall?

The air in your crawl space naturally migrates up into your living space due to the stack effect. Warm air is buoyant and rises which pulls air up from the lowest part of your home. Most building assemblies only restrict the flow of air so during the heating season when the stack effect is strongest the movement of air from the crawl space into the living space is nearly inevitable.

In Alaska we are phasing out passive venting of crawlspaces and using controlled exhaust instead. A small bath fan that exhausts 1 CFM of air per 30-50 square feet attached to a dryer vent is the most common method. We control them with timers, humidistats and other home automation switches to prevent running them in perpetuity. There are several models for sale that are efficient, quiet and designed for continuous use. This might be something you should consider.
 
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Old 10-19-15, 07:40 AM
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With the walls and more importantly the house to foundation and rim joist well foamed you will have reduced much of the direct leakage into that space. But radon and other gasses will pass right through the concrete floor. The stack effect meander mentioned will push that undesirable air up into your home.

A typical home exchanges ALL of its inside air about every 2 to 3 hours.

We all see warm air being pushed up because it is lighter, however, humid air is also lighter than dry air. So any moisture vapor entering the crawlspace acts just like the lighter warm air and heads up into your home.

I don't have numbers for air flow through a home with a foamed crawlspace, but in a typical home, testing has shown that more than half the air those people breathe has passed through the crawlspace or basement.

I won't share the entire story, but a couple of energetic high school seniors looking to raise funds for their project graduation (if they still do that) could go down there with face masks and a couple of shop vacs and be done in less than an hour, probably 30 minutes.

Bud
 
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Old 10-24-15, 11:46 AM
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we're beginning to see that change in atl as more engineers/specifiers rethink themselves i think fresh air exchangers are the key
 
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Old 10-24-15, 12:59 PM
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@stadry As homes get tighter that natural flow of ventilation is going to be reduced significantly so yes, ventilating the crawl space may become necessary. But my feeling is, homes need to address as much of the problem as they can before treating the symptom.

The recent thread from Alaska mentioned they are now requiring ventilation in for crawl spaces. We are now treating the problems caused by the modern improvements.

Bud
 
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Old 10-24-15, 10:01 PM
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First of all, thanks to everyone for spending some time explaining the various issues to me.

Anyway, Meanderthal: above the crawlspace, we have about 25% carpeting and about 75% hardwood. Does this affect anything that is being said?

Bud: I wish I knew some high school seniors!

Just to confirm, is the final vote that I should put in some sort of active exhaust system of 1 CFM / ~40 sq. feet of space to vent the air outside? (I convinced my wife that we should foam spray the crawlspace; I'm guessing she may shoot me if I tell her I now want to vent it outside, but it is what it is!)

Thanks again.
 
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Old 10-26-15, 08:08 AM
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OK, if you're going to vent it, you need to insulate the ceiling.
 
 

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