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Bruce Frey
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| 06/27/2009 3:00 PM |
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Posted By Dcislander on 06/24/2009 7:47 PM
This is what the crawl space company says about their system:
Air flows naturally through your house from bottom to top. This is called the 'stack effect'. Humid air (and everything in it) is sucked up into the living areas of the home from the crawl space. Mold spores, musty smells, moist air and other organisms in the crawl space will create a very unhealthy environment that spreads to your living space and lowers your quality of life. Allergens coming from the crawl space in the form of mold spores, dust mite feces and other materials can cause many people to experience a host of symptoms, sometimes without ever realizing that their dirt crawl space is affecting their health.
This may be true in the dead of winter, but I am not sure that there is enough height in a normal residence to induce much stack effect.
You have not mentioned whether you have an HRV or ERV, so I assume you do not. Have you done a blower door test?
I agree that including the crawl space in the conditioned envelope is one way to deal with the humidity issue. Another possibility is to pressurize the house relative to the crawl space to prevent infiltration.
What insulation do you have above the crawl space? Even a thin layer of closed cell foam makes a good air barrier to provide separation. In that case you would not need to pressurize the house, but in both cases you would want an exhaust fan in the crawl space to keep it ventilated.
Bruce |
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jonr
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| 06/27/2009 9:05 PM |
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My preference would be for a sealed crawlspace with a small dehumidifier in it. Circulating air (inside or outside) into a crawl space that tends to be cooler is asking for humidity problems.
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joe.ami
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| 06/29/2009 8:06 PM |
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Posted By jonr on 06/27/2009 9:05 PM
My preference would be for a sealed crawlspace with a small dehumidifier in it. Circulating air (inside or outside) into a crawl space that tends to be cooler is asking for humidity problems.
Has this worked on many jobs for you?
Dilution really is the best remedy for radon, mold, mildew......
That comes from ventilation. Not isolation.
Even a dehumidifier requires some fresh air. Left alone, an extreme amount of heat would be produced by a dehumi in a sealed crawl space. Probably not a bonus for the surrounding envelope.
J
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engineer
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| 06/29/2009 10:58 PM |
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Good comments about specific hazards / pollutants.
My thoughts:
Radon needs to go away entirely, in other words be sucked from its source before it gets into the envelope, typically via pipes and a small fan unit outside the envelope.
Mold and mildew result from high humidity and should be attacked by lowering humidity. A dehu trades latent for sensible resulting in warmer dryer floors in winter, which is fine. In summer the dehu tradeoff results in more AC runtime to reject its added btus. There's an efficiency hit to operating that way, but in a properly installed setup the dehu sees more operation during shoulder seasons, and the high AC use during mid-summer takes care of the crawl dehu then.
The shoulder season dehu trade for sensible causes a bit more AC use during the shoulders but increased comfort is the likely result. |
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Without data, you only have an opinion. |
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joe.ami
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| 06/30/2009 7:13 AM |
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Posted By engineer on 06/29/2009 10:58 PM
Good comments about specific hazards / pollutants.
My thoughts:
Radon needs to go away entirely, in other words be sucked from its source before it gets into the envelope, typically via pipes and a small fan unit outside the envelope.
Mold and mildew result from high humidity and should be attacked by lowering humidity. A dehu trades latent for sensible resulting in warmer dryer floors in winter, which is fine. In summer the dehu tradeoff results in more AC runtime to reject its added btus. There's an efficiency hit to operating that way, but in a properly installed setup the dehu sees more operation during shoulder seasons, and the high AC use during mid-summer takes care of the crawl dehu then.
While I can't dis agree with anything you say, I was speaking in the context of the sealed crawl with dehu. If radon mitigation were included in the plan it has not been mentioned.
What you propose includes ducting (the hvac system) to the crawl. What Jonr mentioned was a sealed crawl treated only by a dehum. Not sure you'll like those warm dry floors in July.
J
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jonr
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| 06/30/2009 7:55 AM |
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A sealed crawlspace has so little air infiltration that very little dehumidification is needed. It will be cooler than the outside air in summer and since there is insulation between the crawlspace and the living space, it doesn't matter anyway.
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Dcislander
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| 06/30/2009 10:58 AM |
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Hello all. Sorry, been off email for a few days. I am not sure if my blower is an HRV or an ERV. It is 2 stage American Standard. Also, I assume that stage one what the unit runs on in normal conditions and stage two kicks in when the system needs extra power? There is no insulation or barrier of any kind between the floor and the crawl space. It is a small (1200 sq ft) one story home, crawl is about 2.5' high. The floor joists are about 24" apart and the pine floor planks are 2" thick and nailed right into the joists with no subfloor. They are not proposing any ducting either (per se) but instead a 4" fan that pulls air down into the crawl from a closet.
It sounds like most recommend the dehumidifier which would help the house humidity level and to condition the space. Would this then negate the need to include the air circulator in the plan? For cost considerations (this is a 2nd home) I need to pick my poison (dehumidifier or recirculator) and go with it. Originally, I chose the small fan which pulls air from the conditioned interior into the crawl because it was the cost effective option. If the humidifier is a better option, then I would go in that direction as not to be penny wise and pound foolish. Warm floors do not really worry us too much and it sounds like the added electricity costs would be relatively negligible.
Still not entirely sure what the right answer is to accomplish these goals:
1. Lower the humidity level in the home in shoulder season.
2. Keep the crawl conditioned enough that the pipes not freeze in the winter (house above will be set at 55)
3. Keep mold growth to a minimum.
Thanks again for weighing in everyone.
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geo fan
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| 06/30/2009 5:57 PM |
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Given the 2 options I would recommend a hybrid
dehumidifier , and cut a few holes in the floor ( covered by grills ) to allow some passive air exchanges .
also I strongly recommend laying some 4 inch perf. pipe around the floor under the liner , and mechanical vent that to the outside via a radon fan .
I would also suggest a sensor / alarm / alert light . to let you know if the dehumidifier fails . also a condensate pump to pump the condensation to an indirect drain in your plumbing .
the above may seem excessive but is code minimum for sealed crawl spaces in my neck of the woods ( north east ) and seems logical ( unlike some of are other codes ) |
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engineer
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| 06/30/2009 10:27 PM |
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I disagree, and will illustrate with an example using psychrometrics. If 75 deg 50% RH air circulates into a crawl and is cooled to 65 deg by being there, its RH becomes 70% plenty for mold to get going
Here's a link to a report of just how ugly vented crawl spaces get during humid weather: http://www.envirochex.com/Downloads/CrawlspaceCharacterization2005.pdf
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Without data, you only have an opinion. |
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jonr
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| 07/01/2009 8:14 AM |
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It's a good report.
A similar thing happens when people (yes, I did this once) leave their furnace/AC fan (not compressor) running to "get some circulation". Or open the basement windows. The basement RH gets way too high and they get mold and mildew. Seal and dehumidify ground cooled spaces. |
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joe.ami
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| 07/01/2009 8:23 AM |
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Ventilation is always my preference. The crawl is either part of the earth or part of the envelope. Pick one and treat it accordingly.
I don't know where this weally, weally wittle dehumidifier is coming from, but the standard Kenmore will consume as much juice as a window air conditioner.
I'm not a fan of the 4" vent in the closet either.
I suggest you ultimately let the local experts design the project as they will be responsible for results.
Definately more than one way to skin this cat.
J |
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