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My home already has the dirt completely covered and have been tempted to just close the vents and put a piece of foam board in each hole behind them, but have been reluctant to do it.
Interestingly, we have friends with a home built about 2 years ago with no vents and their floor just has plastic on it also, really no better if not less covereage than our 4.5 year old house. They also have a fan under the house that pulls air from a duct in their pantry and blows out under the house, so it is essentially taking conditioned air and putting it under there. Is that OK and sufficient to control the humidity or do we need to put in dehumidifiers also?
What your friends have is a fully sealed and conditioned crawlspace. Expensive and very nice, but not necessary. Your HVAC system would have to be resized to accomodate for the extra volume of space needing conditioned. Sealed/conditioned crawls are most often installed when building a house as it is much less costly and easy to plan for.
You are on the right track sealing the vents with foam board. Take it one step further and once the foam board is in place, spray the perimeter of the vent with "Great Stuff" which is expandable foam in a can. That will seal all voids where the foam board meets the masonry. You can use an appliance grade dehumidifier (we tend to use Frigidaire models from Lowes as they have a well engineered permanent drain connection). Commercial level DH's are about 4-5 times the price with no tangible added benefit. Good luck!
My home already has the dirt completely covered and have been tempted to just close the vents and put a piece of foam board in each hole behind them, but have been reluctant to do it.
Interestingly, we have friends with a home built about 2 years ago with no vents and their floor just has plastic on it also, really no better if not less covereage than our 4.5 year old house. They also have a fan under the house that pulls air from a duct in their pantry and blows out under the house, so it is essentially taking conditioned air and putting it under there. Is that OK and sufficient to control the humidity or do we need to put in dehumidifiers also?
I bought a used 65-pint dehumidifer from craigslist for $75. The important feature of a dehumidifer for this purpose is to have a hose connector for a drain instead of having to empty the tank. Then you can splice the hose into the HVAC condensate drain.
I also bought a remote temp/humidity sensor from amazon for about $50. I installed the remote sensor in the crawlspace (it is hanging from a joist) and the receiver is in the house. This way I can easily keep an eye on conditions in the crawlspace without having to actually go into the crawlspace. Without the dehumidifer running, the humidity can get into the 70% range which I felt was a little too high, so that's why I installed a dehumidifer. I have it set to 55%. (BTW, with the vents open in the summer, the humidity regularly goes into the 90% range - this is why leaving your vents open year-round is wrong.)
This summer the crawlspace temperature never rose above 73 degrees even though we had several days above 100. With the recent cold snap the crawlspace temperature stays in the 50's. The dehumidifer hasn't turned on since October brought in cooler, dryer weather.
Any idea how much it's costing to run the dehumidifier during the summer months?
Great question! It's not enough to notice on my energy bills, compared year-to-year. I'm going to put my kill-a-watt in the crawlspace and start tracking it monthly. I estimate that it runs about 3 hours a day for about 120 days a year (during humid season). And it's 600 watts. So whatever that costs.
There are also energy savings by having a crawlspace that stays within a range of 55-75 degrees year-round and is not drafty due to having open vents.
As far as energy use/savings is concerned, I consider it a wash. And the benefit of a dry crawlspace is obvious, even if it does end up costing a few dollars a year.
What your friends have is a fully sealed and conditioned crawlspace. Expensive and very nice, but not necessary. Your HVAC system would have to be resized to accomodate for the extra volume of space needing conditioned. Sealed/conditioned crawls are most often installed when building a house as it is much less costly and easy to plan for.
Thats the thing. I have seen sealed crawlspaces and theirs is not sealed. It has no plastic on the walls, no taping or anything other than sheets of 6 mil black plastic laid down loosely on the dirt. Plenty of gaps around the edges and spots here and there where there is no overlap. Basically the same treatment that every house got in the last 20 years. Everything I had read said that it should have been sealed much more to go without vents and to condition it like that, so I was a little worried when I was under there installing some speaker wire.
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You are on the right track sealing the vents with foam board. Take it one step further and once the foam board is in place, spray the perimeter of the vent with "Great Stuff" which is expandable foam in a can. That will seal all voids where the foam board meets the masonry. You can use an appliance grade dehumidifier (we tend to use Frigidaire models from Lowes as they have a well engineered permanent drain connection). Commercial level DH's are about 4-5 times the price with no tangible added benefit. Good luck!
I am assuming the minimal expanding foam is better for this application?
I also see Lowes has three Frigidare dehumidifiers, a 25, a 50 and a 75 pint per 24 hours. ANy reason to go with one of the higher capacities? I also see a few people complaiing of it dying pretty fast, one spcifically about power supply boards. Was there a bad batch or is this the typical thing that people don;t post about how awesome stuff runs, just when there is a problem?
That is odd they would condition a crawlspace that is not fully sealed. No real harm in it I guess, but you would lose a lot of the conditioned air without Thermax on the walls and fully sealed vapor barrier.
We have installed probably 100 of the Frigidaires with not a single issue yet. My dad has been operating his for about 7 years. They do give a 5 year manufacturer warranty as well, the same warranty time frame that commercial level DH's do. A Santa Fe DH costs about $1500, the Frigidaire about $200. You could replace it 7 times. We did have an issue with the LG models Home Depot sells. The unit worked fine, but the drain connection was under-engineered and weak. On the frigidaire model, go to the PVC section and get a threaded female to male adapter 3/4". The run the rest of the line with 3/4". You can splice it into an existing condensate line for HVAC if you have one to carry it outside. If not, bring it through a foundation vent or into a positive foundation drain at the lowest corner of the crawlspace. Antyhing that will carry it away from the house. Most existing condensate lines do not extend far from the house either. Recommend digging a 6" x 6" gravel pit for it to discharge into, or put a gutter downspout splash block under the discharge point just to keep everything away from the house.
We put one 50 pint unit in most crawls, but larger footprints or "compartmentalized" crawlspaces we will put two of the 25'ers in. If you use an extension cord, buy a 12/2 gauge as opposed to a standard gauge 14/2. The 50 pint unit draws about 4.5 amps, which would equate to about 495 watts while the compressor is operating.
Vents are not required on new homes with conditioned/enclosed crawlspaces.
Mike
Thanks...I just assumed that they were required but left up to the homeowner to decide to close them or leave them open.
The reasoning behind sealing them off makes a lot of sense, but I'd feel a lot more comfortable closing them off once I stop to occasional seepage of groundwater from standing in the crawlspace.
Thanks...I just assumed that they were required but left up to the homeowner to decide to close them or leave them open.
The reasoning behind sealing them off makes a lot of sense, but I'd feel a lot more comfortable closing them off once I stop to occasional seepage of groundwater from standing in the crawlspace.
You would definitely want to stop that first. Is it coming through the walls in a localized area or random places?
You would definitely want to stop that first. Is it coming through the walls in a localized area or random places?
Just one end of the house where there is a slight decline in the side yard towards that spot.
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We did have an issue with the LG models Home Depot sells. The unit worked fine, but the drain connection was under-engineered and weak. On the frigidaire model, go to the PVC section and get a threaded female to male adapter 3/4". The run the rest of the line with 3/4". You can splice it into an existing condensate line for HVAC if you have one to carry it outside. If not, bring it through a foundation vent or into a positive foundation drain at the lowest corner of the crawlspace. Antyhing that will carry it away from the house. Most existing condensate lines do not extend far from the house either. Recommend digging a 6" x 6" gravel pit for it to discharge into, or put a gutter downspout splash block under the discharge point just to keep everything away from the house. 
