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Crawl space vapor barrier or insulation

Old 01-31-2020, 11:38 AM
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Default Crawl space vapor barrier or insulation

I have an old bungalow that is built up 3’ or so with vents around the crawl space perimeter. The floor is TIG pine on joists/purlins. The humidity can get pretty bad in the house and need to run a dehumidifier regularly. The house is in Tampa.
Would putting a vapor barrier over the dirt help with moisture in the house? I can see both sides of it but mainly that it would help stop that coming up from the ground and there really isn’t water pooling underneath the house.

Another option is icynene insulation sprayed to the underside of the house, would this be a good idea? I know if you spray it in the attic you might create a moisture issue if the attic isn’t completely sealed. Keep in mind the house is old, minimal wall insulation and attic is vented ridges and gabled ends.


what do you guys think is the best approach? I don’t want to call a foam guy yet because they’ll say spray everything....
Old 01-31-2020, 12:30 PM
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Yes, definitely put down heavy plastic sheeting. It should help tremendously. And keep all crawlspace vents open year-round.
Old 01-31-2020, 12:39 PM
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Originally Posted by NC_Native View Post
Yes, definitely put down heavy plastic sheeting. It should help tremendously. And keep all crawlspace vents open year-round.
Agree but would add that if possible you could put perforated drain tile pipe under the plastic and let it "daylight" in a couple locations. That way the moisture will have an exit point. If you can have the pipe higher toward the center and lower at the exit points it will work even better. A couple small fans at the vents might help a lot also.

Remember cool damp air is heavier than warm air is, so keeping the exit point low will help.
Foam would be a last resort. Tyvek stapled to the bottom of the joists would also be cheap and a decent fix.
Old 01-31-2020, 12:48 PM
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Google "crawl space foundation ventilator with dehumidistat".
Several different mfg.
Replaces single existing foundation vent, and really works well.
Simple explanation, when in use create a negative atmospheric pressure in crawl space, creating circulation of otherwise stagnate air and reduces overall humidity.
And definitely install a heavy mil plastic vapor barrier.
Easy fix to your concern.
Old 01-31-2020, 12:52 PM
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Originally Posted by muskrattown View Post
Agree but would add that if possible you could put perforated drain tile pipe under the plastic and let it "daylight" in a couple locations. That way the moisture will have an exit point. If you can have the pipe higher toward the center and lower at the exit points it will work even better. A couple small fans at the vents might help a lot also.

Remember cool damp air is heavier than warm air is, so keeping the exit point low will help.
Foam would be a last resort. Tyvek stapled to the bottom of the joists would also be cheap and a decent fix.
This would be my route. Stop the moisture from coming in, keep it ventilated and then insulate between the joists and add a vapor barrier to them.
Old 01-31-2020, 01:09 PM
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You should definitely insulate the floors if you can. Your AC bill will drop dramatically and so will the humidity. Spray foam is good for that.
Old 01-31-2020, 01:17 PM
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Add a vapor barrier to the ground and walls of the crawlspace. If you feel you need to insulate then insulate the walls of the crawlspace rather than between the joists. The ground temp helps cool the house in the summer and warm it when temps drop below the 60's.
Old 01-31-2020, 01:19 PM
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Do both. Put down a vapor barrier and spray foam the walls with closed cell foam, including sealing existing vents. Add dehumidifier which will hardly ever run, or add a small supply from your HVAC. Do away with floor insulation since this is now a conditioned space. Never have to worry about moisture or mold again. Google crawlspace encapsulation to see it in action.
Old 01-31-2020, 05:51 PM
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We had a damp crawlspace in the last house. I tried the powered vent fans but did not solve the problem of high humidity. We sealed the vents (heavy plastic was already down) and installed a dehumidifier in the crawl space. The dehumidifier did the trick.
Old 01-31-2020, 06:16 PM
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Do a professional encapsulation and see what impact it has on the humidity. It will make a big difference but if you want more .... Add a dehumidifier later. I've had two houses with encapsulated crawlspaces and can state the concept works well. FYI when you encapsulate you cover all of the openings.
Old 02-01-2020, 01:04 AM
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Originally Posted by Toyomo View Post
Do both. Put down a vapor barrier and spray foam the walls with closed cell foam, including sealing existing vents. Add dehumidifier which will hardly ever run, or add a small supply from your HVAC. Do away with floor insulation since this is now a conditioned space. Never have to worry about moisture or mold again. Google crawlspace encapsulation to see it in action.
This.
Fan ventilation from the exterior is a bad idea think about it. What's the average humidity in Fla? You want to keep circulating 93% humidity under there?
Old 02-01-2020, 07:17 AM
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Originally Posted by Tireless View Post
Do a professional encapsulation and see what impact it has on the humidity. It will make a big difference but if you want more .... Add a dehumidifier later. I've had two houses with encapsulated crawlspaces and can state the concept works well. FYI when you encapsulate you cover all of the openings.
We encapsulated and have a dehumidifier at house on Folly and it has worked great but it is expensive
We also had serious moisture problems at our NC mountain house. Since humidity is fairly low there we installed two vent fans and put them on a timer to run in the least humid time of day. They cut on at 10 in the morning off at 5 during the evening and it has worked great.


Old 02-01-2020, 07:19 PM
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Originally Posted by bikem View Post
We encapsulated and have a dehumidifier at house on Folly and it has worked great but it is expensive
We also had serious moisture problems at our NC mountain house. Since humidity is fairly low there we installed two vent fans and put them on a timer to run in the least humid time of day. They cut on at 10 in the morning off at 5 during the evening and it has worked great.
same solution for our house in mount p. Encapsulate, vapor barrier, dehumidifier. No insulation in floor.

Comparing mountain NC to lowcountry humidity is apples to oranges. Our climate, the dehumidifier/encapsulation is the only true fix.

I did the dehu install myself, paid an hourly guy to encapsulate. Cost would've been 5-7k for a 2300 sf house All in. Probably 50/50 parts labor.

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