Crawl space encapsulation.

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Old 08-26-16, 09:25 AM
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Crawl space encapsulation.

Three season cottage in Maine, supported on 3 foot cinder block foundation with vents. Dirt floor. I would like to insulate exterior walls (rigid foam) and encapsulate the crawl space. I plan to leave the ceiling alone and use a crawl space dehumidifier.

1. Should I apply vapor barrier to exterior foundation and insulation on inside, or apply rigid foam first, then attach vapor barrier to foam, and insulate seat the top.

2. If I have a closed encapsulated space (no vents or hvac connection to living area) can I just run a dehumidifier in crawl?
 
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Old 08-26-16, 09:42 AM
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Here's a pretty good introduction to the science and practice of unvented crawl spaces:

Building an Unvented Crawl Space | GreenBuildingAdvisor.com

Properly installed rigid foam on the walls, with all the joints taped, does not require additional vapor barrier.

You do need a substantial vapor barrier on the floor, using heavy plastic or membrane with seams overlapped at least 6 inches and well sealed. The barrier should extend up the walls several inches (before you install the foam) and be mechanically attached to the side walls and sealed with tape and/or mastic.

Even though the crawl space is sealed to the outside, (in fact, because of it) It is recommended that you have positive ventilation between the crawl space and the home. It is not recommended to just run a dehumidifier down there. Positive ventilation means a passive vent at one end and a fan at the other so you have circulation and the crawl space become part of the conditioned space.

Good luck with your project!
 
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Old 08-26-16, 09:43 AM
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A closed encapsulated space should be conditioned. It then needs to be fire rated.
 
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Old 08-26-16, 02:37 PM
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CT and X have covered what you need. If you can find the Dow Thermax it can be left exposed and qualifies as a thermal barrier.

Since you have power you might consider a Radon fan or equivalent for ventilating air out of that space. They are rated for 24/7 for many years. Provide a smallish opening from the crawlspace to the living space above (this is assuming encapsulation) to replenish the air the fan exhausts out.

Which season are you omitting? Winters are getting fun up here.

Bud
 
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Old 08-26-16, 07:53 PM
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I am putting a dehumidifier down there. I'm sure the floor above is not air tight. Do I need a vent or can dehumidifier just test that air. I don't want to blow warm air outside in the winter.
 
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Old 08-27-16, 04:47 AM
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If you insulate the block walls and rim joist, plus a vapor barrier on the dirt floor sealed to the walls, you will (should) not be dealing with a moisture issue, thus no need for the dehumidifier. You could rely on existing leakage from the crawl to above or add a few extra holes to allow some circulation and avoid stale air.

I have read some articles from Alaska about their approach to encapsulating crawlspaces. Here is an example of what they suggest. This is a company selling their product but their explanation is interesting.
Crawl Space Fan System To Ventilate a Crawl Space in Wasilla, Anchorage, Juneau | Dry Your Crawl Space Ventilation Options in AK

Bud
 
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