I am working, doing audits, with a contractor who tends to make basements, crawlspaces into conditioned space. Especially when all or most of the duct work is in the crawlspace. He does this by covering the floor, side walls and piers with heavy plastic. He then uses rigid foam insulation on the walls, foams the joints and above and seals the vents and access hatches. Then we duct seal and re-insulate.

Has anyone been doing this for a while? Any unentended consquences?

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Well, there are so many factors to review with your local code enforcement:

1. 3" gap at top of insulation from foundation wall

2. insulation offset from ground

3. appropriateness of foam board insulation in crawlspace (Thermax, for example, seems to be appropriate across NC, but other states require any foam board to be covered for flame spread, etc.)

4. method of conditioning the space. dehum, hvac, return air from home.

Most of the experts (EPA, DOE, Advanced Energy) recommend conditioning an encapsulated crawlspace using 1CFM per 50 sq ft of crawlspace area from existing ductwork if available. We find that  is more cost effective than buying and running a dehumidifer. Of course, sometimes ductwork isn't available and you have no choicebut to install a dehumidifier. In any case, most local codes require conditioning the space. The subject is addressed on our web site. wwww.yourcrawlspace.com

As always, check your state code.  If you're using HVAC air, you may be required to insulate the walls of your crawl space.  That could lead to higher initial costs than just getting the dehumidifier.

While insulated and conditioned with HVAC air is the ideal, it's not always the best solution.  Tapping into an existing supply line to condition the crawl is robbing air from the living space, and can put more stress and wear on an existing unit.  Also, in some situations it's undesirable to create that slightly positive pressure in the crawl helping lift air into the living space.

In cold climates using the HVAC system to condition the crawl space is in most cases the best way to provide the comfort factor the homeowners expect from a new system. Leaving the crawl space unconditioned leaves the floors cold that can lead to higher setting on the thermostat, uncomfortble seating for the kids on the floor, and generaly less comfort in the home. We always size off the heat/gain calculations to handle the extra load.

In crawl spaces I have encountered upstate NY
6800 HDD and generally wet
walls are stone or block & stand 2ft or more above the exterior grade
Interior grade is less than 2 ft below exterior grade
Limited access to space, no storage
Ducts and pipes may or may not be present

We find it most effective to lay EPDM on the ground in the space and spray 2 or more inches of AC277 appendix X compliant closed cell foam on the EPDM and up the walls to the underside of the subfloor. These spaces are small enough and insulated well enough so the heat load is insignificant and owners report happiness.

If the space is too wet, you can put a drainage plane under the EPDM and lead it to a sump. But that is not necessarily required because the foam will 'set back' slightly which tends to raise the EPDM and create a capillary break in the field of the floor. You may still want a drainage plane near the ground/wall joint because the EPDM will not lift there like it will in the center of the floor.

I don't know how effective it is, but we have been trying to sell an application of boric acid based insecticide on the ground and walls before we do the insulating to defend against bugs.

I have found the lower the interior grade is relative to exterior grade the less you need to insulate the ground. In our area, at -4 relative to outside we are just going to spray the wall and 2 ft inboard of the wall on the ground, we still use foam to seal any seams in the EPDM though. When a space gets to be 4 ft tall, if the space has easy access, we are going to apply thermal barrier paint too.

When the space becomes a basement we spray the wall down to 6 inches off the floor, but not to the floor. If water can get in that wall I want to let it out. It is best to have some water management forethought like a ditch or pre set water path like B-dry or tremco's fiberglass and plastic drainage system. in a difficult spot you can adapt materials meant for exterior drainage on basements to interior use. In basements, the thermal barrier paint is required unless the client is framing and hanging sheetrock.

I'm curious if anyone has data on conditioned crawlspaces in mild climate zones, like California.  It seems all of the responses listed are from areas with more temperature/humidity extremes.

To my knowledge no, though I do have to laugh at "mild climate" & California as that state falls under 7 different climate zones & recall dealing with roads closed due to snow in June.

Like almost everything it depends on where your mechanicals are, location and other factors - got a specific location in mind?

While we are at it - what does Title 24 call for - as I recall they had some info or reqs in there

Should have qualified our location. Climate Zone 2, Santa Rosa where we had 1580 HDD in 2011 and have 1453 HDD to date. 

So given "our mild climate zone," does anyone have data that supports the additional expense for a conditioned crawlspaces?

TWO-PART WEBINAR SERIES: California Crawl Spaces: Integrated Solutions for Healthy Homes & Deep Energy Reductions: http://thousandhomechallenge.com/thc-webinars-retrofitting-ca-crawl...

Great and surprising information.

sounds awesome! probably helps keep down radon as well

Actually the one study I know of pulled the plug on a house in Flagstaff due to Radon levels increasing because it lost the dilution ability of a vented crawl. With that said depending on your area it still can be worth doing & just making sure you install radon control measures via a rat slab or something similar

I have a conditioned crawl space. I think it is well worthwhile. You can read more about why I like it here: http://www.greenspirationhome.com/?p=1033

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