This is a very interesting topic to me, and furthermore, this is a subject that may be of great interest to the many Virginians who own homes with crawl space foundations. Conditioned crawl-spaces have their exterior walls insulated instead of an average home that has R-19 fiberglass batts in between the band area of the floor joists. And a very thick vapor barrier is installed on the crawl space floor and it is taped and caulked with great care to ensure no moisture can intrude. 
Is your crawl space damp? Maybe some puddles after a hard rain? This is a common issue. In particular, crawl spaces that are dug down into the ground have a high chance of experiencing moisture issues. Yes, if the crawl space hole was dug and no home was constructed- then this hole might become a pond. Swimming anyone? And yes, I know drain tile is installed on newer homes, but drain tile can fail. And if your footing was trenched, the drain tile will be at the same level as the bottom of your foundation wall. This can still lead to moisture intrusion. Well, let’s really dissect the crawl space design and look at whether fully insulating and conditioning your crawl space is the right decision for you. 
First off, let me tell you that I just constructed and sold a solar powered energy efficient home that was built on a crawlspace. I didn’t dig a hole. I trenched the footing and poured my concrete with steel reinforcement- but I just dug down to good virgin dirt. I didn’t dig a hole and then trench my footing. Yes, it cost me more money to then construct the brick veneer foundation walls high enough to allow for access to the crawlspace for work under there to take place. And the home sits higher out of the ground than most crawlspace homes. But I feel like that is money well spent for the tall foundation walls and the higher walls add a nice appeal to street view of the home. Constructing a home with almost zero chance of crawl space moisture issues is a good thing. And this starts with not digging a deep hole to save money on brick (most everyone uses cheap concrete block and then switch to brick when they get to surface grade).
Some literature refers to the life-span of a home being 30-50 years. I used to teach a college workforce training class called: “Your role in the green environment”. Sounds cheesy right? Well, the class was based on a book that was published by NCCER in conjunction with VA Tech. And it was well written and informative. This publishing cited the lifespan of a home being 30 years. I’d like to think we can do better than that. And I would guess this figure assumes homes will exist longer than 30 years, but the money invested in maintaining them perhaps equates to the money you would spend on a new house every 30 years. I hope that makes sense to you. Well, I believe we should be spending just a little more money on new construction homes to ensure they last a very long time- and with as little maintenance as possible. And it starts with preventing any moisture from sitting around or under the home. Moisture brings on all the bad stuff- termites, mold, foundation cracking, wood rot, etc. You get the picture. Moisture is the enemy and the cause of homes that may only last 30 years without major, major repairs.
Back to that house I built. Did I encapsulate and condition the crawlspace? No, I didn’t. I did close the crawl space vents that I was required to install, per VA state building code. Crawl space vents in a mixed humid climate? Really? They say you’re supposed to close the vents in the winter and open them in the summer to allow the crawl space to dispel moisture. Well in August in VA you step outside and the humidity has you dripping with sweat almost immediately. Should we believe our crawlspaces are more humid than the outside air that can hover around 80% during many parts of the summer? If so, you have some serious issues, probably including mold and other nasty stuff. So I say close the vents year round. If you have higher levels of humidity in your crawlspace, then you have a moisture issue that needs to be resolved. And encapsulating your crawlspace only puts a band-aid on an existing serious moisture issue. Sure, the home will be healthier if its encapsulated- but if water sits around your footing under the vapor barrier- you may be in for some foundation cracks down the road. And this moisture can migrate up your masonry wall and still make its way into the framing of your building’s exterior walls. I build crawl space homes that don’t need to be encapsulated because there are going to be zero moisture issues. So I just air seal the crawlspace with foam, use zip tape around the perimeter where the sill plate meets the top of the foundation wall, seal the ductwork with mastic, and insulate the floor system. Encapsulating and conditioning a crawl space is expensive and you must have some return air from the crawl space to your HVAC unit. And although many companies call these “clean crawlspaces”, I prefer to only condition space that can be cleaned and dusted. Anybody out there been down in their crawl space dusting lately? I doubt it… Will there be a layer of dust on the floor of a “clean crawlspace” after a few months? Absolutely. Will the return air to the HVAC system be sucking this dust into your ductwork and distributing it throughout your home? Most definitely.
Now I think there are many, many homes out there that are excellent candidates for full encapsulation. Just not the new ones that are being built right. Homes that can benefit are those with moisture issues. A home on a crawl space with no moisture barrier can send 20 gallons of water up through the home every day. Air rises from the crawl space and with it comes the moisture. Sofa feeling a little damp? Is your sheetrock a little swollen and moist? This might be you. First, you need to start off by making sure your gutters are working properly. Attach drain tile to the bottom of your down-spouts and trench the discharge line out at least 5′ from your home. Ensure you have no grading issues where water can puddle against your foundation. Inspect your home for plumbing leaks. After all, the leading source of water issues within a home is leaky plumbing pipes. Sad but true. If you do all this and you still have a soggy crawlspace floor, then you have a couple of options. Option one is paying a whole lot of money to have your foundation perimeter dug down and have drain tile installed. Option two is fully encapsulating your crawlspace. And if you go with option two- be sure your footing and foundation walls show no signs of cracking. Sometimes pouring some areas of the cinderblock walls with concrete can reinforce weak areas that may have sagged and cracked from years of the water living under there.
So say you’ve got a home that has a crawlspace with minor moisture issues, and you don’t see any obvious signs of foundation movement or cracking. Well, you are probably a great candidate for full encapsulation and conditioning of your crawl space. Sure, you will be susceptible to dust entering your HVAC return air, but that’s better than living in a moldy home that may be eaten by termites, right? By the way, did you know termites can only eat wet wood? Dry home, no termites. 
So now you’re going to install 2″ blue board or a few inches of spray foam around the interior perimeter of your crawl space. And you’re going to smooth out and level the dirt under the crawlspace as best you can. Bring in some sand if you want to help with the process. This will leave a nice surface for the installation of your vapor barrier. And not the 6mil plastic required by code. I’m talking about a very thick membrane that won’t easily puncture under stress of workers who will certainly venture down there in the future to do things (cable lines, fix plumbing, etc.). Lastly, you’re going to open up some HVAC supply air and return air to the crawl space. Most companies don’t do this or if they do- it isn’t scientific. They just poke some holes and call it done. But the square inches of supply and return area must be sized based upon crawl space volume. There are charts for that you can look up if it interests you. You see, the HVAC aspect is so important because properly conditioning the crawl space will allow your HVAC unit to operate very efficiently. An air handler trying to heat the home above when the crawl space is 40 degrees is going to run a lot more often than a unit operating in a 70-degree crawl space. So you can’t rely on just duct leakage to condition your crawl. There’s no science or accountability in hoping all the cracks in the ductwork equate to what your crawl needs as far as temperature and humidity. 
Conditioning a crawlspace is expensive. But in cases where there are moisture issues, it makes sense. And I’m not saying to not fix the moisture issue. But some have homes that were built with elevations where some moisture is unavoidable. In that case, go for it! 

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