An Unexpected Benefit of Encapsulating a Crawl Space

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My friend Perry had lived in a nearly hundred year old house for six or seven years when we encapsulated his crawl space in 2005. It was a typical old country house. Originally built on piers, someone had filled in the gaps and turned the foundation into an enclosed, vented crawl space. There was no insulation in the floor over the crawl space. The floor itself was old pine planks nailed right to the floor joists, with no subflooring.

As you might expect, air moved easily between the house and the crawl space. In fact, when we were working on encapsulating the crawl space, I remember being in his kitchen sometimes and seeing the lights we had left on down below shining up through the gaps in the flooring.

And that's not all that came through the floor. Occasionally cave crickets, like the ones in the photo below, would show up in his bathroom. If light and crickets could come through the floor, other stuff could, too.


I was a home performance contractor back then and had started encapsulating crawl spaces. We'd put a heavy duty plastic liner on ground, up the walls, and around the piers. We sealed up the vents in the foundation walls. We air-sealed the band joist. I loved the transformation that happened whenever I did one.

After we finished getting Perry's crawl space all closed up, he started noticing that his house smelled better. But he also got an unexpected benefit. He told me one day that he stopped sneezing every morning. It had been part of his daily life so long he didn't notice it anymore, but he used to wake up each day and go through a little sneezing fit.

Then the crawl space air got better. And Perry's morning sneezing went away.


Related Articles

Solving a Crawl Space Water Mystery

What Is the Best Way to Deal with Crawl Space Air?

Beautiful Duct System in a Beautiful Crawl Space! Was I Dreaming?


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Steven Lefler

It is refreshing to read what has been mandated by Factory builders in HUD Modular or manufactured home industry. They build on a raised foundation as described on a pier and support. The factory encapsulate the underbelly of the sections with fiberglass and thick 6 mil plastic as described.

Allison Bailes

Steve, that sounds like it could lead to problems in cold climates. Putting a Class 1 vapor retarder on the outside of the enclosure could turn that into a condensing surface if indoor air is able to get into the floor system and find the impermeable layer. What am I missing?

pm suds

we have a home on the chesapeake bay in an environment where mold and mildew never sleep. and winter winds rip. encapsulating the crawl space was the best thing that has happened to this (new construction) house. there was an immediate change in the interior environment... no more musty smells, etc. and it really lends a big assist to peace of mind re the possibility of frozen plumbing. this should be code everywhere!

Allison Bailes

Or at least it should be strongly favored in humid climates. It's not so necessary in dry climates.

Craig Langford

Allison, how is the enclosed crawlspace "vented"?

BTW, love reading your articles. I perform IAQ and mold sampling so I'm interested in your work .

Thanks for publishing interesting and educational articles.

Sincerely, CRaig Langford

Allison Bailes

Craig, if you read the article about dealing with crawl space air that I included in the Related Articles, you'll see what's recommended. In Perry's house, we didn't do anything and it performed well, probably because of how much air exchange there was between the house and the crawl through the leaky floor.

Julie Tolliver

We usually start out by "doing nothing" for ventilation then we monitor (check in on) the space with the client every 6 months or so. Many times there are supply ducts down there (we'll leave some leakage down there) so air still moves through them, and they're usually well-connected to the house through the floor, or connected directly to the basement.

I think it's most important to seal these spaces from the outside REALLY WELL (we check every inch with the blower door on to be sure). Encapsulation used to make me a little nervous but now after watching a bunch for a few years I'm completely confident in the method.

I'd be happy to add some ventilation (like in Allison's 'related articles' but in Climate Zone 4, so far, for all we've installed, we haven't needed to do that.

Steve Waclo


We all luv a happy ending!

And with all those gaps in the floor boards, Perry did not have to be concerned about air circulation through the crawl space, either!

Cave crickets. Ewww! I recall my first encounter with those creepy creatures 😟. They are harmless, but I suspect their mothers even found them to be repulsive.

Allison Bailes

Yep! The crawl space performs well, I think, because of the air exchange with the house.

John Mattson

My cats loved them, and moths, until we got things sealed.

Julie Tolliver

We regularly encapsulate crawlspaces in Cincinnati (I did my own a little over a year ago so I live the benefits). The results are always dramatic. Similar story - a client couldn't play with his daughter in the finished basement playroom because he sneezed there. We encapsulated the two connected crawlspaces during a whole home energy upgrade and the sneezing went away completely.

Beautiful space in the picture by the way, I love crawl space before and after pics. I actually love doing crawlspace work, it's really hard work but it's like adding a big layer of "clean and fresh" to someones house. Worthwhile work. Feels like I can breathe easier just thinking about it!

Allison Bailes

Ah, a kindred spirit! I always disliked the initial part of prepping the crawl for encapsulation, but the initial state made the results all the more satisfying.

Don Jackson

Nice looking job, Allison! This reminds me of a story told by Anthony Cox at an ACI presentation. He worked with a retrofit crew to air-seal, insulate, and generally improve the air-quality and thermal comfort of a home in rural Virginia. When he checked back with the homeowner to see how it was going, she said it was great, and that she was no longer getting backaches. Backaches? he asked. Yes, she said, it was so cold before and she had to pile on so many blankets to sleep through the night that she woke up every morning with a backache! Another happy unexpected outcome.

Allison Bailes

Thanks, Don. I hadn't heard Anthony's story about eliminating backaches before, but it makes sense.

John Proctor

One of the experimental houses in the CVRH (Stockton) studies has a crawlspace. It showed obvious signs of moisture, even in the dry climate -- irrigation system soaking around the foundation. When we encapsulated the crawlspace (no venting), the musty smell got worse rather than better. So we had to put a low wattage exhaust fan in the crawlspace that (due to leakage between the house and the crawlspace) became our ASHRAE 62.2 compliant ventilation system.
One experienced factory HVAC guy went into the crawlspace and came out saying: "That is the best smelling crawlspace I have ever been in."

Tyler Kime


I love my closed crawl space, it works great and it gives me great storage!

I am in the HVAC and HP fields and I wanted to know your take on having 80% furnaces in an enclosed crawl space. I have thought that you would need to upgrade to a direct vent for the combustion but I run into some that still have the 80%s. What is your take on this?

Allison Bailes

Tyler, my general opinion is that atmospheric combustion appliances don't belong in any type of conditioned space. According to code, though, they're allowed if there's enough volume of air in the space, or in a connected space. See this article I wrote a while back:

The #1 Question to Ask before Putting Spray Foam in Your Attic

David Butler

For new construction, a basement or slab-on-grade is not only more economical to build, but costs less to heat and cool (as long as provisions are made to bring ducts inside the envelope).

When I lived back east (Charlotte), there seemed to be a strong market bias against slab-on-grade. Only 'low end' tract homes and apartments were built on slabs. Here in Arizona, the vast majority of homes, including mega-mansions, are built on slabs.

Uninsulated slabs provide a passive cooling benefit in warm climates. Just don't forget to insulate the at-grade edges!

Bob Ellenberg

My first sealed insulated crawlspace was in the mountains of Northern New Mexico--definitely a cold climate at 7,200' elevation. Another benefit is heated floors in the winter. Your ducts are in conditioned space and do not need insulation. This was my personal home and I would never have a slab again, cheaper or not.

Scott Ross

In the northern climates that I work in, as an electrician, we se a lot of crawl spaces ( also a lot of full basements) ( Montana and North Dakota). The better higher end homes that have vented crawl spaces generally have vapor barrier (plastic) over an under-layment of 3/4" washed gravel on the flat areas that extend up the concrete foundation sidewalls to the floor system (rim joists and floor joists).
Those sidewalls are sprayed with foam insulation all the way up into the joist cavity. So in essence the whole crawl space is a conditioned space. A lot of the time a furnace would be placed in that space, and duct-work extending around the crawl space hitting each of the rooms on the main floor.
Any thoughts, or comments?

Jeremy. Burke Construction llc

Hello Scott, The conditioned crawlspace is the only way to go if you have a crawlspace.The issuse that we see in North East PA are that most contractors and homeowners miss or partially complete the 4 steps to a conditioned crawlspace.
1 control the groundwater-taking precautions so area will not flood.checking gutter leaders to keep water away from foundation. Installing perimeter interior drains to a tail drain or a good sealed sump pump.A battery backup pump if there is no generator.
2.control the earths moisture- installing a good durable vapor barrier that is sealed 100%! 6 mil just will not do. If you have a good vapor barrier with poured concrete,that will do as well.
3 Stop the outside air infiltration We untilize closed cell foam to seal foundation walls and rimjoist.then you get a good R-value/ thermal break from the exterior. Install a good sealed door,we use cut down exterior doors or if opening is to small a good sealed cover.We see alot of customers that have sprayfoam on there foundation but no sealed door.So the cold air comes right in.
4 condition the space
Once you have a space that is able to be controlled.Do just that! Introduce heat and AC to keep area control.Since there is no outside infueunce in that area it normally will stay around your average ground temperature. We always use dehumidifiers to keep area conditioned and keep relative humidity around 55% so there is no condensation or mold growth.
Again all 4 steps are important to have and utilize a conditioned crawlspace.If 1 is missed or done poorly the system will not function to it fullest potential! Sometimes not work at all and waste money and energy.

Jeff W

Well I have an encapsulated crawlspace that has not turned out well due to The foul odor that occurred afterwards. My whole house ended up smelling like cat urine and I had to open the vents and buy vent fans to keep the foul odor out of my living space. Unfortunately I know several people who have had the same experience. It's not foolproof. No one could ever find the source of the issue. Assume it's the vapor barrier itself as I had no foul smell before. I liked the encapsulation except for that aspect. So I left the barrier and just vent. I assume that is better than nothing but is disappointing since I spent s lot on it. I am not the only one with this issue apparently. Jeff

David Butler

@Jeff, no you're not the only one with urine odor associated with an encapsulated crawl space. The the cause is no mystery, and completely avoidable. See:

Jeff W

Thanks David. That link you sent me opened to a variety of possible web sites so I never found the exact reference to your reply "there is a known cause forvtheccat urine odor". Can you send me the exact link or tell me where to look for it? I still want to address this as I assume it won't go away by itself.

David Butler

Sorry Jeff. I agree that wasn't very helpful (I was in a hurry). The top 2 hits are articles @ Green Building Advisor about chemical odors associated with improperly applied spray foam insulation. Spray foam is typically used to seal the rim bands when encapsulating a crawl. Was spray foam applied in your crawl?

I've seen numerous complaints about odors associated with improperly applied spray foam in green building blogs, most often described as a fish odor, although some people describe as a cat urine odor. Strong chemical odors are normal during application -- installation protocols stipulate occupant evacuation and ventilation during the application. But if spray foam is mixed and applied as per manufacturer instructions, there should be no residual odor from the foam once it cures.

The other possibility, as you mentioned, is the crawl space liner material, or the adhesives used to seal the liner. You you have a poly-vinyl chloride liner (typically black or white), or is it polyethylene (translucent)? PVC liners are commonly used for swimming pools and have a very distinctive odor.

John Proctor

Years ago we tried encapsulated crawlspaces in southern Indiana. This was an area with a very high water table. The energy savings were very nice and there was no smell problem.
Now at one of the Central Valley Research Homes we had a different result. The crawlspace had many signs of being wet -- an irrigation system watered most of the perimeter of the house for plants right up against the foundation. We encapsulated it and the results were good energy wise, but the mildew smell increased (this had an exhaust only 62.2 ventilation system). So we started exhausting only from the crawlspace (sufficient to bring in 62.2 ventilation into the conditioned space). Great results - the smell disappeared and a unknowing experienced HVAC person later came the crawlspace and exclaimed: "best smelling crawlspace I have ever been in."

jim adams

We're going to encapsulate our crawl space (first time) in an early modular home. The crawl space really IS a crawl space -- 17 to 20 inches high, currently with a dirt floor, about 8" fiberglass insulation between the joists, and whoever had the place before us dumped the AC condensate into the crawl space about the middle of the crawl space.

We're considering putting 6 mil plastic on the floors since they will be seldom crawled upon. Should we run it up the walls a little way?

We're looking at filling joist cavities -- about 2 or 3 inches -- with Great Stuff 3X expanding foam using a store-bought tip on the can sprayer (3 for $7)

And some more links: this link shows the triangular spray tips for Great Stuff and a couple other brands of canned spray foam, 3 of these on ebay for $7. I'm gonna practice on a new bath tub we're getting ready to intall. I always wanted an insulated bath tub so i can run a hot bath and have it remain hot for a hour or so in winter

also interesting a how-to youtube of the spray nozzle i list above. After watching, i think i can now use foam effectively spray foam insulation
nightmare -- says that commercial foam appliers get a two part formula from their suppliers. They mix the two on site, spray and leave. It seems that most spray jobs are ok, but if they don't mix the two chemicals well enough (like with epoxies), or if the temperatures are off when mixing or applying -- then there can be problems -- a euphemism for poisonous outgassing which can cause headaches, allergies,generally feeling very poorly and a lot more. And there is a uriney smell, or maybe a fish smell.

On the other hand, small store-bought cans of pre-mixed spray foam don't seem to have a bad rap. Anybody know of problems? I'd like to know before i spray the stuff on a bathtub which will get to 105 to 110 degrees F every time I fill it with hot water.

Brigitte Clark

Hi, once my farm sells I plan to buy a waterfront home on the St. Lawrence River in New York. Most of the places for sale are three season and sit on piers. Unless I find one that is already year round, I plan to convert a three season to four season. As I look at homes, I see that some look easier to encapsulate than others but they all have looked do-able so far. Is there any scenario I need to steer clear of? The only one I can think of is a crawl space so low that it can't be accessed or worked in but I have not actually seen that so far (maybe that doesn't exist.) Thanks!

Brigitte Clark

I was told by a realtor that I would need a 4 foot tall crawl to winterize a cottage, I think as long as there is room to work and install whatever ducting, etc. that is needed would be enough. What do you think?

Jeremy Dr Energy Saver of NEPA

I agree! If there is room to work you can condition almost any space.Not that it will be easy to run HVAC ducting in a low crawlspace.In PA we run into lots of cabins that have walls between the piers.We covert them into sealed areas as well. As long as you can create a good insulated seal from the exterior.You can condition the area! Have fun Jeremy
DR Energy Saver NEPA


Thank you so much!