Crawlspace insulation - be careful to get it right

A crawlspace comes with all the downsides of a basement, but it is just too low a space to be functional either beyond the folded plastic christmas tree and a couple of boxes of baubles. Unless there is a good reason for it, going through the process of pouring a footing, then a concrete wall, then building a subfloor above and only be able to access it on your hands and knees is a lot of effort for the value you will get from that space. If you don't want a full basement, a great alternative is to build a slab-on-grade.

Important note: The following crawlspace solutions are recommendations for how to deal with moisture and heat loss only, and provide only an anecdotal overview of how to deal with radon gas. We would recommend having your air tested for radon, and if it is above Health Canada’s recommended safe levels (200 Bq/m³), please see our pages on radon gas mitigation in crawlspaces .

Fixing a dirt floor crawlspace:

All too often a crawlspace will have nothing but a dirt floor, and probably some un-insulated walls. The result of that is an unending source of humidity as moisture in the ground evaporates into the air and can condense on cold walls and joists where mold can establish itself.

That dank air will inevitably be circulated throughout your home, providing humidity, mold spores and a pretty nasty smell that will be the dominating odour of your home, furnishings, and your clothes, so you will take that aroma everywhere you go. Rotten huh?

Crawlspace vapour barrier ©

For those with nothing but a dirt floor, the single most important thing to do will not cost much - cut off the moisture supply to the air by laying down polyethylene directly on the ground. The standard 6 mil poly vapour barrier will work fine as a DIY solution. Do the best job you can and seal the joints, but remember - if you did nothing else beyond laying it down over most of the floor, even without taping or sealing it, you would reduce the amount of moisture in the air. If your crawlspace access is inconsistent and you can only hope to do 80% of it, do it anyway; you will notice a very big difference.

The 6 mil poly is far from being a complete crawlspace solution, but we’re putting that out there so as not to leave those with really tricky crawlspaces or tight budgets feeling dejected. This is not an 'all or nothing' situation where there is no point doing anything if you can't do it all - do as much as you can, it will help.

Insulating crawlspaces:

The best way to insulate a crawlspace would be to do it from the ground up and consider it just part of the conditioned space as you would with any basement. We make no secret of the fact that we aren't huge fans of spray foam due to the emissions associated with it (with exceptions for eco-friendly spray foams) but this is one place where it can make a lot of sense.

Spray foam is done quickly, it has a high R-value, provides an air and moisture seal and fills all the tiny holes. This is not a DIY job so it will be among the more expensive options, but it will work well, and best of all ... someone else will be slithering around in your crawlspace rather than you. Since it will act as a vapour barrier, there is no need to lay down poly ahead of time; it can go right against the dirt.

When it comes to insulating the walls, there are a couple of ways to go about this – Building Code requires any crawlspace more than 5 feet high to have fire protection covering any foam products, in which case you can frame a 2x4 or 2x6 wall 1 inch away from the concrete wall; sitting it on small chunks of foam will keep it from being in contact with the ground and absorbing moisture. Spray foam is then applied to the walls behind the studs and into the cavities, you can leave enough room to do any wiring if required.

Spray foam insulating basement walls © Bala Structures

The foam needs to then be covered with drywall as fire protection, which can be attached to the stud wall. No vapour barrier is required in addition to spray foam. Building codes change and can vary by region, so confirm this locally, but currently The National Building Code in Canada for example allows foam insulation without fire protection on crawlspace walls that are less than 5 feet in height.

An additional option, for walls only, that you can do yourself is to apply EPS foam boards directly to the concrete using either strapping and tap con screws, or an appropriate adhesive that is suitable for foam boards (it should say so on the tube). EPS can be substituted with XPS foam, but given its much higher greenhouse gas emissions, we prefer to recommend EPS. XPS qualifies as a vapour barrier at 1 inch, and testing shows that EPS foam acts as a vapour barrier (1 perm) at 2 inches or thicker.

Rock wool insulation batts on basement walls © Ecohome

Rock wool (mineral wool) boards are an alternative product you can use and they are made from recycled stone dust, so it does not burn and does not need to be covered for fire protection. But mineral wool is not a vapour barrier, so it is best to apply a poly membrane against the concrete first.

Note: Taped XPS foam is an air barrier, but EPS and mineral wool are not. Air doesn’t leak through intact concrete but it is quite likely that crawlspace walls will leak air. If you have a stone foundation, old cracked concrete or cinderblocks, bringing the poly barrier up the wall and sealing it to the rim joists will provide you with an air barrier.

Preparing a crawlspace for vapour barriers and insulation:

First clear the space as much as possible so you have room to move around. Ensure there won't be any problems with bulk water accumulation by installing a sump pump at the lowest point. Even if you’ve never had flooding in the past, that does not mean you will never have it in the future. We’ve never heard anyone say they regretted installing a sump pump.

Remove any sharp rocks and obvious obtrusions for a more even surface so that you won't tear the plastic as you walk on it.

Should you vent a crawlspace?

No, you should not. Drawing humid air into a cold crawlspace in summer or winter defeats the purpose of stopping the source of moisture. A moisture protected crawlspace will probably be quite cool in summer and should be less humid than outdoor air, so moisture can condense on the cold surfaces of wood and concrete. It's better kept sealed up once you have dealt with moisture and air quality issues. Be sure to monitor the humidity levels once work is completed, you will likely need to include a dehumidifier to keep humidity down.

When shopping for a dehumidifier make sure it can be operated continuously with a drain hose, don't get one that needs to be manually emptied. They can easily fill up at least once in a day, and if you're not there to empty it regularly it will stop working. 

The volume of water that will be produced by a dehumidifier is not so great in quantity that it necessarily needs to be directed to a drain, which likely won't even exist in a crawlspace. You can have it set it up to drain right into a small pit in the dirt below the membrane and it will absorb back into the ground.

Removing radon gas from crawlspaces

We highly recommend getting your air tested for radon gas before beginning work to determine if any measures will be needed to protect your air quality. Air contaminated with radon gas is a serious health hazard and is the second leading cause of lung cancer after smoking. Please see our page on crawlspace radon mitigation for more information.  

Now you know more about how to insulate a crawlspace, reduce humidity and mold. Find more pages about sustainable construction below and in the EcoHome Green Building Guide pages.

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