Mid-Atlantic Waterproofing Blog

Dryness is key when it comes to crawl spaces

by Jim Rooney

We recently had a home energy audit that included a recommendation to "encapsulate" the crawl space under our 12x58 foot addition.  The original part of the house has partially finished basement.  The crawl space is about 4 feet high and has traditional vents and bare dirt.  At one end of the crawl space is a deeper, standing room portion with a cinderblock retaining wall where the oil tank sits behind plywood exterior doors.  We have read that encapsulation is a current trend, but we are not aware of the track record of this approach.  The insulation contractor we used volunteered a proposal to put insulation batts on the crawl space foundation walls, plastic on the dirt base and insulation stuffed in the foundation sill.  What do you think?

If you've read my column over the years, you know that crawlspaces are a detestable bane of my existence.  My last excursion into one to inspect heating and air-conditioning equipment for an older home built over a shallow crawl cost me a broken rib when I rolled over on a piece of scrap pipe.  Lucky for you, all you have to be concerned with down there is the oil tank, and you can easily get to it.
The current approach to crawlspaces is quite simple: get them dry and seal them up.  The vents are there because venting of crawl spaces is a matter of building codes.  But researchers and building scientists have gone over the historical literature and can’t find the original work developing the ratios.  It first came into the building literature in the 1930s with U.S. government home building guides and by the time the post-war home building boom began they were code institutionalized.

Venting crawlspaces makes intuitive sense, so few question it.  But for me it only makes sense if the air outside is approximately equal in temperature and humidity to the air inside the house.  That usually only occurs around here in the spring and the fall.  Bring hot, humid Maryland summer air into cool crawlspaces and you’ll get condensation on materials and soon you have mold farms.  I see it over and over.  Conversely, you don’t want frigid winter air blowing through adding to your heating loads – as your energy audit noted.  Dr. Joe Lstiburek, a building science pioneer from Canada, has been championing closing up crawlspaces for a generation and has even advocated conditioning the air in the crawlspace to a minor degree to help keep things healthy.

There is a cottage industry emerging, especially further south of here, of contractors who go into crawlspaces and do what your insulation contractor has proposed and they’re doing quite well.

But the real key is dryness.  You’ve got to be sure that the crawlspace is dry and will stay that way.  That tends to be a function of how well the addition was built plus gutter, downspout and exterior grading maintenance.  Every drop of water that hits your addition has got to be drained away.  That being done, the next most important issue is the vapor barrier over the bare earth.  Moisture vapor will travel from the earth and go straight up.  It has to be controlled.  I like sheet plastic that is in the 6 mil range of thickness and the joints lapped about a foot and taped tight.  I like to see it run up the perimeter wall about 18 inches and kept in place with furring strips.  Then insulate in the manner your contractor proposed and you’re good to go.  You will save on heating and cooling.

Additionally, If you haven’t had a termite pre-treat of the addition and the soils around it I’d consider having it done.  In putting the vapor barrier down over the dirt and up onto the wall and installing insulation over that provides a potential termite infestation a super highway to your wood framing and they will be hidden from sight.  You’re fortunate that you can view the space easily from the access to the oil tank and I recommend that if you do this, put yourself on a periodic schedule of inspection.  Just take a strong flashlight every couple of months and shine it around the crawl to ensure everything is where it’s supposed to be and there is no moisture showing up.