Ventilated Crawlspaces: A Thing of the Past (Part 2)

Ventilated Crawlspaces: A Thing of the Past (Part 2)

Ventilated Crawlspaces: A Thing of the Past (Part 2)

Be sure to read Part 1 of this 2-part series to learn about the history of ventilated crawlspaces and their potential dangers.

Home energy efficiency is problematic when you have a ventilated crawlspace. Cold air blows in from the crawlspace and hot air rises, leaking through all your attic penetrations. Your home becomes a virtual chimney and loses all your precious conditioned air through the attic leaks. This means your heating and cooling systems have to run harder to keep up. At the same time, contaminated air from the crawlspace are circulated in your actual living space.

So now we know ventilated crawlspaces are a bad thing. How do we fix them?


In new construction, most good builders switched to unvented, conditioned crawlspaces. In existing homes, however, fixing vented crawlspaces is one of those areas that needs to be addressed holistically, taking into account all systems in the home and how they relate to one another. I preach about the “domino effect” in construction: one contractor can do something they think is a good thing, while at the same time creating 5 other problems outside of their trade.

Crawlspaces seem to be especially subject to this issue, so be sure always use a whole house contractor! At Synergy, we’ll take a close look at your home to make sure correcting your ventilated crawlspaces creates the most efficient solution for you. There are 3 main areas that need to be addressed: water (both liquid water and water vapor), air movement, and insulation:


First, we need to make sure the crawl is as dry as possible. This process actually starts outside the home. First, we make sure the grade around the perimeter of the house slopes away. This allows proper drainage from the foundation, ensuring that water does not seep back into crawlspaces. If the draining around the foundation is good and there is still a water problem, it may be necessary to install a sump pump to take water out when it gets in. We’ll also inspect plumbing pipes and drains for leaks and have them fixed if needed.

Once we have the space dry of liquid water, we need to address water vapor, or water that rises from the ground in the form of humidity. We do this by covering the floor with a 6 millimeter layer of plastic sheeting. This sheeting protects water vapor from rising out of the ground. Water will condensate on the back of the sheeting but this is not cause for concern! It just means the plastic is doing its job.


Next, we’ll air seal the crawlspace. The two most important places to seal are the foundation vents and the rim joist, the place where the wooden house meets the concrete foundation. The rim joist is often overlooked, but according to the Building Performance Institute (BPI) 10 percent of a home’s leakage comes from the rim joist area.

In my opinion, the best most cost-effective way to seal these areas is to have a professional contractor install closed cell spray foam to all of the exterior walls. Closed cell foam works very well in areas with higher moisture; applying just 1.5 inches of foam will both insulate and air seal the perimeter of the crawl space. You can get the same effect by installing rigid foam board insulation, but the spray foam expands into every gap and crack, creating a 100 percent continuous seal that is impossible to replicate with rigid foam board alone.


Finally, we insulate the exterior wall of the crawlspace. Again, closed cell foam is the best method for this, but there are a couple of other ways to get the job done. If you are a DIY’er, rigid foam insulation is easy to work with and is completely waterproof, so it is a good product to use in a crawl. Another method is to have a fiberglass insulation contractor to install a vinyl backed fiberglass sheet around the entire perimeter, but this is probably the least desirable way to go because fiberglass and moisture do not mix.


Once you have the crawlspace dry, air-sealed, and insulated, it’s a good idea to have your HVAC contractor install a supply air run off of your system to help condition the crawl. This will give it the same condition as the living space which will stop the negative or positive pressure cycle between the house and crawl.

Addressing the crawlspace should be done before you replace your heating or cooling system because, in many cases, a sealed crawlspace will lessen the burden on these systems, allowing us to install a smaller heating and cooling system which will help offset the cost of improving the crawlspace. The phrase “Two things that when working together are greater than they were as individuals” is what Synergy is all about!

One final comment: there are a lot of contractors out there offering crawlspace solutions. Everyone from termite contractors to crawlspace magicians are jumping into the trade. I would caution any homeowner to make sure that whoever they have work on their home has BPI or some other type of home performance training. In the construction industry, we are slowly learning about the domino effect; I have seen many cases where homeowners spent good money with well-meaning contractors, only to end up creating new problems that no one expected. In most cases, a contractor with a whole house approach could have done the work for less money with better results.

At Synergy, we’re committed to taking your whole home into account and suggesting upgrades that will save you the most energy and money while benefiting all systems of your home.

To learn more about the domino effect and your options for fixing your ventilated crawlspace, Contact Synergy Home today at 859-687-0553 or request service online.