If you’re considering crawl space insulation with a vapor barrier, then you likely have a crawl space in your home that is damp, dark, and leaking moisture-laden air into your living space. You are not alone – crawl spaces, while not common in new construction, are a legacy of previous building techniques and can be found in all climate zones. If you’re stuck with one (or several!) there are ways to make it better including installing crawl space insulation and a vapor barrier.

Why would I need a vapor barrier in my vented crawl space? 

Many crawl spaces have porous floors that allow groundwater moisture to enter the space. This is called capillary flow – moisture wicking up from the ground.  As we’ve written about before, moisture in your home brings problems – poor indoor air quality, mold, mildew, rot, etc. A vapor barrier will help keep this moisture out of the space and protect your crawl space insulation.

Before you install!

Before installing ensure that water is not entering the crawl space from the outside. Water can damage your crawl space insulation, footings, or other key elements of your crawl space. Susceptible areas are the bottom support area of the foundation (footing), between the footing and the walls, through block walls, or through cracks or openings in the crawl space. Look outside your home. Maybe your downspouts need to be extended to keep water away or soil regraded to create a natural runoff. Some groundwater problems might need a bigger fix like a pump and drainage system. Another critical step is to seal up any crawl space vents as they are huge culprits for bringing moisture in. Some rigid foam board with an airtight seal will do the trick nicely. 

How to install a vapor barrier in a crawl space 

Once you’ve addressed outside water sources you can lay down a vapor barrier on the floor. A typical vapor barrier is 6 mil polyethylene. The process is surprisingly straightforward. Clean out any debris on the floor to minimize tears or perforations in the vapor barrier. Unroll the vapor barrier and spread it across the ground and up the foundation wall, sealing at any junctions or overlaps. It’s important that all seams are taped tight in the barrier and there is generous overlap including an extension up the foundation walls. Some folks lay down a drainage mat underneath the vapor barrier to help with any runoff. 

What about encapsulation? 

Ideally, a crawl space, especially one with crawl space insulation, is encapsulated which means bringing your crawl space into the conditioned space of your home via air sealing, insulating and conditioning the air (Installing a vapor barrier is also a crucial step). This can be a big project and is often handled by professionals. If encapsulation is not a realistic option then following some best practices, like installing a vapor barrier will still go a long way.