Better Basement Insulation Detail
One of the best ways to insulate basement walls is by using spray-in-place foam insulation. However, spray foam insulation can be VERY expensive for some projects. That’s why we’ve come up with a hybrid insulation detail that uses a combination of rigid foam insulation and fiberglass insulation.
This detail can vary greatly depending on what part of the Country you live in and what R values are required by your local building codes. The idea for this detail is to install a layer of rigid foam board insulation, carefully seal it to create a vapor barrier adjacent to the concrete, then frame a wall and fill the cavities with fiberglass insulation to come up with an R value that meets the design.
- Install a minimum of 1-1/2 inches of rigid foam insulation board. It’s very important that the insulation be installed from the slab all the way up to the top of the wall including the top surface of the exposed concrete wall. If you use a thinner section of foam board you run the risk of it not performing as an effective vapor barrier.
- Carefully seal all the seams in the foam board. You can use a combination of Tyvek Tape, Dow Construction Tape (or similar) and spray foam in a can (Great Stuff for instance). This step is very important in order to create an effective vapor barrier.
- Frame a wall directly in front of the foam board. Typically we like to leave an inch gap to allow for air flow around the studs. Be sure to use a pressure treated bottom plate to prevent decay. We also like to install the PT bottom plate on top of a piece of composite decking material to prevent any wicking of moisture into the framing.
- Install fiberglass insulation in the wall cavities to create a final composite R value that meets the energy code requirements.
- VAPOR BARRIER – The real question ends up being whether or not to install a vapor barrier over the fiberglass and behind the drywall. Typically we are not in favor of a vapor barrier if you’ve installed at least 1-1/2″ of rigid foam (approx. R9). It is possible if you install a thinner layer of foam that the surface of the foam could be cool enough to promote condensation if water vapor moves from the conditioned room and hits the surface of the foam board. For that reason we recommend a vapor barrier if you’ve used less than the 1-1/2″ of foam. This is not a perfect situation and it’s one we recommend you try to avoid.
Bottom Line On Basement Vapor Barriers
The bottom line really is to stop and think about where the water vapor wants to move. If you think about where the water is coming from you should be able to devise a plan that works effectively and avoids the dreaded mold growth. You can also read more about vapor barriers here.