Basement Vapor Barrier For Basement Insulation
Written by Todd Fratzel.
Basement Vapor Barrier and Insulation
We’ve written several articles about basement insulation and a cost effective approach to basement insulation using foam board and fiberglass insulation. Since writing those articles we’ve received quite a few questions about when to use a basement vapor barrier and when not to. So we thought it might be a good idea to clear up some of the confusion.
Understanding Vapor Movement
First off you need to think of your concrete (or block) walls as a huge sponge for moisture (water vapor). Over time and throughout seasonal changes in temperature concrete will “dry” out releasing a tremendous amount of water vapor. The adjacent sketch shows an unfinished, un-insulated, un-heated basement wall. We’ve shown arrows that indicate where the water vapor goes as the wall “dries” out.
Depending on the time of year it’s possible that all that humidity in the air will turn around and condensate on the cool concrete surface if the dew point is correct. The point here though is how moisture in the form of water vapor leaves the foundation walls and migrates into the basement space or outside above grade.
Bad Basement Insulation Detail
One of the worst basement insulation details that we see on a regular basis involves framed walls adjacent to concrete foundation walls. The cavities of the framed walls are filled with fiberglass insulation and a vapor barrier is installed between the studs and drywall.
This basement insulation detail is a really bad idea. As the concrete dries and gives off the water vapor it get’s trapped between the concrete wall and the vapor barrier. This causes the fiberglass insulation to become saturated with water. Water laden fiberglass is extremely inefficient and very prone to mold growth.
As you can imagine once the fiberglass becomes soaked in water it’s nearly impossible for it to dry out. The fiberglass must be completely removed and disposed of. It’s also very likely that mold will have grown on all the studs, drywall and floor joist framing above.
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.
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