House Design

Weve written several articles about basement insulation and a cost effective approach to basement insulation using foam board and fiberglass insulation. Since writing those articles weve 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.

Before we explain where to use vapor barriers its a good idea to talk about where the vapor comes from which ultimately makes the discussion easier to understand.

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. Weve shown arrows that indicate where the water vapor goes as the wall dries out.

Depending on the time of year its 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.

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 gets 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 its nearly impossible for it to dry out. The fiberglass must be completely removed and disposed of. Its also very likely that mold will have grown on all the studs, drywall and floor joist framing above.

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. Thats why weve 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. Its 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 youve 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 youve used less than the 1-1/2 of foam. This is not a perfect situation and its one we recommend you try to avoid.

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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