Furring Out Walls
"Furring out" is the process of building a wall inside another wall. In renovations, this often takes the form of a new 2x4 stud wall built adjacent to the existing masonry wall. Furring out allows for systems (electric, HVAC, and plumbing) to be run easily and for insulation to be added to increase energy efficiency. Carefully consider the following implications as you are deciding whether to fur out the walls in your project:

1. You will need to drywall the new wall (even if the plaster on the existing exterior wall is in good condition)
2. You will need to install new window sills and casings because the new wall is thicker than the existing
3. You will need to remove and reinstall all the base mouldings and trim
4. The room will be made smaller
5. You may need to move doors, switches, radiators, or other elements that are too close to the existing walls

Benefits include better energy efficiency, ease of installing new systems, you can provide a new flat wall surface, and you can cover damaged plaster easily. Carefully weigh the costs versus the benefits.

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Should You Use a Vapor Barrier?
If you are furring out a full masonry bearing wall, and you live in St. Louis, the answer is usually yes. A vapor barrier will decrease air and moisture infiltration into your rooms and make the home more energy efficient.

If you are working with an existing frame wall or masonry veneer wall, there are additional considerations you should think through before deciding one way or another.

St. Louis is a difficult climate to peg. We have dry, cool/cold winters and hot humid summers. An interior vapor barrier is generally recommended for the latter situation only, so unless you want to install it every summer and remove it every winter (you'd be drywalling all year!), you will have to make a decision one way or the other. Since St. Louis typically spends much more time in the summer extremes than in the winter, it is worth considering an interior vapor barrier in many frame wall applications.

Water condenses on the warm side of a barrier. There is some concern in regards to installing a vapor barrier with a masonry veneer wall This concern is related to the situation where, on a sunny day, water that is trapped in the brick may have a tendency to migrate both into the wall system. When this water vapor contacts the vapor barrier (and the house is air conditioned - cool), it has a tendency to condense and potentially collect in the wall system.

The point of having a "housewrap" like Tyvek outside the sheathing and the vapor barrier inside the insulation is that in the event of condensation or infiltration of vapor into the wall system, the housewrap does allow moisture to escape. Housewrap will also reduce the infiltration of water and water vapor into the wall system from the outside, minimizing the potential for water to condense at the vapor barrier.

If you are concerned about moisture condensing in a frame wall with a masonry veneer, there are three potential solutions.

• Don't install the vapor barrier. This will allow moisture to continue to migrate through the wall system without getting "trapped" at any one point. Downside is that this solution is not as thermally efficient.
• Increase the air space at the brick veneer. Additional air space (say 2" instead of 1") will allow for more air flow up and out of the wall cavity, lessening the potential for water vapor to be stuck in the system. I believe current masonry standards call for an increased air space anyways. Downside, if you are building a new foundation, it may have to be increased for the additional wall width.
• Expanded foam insulation. The closed cell expanded foam insulations are also vapor barriers. Water will not migrate into or through the insulation, so you don't have to worry about condensation at the interior wall surface. Downside is that it may be expensive.