By Paul Bianchina
Most people are familiar with the sight of wall insulation in a framed wall - blankets of fluffy fiberglass insulation tucked neatly between the wall studs to fill up the cavities before the drywall is applied. What becomes a little trickier is when you're faced with insulating a wall that's solid concrete, concrete block, stone or any other solid surface that lacks all those convenient insulation cavities.
The solution is a common building technique called furring - the installation of wood strips on regular centers that create both the insulation cavities and a solid attachment surface for the later application of drywall or other finished wall surfaces. Furring is a pretty straightforward procedure, but you need the right tools and the right materials to get it done correctly.
If you have a wall of solid concrete or concrete block that is relatively smooth, plumb and solid, the furring installation is greatly simplified since you can install the furring directly against the wall.
First, you'll need to determine how deep a cavity you want to create. For the fiberglass wall insulation you see in most new construction, 2 x 6 nominal lumber is used to frame the exterior walls, creating a 5-½ inch deep cavity that will accept an R-21 fiberglass batt. For furring, however, using lumber that large creates some difficulties in attaching the wood to the wall, and also eats up valuable floor space into the basement, shop or other room that you're insulating.
For solid walls, a more common solution is to install 2 x 2 lumber - which is 1 ½ inch thick - then insulate the cavities with rigid 1 ½ inch thick foam insulation. Better grades of rigid foam have an R-value of more than 7 per inch, so you can achieve about an R-11 within that 1 ½ inch cavity. For higher R-values, you can use 2 x 3 lumber, or start with a base of 2 x 2 framing attached directly to the wall, then add additional wood framing over the 2 x 2's to deepen the cavity.
First, the walls need to be clean and dry. If you have moisture problems, mildew, seepage, or other problem conditions, fix them first - do not just cover them over.
The furring begins with a strip installed horizontally along the top and bottom of the wall. Since you are in direct contact with concrete or concrete block, you'll need to use pressure-treated lumber. If you can't find pressure-treated 2 x 2 or 2 x 3, you'll need to rip them out of 2 x 4 or 2 x 6 lumber respectively. You can also use standard non-treated lumber by installing a strip of 15# felt - tarpaper - as a moisture barrier between the wood and the concrete.
The most straightforward approach to attaching the furring is to use a powder-activated nail gun. Similar to an actual gun, this tool uses a 22- or 25-caliber shell to literally shoot a hardened steel nail through the wood and into the concrete. Nail guns can be rented from most rental yards, and they will give you a complete orientation on the proper use and safety procedures. Follow the safety instructions carefully - these guns are easy to use, but they can be dangerous if operated improperly.
Other installation options include gluing the furring to the wall with construction adhesive that is rated for pressure-treated lumber. Do not use this method if you're using standard lumber and felt strips, since you'll only end up gluing the felt to the wall. You can also drill anchors into the concrete and then attach the wood to the anchors using screws.
After the top and bottom furring strips have been installed, start at one corner of the wall and mark the face of both strips on 24-inch centers. Cut additional furring strips to length so that they fit vertically between the upper and lower strips, center each one on your marks, and attach them to the wall.
You have now created a structure that looks similar to a standard framed wall, with wood furring on regular centers for your drywall and cavities in between. The final step is to install the foam insulation, which comes in 4 x 8 foot sheets. First, measure the inside distance between the vertical furring strips, then rip the sheets to size. You can use a utility knife and a straightedge, or simply rip them on a table saw. The strips of insulation are then simply friction-fit between the furring strips - no further attachment is necessary if the wall will be covered with drywall or paneling.
For irregular walls such as stone, the procedure is the same as for the smooth walls, with the exception that you have to shim behind the furring strips to even things out. Using strings or a straightedge, start by finding the point on the wall that protrudes out the farthest. This is your control point, the point that all of the strips will need to be furred out to meet. Then install the strips on regular centers as described above, but install wood shims between the back of the strips and the low points of the wall so that the face of the furring ends up plumb and straight.
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