Insulation for Old Walls
Tom Silva has some ideas on how to retrofit insulation in a late 19th Century house
What is the best way to insulate the walls of a balloon-framed 1880s house? Do I have to remove all the knob-and-tube electrical wiring first?
—Pat Hoover, Everett, MA
Let me answer your second question first: Any knob-and-tube wiring in the walls has to be taken out or permanently disconnected before the insulation goes in. Those old wires are bare copper—there’s no protective jacket like modern wires have; they simply use the air around them to dissipate heat and prevent arcing. Packing insulation around bare wires prevents them from shedding heat and increases the risk of a fire.
Once the old wiring is gone, the insulation can go in. For your house, I recommend dense-packed cellulose, which is made from ground-up newspapers treated with borates to make it fire-, mold-, and insect-proof. It’s not expensive, and you can shoot it into the walls from the outside without damaging interior surfaces. This is a job you do with a helper, using a rented blower from a home center, but if your house is big, hiring an insulation contractor may be a better option.
Either way, attention has to be paid to the joist bays. Unlike in the modern, platform-framed houses that became standard after World War II, the joist bays in balloon-framed houses are open where they connect with the exterior studs. Therefore, before the blowing can begin in your house, blocking has to be nailed across the bays at the foundation and in the attic to contain the insulation within the walls, where it belongs. When that’s done, here’s how to go about blowing in the cellulose.
Remove only enough of one course of siding to expose the sheathing about 3 to 4 feet above the foundation. Locate the center of each stud bay and drill a hole through the sheathing that’s 1⁄8 inch wider than the blower nozzle. Do the same directly above each hole and 18 to 24 inches below the lower edge of the joist bay. With the blower running, stick the hose nozzle in the bottom hole, and shoot insulation into it until the blower motor starts to whine, a signal that that portion of the bay can’t hold any more cellulose. When it’s packed that densely, cellulose won’t settle and it will stop heat-robbing air movement inside the wall. You may need to plug the upper hole with a rag for this step.
Now move the nozzle to the upper hole and fill the stud bay until the motor whines. Some cellulose will be blown into the second-floor joist bay. Repeat this process for the remaining stud bays on the first floor. Then run a bead of caulk around each hole in the sheathing and replace the siding.
For the second floor and above, remove some of the siding in a course 18 to 24 inches above the top of the joist bay, and drill a hole into the sheathing at each stud bay. As before, drill a second hole directly above the first, 18 to 24 inches below the next joist bay. Now when you inject insulation into those holes, insulation will pour into each joist bay, filling it loosely over a horizontal distance of about 2 to 3 feet from the exterior wall. The whining motor will let you know when it’s time to move to the next hole. When you’re done, cover the exposed sheathing with siding, as before.
With all those freshly insulated walls, you’ll certainly be more comfortable in both winter and summer, and your energy bills should be lower too. But don’t skip this last important step: Have the outside of your house scanned with a thermal-imaging camera to see if there are any voids that are missing insulation. An insulation contractor should provide this service as part of the job. Or you can do it yourself with a rented thermal-imaging camera. On a cold night when the heat is on, scan the walls for heat leaks. They’ll show up as yellow or orange splotches on the thermal camera’s screen. Mark the siding at those spots and fill them at the next opportunity.
Shown: When blowing cellulose into a wall, be sure to wear safety glasses and an N95 respirator mask to protect your eyes and lungs from dust.