Insulating Old House Walls

Now that I own a 90 year old house and the electric bill that comes with it, I’m working on ways to make it more energy efficient. Step #1 is to weather-strip my windows and doors and keep the house painted/caulked well. Step #2 will be to insulate the attic and box sills.

Initially, I wanted to insulate the walls. To do this, I would have to breach the plaster walls to blow in insulation, because I have stucco exterior walls and plaster interior walls. After some research, I learned this is a VERY BAD thing. My plan now is to insulate the attic. Here are some “before” pictures. I will post “after” pictures in a year or two, whenever the project is complete.

Pictures of the attic’s existing insulation:

At http://bobyapp.com/blog/2009/06/myths-about-insulating-old-house-walls Bob Yapp wrote:

If your goal is to continue loving your old house, make it energy efficient while keeping your costs down, then you absolutely don’t want to blow insulation into the sidewalls.

One of the top reasons for exterior paint failure, termites and structural damage to old houses is loose cellulose or fiberglass insulation blown into the sidewalls. “Hey, wait a minute Bob, if we can’t insulate the sidewalls, how can we afford to heat our old house?” That’s a valid question but you need to think of air movement in your house as if the house were a chimney. Heat loss primarily happens in an upward movement. So, I want you to insulate your attic space to an R-38 with eave ventilation. You should also friction fit craft-faced (paper faced) fiberglass batting- insulation or foam board into the box sills in your basement (the area where the beams or floor joists rest on top of the foundation). The craft face acts as a vapor barrier and should face the inside.

Most building codes today require that when a new house or addition is built in a northern climate, it must have a vapor barrier. When a new house is going up, they frame the sidewalls and install exterior sheathing. The next step is to go inside and install fiberglass, batting insulation between the 2″ x 4″ or 6″ studs. Before the drywall can be installed over this wall, 4 mil thick plastic sheeting must be laid over the insulation on the entire wall. That plastic sheeting acts as the vapor barrier.

We create warm moist air in our homes by cooking, taking showers, having plants, breathing etc. That warm, moist vapor is attracted to the exterior walls. This vapor enters the wall through hairline wall cracks, outlets, switches and window trim. In new construction, the plastic vapor barrier under the drywall stops the wet air from getting to the insulation and condensating.

In old houses with plaster walls, there is no vapor barrier under the plaster so the wet air hits the insulation and condensates. This wets down the blown-in insulation making it a wet mass at the bottom of the wall cavity creating an inviting place for termites and dry rot. Then the moisture enters the exterior sheathing and wood siding causing permanent exterior paint failure. Since the homeowner, for some “unexplained” reason, can’t keep paint on the house anymore, they call the vinyl siding salesman. This makes the problem even worse as you now have backer board (insulation board) and vinyl siding which in combination creates a vapor barrier on the outside of the wall that stops the free exchange of air, trapping more moisture.

If your house is drafty then tighten it up. Weather-strip your windows and doors, keep the house painted/caulked well, insulate the attic and box sills. This will stop the air infiltration, make you more comfortable and really save money on utilities.