The House Detective: What is best way to insulate an old house?

by Barry Stone, Certified Building Inspector Published: June 7, 2014
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DEAR BARRY: We have an 80-year-old home with lathe and plaster on the inside walls. The house is hard to heat in the winter and becomes very hot in the summer, so we want to add insulation. One contractor recommended injecting insulation into all of the outside walls, but another contractor said this is not as good as removing the plaster and installing fiberglass and drywall. The second method is much more expensive, so we’re not sure what to do. What do you recommend?

— Lewis

DEAR LEWIS: Injected insulation is a popular method for upgrading the energy efficiency of older homes. This type of insulation comes in two forms: expanding plastic foam or particles of cellulose, usually consisting of pulverized newspaper. Injection is definitely the less costly method, but there is one drawback. The wall cavities are not always completely filled when insulation is injected.

Spaces between the studs can be missed because of wood blocks, bracing, and electrical wiring. Some insulation companies try to overcome this problem by injecting insulation through holes along the top and bottom of each wall, but even this approach does not guarantee that all of the wall spaces will be totally filled. However, some contractors follow up with infrared cameras to detect places that were missed when the insulation was installed.

The alternative method is much more costly but leaves no doubt that the walls have been thoroughly insulated. By removing the interior lathe and plaster, all wall spaces are fully exposed. This enables total insulation of the wall cavities and also provides an opportunity to inspect for defects inside the old exterior walls. Wiring can be upgraded, additional outlets can be installed, and evidence of termite damage and dryrot can be addressed. Once the work is completed, the old walls are refinished with new drywall.