Regardless of what type of weatherization or home energy conservation improvements you do, filling cavities with some type of insulation will typically be a part of any treatment plan for older homes built before 1970 — the same cavities used by existing and, especially, new wiring for electrical receptacles and lights. So, it is important to take care of electric BEFORE you insulate with cellular foam…
Thermal and electrical insulation can be at odds with each other
All building envelope insulation can affect existing electric by doing what it’s supposed to do: reduce the movement of heat energy. Wires and wiring methods are designed to move electrons (carry current) without ‘friction’ of electron movement generating more heat than their insulation and terminals can safely withstand. Insulating around some wires or some connection types can take away the original design’s ability to dissipate heat, and cause the wire insulation to heat up and melt, become brittle, or otherwise fail.
To reduce the risk of wire electrical insulators failing due to heat being trapped by building envelope thermal insulation, have an electrician conduct voltage drop tests throughout the home, before applying new thermal insulation to the mix. Voltage drop under designed load (current, typically 15A or 20A) indicates the resistance in the circuit, and indicates the amount of heat that could be generated in the wiring and terminals when heavily loaded, as by a vacuum cleaner, space heater, hair dryer, or window-mount air conditioner. Low voltage drop means there shouldn’t be a problem with wires heating up hotter than the rating of their electrical insulators.
Some types of thermal insulation can prevent new electrical work, or make it much more costly
Some insulation, such as blow-in cellulose, will have little effect on the installation of new wiring — other than to make a bit of a mess as it drains out of openings made during an electrical installation. Rigid insulation types, such as rigid foam board or pressurized expanding-foam insulation (aka celular foam insulation) will encase existing wires, dramatically reducing their ability to dissipate heat (see above), and fill every nook and cranny in a wall with an impenetrable barrier, preventing any new electrical wire or cable from being fished through walls or ceilings treated with this thermal insulation method. To pass a new cable through or into the affected cavity, the finish surface will then have to be cut open, foam insulation carved out of the way, and then the wall openings repaired after cables have been installed. Probably the only great time to do any type of rigid foam insulation is a) on the underside of a roof deck you know to be in pristine condition and beneath a new roof expected to be leak-free for 20 years or more, or, b) when you are completely re-wiring your home (or building it new) or know that it already has all the electric (or telephone, or internet, or home theater…) you could ever want, and that there are no maintenance issues with the electrical system.
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