Last time the Mystery Inspector slapped a Red Flag on the framing contractor for adding too much wood to this wall. Today, we'll take a closer look at the electrical boxes and award an 'Atta boy’ to the electrician: Electrical boxes are air sealed with canned foam

Upon closer inspection of the wall, we see that the electrician has taken some excellent steps towards energy efficiency for this home: the electrical box is bedded in canned foam and backed with a small piece of fiberglass insulation. (We suspect that the electrician did this because the gas pipe is not sealed).

Why does this matter? 
Electrical boxes like these are usually 3 in. x 5 in. holes in the wall. Within this particular stud cavity, there is an outside hole (gas pipe), an inside hole (switch box), and sideways holes connecting to other stud cavities (holes bored for sheathed cable). In other words, if the outside hole is not sealed, then outdoor air can circulate through the entire exterior wall assembly. If there are more holes in the top and bottom plates, then air will circulate freely through the roof, walls, and floors. A three dimesnsional airflow network as the Mystery Inspector's friend, Joe Lstiburek, calles it.

This detail stops air and eliminates cold spots.
The fiberglass backing will help to keep the insulation layer continuous behind the boxes when batts are installed, though it will be extremely difficult or impossible to insulate this cavity well with batts – blown insulation or spray foam would be a superior choice.  Even id spray foam is used, the fiberglass batt pieces will help eliminate cold spots.

As long as we are on the topic of batt insulation, tomorrow we'll look at how the insulation contractor did on the preliminary round in the garage…

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