Before we had our high tech tools, how did we know if there was insulation in the walls? What were the methods?
How funny that I can use a camera but than can't figure out how was it done in the past without drilling holes on the wall and than you had to patch it? That doesn't seem like a cost effective way to bid a job. Or is it another rule of thumb? Like In So. Cal houses built before the 70's we can assume that there is no wall insulation. I am sure on the East Coast that insulation is a MUST.
Well I just simply pop the cover off a few light switches & electrical outlets & shined a light in there. If I didn't see anything I would take a bent coat hanger & use it to fish around in there some.
Generally if we are remodeling a house / gutting a room to the studs I will add in new insulation anyways due to new electrical, plumbing, etc... & the old insulation is probably shot. The only surprise I had was the 1879 remodel we did where it was fully packed with a wool type insulation that stayed in place & was still in great shape.
Common practice in the Minneapolis area for the past 20 years has been to drill a 2” hole in a closet on an outside wall and snap a plastic plug into the hole after verifying the insulation level. This would be done for any home built before about 1970. It is possible for a wall to look insulated using the IR and have only 1.5 inches of insulation in the wall. In our climate it is cost effective to dense pack cellulose into a wall that has an existing 1.5” insulation batt. Sometimes on homes build before 1920, the walls will have no insulation but have back-plaster. Back-plaster is an additional layer of plaster that was installed in the center of the wall cavity, making it more much expensive to insulate. The IR is a great tool but it has limitations.
In reply to probing around ext. wall outlets, the problem is that batt insulation is usually installed poorly. By only probing in these locations you run the risk of determining that no existing insul. is in the wall because it was cut out too big around the box. The other error is compacted insul. around the box because it was not cut at all.
I would agree that the best practice is to drill inspection holes in the closet of an exterior wall and plug them. Of course this is dependent on H/O approval.