I assume that was a exterior wall in the image and if so what type of cladding was present. You did as required by checking with a moisture meter. If you are like most IR trained that are serious about this business when you see a thermal pattern that is not self explainatory you dig until it is.
BTW when posting your images run them thru your software and remove all the un-necessary salad dressing makes you appear more professional:D
A way to check farther into the substrate or in this case, up through the 5/8" or 3/4" subfloor is to drive a couple of shiny new 2" or longer finishing nails into or through the suspected wet/damp subfloor (at the spacing of the meter pins) where they can do no damage to the shower pan (for example: in the area of the raised shower pan perimeter). If the upper surface of the subfloor is wet or damp from leakage/etc, the meter will indicate the moisture.
The visible studs are the studs that frame the upper front wall of the enclosure over the fireplace. This wall does not need insulation unless for some reason, this is not a standard installation. The fact that the drywall is showing up cool is most likely cold air infiltration into the open hollow space above the unit.
I have been called in many times to comment on this situation. In one case, a house supposedly built to “R2000 standards” by an untrained, non-“R2000-certified” contractor had a wood burning “zero clearance” fireplace installed on a north exterior wall with the unit enclosure extending outside the plane of the north wall. On a cold day with north wind, on the floor about 4 feet in the family room from the unit, the temp was only 13 deg C (55 deg F).
Marcel that is typically what I see here in the north country of Okla:D metal flue on the insert and no insulation within the wall cavities surrounding the chase or it has fallen out. The flue piping they use here has a MFG label stating 2 inches to combustibles including building insulation. If it is zero clearance flue the cavity should still be insulated or enclosed at the top of the chase at the top plate level
Marcel I think I understand your image of the shower stall correct me if I am wrong but with the shower curtain in place were you shooting the inside of the stall and if so it appears what you see is where the cold water was hitting the inside wall and running down. I never shoot the inside and I make darn sure not to let any water get past the door while running water into the stall I have even slapped Realtors hands for opening the shower door while I was testing because the door will drip water onto the outer floor. You always want to shoot the outside walls of the stall as many as are available I have in some cases had access to all four wall of a shower stall not often but ocassionall depending on the type of construction. Remember I am the shower pan terrorist and have developed my Companies SOP for testing shower pans and have been very successful at doing so
With the exterior enclosure and chimney chase, the easiest way to complete the top thermal boundary of the enclosure/chase would be to extend the plane of the interior cathedral ceiling (a thermal boundary) out through the chase to its outer wall. This keeps it in a simple straight line plane.
The 3 outer walls of the chase down to and including the floor supporting the unit are then insulated and airsealed to complete an efficient thermal enclosure of the fireplace unit. In this scenario, the inner house wall above the unit would not have to be insulated (redundant). If this protruding chase/enclosure was not airsealed/insulated properly, the inner uninsulated wall would be cooled.
Although Canadian WETT courses teach principals of airsealing and building shell efficiency, the certified WETT technicians are not firstly insulation and airsealing professionals so in many cases that aspect of their work lacks.
A quick peak into the enclosure from the exterior with a borescope would give a better idea of problems with fallen or missing insulation.
Here is what I typically see first pic is from in the attic looking up the chase at the roof penatration and the second pic is looking down at the back side of the fireplace in the living room having significant temp intrusion