Every proper radiant floor, ceiling or wall panel heating system starts with a computer-generated heat load analysis performed by an experienced designer on dedicated software. The heat load will dictated the size of the heat source, e.g. boiler, the PEX tubing size and length, the design water temperature, i.e. the average water temperature needed to satisfy the heat load (coldest few days of the year) and wether or not one needs extruded aluminum emission plates.

In the kitchen remodels we do here in Minneapolis, we go around behind professional and novice alike aswering complaints of cold kitchens when an old radiator was taken out for more cabinets and a "radiant floor" installed below the floor without good effect. Often the addition of aluminum plates will satisfy the load, but just as often some for of supplemental heat must be specified as the kitchen floor/radiator has shrunk with the additional cabinets and the floor surface is simply too small. Thus some math is a must.

The only thing worse than using an ineffective "short-cut" like stapling up raw PEX below the floor and hoping for the best (extra insulation notwithstanding) is to invest in aluminum emission plates and having a cold room anyway. Why guess? If this is DIY radiant floor, it is misnamed as it is should be; DIY shoot yourself in the foot.

GET A HEAT LOAD PEOPLE...before you waste money on PEX, bubble foil, bigger pumps, bigger boilers, more insulation, new thermostats, bigger PEX, etc. etc. etc.

Regarding the IR readings: I find people think the lazer point is the part which is measuring the surface. This could be the measurement problem. The IR probe doesn't have such pinpoint accuracy. the lazer is there to get an idea or amuse your cat.

Laser/no laser, cat/no-cat the emissivity of different materials are different, and will give different numbers for different materials at EXACTLY the same temperature.

Using hockey tape as a highly emissive not-very insulating material is one way to give different objects or surfaces the same emissivity to be able to determine the temperature differences more precisely. Putting the hockey tape on both the cat and the roof may be a more useful way to amuse the cat, when determining the delta-T between a cat and a hot tin roof.

I would check the insulation at the edge of the rim joist wall.  It it very (very, very) important (imporant, important) to have it sealed with foam (foam, foam)!!!

Foaming a rim joist is certainly ideal but not the only way to insulate the rim joist. More important is the quality of air seal between the radiant tube and the floor below, more especially if the adjacent space is unconditioned i.e. cold.