Jean-Paul,
I have sent you a PM.
As has been stated, a professional thermal audit of a building usually requires 160 x 120 pixels or more for efficiency. Though, as I have detailed in my message to you, people need to also consider the field of view that a camera is providing Vs the number of pixels, as a wide FOV can be detrimental to detail. Lower resolution thermal cameras address this with narrower field of view lenses. With a lower FOV the same detail as a higher resolution camera may be achieved, but at the cost of the 'broader view' In some cases however this is of no great importance as the camera is 'scanned' over the building. The Lepton equipped F1G2 lens is sadly quite a wide FOV at over 40 degrees and the latest F1G3 is 50 Degrees horizontal. Resolution on its own can be misleading. The thermal scene detail capture is a combination of microbolometer resolution, lens quality and lens FOV. Sadly the silicon diffraction lens in the Lepton is nothing to get excited about, which does not help matters.
As has been stated, for professional building thermography a decent 320 x 240 pixel thermal camera is recommended. It is really down to what you truly need for the tasks you have in mind. When it comes to commercial thermography the budget is sadly set by the task requirements and not just what can be afforded. You, however, may not have such a demanding requirement. It is easy to spend a lot of money on a thermal imaging camera, but sadly it is hard to spend just a little ! There is limited choice at the lower price end of the market as the parts within thermal imaging cameras can still be expensive to the OEM. This is why the Lepton is somewhat revolutionary..... It is a cost reduced thermal imaging core that uses a lens that would normally be considered useless for Longwave imaging as it is Silicon. Yet FLIR produced a usable, if not wonderful, silicon lens that massively reduced the cores production cost. The much smaller microbolometer die that uses 12um pixels also helped reduce cost, but at a price. That price was image noise. FLIR use their knowledge of thermal image processing to reduce the noise to acceptable levels.
I do not think FLIR designed the Lepton to be a core used in professional thermography though ! It can certainly be pressed into service as a reasonable thermal imaging core, but just do not expect too much from it.
Hope this helps
Fraser