Room temperature is somewhere around 20 degrees give or take so the heat that your block walls absorb will carry on to the outside where its colder
Yes, your wall will gradually heat up to the point where it reaches the same temp as the room, a certain amount of that thermal energy will be lost through the insulation, but you can easily calculate that, so I'm sure you know this is a small percentage. When the heating turns off, and the room starts to cool, the thermal energy stored in the wall will slowly release back into the room, much lower thermal resistance there compared to outside. Yes, you will continue to have some escape through the insulation to the exterior so long as the outside temp is lower than that of the wall, but this will be very low.
With internal insulation, given the lower thermal resistance (unless you're prepared to lose more than 4 inches off every wall), you will have a significantly higher rate of heat loss through the insulation to the outside, where it is lost, with negligible recovery once the heating shuts off.
Good point on the mould , But providing you have a double leaf exterior wall
Ie. Block on edge -cavity- skin of brick or similar build up with timber frame ect...
You would have nothing to worry about if your room is properly vented .
And by properly vented you mean a big hole in the wall to let all the heat out, right? And even then having to regularly open windows to reduce humidity. Most of Europe stopped using the internally insulated cavity a long time ago due to mould problems.
All this air tightness stuff that has been introduced to construction in the last few years is going to cause a huge amount of mould problems in the coming years .
Last few years! It's not even the last few decades, even the Passivhaus-Institut was formed 20 years ago. But that's just how far behind Irish construction methods are.
Even at 25mm ,is alot better than 200mm or 500mm externally insulated .
Forget 200 or 500, what U value are you achieving there versus 100mm external insulation? Kingspan's technical guidance docs states you can only achieve 0.79 W/m2K for their 25mm Kooltherm K5 product. They give 0.5 W/m2K for 100mm external.
Im basing my opinion on 22 years of construction.
it is only my opinion from how these various products and techniques FELT after the install and im sure there are architects, designers and insulation sales people with letters after their names that can back up there arguement with numbers and figures to debunk what ive just said.
The the OP, if any of the builders you approach can only recommend a solution based on 'feel', and can't back it up with real calculations, run away!!