Small temperature differences between rooms may be an OK micro-optimization, but keeping some rooms cold requires adding thermal insulation between the rooms to actually save anything.
Instead, use the same money and effort to put that insulation on the outer envelope of the house -> now you can just keep all the rooms warm with basically the same cost and stop thinking about it.
Really, insulating pays off pretty quickly. You may want to get a thermal camera to look at the discontinuities, to find where the problem is. Most insulation materials also require measures to stop air flowing through them so basically you need to make those vapor proof plastic film / paper / Housewrap / whatever seams completely tight, even small gaps let the wind in your insulation compromising its thermal conductivity value. The material stopping the air flow should be water vapor proof on the inside (typically plastic), but even more importantly, the outside material must let the water vapor through.
OK in theory. The downside of such measures is that you'll also need a ventilation system otherwise you get a large amount of moisture inside the house. This can get really expensive quickly because of the remodelling needed to install the pipes. In the end you can only add a limited amount of extra insulation to an old house (from a financial perspective).
No, you misunderstood what I was saying, or I miscommunicated.
The key is understanding the separation of ventilation and insulation. Both have a reason, neither can replace the other.
Ventilating through insulation materials is never a good idea. If it is happening, it needs to be fixed for both air quality and energy consumption reasons.
Indoor air going out, and outdoor air getting in obviously results in loss of energy. But because we need to vent out moist, high CO2, dirty air and get fresh air to breathe anyway, this is a loss that has a benefit: breathable air and smaller risk of condensation. So we accept the lost energy and vent.
On the other hand, if outdoor air is blown into the insulation materials such as mineral wool through any holes, this air necessarily doesn't even get inside the rooms, or if it does, it carries fiber dust or possible microbiological contamination inside, creating an air quality problem; at the same time, it prevents the insulation material from working, causing excessive energy consumption.
So both from the perspective of theory and practice; also from the perspective of building code, the key is to stop air from moving inside doors, ceilings, floors, insulation materia, making them
airtight (
not vapor tight). At the same time, ventilation must be allowed to happen, in a way or another. Ventilation goes through direct holes, or clean ducts, possibly through HVAC technology designed for it, but definitely not through building materials like gaps in the walls, floors, ceilings, through wool...
Adding ventilation systems containing fans and ducts is completely separate issue, and not necessarily needed. Old houses such as one I have work with
passive ventilation. It's not perfect but the concept is that air is moved in from windows that can be opened, or basically simple ventilation holes made in the walls. Exhaust is in the firewall in this case. Works well when there is temperature difference, i.e. in winter. Doesn't work that well during summer, but you can still open the windows to do manual ventilation. It's OK.
But preventing
heat flow through the envelope pays off
even if you ventilate. Only after your insulation is in a good order, only then the ventilation part starts to be a significant part of energy loss in which case you can open the can of HVAC heat recovery worms, if you want to optimize even further.
What comes to moisture, poor and uneven insulation adds a problem that condensation may occur somewhere in the inner envelope. In any case, the inner envelope should be air tight so that moist indoor air can't get into the insulation wools where it might condensate when it hits colder temperature. Building codes, at least here, nowadays require completely vapor tight (i.e. plastic) material inside of the envelope, but traditionally stopping the air with paper works good enough and has some advantages (depending on who you ask); most important is to prevent large amounts of indoor air getting into structures.
Outdoor air flowing into structures just kills the efficiency of the insulation and must be stopped for this reason. It has no upsides. Similarly, having seriously thin layer (or complete lack) of insulation somewhere has no upsides.