The more thermal insulation, the slower the heat flow.
In simplified equation form,
F = k(T_high - T_low) ,where k is thermal conductivity, T_high is the temperature of the system with the higher temperature, and T_low is the temperature of the system with the lower temperature (see Wikipedia: Thermal Conductivity: Equations).
Note that as thermal insulation ↑, k ↓, and thus F ↓.
Balanced means F_in = F_out.
The house in the image illustrates the system, where the furnace supplies F_in (which is fixed) and the outflow is to the outside.
Inside the system the temperature is T_in and outside the temperature T_out (which is fixed).
Since F_in = F_out, the system is in steady state and at a constant equilibrium temperature T_in.
This causes k ↓ and F_out = k(T_in - T_out) ↓.
Now F_in > F_out and T_in ↑.
But that makes F_out ↑.
Eventually, F_in = F_out again and balanced is restored---but at a higher T_in than before.
In this case, F_in fixes the temperature T uniquely as determined by the Stefan-Boltzmann law for blackbody radiation:
F_in = F_out = σ*T**4 ,where we have Stefan-Boltzmann constant σ = 5.670367(13)*10**(-8) W/*m**2*K**4).
Now if you add thermal insulation for the outflow (but NOT the inflow), you can increase T: i.e., k ↓ T ↑.
So temperature can be made as high as you like. If k=0, the temperature would increase forever.
A hypothetical blackbody radiator gives a lower limit on the temperature of the emitting system in this case.
The situation we are considering is when the planetary atmosphere is relatively transparent to starlight going to the planet surface and relatively opaque to thermal radiation (which is probably rather close to being blackbody radiation) emitted by the surface. The difference transparency is due to the difference of the wavelength of the electromagnetic radiation (EMR).
For example, for the Earth, the inflowing EMR is largely visible light (fiducial range 0.4--0.7 μm) (to which the Earth's atmosphere is rather transparent) and the outflowing EMR is largely infrared (to which the Earth's atmosphere is rather opaque).
The relative opaqueness of the thermal radiation from the planet surface is the thermal insulation.
It causes the surface temperature to rise in order to increase the emission rate of thermal radiation and establish a balance between inflow and outflow of EMR.
This increased temperature due planetary atmosphere is the greenhouse effect.
It keeps the average surface temperature of the Earth well above the chilly surface temperature that a blackbody-radiator Earth would have.
But we want the right amount of greenhouse effect which is the amount the biosphere is used to having since ∼ 9000 BCE (i.e., since almost the beginning of Neolithic and Holocene) (see Wikipedia: Keeling curve: Mauna Loa measurements).
At present, humankind is increasing the atmospheric carbon dioxide (CO_2) (which is a high opacity molecule), and so increasing the greenhouse effect and the average surface temperature.
The increasing average surface temperature could have disastrous effects for the biosphere and humankind.