Underfloor Thermal Insulation; Why? Part II

In the first part of this blog I described the make up of my house extension floor ‘stack-up’, including a 100mm thermal insulation layer. Apart from increasing the cost and construction complexity of the build, what thermal advantage does this bring? To determine that a common methodology is employed, simulate and compare the thermal behaviour without insulation vs. with insulation and quantify the difference.

First thing to note, if all the heat dissipated by the radiator in the room went down through the floor there would be linear relationship between the total thermal resistance of the floor vs. the resulting room temperature. In reality things are a little more complex than that. There are a number of different heat loss paths that should be recognised before appreciating the effect of the under floor thermal insulation.  As shown in the animation, heat rises from the radiator due to buoyancy, hits the ceiling then, as it cools as heat is lost to the ceiling and walls, the air cools and drops (as it is relatively cold) down to floor level where it is then picked up by the radiator up-draft and recirculates. I had initially expected to conclude this blog by showing that the resulting air temperatures where considerably warmer with underfloor thermal insulation compared to without. The thermal conductivity of the insulation layer is about 1/10th that of the concrete (and proportionally much more expensive).

I was somewhat disappointed (initially) to find out that my intuition was lacking correctness. I then took solace in the fact that the subsequent journey of discovery to analyse exactly -why- would be educational (the only way to learn is through mistakes, something Victorian engineers understood, something that has been lost in todays society). The air temperature within the room with (left) and without (right) floor insulation was:

The expected thermal stratification is seen in both cases, this is expected, hot air rises. The average temperature in the volume of the room that would be occupied though is about the same in both cases. Before I got on the phone to the government building regulation agency however I thought I’d better investigate this a bit more. Plotting the surface temperature of the floor did highlight a difference:

Further investigation of the heat loss split, something that is readily available from a CFD based simulation in FloVENT, indicated that out of all the heat being dissipated by the radiator, only about 17% was lost through the floor without insulation, ~10% with. With such a relatively small proportion it’s not that surprising that it has little ‘backing-up’ effect on the room air temperature. What it does do is to keep the cold from the bottom of the floor stack-up at bay, allowing the air to do a better job at heating the floor, if not heating itself up any further.

A lot of the heat was conducted out through the window and out through the cold side walls. As my extension takes shape maybe I’ll come back to this and simulate with and without thermal insulation in the wall cavity. Might also see if window triple glazing is worth it compared to double glazing.

I’m sure there will come a day where such simulation becomes an integral part of EVERY built environment design, right down to where best to place radiators and air conditioning supply terminals.  We waste too much energy heating (or cooling) the outside air through inefficient housing design and resulting heat (or cold) loss to the outside ambient. Simulation software will play it’s part in rectifying that in our green and verdant future.

29th September, Hampton Court.