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Underfloor Thermal Insulation; Why? Part III

Local air temperature plays an important part in the resulting thermal comfort as would be experienced by someone occupying that space; however it’s not the only contributing factor. “Comfort Temperature” is a measure that contains contributions from the local air temperature, the local air speed and also something called the mean radiant temperature. When comparing the effects of including, or omitting, a thermal insulation layer in the floor stack-up, one should look beyond the resulting room air temperatures as a metric and start to examine the comfort temperature distribution.

FloVENT has the ability to predict mean radiant temperature and resulting comfort temperature at all points in the 3D simulation model. The mean radiant temperature is akin to the temperature an object at that point would attain due to radiative heat transfer, divorced from the local air temperature, and can also include the effect of being warmed by (short wave) solar intensity.  Ever wondered why on a sunny day you feel disproportionately cold when the sun goes behind the clouds? This is due to you being warmed by the sun more than by the local air temperature and/or spending summer in England.

Comfort temperature (as defined in FloVENT) is a combination of local air speed , local air temperature and the mean radiant temperature, a combination that is intended to give a better overall indication of… you guessed it… comfort!

Let’s revisit the simulation of my extension but compare air, mean radiant and comfort temperatures together to see if the effects of the use of underfloor insulation has any real benefit apart from keeping the floor warm. The following image paints the air inside the room with the temperatures; walls, floors and ceilings are just shown solid:


The first two images reiterate the previous conclusion that the air temperatures themselves are little affected by the use of underfloor insulation.

The next two images show very clearly the effect the radiator has on the mean radiant temperature in the room. The closer you get to the radiator the warmer you feel. However the radiator isn’t the only thing that’s radiating. All solid surfaces radiate heat. If a surface is colder than other surfaces it can ‘see’ then it will gain heat from those hotter surfaces. Vice versa as well. So as you move closer to the relatively cold floor the mean radiant temperature decreases, especially when you move away from the radiative heating area close to the radiator. When using thermal insulation under the floor, the fact that the floor surface is warmer leads to an increase in the mean radiant temperature close to it -> less blue area near the floor when insulation is used.

Comfort temperature is effectively the combination of the other two images. The biggest differences being near the floor. Without insulation my feet and ankles would be much more uncomfortable. Socks and slippers are the answer but the dog has a nasty habit of eating the latter, maybe I can offset the cost of the insulation with a reduction in required slippers? (Yeh I know, a glittering career in finance does not await me).

Thanks to Cathy Biber for pointing me in the thermal comfort direction in this blog series.

6th October 2009, Hampton Court


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Robin BornoffRobin Bornoff achieved a Mechanical Engineering Degree from Brunel University in 1992 followed by a PhD in 1995 for CFD research. He then joined Mentor Graphics Corporation, Mechanical Analysis Division (formerly Flomerics Ltd) as an application and support engineer, specializing in the application of CFD to electronics cooling and the design of the built environment. Having been the Product Marketing Manager responsible for the FloTHERM and FloVENT softwares he is now Market Development Manager for the Physical Design of Electronics in the Mechanical Analysis Division. Visit Robin Bornoff's blog

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Mr. Bornoff, did you try running the simulation with double the thickness of underfloor insulation? Very efficient buildings use a surprising amount of insulation under the slabs.

10:09 PM Feb 19, 2010

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