heat flowI have a machine that has power output of 2750 watt, which is blasted to a insulator foam with thermal resistance of 2.1 m2 kelvin/watt where the insulator is 5 cm thick and it drives the heat into ventilation and out of the roof.

I want to make sure that the insulator can withstand the heat blast from the machine. So if I know that 2750 watt can output 9383 btu/hr where 1 watt outputs 3.412 btu/hr, how can I be sure that the insulator can withstand the heat without it getting spread out? regards

Edited additional info: exhaust temp is 40 degree Celsius, room temp is 25 degree Celsius, and outside temp is 32 degree Celsius.

Flammability limit: lower limit for hydrocarbon gas (part of the foam) 1.8% by volume upper limit for hydrocarbon gas: 8.5 % by volume flash point of hydrocarbon gas: -84 degree Celsius

Volume of the exhaust = 50x 60 x 200 cm cube = 600,000cm cube

machine exhaust from different angle

  • $\begingroup$ there is not sufficient info here to know the exhaust temperature $\endgroup$
    – agentp
    Dec 26, 2017 at 14:23
  • $\begingroup$ @agentp just add exhaust temp info $\endgroup$
    – ethereal1m
    Jan 2, 2018 at 12:19
  • $\begingroup$ Use fiberglass insulation or batting made by Aspen Aerogels, but put a metal heat shield in front of the fiberglass with an air gap between the insulation and heat shield, you should be OK. $\endgroup$ Jun 3, 2018 at 18:42

1 Answer 1


You need to investigate the flammability properties of the insulating materials : wood can be a good insulator if it is dry, but it burns well etc.

  • $\begingroup$ you mean thermal resistance information is not telling the flammability properties? $\endgroup$
    – ethereal1m
    Dec 26, 2017 at 4:50
  • $\begingroup$ Exactly - metals don’t melt (until you get to a high enough temperature) but they conduct heat very well. Think of a metal spoon in boiling water you feel the hot handle but not with a wooden spoon.... $\endgroup$
    – Solar Mike
    Dec 26, 2017 at 8:10
  • $\begingroup$ but thermal conductivity (is this what you mean by flammability ) is the inverse of thermal resistance, see thermal resistance wiki , so that would be 0.476 watt/ kelvin m2... not sure where to go from there in order to prove if it can withstand the heat $\endgroup$
    – ethereal1m
    Dec 26, 2017 at 9:11
  • $\begingroup$ Thermal conductivity is how heat moves through the material ie the metal spoon in hot water... $\endgroup$
    – Solar Mike
    Dec 26, 2017 at 9:44
  • $\begingroup$ the insulator is polyurethane closed cell foam, it is fire retardant but not fire proof. Is the diagram and current info can be utilized to determine if heat won't spread out to the other side of the wall that doesn't touch the exhaust? $\endgroup$
    – ethereal1m
    Dec 26, 2017 at 11:46

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