Dosrd:

I don’t want to measure the total heat loss, but I want to know if it is possible to measure the radiation heat loss for a certain room (with a certain outside temperature, humidity, etc.).

If your goal is to understand why the room feels less comfortable, it seems to me that the total heat loss would be the right thing to measure. If you have decided not to try to measure that, that’s your business.

When you talk about measuring the heat loss ‘for a certain room’, I assume you are still talking about the heat transfer between a person and the room they’re in.

I can only see a couple of approaches. One is to find and beg/steal/borrow/buy some sort of infrared intensity meter to measure how much heat is being transmitted. I suspect this route would be difficult and expensive. If you managed it you’d need to plot out the intensity over the whole surface of the subject and then integrate that over the area to give you the total heat power being radiated.

The other, much simpler and easier approach is to use an IR thermometer or similar to measure the surface temperature at representative points over the whole area, measure or look up the approximate emissivity of the materials at the surface, measure the temperature over the room’s inner surface in a similar way, and plug those temperature differences, emissivity and areas into the Radiant Heat Loss equation I linked you to yesterday. You won’t be able to do this in one go - you’ll need to conceptually divide the person’s surface up into pieces with similar emissivity and temperature and calculate the heat loss for each piece, then add them up to get the total.

To avoid wasting too much time on this, as the first step I suggest you make worst-case approximations for all these values (round the surface area up, assume the whole surface is at the temperature of the hottest point, assume the emissivity of the whole surface is the emissivity of the highest point) and calculate an upper bound for the radiated heat. You should already have a rough idea how much heat a resting person puts out in total so you can decide whether the radiant heat loss is actually of any relevance to your problem before you try to estimate it more accurately. I already explained that my ‘back of a fag packet’ calc suggest that it would account for less than a tenth of a percent of the total heat loss.