Simple Method to Measure Thermal Conductivity of Insulation

[baha]

Hi,

I'm doing a group investigation on external wall insulation for my school building. The walls do not have a cavity and thus it has been proposed to put insulation over the exterior render of the wall.
This is a fairly common building procedure.

However, my task is to investigate the plausibility of using recycled materials as insulation and once I have decided what to use, I will need to calculate the thermal conductivity.

My proposal is to use a mixture of supermarket plastic bags and other mixed recycled packing plastics, stuffed between the exterior of the existing wall and a new exterior wooden cladding.

With only standard school equipment, how could I measure the thermal conductivity of my custom insulation in order to numerically compare it with existing insulations?

 

[sophiecentaur]

Hi and welcome.
If you can find a set of Lee's Disc equipment in your physics equipment store then that should eo just what you want. To see what it looks like, Google it. I found many hits with pictures and practical details. It is essentially for measuring conductivity of insulating materials.

 

[baha]

Hi and welcome.
If you can find a set of Lee's Disc equipment in your physics equipment store then that should eo just what you want. To see what it looks like, Google it. I found many hits with pictures and practical details. It is essentially for measuring conductivity of insulating materials.

That would be great but I don't believe that my physics store has this equipment. Is there any way to do it by building a box out of the insulator and putting an electric heater inside or something?

 

[sophiecentaur]

You may need to ask the technician. It is not a large thing and could be in the back of a drawer. I was teaching in a UK Physics department from 1991 and they had one - but I was the only person who actually recognised it for what it was. You may need to present them with a picture of it (from the www).
I remember seeing one in about 1960, at my school but never used it. There were questions about it in textbooks though.

Your idea of a heater in a box would, of course, be an alternative. The Lees Disc uses steam and this may have been more suitable 150 years ago than electrical heaters would have been. I'd suggest a light bulb in a copper box as the heat source. Your school will certainly have something similar available.

The things you need to be able to measure would be internal and external (surface) temperatures and heater power. If you wait for the steady state situation, you know the energy flow (it's all getting out at the rate the heater is supplying it) and make sure the temperatures are fairly even all over the surfaces. Two copper boxes with equal spacing between, all the way round.

You need to be prepared to take some time over each measurement and to 'calibrate' your measurement with a know substance. Could be fun.

 

[Chestermiller]

Air has a very low thermal conductivity (lower than any of the plastics you would be using), but, because the wall space is hollow, the air is able to circulate within the wall (by natural convection), and this greatly increases the rate of heat loss. The air flows downward near the cold wall, and upward near the hotter wall. It turns around at the floor- and ceiling studs. So heat is carried by air flow from the hot region to the cold region. If you stuff the walls with plastics, this drastically cuts the air circulation, but the thermal conductivity of the plastic is higher than air. But, although it would be difficult to exactly calculate the equivalent thermal conductivity of the plastic filler, it might be possible to bound the rate of heat loss for design purposes (without the need to do any experiments). The thermal conductivity of the plastic material is probably known for the plastic bags and for any other plastics stuffed in the wall. All you need to know is the amount of plastic stuffed in per unit area of wall (kg/m²). If you divide this by the density of the plastic (also known), this will give you a total thickness of plastic (on average) at any location within the wall. The air in the interstices will provide additional insulation, but, to be conservative, you will neglect that. So you find the rate of heat flow through the plastic part of the space (which is thinner than the total wall space). This will give you an upper bound to the rate of heat loss.

Chet