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cadcoke5
16-01-2014, 03:47 PM
I need to design a method for testing an insulation wall panel 4'x8', to determine its R-value. I aware that there exists a standard method, referred to in ASTM C1363 documents, as well as in ASHRAE [A guarded hot box procedure ... NRCC-34002 by S t e p h e n s o n . Jan 1993]

I am posting because cost is a major issue at this point, and so I need to make the most minimal system I can use, even if some accuracy is sacrificed.

Here is my general idea of how a minimalist system may work;

An insulated box with walls of 4" of urethane foam is created, called a "Hot Box". One of the walls is open, and designed to accept the panel to be tested.


Two ways to calculate the heat loss;

One method is to heat the air in the Hot Box to a specific temperature, and then to monitor its natural decline. After it reaches a steady rate of decline, we know the heat loss rate from the Hot Box.

An alternative method is to maintain the temperature inside the Hot Box using a small resistive heating element, and to monitor the current it requires to maintain the temperature. Once it reaches a steady state, we know the energy going in is equal to the energy going out.


Since only one side of the Hot Box is our test panel, we need to deal with the heat loss through the remainder of the Hot Box walls. I can see two ways to do that.
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HOT BOX WITH GUARD BOX;
11105
The air inside the Hot Box is heated to some temperature, and then permitted to cool on its own. The heat lost is a combination of the heat loss through the test panel, as well as the other walls of the Hot Box.

One way to eliminate the heat loss through the other walls, is to surround those walls within an outer box, which is maintained at the same temperature as the inner "Hot Box". This is called a "Guard Box". Since both sides of those walls are at the same temperature, there will be no heat transfer through them. Only the test panel has any heat transfer.

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HOT BOX WITHOUT GUARD BOX;
11106
I think another, cheaper way, that does not use a Guard Box, is to calculate the heat loss through the hot box walls without the guard box. This might be determined by simply calculating it based on the known R-value of the insulation from published tables. But, it could be directly measured by putting the same insulation where the test panel would normally be installed. The, the heat loss would be the same on all walls. Then, calculate the heat flow for each of the walls, and simply remove the heat loss from the temporary lid, and then you will know the heat loss through the remained of the Hot Box.

For the actual tests, the test panel is installed, and the overall heat loss of the hot box is subtracted from the total, to determine the loss through the test panel.


I know that there are other possible refinements, such as another enclosure on the face of the test panel, so that the temperature is kept constant. But, it would be cheaper to simply monitor the ambient temperature in our building, and later calculate the heat flow based on those temperatures.

Any advice or thoughts on all the above?

-Joe Dunfee