Physics Help Forum Heat Insulation Experiment

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 Jul 7th 2019, 07:01 AM #1 Junior Member   Join Date: May 2019 Posts: 5 Heat Insulation Experiment I'm doing some experiments on heat insulation with a model house (made of styrofoam, 3cm), heated with a 25W light bulb. In some papers I have read that it's important for the box to be two-layered, with only the outer layer being styrofoam and the thin inner layer consisting of material with a lower thermal resistance. Apparently, the reason for this is that the inner layer thus provides a heat reservoir with homogeneous temperature. So it's an homogenous body, and the heat flows over a thermal resistance into the environment, in radial direction. Unfortunately, I don't quite understand this explanation. Why is the temperature only homogeneous with the additional layer? Could someone explain, why the inner layer with the lower thermal resistance is needed? Thanks so much in advance!
 Jul 7th 2019, 09:05 PM #2 Senior Member   Join Date: Apr 2017 Posts: 506 Actually I'm building a house in a hot climate , and some of my understandings may relate to your question .... The outer layer are light insulating blocks , the inner walls are standard higher density , and so higher conductivity .... My plan is to have the windows open at night , till 8am (with mosquito screens ) The cool air will move inside and reduce the temperature of the inner walls which have high mass and high thermal inertia... At 8am the windows are closed , the heat from outside will only enter slowly through the insulating blocks , the high thermal inertia inside will keep things cool untill the windows are open again at 10pm. In the experiment you are performing where the house is permanently closed , layers make no difference , you appear to be trying to keep heat in , and it's all about the insulation properties of the material used. benit13 likes this.
 Jul 8th 2019, 12:20 AM #3 Junior Member   Join Date: May 2019 Posts: 5 Thanks for your response! Interesting thoughts about the subjects. However, I do think that the inner layer does make some difference even with the house permanently closed. For example, measuring a cooling curve, there seem to be clearly different results between the setup with only the styrofoam box and the one with the additional layer (that cannot be explained just by the small additional insulation from the inner layer). Why is that? Last edited by NPhantom; Jul 8th 2019 at 12:23 AM.
 Jul 8th 2019, 02:55 AM #4 Senior Member     Join Date: Jun 2016 Location: England Posts: 1,011 Making a guess Could it be down to the uniformity of the temperature? A box with a central heat source will have hot spots in the centre of the walls and colder areas in the corners. The rate of heat transfer will depend on the temperature difference across the walls; being higher at the central hot spot. The inner high conductivity layer will even out the temperature from the central hot spot into the corners. Have a look at the rate of thermal transport verses temperature. __________________ ~\o/~
Jul 8th 2019, 03:18 AM   #5
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 Originally Posted by NPhantom I'm doing some experiments on heat insulation with a model house (made of styrofoam, 3cm), heated with a 25W light bulb. In some papers I have read that it's important for the box to be two-layered, with only the outer layer being styrofoam and the thin inner layer consisting of material with a lower thermal resistance. Apparently, the reason for this is that the inner layer thus provides a heat reservoir with homogeneous temperature. So it's an homogenous body, and the heat flows over a thermal resistance into the environment, in radial direction. Unfortunately, I don't quite understand this explanation. Why is the temperature only homogeneous with the additional layer? Could someone explain, why the inner layer with the lower thermal resistance is needed? Thanks so much in advance!

If you have a low thermal conductivity layer, the thermal resistance between the outside and inside surfaces of the wall will be low. This means that the interior air will be have a temperature that is more slowly varying than the ambient air.

However, having thick insulating walls is generally prohibitive in terms of cost, so if the problem requires a two-layer solution (a thin insulating layer and a thicker conducting layer), then the question becomes, "where to place the insulating layer?"

Generally, interior wall insulation is typically installed so that air conditioning units only have to treat the interior air; they don't have to treat the interior air and the thermal mass of the thick-wall layer.

That said, where I work, we get some projects where some clients use exterior wall insulation because the climate is very sunny, or install interior active or passive thermal batteries (i.e. concrete blocks) to aid in heat storage. Standards and regulations also play a role.

There are many other factors too, such as cost, installation access and weather-proofing (erosion/water resistance/etc.)

Jul 8th 2019, 10:53 AM   #6
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Thanks for your contributions and ideas. All of your thoughts on the subjects are interesting.

 Originally Posted by Woody Could it be down to the uniformity of the temperature? A box with a central heat source will have hot spots in the centre of the walls and colder areas in the corners. The rate of heat transfer will depend on the temperature difference across the walls; being higher at the central hot spot. The inner high conductivity layer will even out the temperature from the central hot spot into the corners. Have a look at the rate of thermal transport verses temperature.
That does sound like a very plausible explanation.

However, I would still be very happy about some more answers, especially regarding Woody's explanation. Do you agree or maybe there are some other/additional factors relevant?

 Jul 8th 2019, 09:01 PM #7 Senior Member   Join Date: Apr 2017 Posts: 506 It could be the person guiding the experiment didn't want the light bulb coming into contact with Styrofoam and melting it If you show the document , we could resolve it all quickly ...

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