Why is a two-layered design recommended for heat insulation experiments?

[DDesulgon]

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!

 

[Integral]

Are you modeling a house or a Styrofoam box? Houses have multi layered walls to say you are modeling a house implies multiple layers. Most houses have 3 layers, inside is sheetrock, a layer of insulation then an outer layer, wood or brick.

 

[DDesulgon]

Sure, it's not really about creating a realistic model of a house. My question is just about how the physics change if an inner layer of plasterboard or wood is inside the box. And why this may change temperature measurements inside the box.

 

[GreatestPhysician99]

Materials have different thermal conductivities so in a given amount of time , the temperature of the house will change because different amount of heat will "travel" from the hot parts to the cold parts.But if you let it enough time , in both times an thermal equillibrium will be reached.

 

[jrmichler]

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?

I don't understand the explanation for one simple reason: It does not make sense. Where did you find this explanation?

The heat transfer through a wall is described by the equation: Q = U * A * delta T.
Q is the total heat transferred. The heat from a 25 W light bulb is 25 watts, or 85 BTU/hr for those of us who learned those units.
U is the heat transfer coefficient: BTU / hour / square foot / deg F.
A is the area: square feet.
delta T is the temperature difference between the hot and cold sides in deg F.

Note that nothing in the equation calls for a single layer or for multiple layers. The heat transfer coefficient is determined by the wall thickness and thermal conductivity. If the wall has multiple layers, then the heat transfer coefficient is determined from the sum of each layer.

That said, there are some practical considerations for real houses:
1) Styrofoam is extremely flammable. A spark can set it on fire. In house construction, foam must be covered by a fireproof material, such as drywall (gypsum board).

2) A heavy material, such as drywall, adds thermal mass to a wall system. Added thermal mass makes it heat up slower when the furnace turns on, and cool down slower when the furnace turns off. This improves comfort by reducing temperature swings.

3) Styrofoam is weak. It dents easily. It needs to be covered for mechanical protection.

 

[Tom.G]

Just some off-the-cuff thoughts.

You don't say how small your "small" house is. If it is small enough that there is a significant variation in the distance between the bulb and various parts of the wall(s), then a thermally conductive layer will tend to even out the wall temperature... perhaps stopping the styrofoam from melting from the lamp thermal radiation.

Consider also the worst case of a point heat source on an interior wall surface. The predominate heat flow thru the styrofoam will resemble a cone, with a limited area on the outer surface.

To get an accurate measurement of thermal loss thru the structure, you would ideally wait for the interior temperature to stabilize and equalize, with both the air temperature and lamp radiant energy evenly distributed.