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Ran D. St. Clair · 10-11-2009, 10:12 AM

Back to Work

I closed up the back wall of the truck, which is probably the easiest since there is no electrical or plumbing to worry about, just the doors. Everything is insulated with at least 1” of R-MAX foam board. The fit and finish aren’t perfect, but it doesn’t look too bad.

This brings up a question about insulation that I will throw out to the group. I have 1 ½” of space to insulate in the walls, and 4 ½” to 5 ½” in the ceiling. I am planning to use the R-MAXX foam board throughout, and it comes in various thicknesses like ½”, ¾”, 1”, 1 ½”, etc. I am not clear on the detailed physics of how the insulation works.

I understand that there is:
1. Conduction – Heat Transfer through direct contact
2. Convection – Heat transfer through movement of a fluid, like air.
3. Radiation – Infrared radiation (IR), electromagnetic waves, like light, only heat.

I understand that insulation reduces conduction because the insulating material is not a good conductor of heat. I also get that it reduces convection by filling the air space and breaking the air space into tiny convection cells that have to transfer the heat energy from cell to cell via the cell walls which are very thin, but there are lots of them so the insulation value adds up.

What I don’t fully get is the impact of the foil. I get that it’s supposed to be an IR reflector, and that it blocks direct IR transmission, but it also becomes hot itself, and then re-radiates. For example, I know that a tin awning will get very hot in direct sunlight, and will radiate IR on the people below, making its shade value much less than a nice green tree.

I have read that the foil surface somehow increases the insulating value of the airspace next to it, but I don’t know exactly why. In my application I don’t have space to waste on dead air so the foil would seem to be of no value. On the other hand, my intuition suggests that multiple layers of foil with insulation in between should be more effective than just the insulation alone, and probably more effective than insulation with just one layer of foil, on the outside, especially if that foil layer is in direct contact with plywood or some other less than ideal insulator.

Therefore I plan to fill all insulation spaces with at least 2 layers of foil backed foam. That way there will always be at least one foil layer with insulation on both sides. Put another way, there will always be at least 3 foil layers, one each on the inside and outside, and one in the middle where the two inner foil surfaces touch. I am counting those two foil surfaces as one since the aluminum is a good conductor and they are basically touching over much of their surface.

The complicated idea that I don’t know how to handle is the modeling of the IR transmission through these multiple layers. Each foil layer is both reflecting and absorbing and then transmitting some energy to the next layer, which is doing the same, and so on. What I don’t know is the ratio of energy reflected, absorbed, and transmitted in a steady state condition, and even if I did, the math to model it would get fairly complicated.

So, my bottom line question is this… Is there any value to filling my insulation space with multiple layers of foil backed foam, or am I just making more work for myself?

To be Continued…
?

10-11-2009, 10:12 AM	#110
Ran D. St. Clair Senior Member Join Date: Apr 2009 Posts: 212	Back to Work I closed up the back wall of the truck, which is probably the easiest since there is no electrical or plumbing to worry about, just the doors. Everything is insulated with at least 1” of R-MAX foam board. The fit and finish aren’t perfect, but it doesn’t look too bad. This brings up a question about insulation that I will throw out to the group. I have 1 ½” of space to insulate in the walls, and 4 ½” to 5 ½” in the ceiling. I am planning to use the R-MAXX foam board throughout, and it comes in various thicknesses like ½”, ¾”, 1”, 1 ½”, etc. I am not clear on the detailed physics of how the insulation works. I understand that there is: 1. Conduction – Heat Transfer through direct contact 2. Convection – Heat transfer through movement of a fluid, like air. 3. Radiation – Infrared radiation (IR), electromagnetic waves, like light, only heat. I understand that insulation reduces conduction because the insulating material is not a good conductor of heat. I also get that it reduces convection by filling the air space and breaking the air space into tiny convection cells that have to transfer the heat energy from cell to cell via the cell walls which are very thin, but there are lots of them so the insulation value adds up. What I don’t fully get is the impact of the foil. I get that it’s supposed to be an IR reflector, and that it blocks direct IR transmission, but it also becomes hot itself, and then re-radiates. For example, I know that a tin awning will get very hot in direct sunlight, and will radiate IR on the people below, making its shade value much less than a nice green tree. I have read that the foil surface somehow increases the insulating value of the airspace next to it, but I don’t know exactly why. In my application I don’t have space to waste on dead air so the foil would seem to be of no value. On the other hand, my intuition suggests that multiple layers of foil with insulation in between should be more effective than just the insulation alone, and probably more effective than insulation with just one layer of foil, on the outside, especially if that foil layer is in direct contact with plywood or some other less than ideal insulator. Therefore I plan to fill all insulation spaces with at least 2 layers of foil backed foam. That way there will always be at least one foil layer with insulation on both sides. Put another way, there will always be at least 3 foil layers, one each on the inside and outside, and one in the middle where the two inner foil surfaces touch. I am counting those two foil surfaces as one since the aluminum is a good conductor and they are basically touching over much of their surface. The complicated idea that I don’t know how to handle is the modeling of the IR transmission through these multiple layers. Each foil layer is both reflecting and absorbing and then transmitting some energy to the next layer, which is doing the same, and so on. What I don’t know is the ratio of energy reflected, absorbed, and transmitted in a steady state condition, and even if I did, the math to model it would get fairly complicated. So, my bottom line question is this… Is there any value to filling my insulation space with multiple layers of foil backed foam, or am I just making more work for myself? To be Continued… ?