Carpet on the wall ? - Page 3 - Fiberglass RV


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Old 09-29-2012, 02:19 PM   #29
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Originally Posted by GPJ View Post
Regardless of the brand, these products are a thin aluminum foil/coating over a mylar/plastic bubble material. Reflectix makes both single and double bubble material - the R-1.1 I quoted was for the double bubble material ie two layers of bubble wrap with an aluminum coating on either side of the sandwich.

Reflective insulation needs the shiny aluminum to be directly exposed to an air gap for to work as an heat barrier. This creates an insulating effect (ie. it reflects heat back) that is translated into an equivalent R-value for comparison to conventional insulating materials. Reflectix, AstroFoil, etc has an additional insulating effect that comes from the trapped air in the bubbles which works as conventional insulation and gives the R-1.1 (per Reflectix)

If you dig into the tech literature (more and better info on the Reflextix site than the AstroFoil site), you will see that the R-15 or so is the maximum the product can generate in ideal conditions (ie. air gaps), in a certain orientation ie. where the heat flow is down. The product is substantially less effective against horizontal and upwards heat flow. Again, with no air gap directly against the foil surface, you get nothing but the R1.1 from the trapped air bubbles.

Ok, so why do folks have a hard time with this? I guess it is because (like Byron noted above) the walls feel substantially warmer with the Reflectix/rat fur applied. That's because the resin in the fiberglass trailer shell is a great conductor of heat and a bad insulator. Just about anything (ie. R-1.1) will radically improve the situation. Plus, even a very little insulation really helps reduce the condensation on the walls.

The other thing going on here is that molded fiberglass trailers have very few air gaps, so with windows closed the unit is virtually airtight and a small heater is able to keep the unit warm with only a little insulation to break the thermal conductivity of the shell itself.

Byron: I have seen information in regards the size of the air cavity required to provide the maximum insulating effect and am trying to recall where I saw it. I seem to remember that the issue with larger cavities was that they allowed air within the cavity to circulate more easily which defeated part of the insulating effect of the air itself. I think that is part of why fiberglass bats work. The wall cavity with the fiberglass bat is full of air, but the material stops the air circulating ie. stops the wall cavity acting like the air mattress you described.

I don't want to sound too negative about Reflecix/AstroFoil here. I do think it works, but nowhere near as good as the quoted R-values - simply because it is covered. The product would work much, much better if left uncovered. With conventional foam insulation, covering it makes no real difference to the quoted R-values.
"more and better info on the Reflextix site than the AstroFoil site" Your opinion, not necessarily fact. Read the sections at AstroFoil about R values and how various types of insulation work. I know from first hand knowledge that just the foil will slow down infrared heat loss. So I'm more inclined to believe what AstroFoil has to say.

I have seen information in regards the size of the air cavity required to provide the maximum insulating effect and am trying to recall where I saw it. I seem to remember that the issue with larger cavities was that they allowed air within the cavity to circulate more easily which defeated part of the insulating effect of the air itself. The air circulation is called "eddy air currents". This is well known, agreed the smaller the air pocket size and more the better. However, there's probably a size that when you go below you're into "diminishing returns". That is the cost of producing a smaller bubble will not produce a significant better insulation.

At any rate it's better than no insulation. The best insulation I've ever seen was the blue closed cell foam used by backpackers 30 years ago. I used that once around an environmental chamber. The inside of the chamber was -40 (F or C, it's the cross over point) there was no difference in the surface temperature on the outside from outside air temperature which was room temperature. It's a huge 3/8" thick.

Now to compare the AstroFoil with the blue foam in construction.
Closed cell (meaning air cannot get in or out of the material) both cases.
Size of cells. Blue foam smaller than AstroFoil.
Radiant barrier, AstroFoil, not with blue foam.
Thickness AstroFoil approx half the thickness of blue foam.

Enough similar to believe that AstroFoil provides at least some fairly good insulation. R-15 in construction terms, probably not, but still good.

Another thing that I've observed that foil does, it visible light from coming through the fiberglass. Which means it would also at least slow down IR heat transfer.
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Old 09-29-2012, 04:12 PM   #30
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Thanks for the clarification re: eddy currents - for the life of me, I couldn't remember what they were called.

I don't think we are really disagreeing.

"I know from first hand knowledge that just the foil will slow down infrared heat loss. So I'm more inclined to believe what AstroFoil has to say."
I'm saying exactly what AstroFoil says (http://www.astrofoilint.com/stage/faq.html):

Why do I need an air space next to the ASTRO-FOIL or R+HEATSHIELD surface?
When aluminum foil is in direct contact with another surface, it conducts (or transfers) heat at the points of contact. Creating an air space will prevent conduction and allow the foil surface to reflect radiant heat. Aluminum foil reflects 97% of radiant heat.


The air gap is critical to a reflective barrier - without it you have just the insulating value of the trapped air in the bubblewrap.

"At any rate it's better than no insulation."
Yep - I agree.

"Enough similar to believe that AstroFoil provides at least some fairly good insulation. R-15 in construction terms, probably not, but still good."
Well I agree - but I believe that it is nowhere near R-15 when glued to a wall on one side with fabric glued to it on the other. But I'm not worried by that - it is still better than nothing. Even a little insulation makes a big difference. I just prefer foam (and you seem to like the blue backpacker's foam too).

"Another thing that I've observed that foil does, it visible light from coming through the fiberglass. Which means it would also at least slow down IR heat transfer."
This one I'm not sure about - the AstroFoil quote above states that the aluminum foil is a conductor when in contact with other materials, so I'm not sure that blocking visible light makes it an effective IR blocker. Maybe ... but I don't know enough about this to discuss it intelligently.

Anyway, this subject is very interesting to think about and discuss. I want to re-iterate that I'm not against AstroFoil/Reflectix, just believe that you have to leave it exposed to get the reflective barrier benefit that gives rise to the high R-value claims.
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Old 09-29-2012, 05:59 PM   #31
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Fellas, fellas...
Regardless of the pros and cons of either products; and putting aside the thermodynamic details; do we have any consensus on an appropriate, and/or preferred, insulation barrier that can be easily contoured to the complex curves of a single hull fiberglass shell, and to which a fabric wall covering can be applied?
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Old 09-29-2012, 06:39 PM   #32
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Originally Posted by GPJ View Post
Thanks for the clarification re: eddy currents - for the life of me, I couldn't remember what they were called.

I don't think we are really disagreeing.

"I know from first hand knowledge that just the foil will slow down infrared heat loss. So I'm more inclined to believe what AstroFoil has to say."
I'm saying exactly what AstroFoil says (http://www.astrofoilint.com/stage/faq.html):

Why do I need an air space next to the ASTRO-FOIL or R+HEATSHIELD surface?
When aluminum foil is in direct contact with another surface, it conducts (or transfers) heat at the points of contact. Creating an air space will prevent conduction and allow the foil surface to reflect radiant heat. Aluminum foil reflects 97% of radiant heat.


The air gap is critical to a reflective barrier - without it you have just the insulating value of the trapped air in the bubblewrap.

"At any rate it's better than no insulation."
Yep - I agree.

"Enough similar to believe that AstroFoil provides at least some fairly good insulation. R-15 in construction terms, probably not, but still good."
Well I agree - but I believe that it is nowhere near R-15 when glued to a wall on one side with fabric glued to it on the other. But I'm not worried by that - it is still better than nothing. Even a little insulation makes a big difference. I just prefer foam (and you seem to like the blue backpacker's foam too).

"Another thing that I've observed that foil does, it visible light from coming through the fiberglass. Which means it would also at least slow down IR heat transfer."
This one I'm not sure about - the AstroFoil quote above states that the aluminum foil is a conductor when in contact with other materials, so I'm not sure that blocking visible light makes it an effective IR blocker. Maybe ... but I don't know enough about this to discuss it intelligently.

Anyway, this subject is very interesting to think about and discuss. I want to re-iterate that I'm not against AstroFoil/Reflectix, just believe that you have to leave it exposed to get the reflective barrier benefit that gives rise to the high R-value claims.
Yes it has been an interesting discussion. One of your points about contact and conduction of heat. True, however what we're mostly attempting to do is keep heat in, inside the trailer. The contact where the heat is with the "rat fur" which traps a lot of air, providing some insulation. Then the foil which slows down radiation heat loss, then more trapped air in form of bubbles, then foil, and finally the fiberglass shell.

Basic thermal dynamics, heat moves towards cold. That transfer of heat to cold is what you're trying to reduce.

As for an air space between the foil and the shell or metal roof, or any other application, helping the insulation work, I don't think so. Reason being, the air in that airspace is the same temperature as the surface the foil is separated from. The exception would be if you have another air trapping material in between that prevents eddy currents.

IR is just above visible light in electromagnetic spectrum, if visible light is blocked so is IR. Some materials that transfer light, don't transfer IR, widows are one. A bit about my experiences. That last 15 years I worked I was an engineer working with IR thermometers. I was involved in the both the thermometer and manufacturing process equipment design. Some interesting things came out of that activity, not directly associated with the insulation subject except in understand how heat transfer works. As we approach winter, or even remembering winters gone by, at times you might have experienced where there was frost on cars, roads, mail boxes, etc. If you measured the air temperature it could be that air temperature is well above freezing. Look at the sky, it's probably clear. The heat in object exposed to a clear sky will be transferred to space. Depending on how long before heating occurs (sun rises) and air temperature surfaces can get cold enough to freeze while the air is above freezing.

Just a tid bit of information.
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Old 09-29-2012, 07:16 PM   #33
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Robert:

Nicely phrased question. For me, the reality is that either will work just fine. I prefer flexible foam with as high a ratio of closed cells as possible as I think that gives the best combination of insulation and formability as it stretches slightly and can be cut sharply. Really, ensolite was/is pretty good. Cover the foam with vinyl for ease of cleaning or fabric for cozyness.

But, AstroFoil/Reflectix will work fine too - perhaps not quite as good an insulation material (unless you can leave it uncovered), but inexpensive and can be picked up at most home centers. It is harder to form compound curves as it doesn't stretch, and you get a little collapsing along the cut lines as you are bursting the air bubbles, but taping over cut joints will minimize this effect. If I were using AstroFoil/Reflectix I would leave it uncovered in areas behind seat benches, cupboards, closets, etc. (unless a risk of puncturing it) to get the additional reflective barrier advantage in these areas.

I'd probably spend my time doing a good job installing either rather than worrying over which is marginally better.
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