Underbelly Insulation

[Cate&Rob]

Another thread https://www.mygrandrv.com/fo...me-Rails/page2 got me to thinking about this . . . but, I didn't want to hijack that discussion.

Our 2015 Reflection has a layer of (poorly fitted) foil on top of the coroplast. There is no fibreglass insulation as has been referenced in Solitude discussions. I can see the logic of not having the fibreglass "sponge" to soak up water . . . but how much insulation does just a sheet of foil really add?

From what I have read, a reflective barrier does not add any insulation in the sense of "R" value. It does reflect heat. So in the application of foil only, this should reflect heat above the coroplast back up into the underbelly space. I don't know how effective this really is with no insulation backing up the foil as would be there in house applications.

For applications with foil and fibreglass batts, is the foil above or below the fibreglass? If the foil is below the insulation (which I think is the case) this would seem to be backwards. The foil should be closest to the source of the heat.

When I added rigid foam insulation to the underbelly of our reflection, I put it above the foil, leaving the foil against the coroplast . . . I think I did this backwards .

Any other thoughts or theories ??

Rob

[carnolddsm]

Foil is below the insulation. I wonder if the foil is intended more as a moisture barriers or a wick for condensation.

A long explanation of this thought process:

There is a tendency for condensation to get trapped between the foil and coroplast.

We had water in the underbelly once with wet insulation. Was never able to find the source. Blamed it on water intrusion while driving in the rain. That was two years ago.

Having not had any water in the underbelly since the one episode, I wonder from time to time if we’ve been lucky or the wet insulation had nothing to do with towing in the rain.

In addition to replacing the insulation I drilled a few small holes in the coroplast low points to allow water to drain. Didn’t know if the holes would help or not.

In August, a few week old 374 was camped next to us. The owner noticed water dripping from the middle of the coroplast. The owner wasn’t a happy camper, he thought he had a water line leak on an almost new unit. After removing a few screws from the coroplast, I found no leak, water was draining from between the coroplast and foil. And since the owner knew he didn’t drill the weep hole, I’m wondering if this is something the factory does sometimes.

[carnolddsm]

Another thought, if the underbelly is warmer than the outside air, the foil even with insulation above it is reflecting heat.

[FT4NOW]

When I removed the coroplast on our Momentum to replace dump valves i noticed there was water between the foil and coroplast, but the fiberglass insulation on top of the foil was bone dry. I looked and found no evidence of leakage, i imagine it's due to the coroplast not being leak tight and picking up water when it rains. Either way, to answer Rob's question, no I dont think you did your insulation backwards. I've considered adding more fiberglass or rigid foam myself, I just havent gotten to that project yet, it's on my never ending honey-do list.

On a sidenote, GD also placed foil insulation on the floor of our bedroom slide which is a closet/drawers. I installed foil as a liner for our closets during our first winter. I dont recall how much, but I did use a pyrometer on the foiled closet vs the unfoiled and noted a temperature difference.

Sent from my SM-G950U using Tapatalk

[Cate&Rob]

Another thought, if the underbelly is warmer than the outside air, the foil even with insulation above it is reflecting heat.

Hi Colan and FT4NOW,

The foil will reflect the heat, wherever it is in the stack of materials, but most of the parallels that I can find, put the foil on the warm side. This may be because it is also sometimes the vapor barrier (which always goes on the warm side) but in warm climate roof insulation the foil goes on the top side.

Rob

[carnolddsm]

Hi Colan and FT4NOW,

The foil will reflect the heat, wherever it is in the stack of materials, but most of the parallels that I can find, put the foil on the warm side. This may be because it is also sometimes the vapor barrier (which always goes on the warm side) but in warm climate roof insulation the foil goes on the top side.

Rob

I was thinking in terms of keeping the underbelly warm, rather than retaining more heat topside.

[Country Campers]

So 2 questions on this.
1- If the foil is a vapor barrier could the moisture between it and the coro-plast could that be the temp difference between outside and inside?
2-Would the coro-plast be a vapor barrier as well , being it is plastic?
C-When we run our furnace it is pumping warm moist air into the underbelly , could this cause condensation?
D-Under all types of circumstances there could be all kinds of temp differences in the belly, a/c running and cold air dropping to the bottom, filling fresh water tank with colder than ambient temp water causing sweating, doing dishes or taking a shower and running hat water into grey tanks, could these all cause condensation?
Sorry 4 questions , probably my limit for the week.
Good day eh.

Brian

[Cate&Rob]

So 2 questions on this . . .

Hi Brian,

Both the coroplast and the foil are vapour barriers in the sense that they are impermeable. So, just like condensation on the inside of a cold window, condensation will gather on the warm (inside) of the coroplast/foil when it is cold outside. If there is insulation on the warm side of the vapour barrier, it will soak up this condensation. Which is why the vapour barrier in a house always goes on the warm side of the insulation.

Going the other way (colder on the inside than the outside) would cause the condensation to be on the outside of the coroplast/foil.

Any time that I have dropped (or peeked inside) the coroplast there is moisture in there. Some of this is likely condensation for all the reasons that you mention. I have also had my water alarm go off after driving through heavy rain.

So, back to my original concern, I guess it doesn't matter whether additional insulation is between the coroplast and the foil, or inside both. It is important to have openings to drain the inevitable moisture. I would be concerned about having fibreglass batt insulation directly above the coroplast/foil . . . how is this supposed to dry out .

Rob

[Nuffsaid]

I think the foil insulation is grossly overrated by the rv manufacturers. Most claim an R-15 value, yet the engineering sites I have visited only give it about an R-3 if that. It does reflect heat, but makes me wonder if it shouldn't be on top of the fiberglass in the belly to reflect the heat up towards the tanks and floor and allow the fiberglass to offer a thermal barrier.

[carnolddsm]

I happened upon an answer to the where the foil should be installed while researching the R value of air gaps. Having the foil at the bottom, against the coroplast allows the air space below the floor to act as an insulator.

From http://www.smartrate.com.au/media/ar...fying-air-gaps

For an air gap to improve the thermal performance of a building element, it requires the addition of a low emittance surface (shiny aluminium foil) to one or both sides of the air gap. Without the addition of the foil surfaces, the R-Value of the non reflective air gap is small (R0.16). The addition of foil to the building element may also provide a vapour barrier to control condensation if installed correctly.

However, if foil is installed into a building element without a corresponding air gap, it will be of no benefit or only provide a small added benefit if the foil facing is part of a foam or bubble sandwich product.