Welding blanket as thermal barrier?

[MNbusboy]

I was wondering if there would be any value in a thin layer like this acrylic coated fiberglass welding blanket against the metal of the roof and floor before adding foam insulation board. I don't have much headroom, so I am limited to an inch of insulation. The goal would be a thermal barrier to minimize contact transfer of heat from the metal to the insulation, increasing it's effectiveness without adding thickness. I might be able to get a 450 sq ft roll for a few dollars.

https://www.grainger.com/product/3FK...E&gclsrc=aw.ds

[musigenesis]

It will be about as good as wood of the same thickness, since ceramic as a solid material has about the same R-value as wood. Ceramic insulation works the same as fiberglass, rock wool, extruded polystyrene etc. by trapping lots of tiny pockets of air (or blower gases) which has a low thermal conductivity. A welding blanket does not trap air in this way, though, so it will have roughly the same thermal conductivity as the solid material. Wood has an R-value of about 1 per inch, so a layer of this stuff between the steel and your foam board will add R-0.016 to your insulation - effectively nothing at all.

The primary advantage of ceramic over other materials is its ability to resist very high temperatures without being destroyed or catching fire. This is of no value for insulating a bus.

[john61ct]

You need maximum R value per inch.

PolyIso boards.

no air gap, no foil, no other blanket.

Is this against heat?

A different strategy against extreme Arctic cold

[rossvtaylor]

I readily admit that I know only the basics about R values and thermal barriers and such. And some of you here are super knowledgable about all that stuff. So, sort of related to the original question here, how does the ceramic paper like EHP Rollboard or Ceratex fit into the design of an insulation system in a bus? Or doesn't it at all?

As a welder, I don't think the welding blanket would do much. But we've used and have quite a bit of experience with the ceramic papers and they seem to be an effective barrier to heat conduction. You can fan a torch on one side and not burn yourself on the other side of the 1/8th inch paper. I know this is a different situation from having a warm space on one side and a cold space on the other, over a long period of time, though.

My sense, from personal experience, is that having a layer of ceramic paper incorporated into the insulation sandwich is helpful...and my guess is that it's not because of added R value but due to the break of potential thermal bridging and conduction. For a buck per square foot, I personally think it is worth adding and it seems to make a difference...but I've not had a chance to do any real A/B testing, so this is just my gut.

I welcome the thoughts and input of y'all who know much about this stuff.

[MNbusboy]

So I know the blanket wouldn't help against a hot or cold day and regular heat transfer, and I accidentally said roof and floor when I just meant roof. I was thinking that a barrier layer against the roof could help minimize direct heat transfer from a hot metal roof in the sun, to my insulation, then roof, then interior. I know it wouldn't provide "insulation", but I thought it might cut down on the heat getting to the insulation due to direct transfer from hot metal.

[john61ct]

There is no point in using valuable thickness without maximum R value

No need to stop transfer of heat TO insulation

that is the point of insulation itself, not transfer the heat through that space.

If you had an extra inch for an air gap then Reflectix is OK but then two inches of foam board is better.

Also, please answer my Qs

[Danjo]

Reflectix is only a quarter inch and it provides something like R1 on its own, but look at the numbers when a small air gap is involved.

https://cdn.reflectixinc.com/wp-cont...qmnts-0708.pdf

[MNbusboy]

It would mostly be for heat. We live in Minnesota, but we are the kind of Northerners who flee the cold for vacation, not embrace it! We might use the bus up here in winter, but it would be to drive somewhere warmer!

If the metal roof is in the sun and heats up to 120°, that will pretty much transfer straight in and will then be in direct contact with the insulation. If there is a barrier layer the insulation will only be in contact with a lower temperature. If the outside of the insulation is against a lower temperature, wouldn't that mean a lower temperature leaching through the insulation?

The main reason I am looking at this product is that I can probably get 450 SQ ft for $10-20 instead of $575.

Originally Posted by john61ct

There is no point in using valuable thickness without maximum R value

No need to stop transfer of heat TO insulation

that is the point of insulation itself, not transfer the heat through that space.

If you had an extra inch for an air gap then Reflectix is OK but then two inches of foam board is better.

Also, please answer my Qs

[john61ct]

Originally Posted by MNbusboy

If there is a barrier layer the insulation will only be in contact with a lower temperature. If the outside of the insulation is against a lower temperature, wouldn't that mean a lower temperature leaching through the insulation?

No. "R-value" and "thermal insulation" mean the resistance to transmitting heat.

If you had a 4" space available, but did not want to spend so much money on 4" of foam, then an air gap above the foam with a facing Mylar / Reflectix would still be better than your blanket.

Since the polyiso is the maximum resistance per thickness, and your very limited height available, any space occupied by your blanket is better off filled with polyiso.

[MNbusboy]

Interesting. Thanks for the info!!

[Rucker]

Originally Posted by MNbusboy

So I know the blanket wouldn't help against a hot or cold day and regular heat transfer, and I accidentally said roof and floor when I just meant roof. I was thinking that a barrier layer against the roof could help minimize direct heat transfer from a hot metal roof in the sun, to my insulation, then roof, then interior. I know it wouldn't provide "insulation", but I thought it might cut down on the heat getting to the insulation due to direct transfer from hot metal.

Agreed on R-value, but perhaps there's some merit in eliminating direct air-to-metal contact, particularly to reduce condensation. It is difficult to avoid air gaps around the channels or between the Styrofoam and the metal roof, and anything covering the metal would help. Especially if there is good air circulation.

There's a big difference between putting your hand on a cold piece of metal and putting your hand on a piece of cloth on that metal.

I would seriously consider applying it, making sure it is conformal-directly in contact with the metal with some kind of contact adhesive.

[musigenesis]

Originally Posted by MNbusboy

If the metal roof is in the sun and heats up to 120°, that will pretty much transfer straight in and will then be in direct contact with the insulation. If there is a barrier layer the insulation will only be in contact with a lower temperature. If the outside of the insulation is against a lower temperature, wouldn't that mean a lower temperature leaching through the insulation?

As an experiment, buy an infrared thermometer and a small patch of this ceramic blanket and glue it to the inside of the roof panel. Leave the bus out in the sunlight on a hot day (with all the windows and doors open so the entire inside doesn't get super-heated) and measure the temperature on the patch and on other parts of the roof. I think you'll find that the patch is essentially the same temperature as the metal roof (and thus of no value for insulation purposes).

If you do the same thing with foam board, you will find that the temperature on the inside of the foam is much lower than the metal roof (in my bus on a hot day, the roof will reach 135°F but the inside of my 2.25" of foam is 98°F, just a few degrees hotter than the ambient air temperature). This is because the foam board is 1.5" (+) thick and has a very low thermal conductivity, and thus greatly slows the transfer of heat. The ceramic blanket has a relatively high thermal conductivity (about the same as wood but at least much better than steel) but is extremely thing and thus will not slow the transfer of heat much via conduction.

[musigenesis]

Originally Posted by rossvtaylor

As a welder, I don't think the welding blanket would do much. But we've used and have quite a bit of experience with the ceramic papers and they seem to be an effective barrier to heat conduction. You can fan a torch on one side and not burn yourself on the other side of the 1/8th inch paper. I know this is a different situation from having a warm space on one side and a cold space on the other, over a long period of time, though.

Ceramic paper (or this blanket material) is an effective barrier to heat convection but not really heat conduction. Air (or any gas, really) transfers heat by conduction poorly because it has a very low density relative to solids, but it transfers heat by convection very readily because it is free to move around. So it will only insulate effectively when it is trapped as a bunch of tiny air pockets by a solid but very lightweight and sparse matrix, such as that provided by cured foam or a mesh of fibers like rock wool or fiberglass. Since the material is mostly air it transfers heat via conduction very poorly, and since the air is constrained from moving it also transfers heat via convection very poorly.

A torch will give off a huge amount of heat but this spreads out mostly via convection. Ceramic paper will block the flow of this hot air and thus block most of the heat transfer. Regular paper will do the same thing, except not for very long since it will ignite and quickly burn away.

In the case of a bus roof, the heat transfer will be almost entirely due to conduction (unless you have big holes in the roof letting air move in and out), so something like ceramic paper or this matting will not do much. When the metal roof gets extra-hot from direct sunlight, heat transfer via radiation becomes marginally significant, at which point a reflective layer begins to have a small benefit - but only if that reflective layer is adjacent to an air gap of at least 1" (and the air in this gap has to vent to the outside, not to the interior space). The reflective layer + air gap gives an estimated R-2 boost to the effectiveness of the overall insulation (which is why 1/4" Reflectix can claim an R-3 value), but since you need a 1" gap for the reflective layer to have a benefit, you would be better off just filling that gap with foam which would give you an R-5 boost.

The amount of heat transferred via radiation (as opposed to by conduction and convection) is a function of the absolute temperature taken to the fourth power, so by the time you get to the temperatures generated by a wood stove (400°F to 500°F) radiation becomes the dominant factor in heat transfer, which is why you need to have a reflective shield behind them to protect your wall.

[Rucker]

I agree with all the above.

I have to think a little more about conduction versus convection. When I measured surface temperature on the inside of the bus this summer, in the peak of afternoon heat with outside temps around 105 and I had a high of 91 degrees on the inside fiberglass surface (the wall is two layers of fiberglass separated by a cardboard waffle stock), and 147 degrees on the black window frame. Something tells me if I put a layer of fiberglass cloth over the inside of the window frame I'd be lower in temperature, perhaps much closer to the inside wall measurement.

[84chevyguyid]

Good video on insulation, maybe the 1/8th thick ceramic insulation will work for your limited ceiling height issue.

https://youtu.be/hw1UpnSjLV0

[Mountain Gnome]

Originally Posted by musigenesis

Ceramic paper (or this blanket material) is an effective barrier to heat convection but not really heat conduction. Air (or any gas, really) transfers heat by conduction poorly because it has a very low density relative to solids, but it transfers heat by convection very readily because it is free to move around. So it will only insulate effectively when it is trapped as a bunch of tiny air pockets by a solid but very lightweight and sparse matrix, such as that provided by cured foam or a mesh of fibers like rock wool or fiberglass. Since the material is mostly air it transfers heat via conduction very poorly, and since the air is constrained from moving it also transfers heat via convection very poorly.

A torch will give off a huge amount of heat but this spreads out mostly via convection. Ceramic paper will block the flow of this hot air and thus block most of the heat transfer. Regular paper will do the same thing, except not for very long since it will ignite and quickly burn away.

In the case of a bus roof, the heat transfer will be almost entirely due to conduction (unless you have big holes in the roof letting air move in and out), so something like ceramic paper or this matting will not do much. When the metal roof gets extra-hot from direct sunlight, heat transfer via radiation becomes marginally significant, at which point a reflective layer begins to have a small benefit - but only if that reflective layer is adjacent to an air gap of at least 1" (and the air in this gap has to vent to the outside, not to the interior space). The reflective layer + air gap gives an estimated R-2 boost to the effectiveness of the overall insulation (which is why 1/4" Reflectix can claim an R-3 value), but since you need a 1" gap for the reflective layer to have a benefit, you would be better off just filling that gap with foam which would give you an R-5 boost.

The amount of heat transferred via radiation (as opposed to by conduction and convection) is a function of the absolute temperature taken to the fourth power, so by the time you get to the temperatures generated by a wood stove (400°F to 500°F) radiation becomes the dominant factor in heat transfer, which is why you need to have a reflective shield behind them to protect your wall.

I thought you wrote software?
That is a brilliant explanation professor.
Aloha!

[john61ct]

There should be no air gaps anywhere

Just as important as R-value is a tight envelope, no air infiltration between the outer skin and inside living space.

Use strong adhesive between the foam boards and the skin, and infill any gaps using spray foam.

Finally seal with a vapour barrier like plastic drop cloth, do not allow rips, punctures etc

Just behind your inner liner skin.

Condensation on the inside of the outer skin should be physically impossible.

[cadillackid]

Originally Posted by john61ct

You need maximum R value per inch.

PolyIso boards.

no air gap, no foil, no other blanket.

Is this against heat?

A different strategy against extreme Arctic cold

strategy against Artic cold - Hop in the driver seat and hit I-75 SOUTH

[john61ct]

Some go skiing or boarding all winter, camp in area parking lots or

boondocking in Alaska.

But not with only 1"

[jackdebear]

Maybe look into thermal paints. Or insulating paints. They work by having microspheres, tiny bubbles in the paint. I don't know how well they work but research is easy these daze. Also might be some way to use starlight. That would be real deep end diy.