Wall Insulation - combining 3 types..!?

[john61ct]

I strongly disagree, seen lots of IRL tests

but as you say the point is moot anyway.

The plastic dropcloth as vapour barrier is more effective to also create a fully sealed envelope against convection

and nothing beats good closed-cell polyiso for the actual insulation

which = R-value.

Now a Mylar shade tent suspended above shading the living space would be great if no trees around to park under

assuming that won't block solar collection

[musigenesis]

Satellites and launch vehicles in space are a completely different situation from terrestrial vehicles and structures. In space heat transfer is entirely due to radiation; here on Earth, in the normal temperature ranges we experience, heat transfer is almost entirely due to conduction and convection, which a reflective layer is useless against.

[Doktari]

What the heck is vapor pressure?! That’s where I got confused when attempting to figure out insulation. And the paradox of sealing a structure tightly then needing ventilation which might defeat the purpose of insulation if not done correctly. One benefit of controlled ventilation is control of where the condensation is located and drain it away (such as a heat recovery ventilator). I’m still trying to figure out the best insulation strategy and leaning towards foam. One trick the OP might try is using up the “foam board” they already have as filler then spray foam over the entire thing. I did this on a house roof I raised. Fiberglass would never have sealed the structure and spray foam was expensive so I placed 2” foam board in the cavities and sprayed about 4” of foam over that using the spray foam kits available. Even so each kit was around $500 and I needed 3 kits. I’m wondering about using cheap metal studs inside a bus then spray foam the entire inside wall space?. Metal studs would have several advantages: lightweight compared to wood framing; lots of space for wiring and plumbing; space for inner layer of rock wool?; space for foam board filler?

[john61ct]

Originally Posted by Doktari

What the heck is vapor pressure?!

If inside of the living space has 95% humidity, but the insulation air space is dry, nature wants to try to equalise the two.

The vapor barrier envelope being tight slows that down.

>×the paradox of sealing a structure tightly then needing ventilation which might defeat the purpose of insulation if not done correctly.

Ventilation, convection exchange between the living space and the outdoors should be **strictly controlled**.

The water vapour produced by living mammals, and other sources requires varying that ventilation level according to the current ambient conditions

make it close to zero when appropriate, or 30 CFM exchange if you want.

The goal being to avoid wasting fuel not heating or cooling the outdoors any more than necessary.

Or maintaining health and comfort without active measures other than tightly controlled ventilation.

Random levels of "natural" ventilation will make that impossible, and is only appropriate when "following the sixties"

never if you are planning to venture into extreme conditions hot or cold.

> One benefit of controlled ventilation is control of where the condensation is located and drain it away (such as a heat recovery ventilator).

Rare edge case, complex systems.

In the normal course, there is no condensation allowed indoors, the humidity is expelled long before that can happen.

Yes a percentage of heat is lost, but the ability to strictly control the venting minimises that.

...

Yes foam boards are cheaper than spray foam, combining the two is a bit cheaper.

But finding a pro outfit that does your vehicle as a side job on a big construction site, maybe no savings and simpler is always better.

Wood or composite / plastic materials are **much** better than adding metal, unless necessary structurally, bearing loads.

Thermal bridging sucks, even if you cover with foam, those points are thinner, mobile context we're not talking about adding 2" plus 4" interior space getting sacrificed, at least very rarely.

[LargeMargeInBaja]

Originally Posted by o1marc

Rockwool absorbs water like a sponge.

.
Agreed.
.
Our second-to-latest conversion was a retired 40' reefer trailer.
We removed probably at least a ton of smelly damp rockwool.
Ceiling and walls, back to bare aluminum.
.
A crew of four, part-time for a week.
We dumped that crud in a ditch on the acreage, dozed to level.
.
Never again.
I say this with some certainty.

[LargeMargeInBaja]

Originally Posted by Doktari

..the paradox of sealing a structure tightly then needing ventilation which might defeat the purpose of insulation if not done correctly...

.
Instead of 'defeat the purpose', I see ventilation as complementary.
.
By judicious use of opening windows (plural) on opposite walls (plural), heated air can escape, carrying odors from cooking, wet dogs, and questionable choices in spices.
This escaping heated air also carries humidity.
.
If I was me, I would mount my windows high -- eye-level while standing -- so incoming light mimics natural outside sunlight.
The corollary:
* warm air near my ceiling can escape judiciously-open windows (plural).
.
WFTD:
* judiciousnessly
No need to thank me, this's just another fab feature of your lifetime subscription.

[kidharris]

Originally Posted by TomDPerkins

Radiative barrier insulation requires an air gap, not a large one. It is as useful in a mobile application as it is a stationary one. In fact, it is used quite often in such quite mobile applications as satellites and their launch vehicles. Their R value is not 0, their R value does not come from blocking air flow, although most will do so to some degree. Their trouble comes from the fact that if there are enough radiative barrier layers to be effective as insulation, then unless the outer and inner layers are perfectly installed and continuous -- thereby being a vapor barrier -- there will, period, be some layer within the layers on which water vapor condenses. Depending on how permissive the layers are to water vapor, this may be an amount which both makes that layer and the layer adjacent the wet side ineffective, and may accumulate by running down inside the layers as drops to promote corrosion and mold. They are not especially useful as insulation when they are continuous vapor barriers as changes in air pressure as with wind will either force the layers together, or, to prevent that will have to be permissive enough to airflow they permit unhelpful rates of air exchange.


If they are in an void space which is entirely protected from wind/air pressure and water vapor, they can be effective.

satellites are in vacuum situations (outer space, no moisture or vapor or oxygen) where 1 side (facing the sun) is exposed to very high temps while the other side, (facing away from the sun) is extremely cold. Totally different design considerations (no climate - avoiding destruction of machinery) than what us mortals require for preservation of life or comfort on a hourly ever changing water bound planet. Using a satellite as a comparison makes no more sense than using the insulation requirements for a foundry furnace or a cyrogenic freezer for comparison. PERIOD


BTW, studs run vertically.

[jimmythomas]

Some of the space program insulation works better if you drink Tang.

[kidharris]

Originally Posted by Doktari

What the heck is vapor pressure?!

Nerd talk

https://www.chem.purdue.edu/gchelp/liquids/vpress.html


My understanding (formed in 1965-69, hope my memory holds): Everybody knows that the boiling point of water is 212F. Consider a drop of liquid on the floor. Why does it evaporate if the floor never gets to 212F? Lots of people think that swamp coolers (evaporative coolers) don't work in high humidity because water doesn't evaporate well at in high humidity. Then how come the floor still drys when you mop it in Florida when it is near or at 100% humidity?



Why does only some of the water in a pressure cooker evaporate instead of all of it evaporating even though it is at or above 212F?


The answer has to do with vapor pressure.


Consider the earths atmosphere like the inside of a large container. Instead of walls restricting the flow of gasses and liquid like a sealed container, we have gravity doing the same job. Sorry, but that's the best that I came up with, never went to college.


As the water in the container heats up the molecules start vibrating faster and faster. In the liquid form the molecules are constantly bumping into their neighbors transferring energy to each other until some of them have enough energy to escape the water and they go flying around in the air until they hit the walls of the container or the top of the water. This develops a pressure (vapor pressure). No more molecules in the liquid can escape until they gain more energy (heat) than the molecules in the vapor because the vapor molecules either keep pushing the liquid molecules back into the liquid or dive into the liquid whenever a liquid molecule leaves the liquid. The hotter the liquid becomes the more molecules can escape and become vapor. The vapor and the liquid develop a balance based on temperature



Turns out that there is a relationship between the vapor pressure and the temperature. The hotter it is the higher the vapor pressure and the more molecules that are zipping around in the air. That's why they say hotter air can hold more vapor aka, the basis for relative humidity (the same amount of molecules in a hotter air as in an equal sized colder air would result in being different percentages relative to what the theoretical capacity that temperature air could hold)





Back to the drop of water on the floor. Not all molecules of the drop of water are at the same temperature as it is heating up. Some are hotter than others and are ready to leave (evaporate) before others but the water molecules that are already in the air won't always let them evaporate because they are exerting a pressure (vapor pressure) on the top of the drop of water... that is until the pressure or temperatures change. The boiling temperature of a liquid changes with pressure, water boils at a lower temperature with increases in altitude (decrease in atmospheric pressure)


Now to the wall insulation...the air on the 2 sides of the insulation are at 2 different temperatures and different vapor pressures and relative humidity levels. Heat travels from hot to cold, vapor travels from high pressure to lower pressure. Remember that colder temps are going to cause condensation when the vapor pressure is high enough to drive the vapor to the colder, lower vapor pressure side, hence the need for something to stop the vapor transfer aka vapor barrier.


Hope I remembered it all and made sense.

[kidharris]

Originally Posted by jimmythomas

Some of the space program insulation works better if you drink Tang.

Depends on whether there is some vodka in that Tang or
you have some freeze dried icecream to go with it.

[kidharris]

Originally Posted by LargeMargeInBaja

.
Instead of 'defeat the purpose', I see ventilation as complementary.
.
By judicious use of opening windows (plural) on opposite walls (plural), heated air can escape, carrying odors from cooking, wet dogs, and questionable choices in spices.
This escaping heated air also carries humidity.
.
If I was me, I would mount my windows high -- eye-level while standing -- so incoming light mimics natural outside sunlight.
The corollary:
* warm air near my ceiling can escape judiciously-open windows (plural).
.
WFTD:
* judiciousnessly
No need to thank me, this's just another fab feature of your lifetime subscription.

You are just too practical.

[TheHubbardBus]

Originally Posted by o1marc

Rockwool absorbs water like a sponge.

Which rockwool? The kind you start seedlings in, or that purpose-designed as in-wall insulation? Even in the former case, it doesn't absorb water so much as wick water via capillary action.


In the latter case - no - it's a hydrophobic material (to a point). If it becomes waterlogged, it's due to an improperly-designed wall system that doesn't adequately ventilate the space in order to allow moisture a means of egress. Not the fault of the material, but rather a failure in using it properly.

[john61ct]

True but its R-value is much lower than polyiso, and space is at an extreme premium here

[TheHubbardBus]

Originally Posted by john61ct

True but its R-value is much lower than polyiso, and space is at an extreme premium here

Agreed 100%. Just didn't want marc bad-mouthing it without merit. We're using it in our build so I'm emotionally invested

[kidharris]

Originally Posted by john61ct

True but its R-value is much lower than polyiso, and space is at an extreme premium here

The main point about rockwool and perlite isn't that they are greatest insulation or strength, but rather their fire resistance (non combustible). If you don't mind black clouds of toxic gases and rapid fire spread caused by its great insulating properties allowing rapid heat buildup, especially in metal buildings, polyiso is the ticket. You will not find this info on sites selling or making the crap you will need to go to firefighting association websites to get the truth.

[john61ct]

Buildings?

We are talking about a vehicle, a tiny and mobile context here.

If there is a fire large enough to burn through your walls faster than you are getting out

the toxicity of your insulation is rarely going to be your main problem.

But sure, if adding insulation thickness or reduced R-value is a sacrifice you want to make for that extra peace of mind, then sure, up to you.

Have you ever seen a single case documented where that issue actually caused a problem?

[TheHubbardBus]

Originally Posted by john61ct

Buildings?

We are talking about a vehicle, a tiny and mobile context here.

If there is a fire large enough to burn through your walls faster than you are getting out

the toxicity of your insulation is rarely going to be your main problem.

But sure, if adding insulation thickness or reduced R-value is a sacrifice you want to make for that extra peace of mind, then sure, up to you.

Have you ever seen a single case documented where that issue actually caused a problem?

That's pretty much my thoughts as well. I believe rockwool & similar products have their place in preventing ignition in & around potential ignitiion sources. And I'm using rockwool in that capacity & for other reasons. But I'm also using plenty of poly-iso/XPS (and reluctantly a bit of EPS) without undue concern. We're going to great lengths to prevent fire from beginning in, or reaching, the interior of our wall/ceiling assemblies. But IMO once a flame front has compromised a wall assembly to the point it's even starting to spread across the insulation, the battle is lost and you'd better be out no matter what the material is that's burning. Just my opinion, though. I'm no expert for certain.

[kidharris]

Originally Posted by john61ct

Buildings?

We are talking about a vehicle, a tiny and mobile context here.

If there is a fire large enough to burn through your walls faster than you are getting out

the toxicity of your insulation is rarely going to be your main problem.

But sure, if adding insulation thickness or reduced R-value is a sacrifice you want to make for that extra peace of mind, then sure, up to you.

Have you ever seen a single case documented where that issue actually caused a problem?

"We are talking about a vehicle, a tiny and mobile context here." School buses are usually not tiny as far as vehicles are concerned, but the interior is relatively small as far as a fire is concerned and here is where the problem comes in...it will fill up with smoke very fast.

"If there is a fire large enough to burn through your walls faster than you are getting out" what makes you think that a fire has to be large "to burn through your walls faster than you are getting out" Fire doesn't burn thru metal walls no more than sound goes through walls (if it is intense enough the fire may melt them though). What does happen is that the metal heats up to high enough temperature to ignite/melt/vaporize what ever is on the other side, similar to the way sound vibrates a wall/window/etc and restarts the sound on the other side. It is a moot point though because most fires start inside the structure.

"the toxicity of your insulation is rarely going to be your main problem."
People who die in fires are seldom burned alive, they usually die from lack of oxygen, seared lungs, or toxic smoke. In addition the smoke blocks their vision to a point that they can't find their way out. Your statement is completely backwards, the toxicity of your insulation is going to be your main problem. You don't even have to be close to the fire. The fire could start up, go out from lack of oxygen, and the smoke can still kill you.

"But sure, if adding insulation thickness or reduced R-value is a sacrifice you want to make for that extra peace of mind, then sure, up to you." Thanks for giving me your permission.

"Have you ever seen a single case documented where that issue actually caused a problem?" Yes, many times, but I don't have time, right now, to do your searches for you. I will give you a clue though, it is a major problem for firefighters health. Second clue...the rapid heat buildup in metal structures is due to a several things, the burning/combustion temperature of the fuel, size of the fire, and how well insulated the structure is.

BTW, if you see the phrase "self extinguishing" That means that the fumes created don't support combustion, not that there is no smoke/fumes/vapor or that the smoke/fumes/vapor is not dangerous. It means that it usually doesn't burst into flames, although, in some cases, the intensity of the fire can change that.

[Markel]

Originally Posted by john61ct

There is no airgap, waste of precious space.

Forget rockwool, why would you think any combination is needed.

polyiso is what give maximum R-value, read my previous posts if Arctic conditions are likely.

Sprayfoam for gaps and thin spots, do go thick over any thermal bridging metal.

Pretend you're using that for the vapour barrier, belt & suspenders.

Assuming your outer skin is water tight you only need one plastic sheeting vapour barrier on the inside, right after your inner paneling, skin whatever

This protects the inner "steam" from getting into your insulation or condensing on cold metal.

Drop cloth plastic is good, sturdier gets expensive.

In theory if you were using just sprayfoam professionally applied, not needed

but gaps often do occur from driving vibrations.

So I'm back after a break for Chrismas, and we've decided to go with the XPS + Polyiso. We're thinking an application similar to this..

https://youtu.be/Rj0Aa7x-KGI

..but using XPS as the first layer (and filling the gaps created by the poorly cut polyiso with sprayfoam). This will also, inadvertently, give us an air gap when we attach the ply to furring strips.

I forgot to mention that our polyiso is foil faced - does that mean we no longer have to worry about a vapour barrier?

[TheHubbardBus]

Originally Posted by Markel

So I'm back after a break for Chrismas, and we've decided to go with the XPS + Polyiso. We're thinking an application similar to this..

https://youtu.be/Rj0Aa7x-KGI

..but using XPS as the first layer (and filling the gaps created by the poorly cut polyiso with sprayfoam). This will also, inadvertently, give us an air gap when we attach the ply to furring strips.

I forgot to mention that our polyiso is foil faced - does that mean we no longer have to worry about a vapour barrier?

It seems to me that method is inviting potential problems down the road.
There are just too many ways moisture can get into those wall assemblies.
Whether it's from leaking windows, leaking window channels, condensation...
Even if everything's sealed up now, will it always be?
And what happens when moisture gets in there?
Those panels may seem tight to install, but they're not watertight.
In the deconstruction of our bus, there was some rust. Where?
Without exception - in every single case - where one span of something non-porous was pressed tightly up against the interior steel, but not tightly enough to prevent a thim film of water from entering and getting trapped.