Why I'm Choosing Rockwool over Spray and Rigid.

[TucsonAZ]

Sometimes I'm seeking out information and it takes forever and many sources to aggregate the data needed to make a fully informed decision. This is why I'M choosing Rockwool, all decisions are a balance of acceptable risks, rewards, time, finances for the person making them as well as other factors. This may not be the correct decision for you but here's the information I've gathered to lead me to my conclusion and hopefully it helps you.



I'll start with why I'm not using other options:


1) Fiberglass batts. Low R-Value and issues if it does get wet.



2) Polyurethane AKA Spray Foam AKA Soy Something or other (if they're attempting to greenwash it). Two part spray foam is the "Holy Grail" of insulation and, if you can live with the down sides (which are extreme) it's an amazing option. It's seamless, ties things together structurally, has a high R-Value and many other advantages.


The reality of this though is that no matter how hard you attempt to get a perfect install, no matter how good or experienced the installer. You ARE NOT going to get the zero VOC off gassing they're able to do with the tiny samples submitted for testing that actually cures in 48-72 hours. In the real world where a tiny difference of output between the A and B nozzles, more or less humidity, higher or lower temps all play a part in the final product you're looking at a solid 2-3 years of off gassing and that's best case. I'm not going to get into the specific chemicals or health concerns, I'll just say it isn't worth the risk for me in such a tiny space that requires so much effort to build.



This may totally be acceptable for you and your wants, needs, lifestyle, health concerns and so on. This is certainly the best option if you don't have the same worries I do and I can see why it's appealing enough that people choose it even with the incredible risks. And I know many of you don't even notice the off gassing, but that doesn't mean it isn't there.



3) Rigid Foam Board. This of course comes in XPS, Polyiso, EPS and so on. I think it's a good choice and mitigates some risks of 2 part foams, it does have some off gassing over a long time but it's low and within what I would call acceptable. It does still have flame retardants and the dust can be an issue but again this can be mitigated with boards that have a radiant barrier or are being used under subfloor.



My issue with this is getting a good, solid insulation envelope. Let's assume 3 inches of polyiso. If this were not a bus and all walls were flat I would choose it. But in a bus where we're having to accommodate a lot of curves with more cuts and needing so many seams and gaps I think it drastically drops the R-Value from what would be R20 in a 3" thick Polyiso to my speculation maybe an R12 or something. You can fill gaps with 1 part canned foam but I still think you're dealing with huge losses, a higher cost, and a lot of time and energy.



Don't get me wrong, this was my first choice and I still may use it on large, flat spans and will on the floor for sure, but I don't think it should be the only thing you're focused on to the point you're using it in less than ideal situations.



4) Single part spray foam. Can you imagine this, I mean, it addresses a lot of the issues, no risk like in the 2 part spray foams but I think it's best left for gaps here and there and filler. Unless you're insane, and seeing as you own a bus you likely are so maybe give it a shot.


5) Rockwool Formerly Roxul. I've used this a lot, comes in 2.5", 3", 3.5" so it should be easy to work into a build. Cuts easily with an electric bread knife, easy to install, seamless roof install even with the curves and offers you a respectable R3-R4 per inch. It's totally safe and chemical free and really comes with no down sides.



If you go to an insulation cost savings calculator you will see there is certainly a difference with the higher spray foam insulation but insulation has a law of diminishing returns. For example, going from R2 to R4 is HUGE, going from R4 to R6 is big, but those two jumps right there will make more of a difference than going from R6 to R20 and cost you much less. Basically think of it like this, every time you add R2 you double the gains but it's like halfing a number, if you start with 20 and you half it, you end up with 10, 5, 2.5, 1.25, 0.625 and you quickly see that your 50% off isn't making nearly the difference it did in the beginning but with insulation it's costing you time, money, and space. With insulation you hit a point of diminishing returns around R12 before you have to start making other compromises to gain more and those gains start doing very little.


At that point, I think you're better off adding a roof coating, higher SEER AC units, more solar and so on. After all, we have limited space so let's retain as much of it as possible and expend our energy and resources wisely.



I am actually going to try to get R15 Rockwool in the ceiling, I'll see how I feel about that, given the 2" stud depth it will cost me 1.5" of headroom, coupled with an 1" loss on the floor and I can live with that for my height and needs.



Soooo, those are my thoughts, I'll update in a year to see if I'm going to stand behind these decisions but I've built other units and worked with insulation a lot so I'm comfortable enough with this to move ahead given the ease of install and comfort I have in it being chemical free with no off gassing risks. I only got one life, I can buy 100 busses ( :

[cadillackid]

I buy a new car every year or two.. sometimes less.. people told me id be dead by now from the interior plastic off-gassing of "new-car-smell".. im still alive and not dead and I have bought a LOT of new cars....

[PhilipE]

There are two types of rock wool. There is white and brown color. The white bats are a looser type where the brown bats are more ridged. The brown bats do have chemicals in them to help hold them into a firmer type of bat.

Rock wool is fireproof in either style of batting. Rock wool doesn't collect or hold moisture.

Rock wool would make a good insulation choice for a bus.

[musigenesis]

I did read a study a couple of years ago that showed that if improperly installed, foam board could have its R-value effectively cut in half. But the improper installation meant 1/8" gaps between the boards and the studs, and the boards mostly not in contact with either the inner or outer walls; this allowed air to freely circulate all around the foam board, and those convection currents allowed heat transfer to somewhat bypass the foam. If you're cutting your foam board that inaccurately then I don't know what to tell you, although I have seen many builds where people seem to be doing just that.

I cut my boards slightly large and generally hammered them into place with a 2x4 persuader. In most places I don't have gaps that could even take a piece of paper, let alone be wide enough to consider using spray foam. Where I did have wider gaps, I cut slivers of XPS foam for them to ensure a tight fit. And the boards are the same depth as the spaces they're fit into, so there are no gaps on the inner or outer walls.

Fitting the curvature of the roof is definitely trickier, but I eventually figured out how to deal with that by cutting multiple pieces with beveled edges on my table saw, and then again forcing them into the ceiling spaces between ribs such that there are no gaps.

[TucsonAZ]

Originally Posted by cadillackid

I buy a new car every year or two.. sometimes less.. people told me id be dead by now from the interior plastic off-gassing of "new-car-smell".. im still alive and not dead and I have bought a LOT of new cars....

Hmmmm, you don't say, a lot of new cars, I would have never guessed that by your long and articulate response ( : That's just silly saying you're going to die, that's highly unlikely, it has been shown to drastically decrease testosterone though so infertile seems much more likely than dead. Either way, cars have an extremely low R-Value so the suggestion is great but they're not going to work for my build.

[Ronnie]

I have to wonder why you dismiss fiberglass so quickly, it has the same r value per inch (3.7 per inch) as rock wool, no chemicals, nothing toxic about it, and as far as getting wet it does not hold moisture. So seems to be even with rock wool.



If you prefer rockwool, that is fine but you leave the impression fiberglass is worthless

[Rucker]

I love the analysis, even if I might quibble with some of the details.

THIS is how we learn as a community, so thanks for the detailed write-up.

On to quibbling:

Originally Posted by TucsonAZ

<snip>
If you go to an insulation cost savings calculator you will see there is certainly a difference with the higher spray foam insulation but insulation has a law of diminishing returns. For example, going from R2 to R4 is HUGE, going from R4 to R6 is big, but those two jumps right there will make more of a difference than going from R6 to R20 and cost you much less. Basically think of it like this, every time you add R2 you double the gains but it's like halfing a number, if you start with 20 and you half it, you end up with 10, 5, 2.5, 1.25, 0.625 and you quickly see that your 50% off isn't making nearly the difference it did in the beginning but with insulation it's costing you time, money, and space. With insulation you hit a point of diminishing returns around R12 before you have to start making other compromises to gain more and those gains start doing very little.
<snip>

R value is a linear measure of thermal transfer. R6 is half the resistance to thermal transfer of R12, and R12 is half the resistance to thermal transfer of R24. R49 is two percent less resistant to thermal transfer as R50.

So in your example, R2 to R4 is double the value, but R4 to R6 is 1.5 times the value. Maybe I'm missing your point, but I think that's how the math works.

Perhaps you are referring to the total cost of the insulation, the bang for the buck, in which case yes, the labor is constant so the less R value you install the more expensive per...well, maybe the less overall (life cycle) value you get in return.

Agreed?

[TucsonAZ]

Originally Posted by Ronnie

I have to wonder why you dismiss fiberglass so quickly, it has the same r value per inch (3.7 per inch) as rock wool, no chemicals, nothing toxic about it, and as far as getting wet it does not hold moisture. So seems to be even with rock wool.



If you prefer rockwool, that is fine but you leave the impression fiberglass is worthless

I don't think it's worse, they have different qualities and I think for a bus build it's easier to work with the RW in terms of patching in pieces and so on. I also think it's safer in general.

Originally Posted by Rucker

I love the analysis, even if I might quibble with some of the details.

THIS is how we learn as a community, so thanks for the detailed write-up.

On to quibbling:


R value is a linear measure of thermal transfer. R6 is half the resistance to thermal transfer of R12, and R12 is half the resistance to thermal transfer of R24. R49 is two percent less resistant to thermal transfer as R50.

So in your example, R2 to R4 is double the value, but R4 to R6 is 1.5 times the value. Maybe I'm missing your point, but I think that's how the math works.

Perhaps you are referring to the total cost of the insulation, the bang for the buck, in which case yes, the labor is constant so the less R value you install the more expensive per...well, maybe the less overall (life cycle) value you get in return.

Agreed?

Yes, it's like lumen, aperture, and so many other things where you have to essentially double it to see a noticeable difference. I think we're saying the same thing, I'll post a picture I found so others have a visual. I'm basically saying that by R-12 you've gotten the most gains you're going to get given the effort and cost involved. This is even more apparent in a bus build where more may require a roof raise, more framing and so on. When you're at R-12 going to say R-14 is of course a gain but not a "substantial" one.


In short, I'm saying add as much as you can with the least effort, if you're at R-12 by the end of that you should be good. At that point you can put the effort and money into higher SEER, roof coatings or something else.



I'm pretty sure I clarified nothing so I'll just post the picture lol

[rockinmeow]

How'd it turn out?

[TucsonAZ]

Originally Posted by rockinmeow

How'd it turn out?

It's a Skoolie bus, as I guess to be expected major delays on my end so I should be starting on that soon. I had plan changes with the build design and then the monsoons here so I'm waiting till I have slightly more stable weather. I will get too it fairly soon though.

[rockinmeow]

Originally Posted by TucsonAZ

It's a Skoolie bus, as I guess to be expected major delays on my end so I should be starting on that soon. I had plan changes with the build design and then the monsoons here so I'm waiting till I have slightly more stable weather. I will get too it fairly soon though.

I feel ya there, I’m a neighbor in Phoenix who needs to give a 32’ t444e some time shared. Stay excited for what the future has in store.

[TheHubbardBus]

We used rockwool in our build, in conjunction with other products. I'm happy with the choice, but we did use specialty batts (rockwool AFB... "acoustic fire batt") designed for sound insulation & fire proofing, which were very dense, very thin, but still flexible.