Polyiso vs. Rockwool: Which is Better for Your Project? - Rmax

As building codes change and insulation options diversify, designers are left with big decisions when it comes to choosing the right building materials for maximum thermal performance. In this article, we compare two exterior insulation options: polyiso andvs Rockwool.

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Exterior Insulation Options

Exterior insulation comes in many forms, but for this article, we’ll focus on those that are available in board form since it is a common application. Board insulation typically comes in modular sizes such as:

• 2’ x 4’

• 4’ x 4’

• 4’ x 6’

• 4’ x 8’

The thickness of the boards can range between one and five inches in order to achieve the design’s desired R- value (thermal performance). Common board insulation types available today include: mineral wool, polyiso, and polystyrene.

Mineral Wool Insulation

Mineral wool insulation is a broad category of insulation that is made from spinning mineral fibers together. The mineral fibers can include various types of rocks, minerals, and slag waste. Stone wool is one type of mineral wool insulation (made from volcanic rock like basalt). Rockwool is a brand name for one type of stone wool insulation.

Polyiso Insulation

Polyiso insulation is a thermoset, closed-cell foam insulation made from polyisocyanurate foam core that is bonded to various facer materials. Facers can range from cellulose-based paper to aluminum foil facers. Facer material can enhance the properties of the insulation.

Comparing Rockwool and Polyiso

When comparing polyiso to Rockwool, there are pros and cons to each type of building insulation. Let’s do a side-by-side comparison of polyiso and Rockwool.

Similarities

The installation methods and applications for polyiso and mineral wool insulation are similar. Both are available in board form and are installed in a modular fashion by securing boards to the substrate with fasteners and oversized washers. Both materials require finishing, such as exterior cladding or interior drywall. Rockwool and polyiso are both fire-resistant.

Want more information on Rock Wool Insulation Board? Feel free to contact us.

Installation

While the installation methods for polyiso and Rockwool boards are similar, the handling of each material differs. Polyiso can be scored and snapped to cut pieces to fit, while mineral wool requires a serrated knife to cut through the entire length of boards.

Thermal Performance

Mineral wool carries a lower R-value than polyiso. That means to achieve an R13, you’ll need at least 3-½” of Rockwool, while just 2-¼” of polyiso can achieve the same thermal performance. Polyiso’s higher R-value per inch can reduce labor and material costs for large projects.

Air Barrier

Rockwool is a permeable material, meaning air and vapor can travel through it. If an air barrier is required, it must be installed as a completely separate product. Polyiso, on the other hand, is available in versions with varying permeability. For example, Rmax’s EcoMAXci® FR Air Barrier is a foil-faced polyiso that can be installed as an air barrier when joints are sealed.

Weight

Mineral wool is four times heavier than polyiso. Mineral wool insulation adds significant dead loads to your structure. The added weight also slows down installation, and requiresrequiring multiple workers instead of one to install each board.

Acoustic Performance

Acoustic performance is an unexpected benefit of using mineral wool insulation. It is one of the best products to use for sound dampening and sound absorption.

Applications

Mineral wool is typically only installed on walls because of its weight, while polyiso can be used for the entire building envelope, from foundations up to the rooftops.

Environmental Impact

Mineral wool can’t be recycled or reused on future projects, while it’s very common to repurpose polyiso insulation for use inside future buildings. The manufacturing process of mineral wool also requires significantly more energy to produce than polyiso.

Cost

Mineral wool is more expensive per board foot than polyiso. On top of that, it requires thicker layers of mineral wool to achieve the same R-value since it has a lower R-value per inch than polyiso. This drives the price up even further, along with increased labor costs due to the heavier weight of Rockwool.

Polyiso: The Superior Choice

Unless you’re looking for sound dampening, polyiso is the clear winner when comparing polyiso to Rockwool. Polyiso costs less, weighs less, has better thermal performance, and is more versatile than mineral wool. Whether insulating concrete slabs, foundations, crawl spaces, walls, or roofs, polyiso is a superior building insulation suitable for many applications.

Use Rmax Polyiso For the Job

When it comes to making the call on your building materials, it helps to get a second opinion from those who have been in the industry for decades. Rmax has been perfecting polyiso since and has experts on staff to help you make the right call.

Reach out to your local Rmax representative to see how they can help you today!

If you want to learn more, please visit our website Rock Wool Insulation Pipe.

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 ( :

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.

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

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