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Old 12-07-2020, 09:24 PM   #1
Skoolie
 
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Insulating with Polyiso board, need guidance

I originally wanted to DIY closed cell spray foam, but the weather turned cold too quickly on us, so we pivoted and have been laying in layers of 1/2" polyiso board that we can press-bend into the ceiling contours. We are 3 layers deep now between the ribs. Next step will be adding 1"-ish furring strips front to back, and then adding more polyiso between them.

In addition to all of this, we're injecting foam insulation into the hat channel ribs (I found that it did add a couple degrees of warmth versus unfilled, using a contact thermocouple on a treated rib and an untreated rib) and we're going to add a thin layer of insulating tape to each rib before adding the furring strips. I should also note that there will be some small gaps between the roof and the first sheet of polyiso board. We used dabs of adhesive, but just enough to hold the board, not enough to seal with full contact.

Finally, we're going to attach thin plywood panels to the furring strips. There will be a thermal bridge from the ribs to the screw heads attaching the furring strips, but those will be covered by the plywood.

So what's the question? Condensation!!!!! Should I seal up all of the seams with aluminum tape before we get to that final layer of furring strips and polyiso? Should we add a vapor barrier? Should we add vents to the roof between each pair of ribs?

I honestly don't know what the safest course of action is. we will be heating with diesel heaters, not propane. We plan on chasing better weather, but there will be times when we're in colder stuff for short stretches. I don't want to do something that I have to tear apart and redo, so I'm looking for some guidance, especially from those who have been down this road and have had a positive experience using rigid panels rather than spray insulation.

Thank you in advance. I know this is the best community to ask something like this in.
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Old 12-07-2020, 09:36 PM   #2
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Your foam fab work looks top-notch. I'd caulk or tape the seams in the foam to ensure that no warm moist air reaches the roof skin. The thermal bridges by the screws I wouldn't, myself, worry about, though they might sweat a bit.
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Old 12-07-2020, 09:44 PM   #3
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What adhesive are you using for the poly-iso? I sandwiched Poly-iso between the bus floor and my finished plywood and have a squeaky spot or two. Wondering what I should have used.
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Old 12-07-2020, 10:51 PM   #4
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Quote:
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What adhesive are you using for the poly-iso? I sandwiched Poly-iso between the bus floor and my finished plywood and have a squeaky spot or two. Wondering what I should have used.
For the most part we tried to fit the panels so that the ribs provided enough support for the panels to not need any adhesive. But when we did need to use adhesive, it was Loctite 0 sec. Instant grab Power Grab Molding & Paneling adhesive. The reason is that it's the stickiest of sticky stuff that I found. We can put it up, press a panel in, and it stays put.

For the floors, we're going to use Great Stuff Pro Series Construction Adhesive. It doesn't dry nearly as quickly, but it works with the Foamnseal FNS 850 dispensing gun. We're going to go without frame, just a layer of butyl underlayment, topped by insulating panels, thick plywood on that, then framing members attached to the plywood.
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Old 12-07-2020, 10:53 PM   #5
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Quote:
Originally Posted by bus-bro View Post
Your foam fab work looks top-notch. I'd caulk or tape the seams in the foam to ensure that no warm moist air reaches the roof skin. The thermal bridges by the screws I wouldn't, myself, worry about, though they might sweat a bit.
Much appreciated. After hours of searching through past posts, I did not see anything conclusive. Unless there is contrary advice, I will proceed with my current plan of sealing off the layer that comes up to the level of the ribs, then add another layer on top of that to build more barrier between the metal and the interior panels.
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Old 12-07-2020, 11:32 PM   #6
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Checking Home Depot's website... 1" is R-6, so I'm assuming 1/2" is about R-3, and you're talking about 4 layers making it R-12?
Quote:
Rmax Thermasheath-3 1 in. x 4 ft. x 8 ft. R-6
Sounds pretty good to me, without losing too much headroom

edit: added picture & Closure pdf file link https://usa.sika.com/dms/getdocument...structions.pdf
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Old 12-07-2020, 11:40 PM   #7
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Originally Posted by milkmania View Post
Checking Home Depot's website... 1" is R-6, so I'm assuming 1/2" is about R-3, and you're talking about 4 layers making it R-12?


Sounds pretty good to me, without losing too much headroom
The 1/2" is actually rated at 3.2, which I think has something to do with the outside surfaces being covered as well. So We'll end up with 3 layers of R 3.2 and 1 layer of R-6. But polyiso becomes less effective when it gets cold (don't ask me to explain this), so there is some debate as to whether it is an ideal insulator.

Luckily, we are rather short people, (I used to be 5'9, but age and gravity has knocked off a couple inches) my wife is 5'2-ish. We can afford the extra insulation without a roof raise.
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Old 12-07-2020, 11:43 PM   #8
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Quote:
Originally Posted by CoffeeGuy View Post
I originally wanted to DIY closed cell spray foam, but the weather turned cold too quickly on us, so we pivoted and have been laying in layers of 1/2" polyiso board that we can press-bend into the ceiling contours. We are 3 layers deep now between the ribs. Next step will be adding 1"-ish furring strips front to back, and then adding more polyiso between them.

In addition to all of this, we're injecting foam insulation into the hat channel ribs (I found that it did add a couple degrees of warmth versus unfilled, using a contact thermocouple on a treated rib and an untreated rib) and we're going to add a thin layer of insulating tape to each rib before adding the furring strips. I should also note that there will be some small gaps between the roof and the first sheet of polyiso board. We used dabs of adhesive, but just enough to hold the board, not enough to seal with full contact.

Finally, we're going to attach thin plywood panels to the furring strips. There will be a thermal bridge from the ribs to the screw heads attaching the furring strips, but those will be covered by the plywood.

So what's the question? Condensation!!!!! Should I seal up all of the seams with aluminum tape before we get to that final layer of furring strips and polyiso? Should we add a vapor barrier? Should we add vents to the roof between each pair of ribs?

I honestly don't know what the safest course of action is. we will be heating with diesel heaters, not propane. We plan on chasing better weather, but there will be times when we're in colder stuff for short stretches. I don't want to do something that I have to tear apart and redo, so I'm looking for some guidance, especially from those who have been down this road and have had a positive experience using rigid panels rather than spray insulation.

Thank you in advance. I know this is the best community to ask something like this in.
check out the pdf file here, read it closely as I think it addresses your question

https://usa.sika.com/dms/getdocument...structions.pdf

and it came from the link at the bottom of this page
https://www.rmax.com/thermasheath
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Old 12-08-2020, 12:03 AM   #9
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Quote:
Originally Posted by milkmania View Post
check out the pdf file here, read it closely as I think it addresses your question

https://usa.sika.com/dms/getdocument...structions.pdf

and it came from the link at the bottom of this page
https://www.rmax.com/thermasheath
This is pretty much what we are doing. We are staggering the seams, then my plan is to tape off the layer that is approximately level with the ribs using aluminum tape. We may tape off the gaps between the furring strips and additional panels spanning the gaps between the strips, but I'll have to see when we get that far if it seems necessary or overkill.

We are also taping off the panels on the walls where we're fitting in scraps so that we make the most economical use of the materials at hand.

I will confess that I invested in a thermal imaging camera to monitor our progress and effect, but without having heat inside the bus, it's difficult to visualize temperature differential. The camera will be more useful as we get closer to completion and I can also use it in my coffee roasting business for some experiments.
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Old 12-08-2020, 12:26 AM   #10
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Which thermal imaging camera? At what cost?
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Old 12-08-2020, 12:35 AM   #11
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Which thermal imaging camera? At what cost?
FLIR One Pro, around $300 off eBay.
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Old 12-08-2020, 12:44 AM   #12
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Quote:
Originally Posted by CoffeeGuy View Post
This is pretty much what we are doing. We are staggering the seams, then my plan is to tape off the layer that is approximately level with the ribs using aluminum tape. We may tape off the gaps between the furring strips and additional panels spanning the gaps between the strips, but I'll have to see when we get that far if it seems necessary or overkill.

We are also taping off the panels on the walls where we're fitting in scraps so that we make the most economical use of the materials at hand.

I will confess that I invested in a thermal imaging camera to monitor our progress and effect, but without having heat inside the bus, it's difficult to visualize temperature differential. The camera will be more useful as we get closer to completion and I can also use it in my coffee roasting business for some experiments.
Sounds like you're doing the best that can be expected, given the circumstances. And that's what it takes to be able to rest easy at night

Save some coffee for me
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Old 12-08-2020, 06:55 AM   #13
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Your insulation plan is solid, the second-best that can be done with a bus ceiling (the best would be solid insulation flush with the inside of the ribs (i.e. not packed in between the ribs), but that would cost more headroom, of course. I'm doing essentially the same thing as you, just with XPS. I've been considering an additional layer of 1/4" XPS between the furring strips and the ceiling panels, but that might be in the world of diminishing returns.

FYI polyiso board is nominally R 6.5 per inch, but this is only at 75F and warmer. Below that temperature, polyiso's R value starts to decline, such that for cold weather it's often rated similarly to EPS at around R 4 per inch and it continues to decline for really cold weather. This is an unusual physical characteristic for any material, since normally solids have lower thermal conductivities (and hence higher R-values) at lower temperatures (it's unknown why this is the case with polyiso; it's speculated that moisture within the tiny gas bubbles condenses at lower temperatures and allows more rapid transfer of heat). TANSTAAFL.
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Old 12-08-2020, 12:23 PM   #14
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TANSTAAFL... I had to look that one up.
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