Air Gaps and Insulation Questions

[bachataBus]

My wife and I are converting a shortie 1997 International. We're nearing the insulation stage, and are currently planning on doing spray foam. We're planning on doing 2 inches of spray foam on the ceiling/walls.

Given that the hat channels are about 1.5-1.75 inches tall, even if we only have 1/2 inch of strapping that leaves a 1/4 inch gap between the spray foam and the eventual ceiling/walls. I realize that air gaps *are* the insulation, but is there a downside to having these empty gaps? Noise? Structure? Can we have 1 inch of strapping (two peices of 1/2" ply for thermal break) and have a 3/4 inch air gap?

:Thanx:

 

[musigenesis]

Air gaps are not the insulation. The foam is the insulation. You do not want any air gaps at all.

 

[CHEESE_WAGON]

Air gaps are not the insulation. The foam is the insulation. You do not want any air gaps at all.

​

Air gaps will promote condensation and lead to mold and mildew, which you do not want.

 

[waxman]

he is asking about an air pocket between the foam and the wall. typical in residential construction with insulation that has a radiant barrier you need the air gap typically 1/2-3/4" to prevent conductive heat transfer.

https://atticfoil.com/index.php/about-radiant-barrier/why-is-an-air-space-required/

I insulated in between hat channel with r tech sheets, great stuff in the ribs, then spray foam on the front and back fiberglass caps, then fur everything out with treated wood I used Valspar one coat stain and sealer deck seal(idk why but the white base works great and the neutral base is shite) the cans look exactly the same and I wasn't paying attention. once I run my remaining 12v ill be putting my walls up with 1/2 spacing between the wood and foam due to furring strips

 

[musigenesis]

he is asking about an air pocket between the foam and the wall. typical in residential construction with insulation that has a radiant barrier you need the air gap typically 1/2-3/4" to prevent conductive heat transfer.

There's no radiant barrier involved in spray foam. OP is talking about a gap between the spray foam and the inner paneling, which would serve no purpose other than to allow heat transfer via convection and defeat much of the value of the spray foam.

 

[bachataBus]

To be clear, let's take the sides of the bus as an example. I would have the sheet metal of the bus, then the spray foam, then potentially an air gap, then the plywood/whatever wall. In home construction it's not uncommon to have large wall joists (2x6, for example), then only 3-4 inches of foam, with an air gap to the drywall/sheetrock. Is this a different issue in a bus than in a residential construction? I may also have no understanding of the brief literature in residential construction I have poked through!

 

[o1marc]

To be clear, let's take the sides of the bus as an example. I would have the sheet metal of the bus, then the spray foam, then potentially an air gap, then the plywood/whatever wall. In home construction it's not uncommon to have large wall joists (2x6, for example), then only 3-4 inches of foam, with an air gap to the drywall/sheetrock. Is this a different issue in a bus than in a residential construction? I may also have no understanding of the brief literature in residential construction I have poked through!

Every house I've ever built or worked on, if it had a 2 x 6 wall, it got 6" insulation. Never 4" wall insulation in a 6" wall. There's no airspace between the batt and the inner and outer cover.

 

[musigenesis]

To be clear, let's take the sides of the bus as an example. I would have the sheet metal of the bus, then the spray foam, then potentially an air gap, then the plywood/whatever wall. In home construction it's not uncommon to have large wall joists (2x6, for example), then only 3-4 inches of foam, with an air gap to the drywall/sheetrock. Is this a different issue in a bus than in a residential construction? I may also have no understanding of the brief literature in residential construction I have poked through!

I'd have to read whatever you read to respond to it properly, but generally "air gap" refers to the space needed for a reflective radiant barrier to work properly, as waxman mentioned. Apart from that situation (and even in that situation, really), in any kind wall insulation (of whatever type) in a space is going to be more effective at slowing heat transfer than a free air pocket would be.

So in your case, just spray the foam flush with the strapping and your insulation will be as good as it can be.

 

[bachataBus]

It is my understanding that, given the air-barrier properties and insulation properties of spray foam, it is not required to fill the entire cavity. [source].

 

[bachataBus]

I'd have to read whatever you read to respond to it properly, but generally "air gap" refers to the space needed for a reflective radiant barrier to work properly, as waxman mentioned. Apart from that situation (and even in that situation, really), in any kind wall insulation (of whatever type) in a space is going to be more effective at slowing heat transfer than a free air pocket would be.

I grant that it would be more effecient to fill the air gap with more spray foam, but the reason that we're going to two inches is cause they really only sell the kits in 600 bf or 200 bf, and they're expensive as all heck. With 600bf we can get the walls/ceiling to two inches, but I feel like we should have at least 1/2 inch framing to attach the ceiling to. I don't know how I feel about a ceiling/walls attached to a 1/4 inch of framing...

 

[musigenesis]

I grant that it would be more effecient to fill the air gap with more spray foam, but the reason that we're going to two inches is cause they really only sell the kits in 600 bf or 200 bf, and they're expensive as all heck. With 600bf we can get the walls/ceiling to two inches, but I feel like we should have at least 1/2 inch framing to attach the ceiling to. I don't know how I feel about a ceiling/walls attached to a 1/4 inch of framing...

Have you done spray foaming before? I feel like you're going to have some difficulty getting a layer to be exactly 2" thick. Generally, people try to spray it so it comes out a bit extra-thick and then they shave it flush with their studs or strapping (whatever the paneling is going to be attached to).

 

[bachataBus]

Have you done spray foaming before? I feel like you're going to have some difficulty getting a layer to be exactly 2" thick. Generally, people try to spray it so it comes out a bit extra-thick and then they shave it flush with their studs or strapping (whatever the paneling is going to be attached to).

I have not spray foamed before, but then again, I haven't ever done any of this before! I suppose I was planning on spraying very conservatively over the whole bus, then going back and trying to do that again, until I run out of spray foam. Not very scientific, and I am talking to a couple of contractors to see if we can get it professionally done, but that's what we've got. We don't really have the money for a ton of extra foam.

 

[musigenesis]

I have not spray foamed before, but then again, I haven't ever done any of this before! I suppose I was planning on spraying very conservatively over the whole bus, then going back and trying to do that again, until I run out of spray foam. Not very scientific, and I am talking to a couple of contractors to see if we can get it professionally done, but that's what we've got. We don't really have the money for a ton of extra foam.

FWIW I'm going with XPS foam board, specifically because a) spray foam is so expensive and b) I rarely do anything right the first time. If I were going to use spray foam, I would only use it on the ceiling because it can handle the curvature of the ceiling so well; I might even use it only on the corners since the middle part of the ceiling is relatively flat and you can bend XPS to fit it. The flat parts like the walls and floors are very easy to cut and fit rectangular pieces for.

The insulating value is the same either way; the big advantage of spray foam is that (if done correctly) you also get an excellent vapor barrier out of it.

 

[bachataBus]

FWIW I'm going with XPS foam board, specifically because a) spray foam is so expensive and b) I rarely do anything right the first time. If I were going to use spray foam, I would only use it on the ceiling because it can handle the curvature of the ceiling so well; I might even use it only on the corners since the middle part of the ceiling is relatively flat and you can bend XPS to fit it. The flat parts like the walls and floors are very easy to cut and fit rectangular pieces for.

The insulating value is the same either way; the big advantage of spray foam is that (if done correctly) you also get an excellent vapor barrier out of it.

Yah, that is totally reasonable. I'm also hoping that the foam will help with road noise/wind noise/air leaks. It is totally airtight, if applied correctly. Perhaps I'm hoping for too much from the foam...

 

[o1marc]

How big is the bus you are foaming and how many windows will it have? Go to the UserCP and fill out your profile with your location and bus info. Helps address your concerns.

 

[Native-SKO]

600 BF is 600 sq ft at 1" thickness.


You are saying that you can cover your walls and ceiling with 600 BF. This means you have 300 sq ft to cover with the insulation. In rough measure a typical school bus is 6' high (inside) and 8' across the ceiling. This is 20 sq ft per ft.


Thus, you must have a 15 foot area to cover ... making it a 5 or 6 window bus?


To answer your concern about furring strips ... you can always attach 2x2's to the sides of the hat channel and have the foam sprayed right out to the edges of the furring strips (wall studs). This will give you a thermal break since the wood is attached to the sides and thus making no thermal bridges. You would also get about 1/2 inch of insulation on the edges of the hat channels for good isolation as well.

 

[bachataBus]

Thus, you must have a 15 foot area to cover ... making it a 5 or 6 window bus?

Woah, Native with the quick deductions. Yah, 5 window. Did the math today, I found it's about 240 sqft. Did you guys insulate your doors (rear door, handicap door, etc)? At ~250sqft, I have a touch of slack at 2 inch depth with a 600bf kit.

 

[Rucker]

Yah, that is totally reasonable. I'm also hoping that the foam will help with road noise/wind noise/air leaks. It is totally airtight, if applied correctly. Perhaps I'm hoping for too much from the foam...

A radiant barrier only works OUTSIDE the conditioned space and is for heat, not cold. It simply blocks and to some degree reflects the heat back/away from the living space. It is always installed in the unconditioned space of an attic, not within the building envelope. In the skoolie world you accomplish this with Tropicool which is applied to the exterior skin of the bus. Or you can park under a tree. :-')

If you plan to operate your bus in outside temperatures below 45 degrees Fahrenheit you must plan for condensation. You are absolutely correct that a thermal barrier is needed over the ribs because colder conditions outside will conduct through that direct-contact metal, dropping the humid air inside below the dew point, resulting in condensation on those surfaces.

You do not want any air gaps between your finished walls and ceiling of the bus and the exterior skin of the bus. The humidity of the air inside the bus is extremely variable and will frequently hit the dew point on windows, metal surfaces, anything that has high conductivity to outside air.

You can't absolutely prevent condensation, but thermal barriers (and no air gaps) reduce the chances of it occurring behind your finished interior surfaces.

A bit of a lecture, I know. Just trying to build the knowledgebase a bit so we are more equipped to build a better bus.

 

[GreenGoddess]

Ispray foam nsulatiom

I just spray foamed the inside of my short School Bus. It was an enormous job. It is not like spray foaming in a cavity.
The foam melted if the metal was hot, and dripped down from the ceiling.
I filled the large spaces with solid foam insulation, and spray foam around the edges but that was also hugely difficult on the ceiling because of the Curve

Although the insulation properties are superior, I will never do that again

 

[GreenGoddess]

Ispray foam nsulatiom

I just spray foamed the inside of my short School Bus. It was an enormous job. It is not like spray foaming in a cavity.
The phone melted if the metal was hot, and would not stick to the ceiling as it is a curved surface.
I filled the large spaces with solid foam insulation, and spray foam around the edges but that was also hugely difficult on the ceiling because of the Curve

Although the insulation properties are superior, I will never do that again