Putting the metal ceiling back up after insulating?

[Ronnie-SKO]

You should realize that whether you use a rubber or wood spacer the sheet is going to come down further on the sides than original so centering it should probably be a priority so that the ends wind up looking the same on both sides (consider 2 concentric arcs/circles - the outer arc is longer so when you take the metal from the outer arc and put it in the inner arc the extra length will need to go down the 2 sides unless you trim the sheet). I like the idea of fastening the spacer to the ribs, then fastening the metal to the spacer. Sounds like it would be easier, more flexible, and still provide the thermal break. Much better than the plastic rivet idea. The rubber would match the curve better then wood, but will it hold a screw if you screw the metal to the rubber?


If the ribs are hat channel, then the foam is probably not inside of them, however there may be some fiberglass insulation inside of them from the factory. Personally I would find an adhesive/caulk for the thermal break (maybe glue a thin strip of rubber to the rib) and apply the metal over the adhesive/caulk/rubber and be done with it. The ribs are going to cause a weak place in the insulation no matter what you do. Anything separating the broad face of the hat channel making a metal to metal contact with the ceiling sheet is going to help.



It would be interesting to see a thermal image (FLIR?) comparison of different techniques to see what you actually get for all of the extra cost and work of different methods.

I do not have a thermal image but I have used an infrared thermometer and my ribs showed 10 degrees cooler then the surrounding sheet metal. This extended about an inch to each side of the rib. Measured when it was in the 20's outside, and 65 inside.

I have to say if I did anything with roof insulation I would add 2" rigid foam board to the inside, and put wood over that. So it would be from the outside in, outer metal roof, fiberglass 2", metal ceiling, foam board, wood ceiling.

I do have a high roof so there is room to do this for me.

 

[EastCoastCB]

I think what Kidharris is saying is the body is built as a complete unit able in it's self to hold up in an accident, hence "unibody". He does say mounted on a frame, the bus chassis. Considering that the body is meant to slide on the chassis in a head on collision, this makes sense. It may not be the best choice of words however not sure how to say it better.

In a roll over a bus stays in one piece, in a roll over a motorhome gets flattened. I am sure that removing the metal ceiling will have little or no effect for normal driving. However in a roll over then it does come into play as a structeral member. I hope to never roll over. However one of the reasons I want a bus and not motorhome is strength, and durability.

Bluebird's Wanderlodge is a school bus body with half as many ribs and a cardboard ceiling and they're still WAY overbuilt for the purpose.

 

[Warthog-SKO]

Why? It ups the insulation R-value hugely without all the hassle of dismantling. No, it isn't "as good" as the more extensive measures, but...........it is one heck of a lot better than doing nothing, and a heck of a lot less work.

 

[EastCoastCB]

Why? It ups the insulation R-value hugely without all the hassle of dismantling. No, it isn't "as good" as the more extensive measures, but...........it is one heck of a lot better than doing nothing, and a heck of a lot less work.

If you're just filling the steel with foam its not really doing much to insulate. Better for noise insulation than thermal.

https://tinyhousedesign.com/how-to-defeat-thermal-bridging/

https://www.bautexsystems.com/blog/how-to-reduce-thermal-bridging

https://www.rhinobldg.com/beating-thermal-bridging-in-metal-buildings/

 

[Warthog-SKO]

I've a PhD in chemistry....I understand thermal bridging quite well, thank you.

Insulating the air space with a foam is still drastically increasing the R-value by cutting out convection and radiation heat transfer across and in the air gap between the inner and outer steel walls. All that remains is transfer by direct conduction in the thin metal strips that comprise the spacer beams.

Yes, that conduction is non-zero, but far less than the original total.

 

[EastCoastCB]

I've a PhD in chemistry....I understand thermal bridging quite well, thank you.

Insulating the air space with a foam is still drastically increasing the R-value by cutting out convection and radiation heat transfer across and in the air gap between the inner and outer steel walls. All that remains is transfer by direct conduction in the thin metal strips that comprise the spacer beams.

Yes, that conduction is non-zero, but far less than the original total.

Do you have a phd is building buses?

I don't but I know that this is an exercise in futility but skoolie is all about reinventing the wheel so please- drill a million holes into your metal headliner and have it professionally foam injected... No one's gonna stop you.
Happy Turkey Day!

 

[Ronnie-SKO]

Do you have a phd is building buses?

I don't but I know that this is an exercise in futility but skoolie is all about reinventing the wheel so please- drill a million holes into your metal headliner and have it professionally foam injected... No one's gonna stop you.
Happy Turkey Day!

Oh my I can not help but laugh thinking about drilling a million holes in the roof.

as a boat builder I have foam filled plenty, and would no way I would try to foam fill the ceiling, really great risk of bulging the sheet metal, and also having cavities unfilled.

Not saying it should not be insulated, just foam filling would be the least desirable way to do it from my experience with foam filling.

 

[banman]

I've been wondering... so I might as well invite the abuse on this thread (I think the OP's original Q has been long answered...)

Why not use fiberglass bat instead of ridged foam board like the factory does?
Fiberglass is much cheaper, and lighter. The thermal bridging issue of the ribs is a separate issue -- I'm just wondering why no-one likes the fiberglass bat?

Happy Thanksgiving!

 

[Ronnie-SKO]

Because it is written in the skoolie rules we must tear down our ceilings...

Ok having said that in good humor this is something that one must decide if it is of value to each person. My ceiling has 2" fiberglass batts in good shape. I needed to take down a few ceiling panels for rooftop a/c so that gave me a chance to see what was in there. I kept the original and closed it back up.

If I found no insulation, then foam board, or spray foam does give slightly better R value. Although it is a lot more work to put in. So my walls I did use fiberglass batt because it is soo much easyer, and the r value difference for my use is just not worth it.

 

[EastCoastCB]

I've been wondering... so I might as well invite the abuse on this thread (I think the OP's original Q has been long answered...)

Why not use fiberglass bat instead of ridged foam board like the factory does?
Fiberglass is much cheaper, and lighter. The thermal bridging issue of the ribs is a separate issue -- I'm just wondering why no-one likes the fiberglass bat?

Happy Thanksgiving!

There really hasn't been any "abuse".

 

[EastCoastCB]

Because it is written in the skoolie rules we must tear down our ceilings...

Ok having said that in good humor this is something that one must decide if it is of value to each person. My ceiling has 2" fiberglass batts in good shape. I needed to take down a few ceiling panels for rooftop a/c so that gave me a chance to see what was in there. I kept the original and closed it back up.

If I found no insulation, then foam board, or spray foam does give slightly better R value. Although it is a lot more work to put in. So my walls I did use fiberglass batt because it is soo much easyer, and the r value difference for my use is just not worth it.

I'm leaving my metal headliner for now. But have zero plans to live in the bus.

 

[banman]

There really hasn't been any "abuse".

I'm leaving my metal headliner for now. But have zero plans to live in the bus.

This statement/Q should probably go to my build thread but...
My metal ceiling will come down because I want to inspect for leaks/rust/mold/vermin...? all the things... *and then I'll probably use all that metal to patch my floor which I know is gonna need some attention...

Won't be living in the bus -- week long trips at most... LOW budget "toy" but with a classy bar room within.

Fiberglass bat covered with ¼" bead-board paneling to buffer the thermal bridging a bit... Possibly cover the bat with ¼" foam board to act as vapor barrier and improve the thermal bridging reduction -- I don't want to loose much ceiling height.

Sounds like I have my answer from Ronnie.

 

[brokedown-SKO]

That fiberglass bat has an advertised R per inch value when installed according to the guidelines. Typically this means in a 4 inch deep area, with an R value around 3.2 per inch.

We have around 1.75 inches to work with. Fiberglass gets it's insulation value from the air trapped inside it, if you smoosh it you lose effectiveness fast. Rigid foam has an R per inch value around 5 without requiring a bigger space.

 

[Warthog-SKO]

And what actual DATA do you have of the SPECIFIC instance of putting foam in a bus ceiling that proves said futility???

Not handwaving theoretical non-specifics largely unrelated to the case in point. Has anyone here actually done it?? My understanding of the heat transfers involved says it would be beneficial.

If I can get 90% of the benefit with 10% of the effort, I would think it worthwhile.

But Happy Thanksgiving anyway.

 

[brokedown-SKO]

If I can get 90% of the benefit with 10% of the effort, I would think it worthwhile.

Maybe near 50% of the insulation factor, if your insulation is in good shape and your roof hasn't leaked. The fiberglass stuff stars out yellow of pink but it's very common to be black and wet when it comes out. Wet fiberglass doesn't insulate, and trapped moisture will mold the paper backing quick... And if you're installing a thermal break it's going to more or less fall out in the process anyway. In my mind, removing the ceiling panels makes replacing that insulation an obvious step not worth skipping.

 

[banman]

And what actual DATA do you have of the SPECIFIC instance of putting foam in a bus ceiling that proves said futility???

Not handwaving theoretical non-specifics largely unrelated to the case in point. Has anyone here actually done it?? My understanding of the heat transfers involved says it would be beneficial.

If I can get 90% of the benefit with 10% of the effort, I would think it worthwhile.

But Happy Thanksgiving anyway.

No data but -- IF the bus manufacturer's bothered to add fiberglass bat w/out addressing the thermal bridging -- it's worth doing. NO MANUFACTURER ever added unnecessary steps or parts to their product because they were feeling generous in their design builds, etc... so if the fiberglass bat was good -- spray foam would be better...
(at least not in a capitalist economy)
Therefore -- sprayfoam would of course be a benefit. I think you'd first have to remove the metal paneling to remove the fiberglass bat for the foam to be able to spread into the space though...

 

[o1marc]

I do not have a thermal image but I have used an infrared thermometer and my ribs showed 10 degrees cooler then the surrounding sheet metal. This extended about an inch to each side of the rib. Measured when it was in the 20's outside, and 65 inside.

I have to say if I did anything with roof insulation I would add 2" rigid foam board to the inside, and put wood over that. So it would be from the outside in, outer metal roof, fiberglass 2", metal ceiling, foam board, wood ceiling.

I do have a high roof so there is room to do this for me.

R13 batts (3-1/2")compressed to 1-1/2" drops it to R7, equivalent to rigid board @7.5.
They say compressing it gains R value, but the loss in thickness reduces the R value. I wonder what the percentage is. Cost difference is around $.40sf for batt as opposed to around $.62sf for rigid

 

[banman]

That fiberglass bat has an advertised R per inch value when installed according to the guidelines. Typically this means in a 4 inch deep area, with an R value around 3.2 per inch.

We have around 1.75 inches to work with. Fiberglass gets it's insulation value from the air trapped inside it, if you smoosh it you lose effectiveness fast. Rigid foam has an R per inch value around 5 without requiring a bigger space.

Ageed that if you "squish" compress the fiberglass bat you greatly diminish it's value. I didn't think the R value per inch was that low on the fiberglass though.
I know there's a "high density" fiberglass bat with a higher R rating but depending on cost if it approaches the foam than why bother with it...

The foam board will certainly be easier to deal with when future maintenance issues occur.

There is also fiberglass bat with plastic cover on both sides for vapor barrier and dust free applications but again with an increase in cost -- haven't priced it out yet...

 

[musigenesis]

NO MANUFACTURER ever added unnecessary steps or parts to their product because they were feeling generous

Somebody posted something awhile back that said buses were factory specced to have insulation worth a 20-degree difference from the outside temperature. Adequate in a running bus with industrial-strength heating and AC but nowhere near enough to make living in it comfortable.

 

[brokedown-SKO]

Ageed that if you "squish" compress the fiberglass bat you greatly diminish it's value. I didn't think the R value per inch was that low on the fiberglass though.

The stuff they sell at the hardware store is R11 or R13 total, for a 4 inch space. They also have R19, which requires a 6 inch space... etc