To Foam or not to Foam?

[Roadie]

I have been all set to spray foam my bus and then I've started to consider an alternative. Has anyone used solid board insulation in the walls of their bus? I have 30"s between the hat channels by about 6' tall My intention would be to cut 2"X 30"X 6' sections of foam and use construction glue to adhere them to the inside walls any gaps could be filled with a Great stuff type gap filler. This would be far more economical and I think have a higher R value, R10, than if I have a professional spray it. I would like to hear for and against opinions of insulating this way.

[o1marc]

You pretty much have it figured out. The foam is nice, but messy for a DIY, and expensive. The hat channels will accept 1-1/2" of rigid flush.That is most often used when budget is considered.

[peakbus]

Seems like a great plan. I used rigid foam under the windows on my fist bus for the same reasons. Save $ there and spray the curvy parts.

[musigenesis]

Spray foam and foam board will have the same R-value per inch (assuming they're the same substance, either polystyrene or polyiso). Polyiso (either board or spray foam) has a nominally higher R-value than polystyrene (R-6.5 vs. R-5) but this is only true in hot weather; in cold weather polyiso has the same R-value (or lower) as polystyrene. Most spray foam kits are polystyrene (although polyiso is available).

The big advantages of spray foam are that if properly applied it creates an excellent moisture barrier, and it's easy to handle the curvature of the ceiling. For fitting foam board into the rectangular spaces between the ribs (on the walls), you really shouldn't have any gaps to worry about. Just cut the piece very slightly larger than the opening and push it in.

[WhatBus]

I haven't used either in a bus, in fact I have yet to buy a bus. So everything I say is either theoricial (as applied to a bus), or learned from other circumstances. Also of note, in regular house insulation, every time I have considered rigid foam vs spray foam, I have chosen rigid foam board with a few extra air sealing details.

First, with rigid foam, do not use any old construction adhesive ... some of them are incompatible with/will dissolve foam board. Look for one that is listed as compatible, or better yet use a specific board board adhesive.

Second, rigid foam can be the same thing as 2-part closed cell spray foam and will have the same R-value, so long as they are the same material.

Third, the plusses, as I seem them, for spray foam are air sealing, contour following and noise reduction; rigid foam is not the greatest noise blocker, but spray foam adheres to almost everything and will add stiffness locking everything in place and reduce the chances of a rattle.

Fourth, the cons of spray foam, again as I see them, are cost, mess, and skill level required.

In my future, hypothetical bus, I think I have narrowed it down to one of two options:

1) What I have heard called flash-and-batt. This is where a thin layer of 2-part spray foam is used as an air sealing layer, then a traditional batt insulation is used. Mineral wool interests me, while it is a lower R-value it is less prone to mold when exposed to moisture (liquid or humidity) and it is better at addressing sounds from outside.

or

2) Used rigid foam as the primary insulation. This would have three special considerations for me (all learned while researching how to properly insulate my basement from the inside).

A) Ensure that the rigid foam is thick enough for my climate zone that the "condensation plain" (the location within the assembly where the temperature is such that condensation would occur) is within the rigid foam and not on either face. (For my basement, which is the last place I used rigid foam, this was in the 1.5 to 2 inch range, I forget the exact point).

B) A healthy bead of foam board adhesive around the perimeter for the board. This is an air sealing step that prevents air from circulating behind the board. I am in a heating dominated climate and this circulation will result in condensation behind the insulation and mold growth (in a bus I suppose rust is a threat).

C) 2-part spray foam all penetrations through the rigid foam and joints and edges of the rigid foam. This is also an air sealing step, preventing air from getting behind and between the boards and condensing.

D-Bonus) If, multiple layers of foam board are used, say a 1.5 between structural elements and a 1/2" over the frame elements (to somewhat address thermal bridging), stagger the seams so that they do not overlap as much as possible.

[JackE]

I was recently in the attic of several new construction homes. I was a little surprised at the method of insulation chosen. All of them had about 18" of blown in insulation on the top side of the ceiling, but bare rafters. Each one did have a thermostatically controlled vent fans. The most recent one I was in was in southern Utah when it was about 110* outside. Surprisingly, the attic area was cooler than the garage where I climbed the ladder to gain access to the attic. This got me thinking about my plans for my bus.

I am doing a roof raise. I am planning on going with a flat ceiling, vaulted side look which will leave some room for ducting the roof top A/C units, as well as plenty of space for running wire front to back, all while eliminating the need to make a curved ceiling. Any thoughts on using the house construction idea of insulating the ceiling, but not the roof, and installing some kind of vent fan to exhaust the heated air in-between the two?

[o1marc]

Originally Posted by JackE

I was recently in the attic of several new construction homes. I was a little surprised at the method of insulation chosen. All of them had about 18" of blown in insulation on the top side of the ceiling, but bare rafters. Each one did have a thermostatically controlled vent fans. The most recent one I was in was in southern Utah when it was about 110* outside. Surprisingly, the attic area was cooler than the garage where I climbed the ladder to gain access to the attic. This got me thinking about my plans for my bus.

I am doing a roof raise. I am planning on going with a flat ceiling, vaulted side look which will leave some room for ducting the roof top A/C units, as well as plenty of space for running wire front to back, all while eliminating the need to make a curved ceiling. Any thoughts on using the house construction idea of insulating the ceiling, but not the roof, and installing some kind of vent fan to exhaust the heated air in-between the two?

Definitely doable with a roof raise. Vent in front and rear with a fan.

[waxman]

i used foam board for 90% of my insulation (r tech brand) my hat channels are 1.75 inches deep so i used two layers of 1/2 and one 3/4. if you use thinner sheets they flex really nice and you can match the ceiling curve rather easily. put the thicker layer in first as the curve is tighter as you move in. cut your pieces 1/8-3/16 oversized so you can friction fit your foam board this will cut down on air gaps i tried a couple different spray adhesives and found 3m supper 77 to work the best and not dissolve the foam if it contacted where there is no foil or plastic backing