Hey y’all, happy to be a part of this awesome community ! Just jumping into the skoolie life and seems there isn’t much detail out there for designing a proper assembly that is resilient against moisture and mold. I’m also seeing a LOT of folks using materials and methods that would trap moisture and are not accounting for any vapor transfer.
Excuse the length of this post, but I feel this topic is super important due to the apparent wide spread confusion of materials, vapor barriers, and the amount of work and money people are investing into this process. Would love to hear the input of other experienced builders (especially with bus experience) to come up with the best practice for insulating and sealing a skoolie. I’m an architect and builder (for regular homes) and have done a lot of research on building science and the thermodynamics of typical floor and wall assemblies. A bus is obviously a different ball game, having a painted steel shell (continuous vapor barrier) which throws a little more of a challenge as far as managing vapor transfer, mold and condensation than a conventional home. It’s also a massive thermal bridge, allowing heat or cold to transfer to the interior via conduction of the metal. This is a huge concern that seems to be overlooked in most bus builds, and most certainly needs to be addressed to avoid condensation and a comfortable interior.
For any floor or wall assembly, it’s important to maintain a clear path for vapor transfer to allow the wall or floor to dry out to both the interior and exterior (depending on the climate and season).
Considering this will be mobile, it’s hard to plan for a specific climate due to it always changing, so the more forgiving your assemblies are with vapor permeability, the better. In this type of situation (mixed climate with alternating paths of vapor drive) you would want to avoid any vapor barriers to allow the assembly to dry out in any direction. Obviously this isn’t possible for a bus since the painted metal vapor barrier is pre existing, but you most certainly don’t want to be adding additional vapor barriers within the assembly because you’re only option is to allow the assembly to dry toward the interior.
The most ideal situation would be to have exterior insulation to eliminate the dew point from occurring inside the assembly (and allowing condensation to form on the interior side of the metal), but this would be pretty hard to achieve while preserving an existing outer shell.
It seems the norm for these builds is to use rigid foam insulation (usually with a foil faced vapor barrier) directly over the metal floor and walls. This seems problematic because there is no venting behind the insulation, and does not allow the moisture to dry out when condensation is formed due to warm air reaching the cold metal surface. Not only is there not an air space for drying, your trapping the moisture between 2 vapor barriers (the foil faced foam and the painted metal). This will result in a build up of moisture causing mold, rot and rust. This problem can be even worse when batt or loose fill insulation is used, which would most certainly saturate the insulation with moisture and in a short time have a minimal resistance to heat transfer. (Batt insulation ALWAYS needs to have an air space to dry out if placed against an exterior vapor barrier, such as a roofing membrane or your painted metal skin)
Just like any regular home, the only way you can properly insulate a closed unvented assembly (with an exterior vapor barrier) and prevent the surface from reaching the dew point temperature and forming condensation is by using spray foam insulation directly over the interior side of the metal, or rigid foam on the exterior side. This will keep the dew point occurring in the middle of the insulation and can allow the vapor to dry out of the assembly.
So clearly some kind of spray foam insulation (I like airkrete because it is a sustainable foam with the same r value as petroleum based foams minus the off gasing) is pretty much the only option for walls and ceiling if you don’t want condensation forming inside your assembly. You would also need some kind of thermal break in addition, as leaving the ribs uninsulated without any thermal break would surely cause a large amount of heat transfer and condensation. The only alternative for the ceiling that I can see is to install rigid foam over the exterior roof, then installing a support structure over the foam for solar or a roof deck. This saves time and money for leaving the original ceiling intact and eliminates the massive thermal bridging that occurs in these buses.
For floors, I really like the idea of spray foaming the exterior underside of the metal floor as this will save head clearance, stop your metal from reaching the dew point temperature and forming condensation, and will act as a secondary barrier for allowing moisture to penetrate through the floor. This would require some extra work with prepping your metal so the foam can adhere properly (sand blasting and then using a rust encapsulating paint like por 15) , but would undoubtedly be a superior product in the end. Has anyone have any insight or experience with these methods?
My second choice would be to use a NON FOIL FACED foam like XPS to allow the moisture to dry out to the interior. It’s important to get a continuous uninterrupted layer of insulation over the floor to eliminate the thermal bridging. I don’t understand why people are using furring strips over the metal and insulating in between . This is only causing more thermal bridging, less insulation, and allowing areas to form condensation and a nice organic medium for mold growth. Rigid foam is designed to withstand pressure when evenly distributed, so you can just put your plywood directly over the foam. If your putting an oil based coating over your finished floor or using vinyl or some other similar non permeable surface, be sure to leave areas around your floor (such as under cabinets or furniture) that are permeable like just painting with acrylic latex paint.
Any professionals out there or someone with experience in properly insulating vehicles please chime in !! Thanks for reading my long ass rant lol
Excuse the length of this post, but I feel this topic is super important due to the apparent wide spread confusion of materials, vapor barriers, and the amount of work and money people are investing into this process. Would love to hear the input of other experienced builders (especially with bus experience) to come up with the best practice for insulating and sealing a skoolie. I’m an architect and builder (for regular homes) and have done a lot of research on building science and the thermodynamics of typical floor and wall assemblies. A bus is obviously a different ball game, having a painted steel shell (continuous vapor barrier) which throws a little more of a challenge as far as managing vapor transfer, mold and condensation than a conventional home. It’s also a massive thermal bridge, allowing heat or cold to transfer to the interior via conduction of the metal. This is a huge concern that seems to be overlooked in most bus builds, and most certainly needs to be addressed to avoid condensation and a comfortable interior.
For any floor or wall assembly, it’s important to maintain a clear path for vapor transfer to allow the wall or floor to dry out to both the interior and exterior (depending on the climate and season).
Considering this will be mobile, it’s hard to plan for a specific climate due to it always changing, so the more forgiving your assemblies are with vapor permeability, the better. In this type of situation (mixed climate with alternating paths of vapor drive) you would want to avoid any vapor barriers to allow the assembly to dry out in any direction. Obviously this isn’t possible for a bus since the painted metal vapor barrier is pre existing, but you most certainly don’t want to be adding additional vapor barriers within the assembly because you’re only option is to allow the assembly to dry toward the interior.
The most ideal situation would be to have exterior insulation to eliminate the dew point from occurring inside the assembly (and allowing condensation to form on the interior side of the metal), but this would be pretty hard to achieve while preserving an existing outer shell.
It seems the norm for these builds is to use rigid foam insulation (usually with a foil faced vapor barrier) directly over the metal floor and walls. This seems problematic because there is no venting behind the insulation, and does not allow the moisture to dry out when condensation is formed due to warm air reaching the cold metal surface. Not only is there not an air space for drying, your trapping the moisture between 2 vapor barriers (the foil faced foam and the painted metal). This will result in a build up of moisture causing mold, rot and rust. This problem can be even worse when batt or loose fill insulation is used, which would most certainly saturate the insulation with moisture and in a short time have a minimal resistance to heat transfer. (Batt insulation ALWAYS needs to have an air space to dry out if placed against an exterior vapor barrier, such as a roofing membrane or your painted metal skin)
Just like any regular home, the only way you can properly insulate a closed unvented assembly (with an exterior vapor barrier) and prevent the surface from reaching the dew point temperature and forming condensation is by using spray foam insulation directly over the interior side of the metal, or rigid foam on the exterior side. This will keep the dew point occurring in the middle of the insulation and can allow the vapor to dry out of the assembly.
So clearly some kind of spray foam insulation (I like airkrete because it is a sustainable foam with the same r value as petroleum based foams minus the off gasing) is pretty much the only option for walls and ceiling if you don’t want condensation forming inside your assembly. You would also need some kind of thermal break in addition, as leaving the ribs uninsulated without any thermal break would surely cause a large amount of heat transfer and condensation. The only alternative for the ceiling that I can see is to install rigid foam over the exterior roof, then installing a support structure over the foam for solar or a roof deck. This saves time and money for leaving the original ceiling intact and eliminates the massive thermal bridging that occurs in these buses.
For floors, I really like the idea of spray foaming the exterior underside of the metal floor as this will save head clearance, stop your metal from reaching the dew point temperature and forming condensation, and will act as a secondary barrier for allowing moisture to penetrate through the floor. This would require some extra work with prepping your metal so the foam can adhere properly (sand blasting and then using a rust encapsulating paint like por 15) , but would undoubtedly be a superior product in the end. Has anyone have any insight or experience with these methods?
My second choice would be to use a NON FOIL FACED foam like XPS to allow the moisture to dry out to the interior. It’s important to get a continuous uninterrupted layer of insulation over the floor to eliminate the thermal bridging. I don’t understand why people are using furring strips over the metal and insulating in between . This is only causing more thermal bridging, less insulation, and allowing areas to form condensation and a nice organic medium for mold growth. Rigid foam is designed to withstand pressure when evenly distributed, so you can just put your plywood directly over the foam. If your putting an oil based coating over your finished floor or using vinyl or some other similar non permeable surface, be sure to leave areas around your floor (such as under cabinets or furniture) that are permeable like just painting with acrylic latex paint.
Any professionals out there or someone with experience in properly insulating vehicles please chime in !! Thanks for reading my long ass rant lol
