Believe it or not how to insulate your bus is one of the more technical decisions you will make early on in your school bus conversion. 

 

For insulation there are the following options and each has very significant pros and cons. 

 

Closed Cell Spray Foam

Rigid XPS Insulation

Polyiso Insulation

Natural Wool Insulation

 

But before we talk about how to insulate your bus we need to talk about why you would insulate your bus and help you decide if you even need to insulate it at all. Once again how you plan to use your Skoolie comes into the forefront of your decision making. If you plan to spend a major portion of your bus life living in or traveling through a cold climate then you should likely plan for insulation. You will be heating your bus from the inside and the insulation will help absorb that heat, keep it inside of the bus, while slowing the cold air from coming in. Because warm air rises you need to focus your insulating on the floor of your bus which will be cold and uncomfortable if not insulated regardless of how warm the air at your head is. For a cold weather bus you also need significant insulation in the walls, and ceiling to keep that heat in, and you will be most comfortable with your school bus windows replaced with more efficient windows.

 

However if you plan to be in hot weather for example year round living in Florida your needs will be slightly different. You do not need to prioritize insulation in the floor as highly, and having a white reflective roof, paint job, and shade through solar panels or a deck may be just as important as insulation. But as a general rule if you will be in a hot enough locale that you plan to frequently condition your space you also need to insulate. In this scenario the insulation in your roof is still important, and so is the insulation in your walls. Again, for maximum comfort removing the majority of your windows and replacing them with dual pane will give you the best results in terms of efficient cooling and physical comfort. 

 

If you are planning a weekend camper, or a low budget short term home, or you plan to constantly chase ideal weather than the rewards of insulation begin to diminish. If you don’t plan to remove windows and heavily insulate then the hot or cold air that will be constantly flowing into your bus from the drafty old school bus windows will offset the advantages of the insulation significantly no matter how well you do it. On that note technically you can “camp” inside of your school bus and have a fun adventure and do no insulating at all. You will still be more comfortable than if sleeping in a car or a tent. You can also plan to be plugged in to 50 amp shore power in warm weather and air condition the heck out of your bus while only prioritizing roof insulation, although you will need efficient AC appliances to avoid a significant power bill if it’s even avoidable at all. If you’re finding yourself temporarily homeless in a Northeast winter and a school bus is your shelter and you have enough income for propane but not to insulate a school bus by all means heat up your bus and skip the expense. As you can see whether or how to insulate your bus is once again a decision dictated by your goals and situation. 

 

We also can’t talk about insulation without talking about condensation. Condensation occurs when the exterior of your bus is cold and you make the interior of your bus warm, even if it’s just with your bodies, breathing, cooking, or the sun etc. and certainly by heating. As the cold exterior interacts with the warm interior surfaces in your bus, condensation  will occur, especially in the morning. There are a few ways to counteract this for example you could run a dehumidifier nonstop if you are plugged in to grid power. You could use a dry heat source such as a wood burning stove or a diesel air heater, and you can even use some disposable products to absorb the moisture in the air such as “Damprid” to dry out the inside of the bus but the reality is condensation is always a consideration when building a Skoolie. Consider what could happen if you decide not to remove your metal bus ceiling, do not create a thermal break between the metal bus exterior and your interior, and don’t insulate and then warm up the inside of your bus with propane (which is a heat source that creates additional moisture). You may actually find your ceiling raining little bits of condensation which will definitely not be comfortable long term. So while some builders can get away with minimal or no insulation, those building a long term full time home need to give serious consideration to their climate and needs, and then prioritize creating thermal breaks between the inside and outside of the bus and install quality insulation. 

 

Finally before we talk about your insulation options we need to talk about R-Value and the regional recommendations made by the Department of Energy in the USA so that you know how much insulation to aim for. Given the different climates in the continental US the recommendations will change slightly. But I can summarize them easily for your bus given that for the most part you won’t be able to meet the minimum recommendations given by the DOE in the space available to you in a school bus. 

 

Wall Insulation in all zones: R13  

Floor Insulation in almost all of the US: R25 

Floor Insulation in Florida, Louisiana, and South Texas: R13

Attic Insulation in the coldest parts of the US: R49

Attic Insulation in the Northeast and Central US: R38

Attic Insulation everywhere else: R30

 

So to achieve these recommended insulations values let’s assess our insulation options to determine how or if we can achieve them. 

 

Closed Cell Spray Foam: R6.5 per inch (when purchasing the highest quality product and installing it correctly)

 

So let’s talk about Spray Foam. This is your highest r-value per inch of insulation and while there are two versions: open-cell and closed-cell spray foam insulation, closed cell is the only one that you should consider. Spray foam may also be the most expensive and potentially the most toxic if not installed correctly. By furring out the walls 1.5 inches an insulation value of R9.75 to R19.5 (between the metal framing of your bus walls for the higher value) can be achieved. I do not recommend furring out the walls any more as this will impede on interior space. The same insulation value could be achieved in the ceiling, however if you plan to lose 1.5 inches of headspace to your furring strips you may need to consider a roof raise as you will soon be running out of headspace and remember we still need insulation in the floor. To achieve almost R30 or higher you would need furring strips on your ceiling at least 3 inches thick. In Gilligan we used furring strips that were 1.5 inches on the walls, and .75 inches in the ceiling for R9.75 to 19.5 and R4.875 where the metal ceiling structure was to 14.625 for the vast majority of the ceiling. This was a compromise as we were planning to chase nice weather, or be plugged in to 50amp shore power. This is still a massive improvement over not removing and insulating the walls and ceiling as with the furring strips and insulation we created a very effective thermal break from the outside. There will be no condensation on the walls or ceiling of Gilligan Phantom. 

 

There are additional benefits to spray foam insulation. For one it is it’s own vapor barrier. Another advantage is that it improves the rigidity and structural stability of your school bus, and it certainly won’t squeak when bouncing down the road like rigid insulation might. However we absolutely must address the toxicity of spray foam. Spray foam insulation when applied correctly involves mixing two extremely toxic chemicals at a specific PSI and when it cures as intended it is supposed to be inert. This means that while applying spray foam the utmost care should be taken to protecting your lungs, and skin and all people and animals who are not protected need to stay far away until it has cured (when it cures correctly it should take less than 24 hours). But what happens if it doesn’t cure correctly? It remains extremely toxic, is basically impossible to remove, and makes your bus uninhabitable. If you plan to use spray foam insulation please take some time to research and understand this topic as it is beyond the scope of this article. If you do plan to use spray foam my personal opinion is that you should hire a very qualified contractor to apply and trim the foam. Yes it will be expensive, however this is not something you want to go wrong. If you’re determined to DIY, there are spray foam kits for sale and I’ve seen bus converters successfully use these themselves. While I do sometimes worry about the unknown long term effects of spray foam insulation on our health, we did have ours professionally installed, there is every indication that it cured correctly, and my only regret is that I chose to trim it myself. It would have been worth it to pay extra to have the installer trim my spray foam and not expose myself to all of the dust created in trimming it. While I took precautions to protect my eyes and lungs, after 5 days of trimming some exposure was inevitable. 

 

Could you put spray foam insulation in your floors? Yes, you could, and some have done it, however I don’t think the advantages of a higher R value per inch outweigh the rock solid subfloor that we made out of XPS insulation, plywood, and a specific foam board construction adhesive which I will describe in this article.

 

Rigid XPS Insulation: R5 per inch 

 

Rigid foam insulation is also a great insulator for your Skoolie. You can buy it in various depths (not all big building stores will carry all options however so you may have to search lumber yards or special order) to perfectly fit cavities in your bus’s walls and ceilings. It is easy to cut into any shape you may need to insulate, and so it is very DIY friendly and is much cheaper than professional spray foam. However rigid foam alone will not perfectly seal and insulate walls and ceilings. To fill the little gaps left behind, you can use Great Stuff Spray Foam which is a one part closed cell spray foam. Or you can use aluminum tape to reduce drafts coming through the gaps. You may want to use a foam board adhesive behind it as you push it into ceiling and wall cavities so that it doesn’t bounce around and make noise while you’re driving down the road. You can still use foam board with furring strips, however you may want to run furring strips on top of steel members which is less of a thermal break than running them perpendicular to steel members, but still better than nothing. 

 

The place where rigid insulation shines however is in the subfloor of your Skoolie. While many builders for whatever reason decide it necessary to build a frame in the floor to support the subfloor and cabinets I find it to be completely unnecessary. This is additional work and creates screw penetrations through the metal subfloor of your bus inviting more thermal conductivity and moisture. A better choice I believe is to glue 25 PSI XPS Insulation (I’ve found both 15 PSI and 25 PSI Insulation and you will want the latter. It is more rigid. In fact it is made to be used underneath concrete slabs in basements and crawlspaces. Yes it can stand up to the weight!) straight to your metal subfloor. 

 

With 2 inch insulation panels you can have an R10 floor, and with 2 layers of 2 inch insulation stacked and glued to each other you can have a 4 inch deep R20 floor. This is the level of insulation that you may find yourself needing for full time Skoolie dwelling in Vermont for example and as usual plan for a roof raise when losing this much vertical space to insulation (unless you’d like radiant in floor heating which we will talk about shortly in which case a 2 inch thick subfloor may be a worthy compromise). 

 

Polyiso Insulation: R6-6.5 per inch

 

Polyiso insulation comes in a rigid board like XPS and it is a great insulator and cost effective. HOWEVER this insulation actually loses it’s performance in cold weather. Additionally if you plan to put it in your floor you will need to build a frame to support your subfloor as this foam board will not stand up to compression. If you’re building a cold weather Skoolie, the XPS is the obvious choice, however if you find a great deal on Polyiso insulation and are living in a warmer climate, don’t be afraid to use this insulation!

 

Natural Sheep’s Wool : R3.6 per inch

 

Lastly we have Wool Insulation and the major wool retailer in the US is a company called Havelock. Havelock wool has a lower R value than some of the other insulations described above but it is the only one that is completely natural, creates no toxic off-gassing, and is a sustainable product. It manages moisture well unlike any of the other options and helps with sound deadening. The big reason this insulation is a winner is it is an easy, diy friendly install and is pretty much guaranteed to not bother those who are chemically sensitive or worried about spray foam off-gassing. If you are building a warm-weather skoolie I think it deserves some serious consideration and is the insulation that we used for the second bus that we built: @ourwildcaravan for our friends Katy and Sam.