I recently bought a 2006 Ford E-350 which I am converting into a tiny home for full-time dwelling. My problem is once that I have about 2 inches of vertical clearance from the ceiling.

My options are:

  • Rip out the existing floor, build a 1 inch frame, lay insulation, then put a new floor in. This sucks because my current floor is a really nice custom solid wood floor left in by the news crew. It's also glued in with LOTS of glue, and it will be hell taking it out.
  • Leave the existing floor in and just build on top of it. This will leave me barely be able to stand up straight in my home. Deal-breaker.
  • Put the insulation in the undercarriage.

Here's what I'm thinking. First of all, I'll stay well clear of the exhaust pipe, which gets super hot. I'll paint the entire undercarriage with a protective coat. Then, I'll bolt sheet metal to the bottom beams. Then, I'll spray expanding waterproof insulation spray into the gap between the sheet metal and the bottom. I won't be able to insulate where the gas tank is (it's too close to the bottom) or the exhaust. In addition to insulating from convective heat loss, I'm also considering using a layer of reflectix in there somewhere to reflect radiant heat from hot pavement.

Is this a terrible idea? What could go wrong? If it's doable, what materials would you use?

  • P.S. if anyone has 150+ points, I couldn't create a tag for 'insulation'.
    – crypdick
    Jan 31 '17 at 3:30
  • 2
    Since you don't really need the insulation while you are moving, why not build a quick to install temporary skirt around the base of the van. Then you will have an enormous air gap under the car. Park the van, set up the skirt and use the still air as you insulation.
    – enderw88
    Jan 31 '17 at 4:40
  • 1
    @enderw88 that helps, certainly, but it's not enough for comfort in winter.
    – Chris H
    Jan 31 '17 at 9:20
  • Is replacing the roof with a higher one or extendable roof an option? That would solve the clearance issue, and give more space for cupboards as well. You can always use more storage space, esp. if you live in the van full-time.
    – Hobbes
    Jan 31 '17 at 12:40
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    Re @enderw88, what about an actual airdam and side skirt? Would help to create dead airspace under. Regarding the reflectix, you may want to also add some noise insulation on the larger panels. They have that sticky tar sheets - you could stick to the body metal and put the reflectix over that. Sounds cozy, but not sure how much the reflectix will dampen street noise at night. Jan 31 '17 at 20:51

I'm tall and can't quite stand up straight in my transit, or at least not with my feet together. It's not as bad as you think, even when cooking. Even to get this much height I had to be mean with the insulation. Mostly I think about insulation against the cold as I use the van in the UK/France and it doesn't have a/c. There isn't much difference when it comes to insulating against heat.

That said, I've looked into this myself and there are a few things to take care of:

  • The insulation needs to not absorb water thrown up from the road. Expanding foam isn't generally completely closed-cell, and relies on a thick layer to block water. So you need good seals, and/or really good foam (like the stuff used in flotation compartments in canoes; it's more expensive).
  • On the other hand, any water that does get in there will stay there -- this could even be from spillages inside/washing the floor if you have cable holes through the floor. Then it will cause rust.
  • Expanding foam sticks to everything and makes rework hard.
  • Thermal expansion is significant and can open up unexpected gaps. Conversely if you install stuff when it's hot, differential contraction can cause buckling in the cold. Both of these are much worse on the outside.

If I could get the compartments perfectly sealed (and there would be a lot of little compartments in my case) I'd use glass fibre (rockwool) held in by sheet aluminium. It's heat proof enough to use near the exhaust.

I don't see a need to reflect more heat than the sheet aluminium would -- when parked the surface under the van will be mostly in the shade, and the insulation will stop a lot of the heat coming up anyway..

Even a thin layer of something insulating over the fuel tank would help -- perhaps you could buy a sheet of 10mm (~1/2") closed-cell foam mat. OVer the exhaust, either find some heat-proof insulation or do something like floor -- insulation (thin) -- aluminium sheet -- air gap -- aluminium sheet -- heat shield -- exhaust. Both of these are about avoiding big cold spots in the floor, though the fuel tank itself will help to some extent, and the tiny air gap above it

Some thermal bridges are inevitable with van construction, and you will get cold spots where the walls meet the floor with this setup. But you may be able to fix that on the inside (e.g. inside cupboards, or with carpet), and besides, a good layer of insulation underneath will help so much it's almost certainly worth it.

  • I didn't realize that the expanding foam still absorbed some water. Would you change your answer if you couldn't protect the foam from the elements? This is a DIY job and I have no prior experience, and I'd hate to turn my new van into a rust bucket because of trapped moisture. The big complication is that the gaps in the undercarriage aren't nice rectangular areas-- there are car components here and there, so I'd have to do a huge effort shaping the sheet aluminum like a jig-saw puzzle. Is the kayak foam you mentioned expanding foam?
    – crypdick
    Feb 1 '17 at 23:57
  • There is an expanding foam used in canoes, I'll try to track it down. You'd need to find a way to attach it securely as well.
    – Chris H
    Feb 2 '17 at 6:58
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    Here's the thread I was thinking of in which it's stated that the foam will eventually absorb water and fail (under a vehicle you'll get all sorts of muck thrown up; that won't help. Boat-building foam which is "94% closed cell". (This brand is US, I'm in the UK so don't know more than I've seen on the web). Here's a cautionary tale
    – Chris H
    Feb 2 '17 at 8:57
  • I didn't realize it was liquid-- sounds very difficult to work with, unless I flip the van upside-down. In any case, I'll make sure not to use any of it in my living room, lol
    – crypdick
    Feb 4 '17 at 2:52
  • The pro stuff is mixed as liquid that expands to foam very quickly. It's the same chemistry in the spray cans, but the mixing happens at one end of the nozzle and by the time it reaches the other end it's foam.
    – Chris H
    Feb 4 '17 at 7:50

I don't think putting part of the insulation outside the bodyshell is going to be very effective. Metal is pretty good at conducting heat/cold, so your floor will tend to have the same temperature as the outside walls and roof: When it's cold outside, the interior heat will be transported from the floor to the walls, cooling down the interior.

You have to insulate either the entire inside surface, or the entire outside surface.

Most of the floor space in the van will not be places you can stand up in: cupboards, bed, seats. You could put insulation on top of the floor in those places, and rely on a nice thick rug for the places where you can walk. In one campervan I've used a few times, the seating area floor is raised slightly (~+8 cm) above the rest of the floor.

  • That is an interesting idea! I wonder if that will cause problems with snow and ice melting into the carpet after a day of snowboarding. I'm not sure how much of a concern that should be, I grew up in FL and don't have much experience spending winter in a cold place :P
    – crypdick
    Jan 31 '17 at 21:19
  • The boots are simple: use a doormat. If you want to use this van in the winter, check out how campervans are winterized. In addition to insulation, you can get e.g. heated floors. You also need to make the water systems suitable for winter use (e.g. the wastewater tank needs a heater).
    – Hobbes
    Feb 1 '17 at 8:13
  • 1
    The amount of heat conducted along a thin sheet of steel is really quite low (how hot does the handle of your soup spoon get?) because steel has low themral conductivity for a metal and the cross-sectional area in the direction of heat flow is small. The floor will only be cold near where it meets the walls.
    – Chris H
    Feb 1 '17 at 13:25

An old thread, but still, ...in houses it is accepted as BEST practice to insulate OUTSIDE the existing brick walls, rather than inside

If you insulate inside you will inevitably get cold spots, and that's where the moisture will condense, causing damp, mould, etc. etc. all in hidden little corners where it's difficult to see and difficult to deal with.

If you put your insulation on the outside then then condensation isn't a problem because you raise the temperature of the inside of the walls above the dew point.

In a vehicle, the same applies: insultate inside and you have to be VERY careful to stop ANY air contacting the metalwork, especially in hidden places like under the flooring, because you'll get massive amounts of condensation which will form liquid water in places where it is very difficult to dry out. Closed cell spray foam allows provides insulation with total exclusion of air from the metal shell. It's not cheap, it's potentially messy, and it's not a DIY job, but it works.

Now in theory, you could use closed cell spray insulation on the outside of a van, and that would work very well, but it would look very strange. Under the floor and on the roof it could be applied without necessarily looking too bad.

The space under a vehicle is often in fact a rather calm and peaceful place. Usually quite dry and always with a very stable temperature. You can keep your beer under there on a hot day and it will stay pretty cool.

The problem under the van is that there are all kinds of pipes and components and so on which you need to keep clearances around. It's still worth insulating the areas you CAN insulate though - it never hurts. Closed cell foam wil protect metalwork just like vehicle underseal I reckon, and you've already got the underseal on there so I really don't anticipate any issues with moisture or corrosion. And if you later find you need more space to service components, the foam can always be cut away. I wouldn't enclose it in metal sheeting or anything like that; I would treat it as thick foam underseal.

But to save headroom, why not use closed cell spray foam on top of the roof? That will GREATLY reduce the risk of water droplets falling onto you on a cold night from the ceiling, and will make a massive difference on a hot sunny day.

Applied by professionals it usually ends up as a fairly even and smooth layer. It's really quite tough, and you can paint it body-coloured to protect it from the elements. UV will be more of a problem than rainwater I reckon, but paint will deal with that issue. Just make sure that water can always run off OK.

The problem with insulation foam becoming waterlogged (in boats and so on) generally only occurs in confined spaces where a cold surface promotes condensation from moist air and the foam then soaks up more and more of that water over time in a plce where there is insufficient ventilation for it to evaporate off again. A van rooftop is very unlikely to ever trap moisture like that, ...unless you want to go down the turf roof route, but I don't recommmend that because of the weight of the soil.

  • Have you got any references to back up your first assertion? I've never heard of anyone insulating outside the walls of a house, let alone it being best practice. Here in the UK it is standard to insulate the cavity between the two layers of brick, so the structure becomes: outside-brick-insulation-brick-plaster-inside
    – Nick C
    Feb 16 '19 at 18:08

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