The Problem Isn't Cold

Insulation gets discussed as though the job is keeping heat in. In a box this size that's the easy half. Two adults breathing overnight put out something on the order of a pint of water each, and it has to go somewhere. In a sealed cabin it goes to the coldest surface it can find, and it will find one whether or not you planned which.

So the real job is two jobs. Slow the heat down, and give the moisture a way out that isn't the inside of a panel. Nearly every decision below is about the second one, and it's the reason a fan matters more than a rating on a foam board.

Where Ours Has It, and Where It Doesn't

Most insulation advice assumes a stick-framed wall with bays waiting to be filled. Ours isn't built that way, and once you look at where the cavities actually are, the list gets short.

The Envelope Is the Bed, Not the Trailer

That last one is worth sitting with, because it organizes everything else. A teardrop looks like one object, so it's natural to think of insulating the trailer. It isn't one space. There's a cabin you sleep in, a galley you cook in, and a bulkhead between them.

The galley is outside the envelope on purpose. It's open to the weather whenever the hatch is up, which is exactly when you're using it. Insulating a lid that spends its working life propped open buys nothing and costs weight, thickness and fiddly curved fitting. The heat you'd be holding in isn't there to hold.

What that does mean is the bulkhead earns its keep. It's the actual boundary of the heated space, and any gap through it — a wiring pass-through, a cabinet back, a hatch seal that doesn't quite land — is a hole in the envelope in a way an uninsulated hatch panel isn't. Seal the partition properly and the hatch can stay a cold box with a clear conscience.

The Roof Pays Twice

The roof rewards the work in both directions. Warm air rises to it on a cold night, and on a hot afternoon it takes direct sun with nothing between it and you. It's also the only place the framing hands you ready-made bays.

The complication is the curve. Rigid board does not want to follow a radius, and there are three ways around it. Kerf the board — parallel slots cut partway through the back so it bends — and it will follow a gentle arc, at the cost of slightly less thickness along every cut. Score it into narrow strips and fit each one flat between the spars, which handles a tighter curve but leaves you a great many perimeter joints to seal. Or spray it, the one case where closed-cell earns its mess: it follows any shape, bonds to the skin, and leaves no gaps at all.

Whichever way you go, the roof is also where the fan, the solar cable gland and the interior lighting pass through. Plan every penetration before the foam goes in. Cutting a hole through finished insulation leaves a gap you can't reach to seal, behind a skin you've already screwed down.

The Floor Is the One People Skip

The floor gets skipped because it's built first, when insulation still feels like a later problem. It's also the least forgiving surface on the trailer.

Ground temperature runs below air temperature for most of a cold night, so the floor loses heat to something colder than the weather report. Underneath it is open air and a steel frame that conducts heat better than anything else in the build, and you're lying a couple of inches above it for eight hours.

There's a second reason, and it ties back to the top of this post. The floor is a strong candidate for the coldest interior surface, and the coldest surface is where the moisture lands. Insulating it isn't only about warm feet — it's about not waking up to damp under the mattress.

Which is what the trailer in these photos gives up. Its floor is storage, reached through hatches in the deck, and foam in those compartments means storing nothing there. We took the storage. You feel it underfoot on a cold morning, and a good mattress pad does more about that than three-quarters of an inch of XPS would have. If you don't need the compartments, insulate the deck while it's open — after that you'd have to undo the trailer to reach it.

What Bare Walls Actually Cost

Honest accounting: leaving the walls uninsulated isn't free. Wood is a mediocre insulator, and a plywood side panel is a genuine path for heat out of the cabin. On a cold night the walls are a strong candidate for that coldest-surface title, which makes them the surface most likely to show condensation.

The answer isn't a second wall to hold an inch of foam. It's getting the moisture out before it finds anything to condense on. That's ventilation — a fan on low all night and a window cracked, even when it feels wrong to open one. A dry cabin at 50°F beats a sealed one at 55°F running with water.

If you want the walls in the envelope, decide it at the panel stage. A foam-core sandwich wall — two thin skins with foam bonded between them — gets you insulation and structure in one assembly. Retrofitting a cavity into a finished single-skin wall is a much worse project.

The Materials Worth Considering

Ignore the exotic stuff. In a roof bay an inch and a half to two inches deep, or a floor deck at three-quarters, the choice between R-5 and R-6 per inch barely matters. Fit, gaps and moisture handling should decide it instead — a bay filled 90% with the best foam on the market performs worse than one filled 100% with the cheapest, because the empty tenth is where air moves and heat follows.

Thermal Bridges, and Why Steel Is Worse

Every spar that runs from the outer skin to the inner ceiling is a path for heat straight past all that foam. In a roof with bays every foot or so, that's a lot of wood, and wood conducts considerably better than foam does.

The floor is the harder case, because the members there are steel. Steel moves heat hundreds of times better than wood, so a frame rail touching cold air below and warm deck above does real damage no matter how well you fill the space beside it. The fix is a layer that crosses the frame rather than sitting between the rails — even a thin continuous sheet under the deck breaks most of the bridges at once.

The bridges people miss: door frames, the hatch surround, window cutouts, and any aluminum trim passing through a panel. Small individually. On a cold night with the heater running they add up to a draft you can feel.

Putting It In

The install isn't complicated. It rewards patience and punishes a rushed afternoon.

Vapor, and Why Sealing Harder Isn't the Answer

The instinct after reading about condensation is to seal the interior completely — vapor barrier, taped seams, no path for moisture anywhere. In a house that's often right. In a trailer it's a trap, because the shell gets wet from the outside too, and a perfectly sealed interior means anything that does get in has no way back out.

What holds up in a small trailer is to slow moisture down rather than stop it, and then remove it from the cabin before it goes anywhere. Closed-cell foam already resists vapor well on its own. Add a fan that actually runs at night, crack a vent, and the humidity leaves through the roof instead of migrating into a panel.

Practically: sleep with the fan on low and a window open an inch, even when it's cold out. It feels like undoing your own work. It's the difference between a dry cabin and a wet one.

In the Build

What We'd Tell You Not to Do

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The foam itself is a local buy — 4x8 sheets don't ship at a price worth paying, and every lumber yard stacks them by the door. What's here is everything that goes around the foam, which is the part people forget to budget for.

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