The most common winter complaint from DIY tiny house builders isn't that they didn't insulate — it's that they insulated well and the house is still cold. R-23 mineral wool in the walls, a high-BTU heater, and the floor is still freezing and drafts are coming from somewhere. This is a predictable outcome when insulation is treated as the only layer of the thermal envelope, rather than one component of a system that also requires air sealing and thermal bridge management.
A 1-inch gap in the building envelope can neutralise the effectiveness of an entire insulated wall cavity. In a small structure — especially one that moves at highway speeds and has trailer frame penetrations, wheel wells, and dozens of construction joints — those gaps accumulate quickly if they're not specifically addressed. Here's how to build a thermal envelope that actually holds heat.
1. Thermal Bridging: Why Your Studs Are Leaking Heat
Heat follows the path of least resistance. In a wood-framed tiny house, the studs are that path. Insulation between the studs might have an R-value of 15, but wood itself has an R-value of approximately 1.2 per inch. Since studs typically account for 15 to 25% of total wall surface area, a meaningful fraction of the wall's heat loss travels directly through the framing regardless of what's in the cavities.
The fix is continuous exterior insulation — a layer of rigid foam (EPS or XPS) or high-density mineral wool board applied over the outside face of the framing before siding goes on. This wraps the entire framing including the studs, breaks the conductive path, and keeps the framing members warm enough that the dew point doesn't reach the interior wall cavity. It's a separate layer from the cavity insulation and needs to be specified as part of the wall assembly from the start.
2. Air Sealing: Where Most Heat Actually Escapes
In the building industry, the standard is "build tight, ventilate right." Most tiny houses are leaky — not through their insulated wall cavities, but through the construction joints that don't get sealed. Air enters through the trailer-to-subfloor connection, around electrical boxes, at the top and bottom wall plates, and through improperly detailed window and door openings. That moving air carries heat out of the insulation fibers and out of the building.
An air barrier addresses this. High-performance tapes like Siga or Pro Clima products, applied to every seam in the subfloor sheathing and wall sheathing, create a continuous air-resistant layer that the insulation alone can't provide.
The Specific Points That Cause the Most Problems:
- The trailer interface: The connection between the subfloor and the steel trailer frame is the primary draft source in most THOW builds. Closed-cell spray foam or high-expansion gaskets seal this joint correctly. It's not a glamorous detail but it has a disproportionate effect on comfort.
- Top and bottom wall plates: Acoustic sealant or sill seal foam under the plates before they're fastened prevents air movement through the framing-to-sheathing joint.
- Windows and doors: Flash and seal with low-expansion window and door foam. Standard high-expansion foam like Great Stuff can bow the window frame during cure, permanently compromising the seal it's meant to create.
3. Comparing Insulation Materials for a Mobile Structure
Fiberglass batts are a poor choice for a tiny house on wheels. They settle under road vibration, leaving gaps at the top of wall cavities — voids where insulation should be but isn't. The three materials that perform better in this context:
| Material | R-Value (per inch) | Pros | Cons |
|---|---|---|---|
| Closed-cell spray foam | R-6.5 to R-7.0 | Best air seal of any insulation type; adds structural rigidity to the framing. | Expensive; requires professional application; difficult to modify after cure. |
| Mineral wool (Rockwool) | R-4.0 to R-4.3 | Non-combustible; hydrophobic (doesn't absorb water); excellent sound dampening. | Heavy; requires a separate air barrier layer to perform well. |
| Sheep's wool | R-3.5 to R-3.8 | Renewable; manages moisture naturally through absorption and release. | Lower R-value per inch; higher cost than mineral wool. |
4. Moisture and Vapor Control
A well-sealed tiny home creates a moisture management challenge that a leaky one doesn't have. A single occupant exhales roughly one litre of water per day. Cooking, showering, and breathing together produce significant interior humidity in a small sealed space. Without a correctly positioned vapour retarder, that moisture migrates into the wall assembly and condenses when it meets a cold surface — which creates mould conditions inside the wall where it can't be easily detected or remediated.
In cold climates, the vapour retarder belongs on the warm side of the insulation — the interior face. This keeps warm, humid indoor air from reaching the cold exterior sheathing. Smart vapour retarders, which adjust their permeability based on ambient humidity levels, improve on this by allowing the wall assembly to dry outward when conditions allow, rather than trapping any moisture that does enter the cavity.
5. Mechanical Ventilation: Required, Not Optional
A properly sealed tiny house will not ventilate itself. Without mechanical ventilation, CO₂ levels rise during sleeping hours, affecting sleep quality and cognitive performance, and moisture accumulates. Opening a window in January to address this wastes most of the energy the insulation system is working to retain.
An ERV (Energy Recovery Ventilator) or HRV (Heat Recovery Ventilator) is the correct solution. These units continuously exchange stale indoor air for fresh outdoor air while transferring up to 80% of the heat from the outgoing air to the incoming air. The house stays fresh and the heating load stays manageable. In a well-sealed tiny home, this is not an optional upgrade — it's part of what makes the air-tight building strategy work safely.
Build Checklist: Four Things That Actually Make the Difference
- Continuous exterior insulation: Wrap the framing from outside to break thermal bridges at every stud.
- Tape every sheathing seam: High-performance construction tape on all subfloor and wall sheathing joints creates the air barrier the insulation can't provide on its own.
- Spray foam the floor joists: Closed-cell spray foam in the floor joist bays stops the road drafts that make the floor cold regardless of what's in the walls.
- Install an HRV or ERV: Mechanical ventilation is the mandatory complement to a tight building envelope.
Thermal performance isn't about the R-value on the insulation label. It's about the discipline of the seal — every joint, every penetration, every transition between materials. Get those right and the insulation works as advertised. Leave gaps and the insulation value on the label is irrelevant.
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