Cooling a mobile or manufactured home

Manufactured homes get treated in cooling advice as though they were small houses. They are not, quite. The construction differs in ways that change which fixes pay back, and generic advice misses most of them.

Four things, and they compound:

1. The roof is directly over the living space. Most manufactured homes have a shallow or nonexistent attic cavity, so there is no buffer between the hot roof deck and your ceiling. In a site-built house an attic absorbs and delays that heat; here it arrives.
2. Wall assemblies are thinner. Less depth means less insulation, so the rate at which outdoor heat leaks in is higher for the same temperature difference.
3. There is less thermal mass. A house with a slab, plaster and masonry stores heat and releases it slowly, which flattens the daily swing. A lightweight structure follows outdoor temperature much more closely — hotter in the afternoon, and cooler at night.
4. The floor may be over open air. If the underside is not skirted and insulated, the floor is exposed on both sides to whatever the weather is doing.

The third of those is genuinely useful rather than purely bad. Low mass means the structure responds fast in both directions, which is why night ventilation works unusually well here — the night purge.

This is the practical failure that sends more units back than any other, and it is entirely avoidable.

Manufactured-home windows are frequently narrower and shorter than site-built ones, and many slide horizontally rather than vertically. A conventional window air conditioner needs a vertically sliding sash to rest on top of it.

Measure three things: the opening width between the jambs, the opening height when the sash is fully up, and how the window opens. If it slides sideways or cranks outward, a conventional unit will not fit and you need a different product entirely — casement and sliding window units.

In a site-built house, windows usually dominate the heat gain. In a manufactured home the roof competes hard, because there is so little between it and you.

The interventions worth knowing about, roughly in order of return per effort:

• Shade whatever you can from outside. An awning, a carport, a porch roof or a tree removes the sun before it reaches the surface. Cheaper and more effective than anything you do indoors — shading and film.
• Ventilate the roof cavity if there is one. Even a shallow cavity benefits from moving air, and a solar-powered vent adds no wiring — solar attic fans.
• Skirt and insulate underneath. Closing the underside stops the floor being a second exposed surface, and it helps in winter as much as in summer.
• Seal the obvious leaks. Around windows, doors, and where services enter. Low-mass, leaky construction rewards this more than a tight house does.

Manufactured homes are long and narrow, usually with a living area at one end and bedrooms at the other, connected by a corridor. That layout resists whole-home cooling from a single point.

A single large unit in the living area will make that room cold and leave the far bedroom warm, because air does not travel down a corridor without help. Two correctly sized units do the job better and cost less to run than one oversized one.

Where you are not putting a compressor, use air movement to distribute what you have. A fan in the corridor moving air toward the warm end does real work for a couple of watts — where to put a fan, and circulators built for moving air across distance.

A manufactured home on piers with an unsealed underside shares a problem with crawl-space houses: moisture from the ground below finds its way up into the floor assembly and the living space.

That shows up as a home that feels clammy even when the temperature is reasonable, and over time as soft spots in flooring. Skirting helps; a ground vapor barrier underneath helps more; and a dehumidifier addresses the symptom directly — crawl-space dehumidifiers and why humidity makes the same temperature feel worse.