Why You Must Air Seal a Modular Home

One of the most important steps that a general contractor can take to make a modular home energy efficient is to air seal the gaps between modules. The manufacturer can control air infiltration within each of the modules, but when the modules are placed side by side or stacked on top of each other, significant gaps are created. It is just not possible to bring two modules together tightly when a cable is wrapped around each one while being lifted into place. In addition, even when the framing of one module is tightly butted up against the framing of another module, it is not possible to make the joint airtight. Very few set crews completely seal these gaps, so the GC should assume the job.

How to Properly Air Seal a Modular Home

A modular home that is properly air sealed is significantly more energy efficient than a typical site-built home. When a modular home is poorly sealed, however, it will leak more than a site-built home. The GC should make every effort to seal the following:

• The basement and attic marriage wall
• The interior marriage wall wherever there is a passageway, door, or clear-span opening
• The exterior marriage wall along the gable ends
• The exterior band between floors on two-story homes
• The exterior sill plate where the first-story modules sit on top of the foundation

On the interior, the best way to air seal large gaps is with expandable foam. On the exterior there is a special technique that should be used before the GC completes the siding. Most manufacturers hold back a small piece of the exterior sheathing where two or more modules join. The GC must insert a piece of sheathing that bridges the space between the adjoining modules. The sheathing connects the modules structurally and creates a flat surface for the siding. By itself, however, the sheathing does not do a good job of reducing air infiltration, even when combined with the sill seal installed between modules by most set crews. But if the GC applies two beads of caulk to the back of the sheathing before nailing it to the modules, this will create what in effect is a gasket seal. The GC can then finish by caulking the outside edges of the installed sheathing to close the remaining gaps.

The GC should carry out a similar procedure where the bottom of the first-story modules joins the sill plate and where the top of the modular wall connects to the roof overhang at the eave. He should also foam seal the gable-end triangle sections under the roof; he might first need to brace the sections from inside the attic. Completing all of these steps will create a significantly more energy-efficient and comfortable home, for a negligible cost.
For more information about why you must air seal your modular home, see The General Contractor’s Responsibilities for Building a Modular Home in my book The Modular Home.