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2018 National Home Improvement Estimator, All Rights Reserved Page 82

Older homes develop thousands of tiny air leaks around doors and windows, at the sill, around electrical
outlets and where interior and exterior walls have cracked. To be truly effective, the vapor barrier should
be continuous over the inside of studs and joists. That’s easy in new construction. In home improvement
work, applying continuous vapor barrier is practical only when interior wall cover is being replaced.

If removing the ceiling or wall finish is part of the job, staple vapor barrier directly to the studs or joists.
Lay vapor barrier on a subfloor directly under the new floor finish. Be sure to lap joints at least 2” and
avoid unnecessary punctures. Nails or screws driven snug against the vapor barrier don’t do any harm.

Vapor movement through any barrier is measured in perms. The lower the perm rating, the more
effective the barrier. Materials with good perm ratings include polyethylene at least 2 mils thick,
asphalt-impregnated and surface-coated Kraft papers, and asphalt-laminated paper.

Blanket Insulation With Vapor Barrier
Most roll (or “blanket”) insulation has a vapor barrier on one side that extends beyond the edges of the
roll to form a stapling tab. Lap these tabs over the 2” thickness of studs and joists. (Figure 5-3). Tabs on
adjacent blankets should overlap. Your drywall crew probably prefers to see these tabs stapled to the
inside face of studs or joists. That makes it easier to hang drywall with a smooth, even surface. But
stapling tabs inside the stud or joist cavity will also reduce the effectiveness of the vapor barrier.
Moisture will seep through openings between the tabs and framing members. It’s better to make the
drywall finishers work a little harder and protect the studs and joists from decay, mold and termites.

Vapor-Resistant Coating
If walls and ceilings don’t have a vapor barrier, the next best thing is a vapor-resistant interior coating
such as aluminum primer. Two coats of aluminum primer covered with acrylic latex offer moderate vapor
resistance, though not as much as a true membrane.

Crawl Space Ventilation
Damp soil in a crawl space will transfer moisture to the floor framing and even to occupied rooms above.
Good ventilation helps keep the crawl space dry. Promote good cross-ventilation by installing a vent at
each corner of the foundation. Taken together, foundation vent “free circulation area” should be 1/150th
of the crawl space surface (10 square feet of vent for a 1,500 square foot crawl space). Free vent area
excludes space occupied by the vent frame, screen wire and louvers. Most manufactured vents are
identified by the overall dimensions and the net free area. If there’s a partial basement, the crawl space
can be vented to the basement instead of the exterior.