Proper Air and Moisture Control in Homes, According to the US Department of Energy

Moisture Control in Homes

Air and moisture control in homes, says the US Department of Energy, begins with a discussion of what used to be called “vapor barriers,” but are now referred to as “vapor diffusion retarders.”

Vapor diffusion retarders can aid in controlling moisture within “basements, ceilings, crawlspaces, floors, slab-on-grade foundations and walls,” and accomplish this by cutting down on how quickly “water vapor moves through a material.”

Permeability of Vapor Diffusion Retarders

Water vapor diffusion is measured in units called “perms,” for permeability.”

There are three types of water vapor retarders acknowledged by The International Residential Code:

1) CLASS I RETARDERS, OF .1 PERM OR LOWER, which are composed of “glass, sheet metal, polyethylene sheet, or rubber membrane.”

2) CLASS II RETARDERS, OF BETWEEN .1 PERMS AND 1. PERMS, and which are made up of “extruded polystyrene, 30 pound asphalt coated paper, plywood, or bitumen-coated kraft paper.”

3) CLASS III RETARDERS, OF BETWEEN 1. PERMS AND 10 PERMS, and which are composed of “unfaced fiberglass insulation, cellulose insulation, board lumber, concrete block, brick, 15 pound asphalt coated paper, or housewrap.”

For air and moisture control in homes, these vapor diffusion retarders are usually manufactured as “membranes or coatings,” which are “thin and flexible.” But, this is not always the case. Some retarders–known as “structural”–are made of “thicker sheet materials,” like “rigid foam insulation, reinforced plastics, aluminum and stainless steel.” These thicker and stiffer retarders are fastened by some mechanical means and “sealed at the joints.”

Thinner, bendable retarders are usually made in rolls and include “polyethylene sheeting, aluminum or paper faced fiberglass roll insulation and foil-backed wallboard.” Thicker, paint-like substances can also act as vapor diffusion retarders.  

Permeability, Climate, and New Home Construction

For those building new homes in mild climates, products such as “painted gypsum wallboard and plaster wall coatings” may be sufficient retarders.

For new-home construction in extreme heat and/or cold, vapor diffusion retarders with a higher permeability work best, and should be installed “closest to the warm side of a structural assembly” and facing the “interior of the building in cold climates” and “the exterior in hot/wet climates.”

In extremely cold and hot/humid environments, your retarder installation must be as accurate and thorough as possible. Make sure that every tear, opening, or puncture that occurs during the construction is completely re-sealed. Otherwise, you could end up with “condensation within the cavity” and moist insulation. Furthermore, wet insulation assuredly leads to problems such as wood rot and mold.

Existing Home Air Control and Renovation

It’s not easy to add vapor diffusion retarders to an older home. Getting a professional “energy audit” and “thoroughly sealing any leaks” is often the best way solution for air and moisture control of a preexisting building.

And, if you don’t reside in an extremely cold climate, “numerous layers of paint on the walls and ceilings” can certainly help slow down moisture movement. In choosing a paint for vapor barrier purposes, consult the label for the perm rating. If it’s not on the label, ask store personnel about the paint’s formula. The higher the “percentage of solids and thickness” of that formula, the better the moisture/air retardation effects. Glossy and acrylic paints act as better retarders than flats and latex. When in doubt, apply many coats and look for paint expressly labeled “vapor diffusion retarder.”  

Importance of Air and Moisture Control

To cut down on moisture within homes, air gaps must be sealed. Air leakage into and out of a structure makes up almost “30% of…heating and cooling costs. An air barrier is a great solution for increasing air tightness.

House wrap is not the best solution for an air and moisture control. It can be easily damaged during construction defeating the whole purpose of an air barrier to begin with. Every seam or staple hole allows for unwanted air intrusion.

A continuous air barrier and moisture barrier ensures airtightness. A high performance barrier resists air movement, and therefore moisture, by increased air tightness. Air barriers and moisture barriers that come in fully adhered sheets, are particularly useful due to ease of application.

Before construction on a new building or renovation of an old one, it’s best to employ the “whole systems approach,” and analyze how air/vapor retardation “materials and techniques” will interact with the structure as a whole. Considering the perms needed depending on the local climate will ensure proper air and moisture control, and therefore building longevity.