One of the most important factors in energy efficiency is home insulation. And one of the most important parts of insulation is keeping the material dry. There are two reasons for this:

• Almost all types of insulation suffer efficiency losses when they are wet, though natural wool insulation is a notable exception.
• Moisture retained in insulation can cause mildew and mold growth as well as slowly seeping into any timber or materials it contacts, causing rot and eventual structural collapse.

The first line of defense against moisture is to reduce moisture production in the home. Make sure the kitchen and bathroom exhaust fans run when heat is generated in either location, mulch your plants, cover the aquarium, line-dry your clothes outside and make sure that appliances are appropriately vented or ducted.

With that done, let's look at the two main approaches to moisture control: vapor barriers and housewraps.

Vapor barriers

Usually consisting of a 6-mil polyethylene sheet stapled to the frame, and overlapped with other sheets to reduce air filtration, a basic vapor barrier is placed on the "warm" side of a wall. For cool or cold climates, that means it goes on the inside, typically just under the drywall or interior paneling. In hot, humid regions such as certain southern US states, a vapor barrier works better on the outside of the wall, under the siding, where it prevents moisture from getting in.

Vapor barriers aren't limited to walls. In homes with crawl spaces, they are useful in preventing ground moisture from penetrating; if your home uses a concrete slab foundation (especially if it's used as radiant floor heating), a vapor barrier is extremely useful when installed under the slab.

Attics are the one place that most homes do not need a vapor barrier, as there is usually plenty of space for moisture to escape. However, if your home's attic is very humid, you should install attic vents (ridgeline and soffit) or even a solar-powered roof vent (a fan that extracts moisture year-round and runs off a small PV panel).

It is important to note that vapor barriers are not required (or desired) in natural homes which use insulation such as wool, cotton, or straw. Installing a barrier in those homes will potentially damage the structure in the long term, as it will retain moisture where the materials should be allowed to breathe.

Housewraps

A housewrap is an exterior sheathing stapled onto the outside of the house. It's a thin, lightweight plastic made of polyethylene or polypropylene which repels water coming from the outside but allows moisture to pass through it from the inside. The practical result is that it keeps your insulation dry year-round.

Some local building codes require a housewrap on standard stud-frame houses, while others do not, usually depending on the climate. Housewraps work well in moist climates, whether hot or cold, and are usually required wherever shingles, shakes and board sidings are used.