Breaking down energy needs: Insulation (Part 2)

In the last post we discussed the common industry insulation measure known as the R-value. Focusing solely on a material’s R-value can be a major mistake, however.

First, R-values are determined under very specific laboratory conditions: in a warm, dry, wind-free environment. Of course, every home is subject to wind and precipitation, and even protected layers of insulation will be affected by weather. Fiberglass insulation loses nearly all of its thermal properties when wet, and allows air to flow through it easily even when dry.

In order to properly insulate your house, you need to seal your home against moisture and wind. Some go so far as to recommend against fiber insulation altogether.

Vapor barriers should be placed on the warmest side of the insulation. This is obvious in either very hot or very cold climates, but in places where the temperature varies wildly throughout the year the placement isn’t always readily apparent. No matter what the case, you should never put moisture barriers on both sides of the insulation – that will prevent the moisture that forms (and it always will to some degree) from escaping. Trapped moisture can both ruin insulation and lead to mold growth.

Solid insulation has a lot of benefits over fiber insulation. In general, it prevents airflow and is more resistant to moisture. Spray-on insulation (such as polyurethane) is better than pre-formed sheets as it allows a complete seal, but petroleum-based insulation introduces several Peak Oil/environmental concerns. Someday your insulation will need replacement, and the old waste insulation will need to go somewhere. In addition, such materials won’t be available post-Peak Oil, and if you didn’t make accommodations for natural insulation you might lose valuable home thermal properties you counted on in your initial design.

Properly sealing your home is critical. This includes horizontal sealing between studs; sealing around outlets, windows, and doors; and sealing between the walls and vertical connections. Eliminate air gaps at all points (such as at the top of walls), as air has a much smaller R-value than insulation.

So great, your home is now fully insulated and sealed all around with an R-value of 70…and now you can’t breathe because you have no air exchange. You need ventilation, and that will be the focus of a future post.

References:
Monolithic Dome Institute
U.S. Department of Energy
Kansas State University
Ecobuild Network