R-Values and Air Infiltration

An R-value is a measure of how much time it takes 1 Btu of energy to pass through a given material, which is important. But it is also critical to think about insulation's ability to prevent air and moisture infiltration.

When evaluating an insulation product, it is important to look at more than just the R-value ratings as R-values do not account for the affects of air movement.  For example, polyurethane foam provides much higher performance than its actual R-value when properly installed.

Although fiberglass insulation is still the most common product used, spray foam or cellulose insulation are much more effective products. For fiberglass to perform well, it must be installed correctly. If voids and gaps are left unfilled, heat and sound will travel. It can only achieve its assigned R-value of 3.5 per inch, if tested in an absolute zero wind and zero moisture environment.

Insulation levels throughout the house are where zero energy buildings diverge from conventional energy code requirements most dramatically. To reduce net energy use to zero, houses need significantly higher insulation levels than conventional buildings that rely on abundant and inexpensive fossil fuels.

How Much Insulation is Appropriate

It depends on climate. The real issue is now to get the heating and cooling loads so low that a homeowner could heat the house with the refrigerator, a computer or two, and the dogs. Low heating and cooling loads are the magic bullet that allows zero energy houses to be cost-effective in any climate.

Insulation types and location determine how well the home will perform through the diversity of climate conditions and future challenges. For example, using spray foam to air-seal the house makes subsequent  insulation strategies easier than trying to air-seal the house with just fiberglass.

Note: Fiberglass insulation that is installed over a couple of inches of spray foam is one way to make the house airtight and still affordable.

Types of Insulation

Insulation R-Value Quick Chart

Insulation R-Values
Type of Insulation R-Value
Fiberglass 3.5 per inch
Cotton Batts 3.5 per inch
Cellulose 3.5 per inch
Closed-cell Foam 6 per inch
Open-cell Foam 3.5 per inch
Note: These numbers are estimates and the final R-value depends on density and installation.

Fiberglass

Conventional installation of fiberglass doesn't yield the performance required for a zero energy house. Typically, fiberglass is stuffed into wall cavities without regard for wiring, plumbing, or other obstacles already installed. Fiberglass requires conscientious air sealing before installation:

  • All vertical and horizontal penetrations from the envelope need to be foamed to prevent air movement
  • Attic/second floor connection requires perfect air sealing because fiberglass itself does very little to resist air movement
  • Batts must be cut around every wire, electrical box, and pipe in the wall

Cotton Batts

Cotton batts are made from recycled materials. To work effectively, they require perfect installation around all obstacles in a wall cavity. The material is very difficult to cut: use an electric knife and a manual hedge trimmer for straight cuts.

Cellulose

Cellulose insulation has a high recycled content and is more affordable than foam. Make sure to use only borate-treated cellulose, as any moisture that gets into ammonium sulfate-treated cellulose will cause it to off-gas ammonia.

Note: It is recommended that you use a professional to install cellulose.

Cellulose can be installed two ways:

Dry as loose fill in wall and is kept in place behind netting. But be careful. Too dense, it fills out the netting and makes drywall difficult, if not impossible to install.

Sprayed on. It is mixed with an acrylic binder that holds it firmly in the wall cavity. It can be sprayed in as deep a wall cavity as necessary and stays stable. It has the advantage that it fills around wiring, plumbing, and other obstacles.

Spray Polyurethane Foam

Spray foam tends to be the insulation of choice for zero energy home builders. It has superior air-sealing properties, fills around pipes and wires in the walls for a perfect seal. There are two forms of spray polyurethane foam:

Closed-cell - has a higher R-value, 6 per inch, and is more water-resistant than open-cell foam. It is actually a commercial roofing product with a permeability of less than 1. It has structural properties that increase the strength of a wall by 30% and improves the connection between walls and roofs four times over toe-nailed trusses.

It is a great product for high-wind load areas or for tornado- or hurricane-prone zones. Closed-cell foam should be sprayed only 2 in. to 3 in. per pass rather than all at once because of the heat it creates as it sets.

Note: For cost-savings, you may consider using one pass of closed-cell foam for air sealing and then fill the rest of the cavity with cellulose or fiberglass.

Open-cell foam - has a density of 1/2 lb. per cubic foot. It expands rapidly - up to 100 times its initial volume when applied - but as it is expanded with water, there is no off-gassing. It is vapor permeable, which means it dries faster, but unlike closed-cell foam it is not water resistant. It's R-value is only 3.5 per inch(about the same as fiberglass or cellulose) but its advantage over fiberglass or cellulose is that it still performs as a great air barrier.  Open-cell foam is priced considerably lower than closed cell and can be sprayed to fill the entire wall cavity.

Want to know more about Insulation and Green Building Practices

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