Breaking down energy needs: Insulation (Part 1)

An often-mentioned number in home building is “R-value” or “R factor”. The R-value is a measure of a material’s resistance to heat transfer and is used to compare different types of insulation. It is calculated using the thermal conductivity constant, k:

R = [thickness (in inches)] / k
k = [BTU * inch]/[ft^2 * hours * ˚F]

Insulation must be treated as a whole-structure concept. Just as the amount of insulation in your home means little if you leave your door wide open, using very high-R material in the walls may provide little return if you use low-R windows. It’s all in the rate of heat transfer.

The following table provides some common measurements for R-values compiled from various sources. A few things to note: R-values are calculated under ideal laboratory conditions and may vary considerably “in the field” (more on this later); these R-values are valid for wall, roof, or floor calculations; R-values for green roofs can be complicated and will also be discussed later.

Material	R-value per inch thickness	R-value per unit
Metal	0.0
Concrete	0.08
Gypsum	0.6 – 0.9
Hardwood (oak, maple)	0.91 - 0.94
Softwood (fir, pine)	1.3
Plywood	1.25
Fiberglass	3.0 – 3.8
Strawbale	1.45
Sand/gravel	0.6
Stucco	0.2
Brick	0.1 – 0.2
Asphalt shingles	0.44
Aluminum/Steel Siding		0.61/panel
Loose cellulose	2.8 – 3.7
Loose fiberglass	2.2 – 4.0
Loose rock wool	3.1
Loose vermiculite	2.2
Molded polystyrene	3.6
Extruded polystyrene	3.6
Polyurethane foam	5.6 – 6.2
Sprayed cellulose	3.0 – 4.0
Carpet (with padding)		1.2 – 2.1/layer
Air (3/4” – 4”)		0.9
Windows
- Single		0.76 – 1.1/unit
- Double		1.2 – 2.2/unit
- Triple		1.3 – 2.6/unit
- Double, low emissivity		1.3 – 2.9/unit
Cement mortar	0.2
Vinyl		0.05/layer
Soil (fine, 20% moist)	0.08

Coming up next: The Fallacy of R-value Tunnel Vision

References:
Roofhelp.com
Oswego University
Grassroots