Article #139 - Insulation Overview
Insulation reduces heat flow through the building envelope. Gaps in insulation waste energy and can lead to condensation which can damage building materials and cause growth of molds, dust mites, and other biological contaminants.
The effectiveness of insulation is measured by its R-value - the resistance to heat flow. The higher the R-value, the greater the insulating value. The recommended amount of insulation depends on the building design, climate, price of energy, and cost of materials and labor. Choose insulation materials based on the installed cost per R-value per square foot.
While the amount of insulation installed is important, so is the quality of installation. Even small gaps and compressed areas can reduce insulating levels significantly. A study of attic insulation found that just 5% voids in the insulation - typical in many homes - could reduce the overall R-value by over 40%.
It is important to seal air leaks before insulating. Commonly used insulation materials, such as batt and loose-fill products, do not stop air leakage. As air leaks through these materials, it lowers the R-value. For most affordable home designs, materials other than insulation will form the air barrier. There are some insulation products, such as rigid foam sheathings and spray-in-place materials, that can reduce air leakage as well as insulate.
The 2006 IECC requires the following insulation values:
| Climate Zone | Fenestration U-factor | Skylight U-factor (2) | Glazed Fenestration SHGC | Ceiling R-value | Wood Frame Wall R-value | Mass Wall R-value | Floor R-value | Basement Wall R-value (3) | Slab R-value, Depth (4) | Crawlspace Wall R-value (3) |
|---|---|---|---|---|---|---|---|---|---|---|
| 1 | 1.20 | 0.75 | 0.40 | R-30 | R-13 | R-3 | R-13 | R-0 | R-0 | R-0 |
| 2 | 0.75 | 0.75 | 0.40 | R-30 | R-13 | R-4 | R-13 | R-0 | R-0 | R-0 |
| 3 | 0.65 | 0.65 | 0.40 (5) | R-30 | R-13 | R-5 | R-19 | R-0 | R-0 | R-5/13 |
| 4 except marine | 0.40 | 0.60 | NR | R-38 | R-13 | R-5 | R-19 | R-10/13 | R-10, 2 ft | R-10/13 |
| 5 and marine 4 | 0.35 | 0.60 | NR | R-38 | R-19 or 13+5 (7) | R-13 | R-30 (6) | R-10/13 | R-10, 2 ft | R-10/13 |
| 6 | 0.35 | 0.60 | NR | R-49 | R-19 or 13+5 (7) | R-15 | R-30 (6) | R-10/13 | R-10, 4 ft | R-10/13 |
| 7 and 8 | 0.35 | 0.60 | NR | R-49 | R-21 | R-19 | R-30 (6) | R-10/13 | R-10, 4 ft | R-10/13 |
- R-values are minimums. U-factors and SHGC are maximums. R-19 shall be permitted to be compressed into a 2x6 cavity.
- The fenestration U-factor column excludes skylights. The SHGC column applies to all glazed fenestration.
- The first R-value applies to continuous insulation, the second to framing cavity insulation; either insulation meets the requirement.
- R-5 shall be added to the required slab edge R-values for heated slabs.
- There are no SHGC requirements in the Marine zone.
- Or insulation sufficient to fill the framing cavity, R-19 minimum.
- "13+5" means R-13 cavity insulation plus R-5 insulated sheathing. If structural sheathing covers 25% or less of the exterior, insulating sheathing is not required where structural sheathing is used. If structural sheathing covers more than 25% of exterior, structural sheathing shall be supplemented with insulated sheathing of at least R-2.