New IBC rules for insulation in unvented enclosed roof framing assemblies

Authors’ interpretation: An air-permeable insulation, such as a fiberglass batt, on the underside of the roof deck must be used in conjunction with insulation above the roof deck (i.e. ‘split insulation’ assembly). Table 1203.3 lists minimum R-values of air-impermeable insulation to be used for each climate zone to limit potential for condensation within the roof assembly (Figure 5).

The values in Table 1203.3 must be used in conjunction with the total R-value required in IECC. Also, this provision does not presume or exclude a vapor retarder applied to the top of the roof deck. Some roof assemblies with split insulation require project-specific analysis using hygrothermal computer modeling software to predict whether a vapor retarder should be included on the structural deck or not.

5.1.3. Where both air-impermeable and air-permeable insulation are provided, the air-impermeable insulation shall be applied in direct contact with the underside of the structural roof sheathing in accordance with Item 5.1.1 and shall be in accordance with the R-values in Table 1203.3 for condensation control. The air-permeable insulation shall be installed directly under the air-impermeable insulation.

Authors’ interpretation: Where both air-impermeable and air-permeable insulation are below the roof deck, the air-impermeable insulation must be tight against the roof deck. The minimum amount of air-impermeable insulation required is listed in Table 1203.3. The air-permeable insulation must be tight to the underside of the air-impermeable insulation (Figure 6). The need for a vapor retarder in this assembly is discussed in Item 4 above.

5.1.4. Alternatively, sufficient rigid board or sheet insulation shall be installed directly above the structural roof sheathing to maintain the monthly average temperature of the under-side of the structural roof sheathing above 45 F (7 C). For calculation purposes, an interior air temperature of 68 F (20 C) is assumed and the exterior air temperature is assumed to be the monthly average outside air temperature of the three coldest months.

Authors’ interpretation: Adding insulation above the structural roof sheathing increases the temperature of the structural roof sheathing during cold winter months. Designers who comply with 5.1.4 may use a combination of insulation—when used with rigid board insulation—if a computer software model can demonstrate the average temperature of the roof sheathing meets the stated criteria. Such simplified and prescriptive criteria is an indicator of resistance to condensation on interior surfaces given relative humidity (RH) within the operating parameters described earlier in the section (e.g. less than 35 percent). Air at 20 C and 35 percent RH has a dewpoint of 1 C (39 F), which is safely lower than the 7 C threshold required by this provision. The given boundary conditions capture a reasonable degree of situations for non-humidified buildings. Humidified buildings are excepted from these provisions and require their own project-specific analysis.

5.2. Where preformed insulation board is used as the air-impermeable insulation layer, it shall be sealed at the perimeter of each individual sheet interior surface to form a continuous layer.

Authors’ interpretation: The seams at the interior surface of an air-impermeable board insulation placed below the structural deck must be sealed to create a continuous air barrier at the interior surface, thereby preventing interior conditioned air from penetrating through or partially through the insulation layer at seams.

Note the 2015 IBC errata issued on November 16, 2015, changes the text in Item 5.2 from “air-permeable” to “air-impermeable.”

Exceptions

1. Section 1203.3 does not apply to special use structures or enclosures such as swimming pool enclosures, data processing centers, hospitals, or art galleries.
2. Section 1203.3 does not apply to enclosures in Climate Zones 5 through 8 that are humidified beyond 35 percent during the three coldest months.

Authors’ interpretation: Special-use buildings or those with elevated interior RH require special engineering. Prescriptive guidelines of this section do not apply. For instance, interior RH can create a strong vapor drive from interior to exterior in winter, creating the need for a Class I winter-warm side vapor retarder, omitted from this section.