New IBC rules for insulation in unvented enclosed roof framing assemblies

Options for contemporary unvented roofs

Designers may be confused by the numerous and lengthy requirements contained in section 1203.3. To help, below are some examples of roof assemblies for unvented rafters/trusses or unvented attics meeting the new code requirements, illustrated for simplicity. The examples below are only meant to illustrate code requirement of section 1203.3, and do not address any other rules (e.g. applicable fire code requirements) that may be triggered.

Insulation entirely above the roof deck

Placing all of the IECC code-required insulation above the roof deck with a vapor retarder will provide acceptable hygrothermal performance in all climate zones (Figure 7). This meets the requirements of Section 5.1.4.

Insulation entirely below the roof deck

Section 1203.3 pushes designers away from having all the insulation between ceiling finishes and roofing in an unvented assembly. Designers pursuing this arrangement of insulation must be aware of several caveats. If insulation is entirely below the deck, some of it must be air-impermeable, typically an expansive sprayfoam. If only providing air-impermeable insulation, it must be applied directly to underside of roof sheathing, per Item 5.1.1 (Figure 8). Continuity of the insulation layer in these assemblies is very critical to performance. Note that sometimes continuous insulation below the roof deck may conceal or trap water leakage resulting from a defect in the roof covering.

Alternatively, insulation entirely under the roof deck can be composed of a combination of air-impermeable and air-permeable insulation in order to meet the requirements of 5.1.3. The impermeable insulation must be in direct contact underside of deck, while air-permeable insulation is installed directly below (Figure 9). Table 1203.3 describes minimum insulation value of the air-impermeable layer of insulation for condensation control when combining air-impermeable and air-permeable insulation below the deck that meet the prescriptive IECC code-required insulation thickness. For most practical combinations of insulation layers, this arrangement will protect the assembly from condensation. However, adding a large amount (i.e. well beyond the prescriptive IECC code-required insulation thickness) of air-permeable insulation, which is often not a vapor retarder, may draw the dewpoint inward during cold, winter months, potentially increasing the risk for moisture accumulation.

Insulation both above and below the roof deck

Split insulation assemblies are subject to sections 5.1.2 or 5.1.4. The code offers prescriptive requirements in Table 1203.3, that clarifies the minimum amount of insulation above the roof deck based on the climate zone. It is important to note the code requires progressively more insulation above the roof deck as the design climate zone becomes colder, thereby keeping the plane of the structural deck sheathing above the dewpoint (Item 5.1.2).

Alternatively, less insulation can be provided above the roof deck, provided a performance-based analysis illustrates the temperature of the interior surface of the roof sheathing is maintained above 7 C under normal operating conditions (Item 5.1.4).

Designers must comply with both the insulation values above the deck and total insulation amount prescribed in IECC, which anticipates that insulation categorized as “attic and other” will be interrupted by studs, so designers may rearrange some portion of the total insulation to be on top of the structural sheathing deck while still adding insulation amounts in series (i.e. simple addition of R-values rather than a U-factor calculation that calculates area-weighted contributions of thermally different members in parallel).

The code does not prescribe or bar use of a vapor retarder on the structural deck sheathing in this instance. Designers use their judgment to determine whether the vapor retarder is required to limit vapor diffusion and potential formation of condensation. Figure 10 shows an example roof assembly for a roof in Climate Zone 4a.

Conclusion

These new requirements in section 1203.3 place restrictions on roof assemblies with insulation below structural roof decks in unvented spaces, as the code recognizes the risk of condensation on interior surfaces. In many cases, simple rearrangement of insulation to the top of the structural deck will satisfy these requirements. When insulation must be placed in an unvented attic, designers would have to provide supporting analysis demonstrating materials inside the building enclosure remain above the dewpoint. For any arrangement of roofing and insulation materials unfamiliar to the designer, computer modeling and further consulting is a prudent choice regardless of new requirements in section 1203.3.

Anthony J. Nicastro, PE, is a senior project manager with Simpson Gumpertz & Heger, Inc. in Washington, DC. As a member of the building technology group, he has national experience in the investigation, design, and construction contract administration of building enclosure components for major institutional, commercial, educational, and residential buildings. Nicastro can be reached at AJNicastro@sgh.com.

Megan A. Toumanios, PE, is a consulting engineer with Simpson Gumpertz & Heger, Inc. in Washington, DC. She works in the building technology group on projects involving building enclosure design, rehabilitation, and investigation. Toumanios can be reached at MAToumanios@sgh.com.

FOLLOW
Insulation