Insulating the Wall Between the House and an Attached Garage
Yes. The shared wall with an unheated garage is a demising partition, so it takes the mandatory minimum rather than the new-home table value. That is U-0.095 for 2x4 framing, usually R-15 batts at 16 inches on center, and U-0.069 for 2x6 framing, usually R-21. Converting the garage to living space changes the rules.
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Why this wall is a demising partition
A demising partition separates conditioned space from enclosed unconditioned space. An unheated attached garage is exactly that. The compliance manual says demising partitions and knee walls are not held to the Table 150.1-A values. They must meet the mandatory wall insulation in Section 150.0(c): 2x4 framing at U-0.095 and 2x6 framing at U-0.069, which wood-framed assemblies meet with R-15 or R-21 batts at 16 inches on center. The tighter new-home U-0.048 for exterior walls does not apply here.
Details that catch inspectors
- No sheathing. The garage face usually has none, so batts with stapled flanges are common. Inset stapling may leave a void, but not more than 2 inches from the stud on either side.
- Cavity sealing. Where QII applies, stud cavities are sealed against the garage and penetrations through the plates are sealed.
- Equipment closets. An interior closet for HVAC or water heating equipment that needs combustion air is insulated to at least the demising-wall R-value, or as the energy report states.
- Air paths. The ventilation rules ask that walls and openings between the house and an attached garage be sealed or gasketed.
The door in the wall
Swinging doors between the garage and the house that must be fire-protected are excepted from the door U-factor in Section 150.1(c)5. Other doors to unconditioned space are held to the table value of 0.20 in new construction. See exterior doors for how that value is shown.
When the garage becomes a room
Once the garage is conditioned, the shared wall is an interior wall, and the garage's outside walls become the thermal envelope. In the manual's addition example, existing wood-framed garage walls must reach R-15 in 2x4 framing or R-21 in 2x6 framing, and QII header and air-sealing items drop out when the cladding stays on. The conversion rules are on the garage conversion answer and legalizing an old conversion.
Space types are defined on the unconditioned space page, and exterior framed walls on wall insulation. See our residential reports.
Common Title 24 Questions
- When is Title 24 required for a home?
- Does a remodel need Title 24?
- Does a room addition need Title 24?
- What plans are needed for a residential Title 24 report?
- How much does a residential Title 24 report cost?
- How long does a residential Title 24 report take?
Frequently Asked Questions
State the mandatory minimum for the framing depth, R-15 for 2x4 or R-21 for 2x6 at 16 inches on center, so the plans match the energy report.
No. That is the new-home exterior wall value. A demising partition is held to the mandatory minimum instead.
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