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Garage insulation – attic space options?

funkyhunky | Posted in Green Building Techniques on

When I moved in my garage was unfinished and basically unusable in our hot CA valley summers. Right now I’m finishing the garage walls with insulation and am wondering how to insulate the attic space above once I get ceiling sheetrock up. 

Right now the garage is firewalled up to the exposed rafters and there are no roof vents. If I put up ceiling drywall as is, the attic would be unvented and get real hot. To help keep it cooler I was thinking about adding a few vents to get some air flow, maybe a gable fan, and throwing some thick bats on the ceiling sheetrock and calling it a day

But since I plan on reroofing within the next year, my buddy said to look at CA title 24, and I see that they prescribe putting insulation directly against the roof decking. Which would be easy for me to do right now while the roof and support members are totally exposed. But at the same time I am reading that insulation against roof decking can cause a lot of issues unless it’s very well ventilated. Right now there’s no soffit, no vents, just exposed rafter rails with blocking between them. 

My first question is when I do reroofing for the whole house, do I have to follow title 24 for roof area above my garage, that is firewalled off from the rest of the attic and doesn’t contain HVAC equipment in the attic?

My second question is what do you think I should do regarding the garage attic, to help keep it cool in hot CA summers?

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  1. Expert Member
    BILL WICHERS | | #1

    If you can add vents, you need a ridge vent and soffit vents. That's the most common way. What will NOT work is just a few vents on top of the roof. The vents up high (ridge vent) are the "out", but you still need an "in" down low (soffit vents). You need air FLOW, not just a single vent. Don't use any power vents (gable fans, etc.).

    If possible, I'd vent the assembly and use loose fill insulation on the attic floor. That's usually the best/cheapest way to go, but assumes you have a truss-style roof, or rafter ties, something to make a flat attic floor. If you have a sort of catherdral ceiling in your garage, then you either need to build vent channels and use batts (or dense pack cellulose), or you need to go with closed cell spray foam against the underside of the sheathing for an unvented assembly.

    I wouldn't expect title 24 to apply to a garage, since the garage isn't living space and is usually not even conditioned space, but I don't really work with California energy codes so I don't know for sure.


  2. funkyhunky | | #2

    Yes, it's a truss style roof, and I'll 100% put insulation on the attic floor/garage ceiling and vents on the eaves.

    Thing is Title 24 appears to also require insulation against the roof decking, which requires a robust soffit/ridge vent system to prevent moisture buildup where the insulation meets the decking. But this seems excessive for a unconditioned garage.

    If I don't have to do this, I could put some simple eave screens and roof vents in, install the ceiling/insulation/lighting and be done with it.

  3. mr_reference_Hugh | | #3

    I am not a registered professional in any jurisdiction in the world :). I just love reading technical documents and learning. Hoping that my reading of Title 24 helps.

    Title 24 definition to know about

    RESIDENTIAL SPACE TYPE is one of the following:
    Garage is a nonhabitable building or portion of building, attached to or detached from a residential dwelling unit, in which motor vehicles are parked.
    HABITABLE SPACE is space in a building for living, sleeping, eating or cooking, excluding bathrooms, toilets, hallways, storage areas, closets, utility rooms and similar areas. (See also “occupiable space”.)
    OCCUPIABLE SPACE is any enclosed space that is intended for human occupancy, including all habitable spaces as well as bathrooms, toilets, closets, halls, storage and utility areas, laundry areas, and similar areas. (See also “habitable space”.)
    Utility room is a nonhabitable room or building which contains only HVAC, plumbing, or electrical controls or equipment; and which is not a bathroom, closet, garage, or laundry room.

    *************Back to the issues you raised in sequence you raised them****************

    $$$ Isssue #1 - "To help keep it cooler I was thinking about adding a few vents to get some air flow, maybe a gable fan, and throwing some thick bats on the ceiling sheetrock and calling it a day"

    My comments: You may not need to pay for the co$t of the fan, the co$t of the install, the electricity that it co$ts to run a fan. If you really don't, then why would you????

    You could read this Energy Vanguard article which is about mechanical venting of attics with fans. It is not directly applicable to your situation but it explains the issue about keep an attic (or spaces open to an attic) cooler. I should also address your idea for a very simple solution of batt insulation and drywall which I would support (I am not an expert :)

    $$$ Issue #2 - "But since I plan on reroofing within the next year," AND "when I do reroofing for the whole house"

    Choosing a light colour for your roof is going to make a big difference. Reference the same article as above (link repeated). It speaks specifically to the impact of the colour of the roofing materials.

    The article mentions choosing a reflective finish like galvanized or galvalume, if and only if you install a metal roof. Here is a webpage that compares both products.

    $$$ Issue #3 - "look at CA title 24, and I see that they prescribe putting insulation directly against the roof decking".

    My comments:
    (But see my comments under Issue #4 also)

    Title 24
    (a) Roof deck, ceiling and rafter roof insulation.
    4. Insulation shall be installed in direct contact with a roof or ceiling which is sealed to limit infiltration and exfiltration as specified in Section 110.7, including but not limited to placing insulation either above or below the roof deck or on top of a drywall ceiling.

    Circle back to the article I reference in Issue #1. This article also covers your concerns about insulation directly against the roof decking. Also read through my comments on Issue #4.

    Link repeated =

    $$$ Issue #4 - Does Title 24 apply to the residential garage.

    My Comments:
    Consider that Title 24 covers almost all aspects that a building code could cover. So the question is very broad. Consider insulation, lighting controls, fire code requirements that fall under Title 24.

    *********Lighting controls***************

    Title 24 for example covers requirements for lighting controls under Section 130.


    Partial-OFF occupant sensing controls. Partial-OFF occupant sensing controls are required for specified stairwells and common area corridors, parking garages, parking areas ...

    Interior areas of parking garages are classified as indoor lighting for compliance with Section

    ********Insulation and other related***********

    Title 24 covers insulation and related issue under Section 150.0 (a)

    Garages like yours appear to be EXEMPT from insulation/air sealing requirements based on Title 24 Sub Chapter 7 Section 150.0 (a) because the space is not conditioned and it does not house HVAC equipment (utility closet). Note that Section 150.0 says to see Section 150.2 for "alterations" but section 150.0 (a) exempts you (I am not an expert) so it appears pointless to read 150.2.



    Single-family residential buildings shall comply with the applicable requirements of Sections 150(a) through 150.0(v).

    (a) Roof deck, ceiling and rafter roof insulation. The opaque portions of roof decks separating attic spaces from ambient air, and ceilings or rafter roofs separating conditioned spaces from unconditioned spaces or ambient air, shall meet the requirements of Items 1 through 4 below:

    $$$ Issue #5 - The size of your garage

    My comment: Are you lucky enough to have a garage for 8 or more vehicles? Unlucky for you, Title 24 treats these differently. See below:

    "5. Residential garages for eight or more vehicles. Lighting for residential parking garages for eight or more vehicles shall comply with the applicable requirements for nonresidential garages in Sections 110.9, 130.0, 130.1, 130.4, 140.6 and 141.0."

    If you managed to make it to the end, congratulations. I am going to have dinner!

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