GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Insulation Between Rafters AND on the Attic Floor

itseasybeinggreen | Posted in Building Code Questions on

I am redoing the insulation in my attic, cleaning out all of the old, air sealing, and then putting in new. I live in the Los Angeles area.

In my research I came across Title 24, California green requirements for new construction and certain remodeling and such.

I noticed that in my zone, and many others, they are now requiring both roof and attic floor insulation for vented attics with ducts in the attic.

Under the roof deck, between the framing, it’s either R-18 when the roofing has no air space (asphalt) or R-13 when there is an air space (tile), and then on the floor it’s I think it’s now a minimum of just R-19. Lots of details depending on how you’re building and the zone, like in some cases a radiant barrier is required along with both roof and attic floor insulation. But those don’t apply to me.

So now that I’ve seen this I’m wondering if I should go ahead and insulate between the rafters, putting in R-19 (2×6 rafters), or maybe R-13 to give some breathing room under the deck, and then probably R-49 on the floor.

This is the first time I’ve seen anywhere on the interwebs to insulate both roof and floor in a vented attic. Usually the recommendation is to seal the attic when you’re doing such a thing.

What say you? I’m confused.

Here is the link, page 44:

Thanks all.

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. Expert Member
    BILL WICHERS | | #1

    The only thing I can think of is that they’re trying to keep attic temperatures down to help keep attics from radiating into the living space. Vented attics can still get VERY hot on a sunny day. A radiant barrier is probably your friend here, if applied under the rafters in such a way that it would have the required air space above it.

    This might be a great place to use foil faced polyiso. Nail the sheets up under the rafters and you have a radiant barrier from the foil, continuous insulation so no thermal bridges, and a nice vent space between the sheathing and the insulation too.


  2. Jon_R | | #2

    I find it interesting that they didn't consider ducts in the attic enough of an energy loss to ban it completely. My guess: they have data to back this up. Ie, done right and with their specs, it's OK.

    1. severaltypesofnerd | | #25

      My guess is they wanted to ban ducts in the attic, but feared the pushback....

  3. itseasybeinggreen | | #3

    Thanks Bill. I hear you. If I were to follow the guidelines I wouldn't need a radiant barrier, but would need R-18 between the rafters, so really R-19 for a 2x6 using batts. To get that in polyiso I'd need 3 inches of the stuff I think, which is pretty thick, and expensive.

    I'm sure I can get away with just redoing the floor, who is going to check, but if following the rules is doable and not too much more expensive, then why not. I am just confused by the rules.

    1. Expert Member
      BILL WICHERS | | #5

      If the title 24 rules allow you to also go by assembly U factor, you might be able to use less total R value of continuous insulation due to the lack of thermal bridging.


  4. RussMill | | #4

    Ive seen this done, voluntarily in Cali, South Africa AND Australia. Without ducts in the attic even. I concur with the first response to your question. BAN ALL ATTIC HVAC DUCTS unless fully conditioned!

    1. itseasybeinggreen | | #6


      In my mind it would be helpful to have an airway between the roof and the insulation, but at R19 there wouldn't be. Also, could I just put up kraft faced fiber? I mean once I'm done with the attic I hope to never have to go up there again. Or would I be better off with say Roxul?

    2. JC72 | | #8

      That'll never happen because it'll just raise the cost of construction due to the extra space and design constraints of moving the HVAC/Furnace inside the housing envelope.

      I suspect builders would rather just be required to use unvented attics instead (spray foam the underside of the roof deck) in milder climates.

      1. RussMill | | #12

        Either or i could go along with.

  5. GBA Editor
    Martin Holladay | | #7

    Code writers have been frustrated for years by the difficulties associated with convincing HVAC contractors to locate ducts inside the conditioned space of a house. This new code mandate is an innovative way to require at least some insulation between the rafters when builders make the blunder of locating ducts in the attic.

  6. Expert Member
    Peter Yost | | #9

    High performance architect Steve Baczek has an interesting way of expressing this:

    How come any architect can routinely find room for a powder room in a home but can't design for ducts in conditioned space? It's just an additional design constraint to incorporate into your work as an architect.

    I know this string is about renovation but quite often mechanical systems--particularly distribution--are NOT part of design but superimposed after the fact.

    When working at Building Science Corp in the Building America program, I once heard an HVAC contractor say: You want to know my definition of non-structural? Anything that's in my f_____g way...



    1. Expert Member
      BILL WICHERS | | #11

      >”When working at Building Science Corp in the Building America program, I once heard an HVAC contractor say: You want to know my definition of non-structural? Anything that's in my f_____g way...”

      I’ve seen this too. I was on a big renovation project a long time ago in a catholic school. The architects drew the flue for the boiler (flue was maybe 24” diameter) going through a massive concrete beam. The coring contractor said “no f***ing way!” To the GC when he took the time to see what that beam was holding up: an approximately 75 foot high masonry bell tower. Obviously the architect never bothered to check.

      I think it’s just laziness on the part of the design team. The same goes for all mechanical spaces, commercial and residential: code minimum clearances are often not really big enough, but the architects don’t want to “waste” any space on the non-pretty areas.

      All mechanical spaces should be designed with serviceability in mind. Leave enough room, leave a service outlet for powering tools, and put a decent amount of light. All the repair guys will appreciate it, and the owner of the house will spend less on labor hours since repairs can go more quickly. This is another reason to keep mechanical systems out of attics!


      1. brendanalbano | | #13

        We've already thrown contractors and architects under the bus, so lets finish spreading the blame around. Clients also don't want to "waste" any space on the non-leasable/programmable floor area. :)

        1. Expert Member
          MALCOLM TAYLOR | | #14

          The house design I just finished is a good example. I provided a small conditioned attic for the HRV and the duct runs. The owners found out the code allowed an exhaust only ventilation system using the bathroom fans, and that attic will now probably house Christmas ornaments and old photos.

    2. Aedi | | #15

      This is only tangentially related, but your anecdote reminds me of a story I stumbled across on the internet: A thread by a DIY homeowner who decided what his bathtub really needed was a sunken rock garden around it, and wasn't going to let any pesky TJIs get in the way:

      I attached the image below as an example. Be sure to read through for the homeowner's self-engineered proposed fix!

      It just goes to show even the best laid plans of designers and builders are no match against a homeowner with a Sawzall.

      1. Expert Member
        BILL WICHERS | | #16

        That’s just scary. I’ve see too many and/or too big holes drilled through the web plenty of times, but I haven’t seen sections of the chords cut out before. From the look of the room and the placement of that recessed can light, it looks like this might be somewhere midspan in the floor too.

        He might have some problems when he fills that tub...


      2. Expert Member
        MALCOLM TAYLOR | | #19


        Tangentially related perhaps, but a very enjoyable read!

  7. Expert Member
    Dana Dorsett | | #10

    >"...they are now requiring both roof and attic floor insulation for vented attics with ducts in the attic."

    Title 24 would allow roof-only insulatoin if the attic were UNvented and the roof insulation met the prescriptive performance levels.

    >"...would need R-18 between the rafters, so really R-19 for a 2x6 using batts. "

    When compressed to 5.5" in a 2x6 framing bay an R19 batt performs at R18, which is probably why it was written that way.

    Note that with an air space between insulation and roof deck they would allow R13, so with rafters deeper than 4.5" you could get there with contractor roll R13s leaving at least an inch of space between the fluff and the roof deck.

    >"To get that in polyiso I'd need 3 inches of the stuff I think, which is pretty thick, and expensive."

    Reclaimed fiber faced roofing polyiso would need to be 3.25" or thicker to hit R18, but is usually pretty cheap compared to batts. R13 with most roofing would only take 2.5". Both 2.5" and 3.25" are common thicknesses (as are 2" and 3".) Mounting it to the underside of rafters, thermally breaking the rafters is higher performance and less work than a cut'n'cobble job stuffing it between rafters.

  8. itseasybeinggreen | | #17

    The way I read it, the air space is between the roofing materials and the deck, not under the deck and between the insulation. So in my case, with asphalt shingles, I'd need R-18 between the rafters. You'd need a tiled roof or other to be able to use R-13.

    Regarding the polyiso, sounds like an interesting solution except I have no way to get it into the attic. I'd have to cut the 4x8 sheets in half lengthwise to get them through the attic access.

    Regarding using batts, should these be faced or unfaced? With there being no finishing material over the rafters I'm concerned about the kraft being exposed. Or could I use unfaced with strapping or insulation wire?

    Thank you.

  9. cr0ntab | | #18

    I found the same title 24 stuff, but I did it voluntarily.

    I'm in the process of DIY-ing the insulation install in the attic. It's been far more work than I gave it credit for:

    I regret not just paying the extra money for spray foam, but it is what it is.

    I have a concrete tile roof on my home in CA and the trusses are 2x4 so not very deep. I ended up with the mineral wood insulation and it's worked out well.

    I can see huge differences in heat with my Flir camera on areas with/without the insulation.

    It's starting to warm back up so I don't know how much more progress I'll make over the summer, but slow and steady.

    1. itseasybeinggreen | | #20

      Same conclusion, Robert. I decided to clean it out myself (old dirty faced batts on top of a few inches of matted down blown fiber, coupled with construction debris, and other random odds and ends) and it's proven a chore for sure.

      Still not sure I'd hire it out. My experience with contractors over the years hasn't been stellar, particularly when they think you know nothing about what they are doing. And not many people know much about what's going on in their attic. Plus I have a hard time paying a premium for unskilled labor.

      Regarding the roof batts, I see you went unfaced, no vapor barrier. Did you do the same for what's on the ceiling? I keep reading differently about a vapor or no vapor in my zone. I'm in zone 3. BuildingScience says no vapor in zones below 6. Others say vapor on ceiling, etc.

      The confusion continues.

      1. Expert Member
        Dana Dorsett | | #21

        >" BuildingScience says no vapor in zones below 6. Others say vapor on ceiling, etc."

        In a VENTED assembly in zones 5 & lower no vapor barrier is necessary, providing the venting was done reasonably. The same would be true for a vented roof deck. But without soffit to ridge venting between the roof insulation and the exterior the IRC calls out a minimum of R5 air-impermeable insulation directly against the roof deck if unvented.

        But if the attic space between the floor insulation and roof insulation is VENTED the risk is vanishingly small.

        Depending on density/air retardency you may need an interior side air barrier (probably not needed- are those cellulose batts?), that would have to be at least semi-permeable to water vapor. Using PERFORATED aluminized fabric type radiant barrier to restrain & contain the batts could perform that function. That also protects the batts from mechanical damage while working in the attic, and keeps them from coming down in an earthquake. Perforated RB runs ~5 perms- definitely somewhat vapor retardent, but far from vapor barrier territory- the roof deck could still dry into the attic space.

        1. itseasybeinggreen | | #22

          Dana, I that all makes sense to me.

          Current ventilation is 2 gable vents, 2 whirlybirds, and a few soffit vents. Very little soffit area on this roof.

          From what I've seen, even with faced fiber laid on top of blown fiber, who knows how many years ago, the doesn't seem to be any moisture problems.

          I plan on adding as any soffit vents as possible while I'm at it. Soffit to ridge baffles will be installed, but would have to stop short of the ridge to maintain air flow.

          I already have some RB I bought long ago that I planned to install, so I could easily staple that over the rafter batts. I'd be using fiber batts if I end up doing them.

          One last thing. So is there a problem using faced batts on the ceiling/floor, or would unfaced breathe better?

      2. cr0ntab | | #23

        I'm with you, I wouldn't hire it out. I don't trust contractors to be as thorough as I am.

        In terms of paying extra money for spray foam, this is what I meant:

        In terms of insulation, this is what I went with:

        I forget which matters in which context so I just list them all....

        California Climate Zone 10
        USDA Climate Zone 9b
        ASHRAE Climate Zone 3b

        I went off of this:

        Which says:

        R13 below roof deck (I have concrete tiles so I have an air space)

        R38 ceiling insulation - I haven't completed this still doing the roof deck then I'll revisit the ceiling. Though the house did come with some pretty thick fiberglass insulation, haven't looked at the R value of that.

        No radiant barrier required - Though I will likely put one in over the insulation. I already have a roll of this stuff laying around from another project:

        I have a vented attic that includes soffits and gable vents. All my HVAC equipment is up there so I wanted to create a better environment for that equipment.

        1. itseasybeinggreen | | #24

          Your attic sounds just like mine except the framing.

          I plan on using fiber just because of the weight of Roxul and it seems I can get a much bigger utility rebate on fiber vs Roxul, but need to verify. From what I've seen Roxul is 3 to 5x heavier than fiber. I calculated I'd be putting in something like 4,000 lbs of the stuff or something. I don't remember exactly. But it was a lot and I didn't feel comfortable with that. And that was before I thought about doing the roof.

          Thanks for your input. Good luck!!!!!

  10. severaltypesofnerd | | #26

    Just sayin' you won't get title 24 credit, but a great way to reduce your attic temperature is with a metal sheet on the south facing roof, about 4-5 inches above the deck.... that happens to also be solar panels.

  11. Kirk_Ellis | | #27

    I just found these Title 24 requirements for attic insulation and none of the "options" they list seem to allow what I had planned for my new build in the local mountains Title 24 Climate Zone 16. My intent was to build a two story box, sheath it all, including an attic floor above ceiling joists, then wrap it all in peel and stick Grace Select, then frame a roof on top, including 2' high walls at the eaves and rake walls starting from 2' rising to 10' at the ridge. Install soffit vents with baffles between rafters and ridge vent, then cover the floor with 20" of cellulose for an R60. No equipment or ductwork in the attic, the equipment will be in a room on the second floor inside the conditioned space. Ductwork would run through the ceiling joists.

    This is how I've always envisioned a cold roof, vented attic, per Joe Lstiburek, Steve Baczek, et al. But it sure sounds like this Title 24 wants insulation under the roof sheathing even for a vented attic. My plan would be about $8K less in materials than a conditioned attic with rooftop insulation I have no use for. I could understand RB under the rafters to prevent wind washing the blown-in insulation and reflecting radiant heat back up through the roof, but actual batts between rafters ? What ever happened to not leaving voids in insulation ? Now they want a big empty space between insulation in the rafters and on the floor ? It also says insulation must be in contact with the ceiling drywall, assuming there is no attic floor deck. They say against the "infiltration barrier", so does an attic floor deck still meet that definition, they just don't mention it ? Are attics with floors so uncommon ?

    1. Jon_R | | #29

      > Title 24 wants insulation under the roof sheathing even for a vented attic

      Option C allows all of the insulation to be on the attic floor. The other options apply if there are ducts in the attic.

  12. user-6184358 | | #28

    The Wildland compliant vents are $$ - limiting soffit vents to helps with cost- most are going with on the roof vents placed high and low on the slope. I don't like a continuous ridge vent due to the seismic diaphragm issues. and lack of ridge blocking.
    To answer you title 24 question - find a title 24 consultant to work with and as them about your plan.

    1. Kirk_Ellis | | #30

      I don't trust those roof vents in mountains where snow can pile 3' high on the roof. Or even have wind driven snow end up in the attic wetting my cellulose. The ridge seems to stay clearer of snow than lower roof especially with the warmer air exiting along the ridge. I intend to have the roof decking extend 1 1/2" past the fascia and wrap 1/16" stainless steel screen from decking down to fascia, along with a 1/4" mesh 1/2" in front of that to prevent birds from poking holes in the stainless screen. All of which would be hidden behind the drip edge. 1/16" screen I think meets the fire marshal recommendation for an ember-stopping vent without any expensive special products. As far as ridge blocking goes, can't the opposing rafters be blocked for their full height except for the top 1 1/2" and the decking stop 3" short of the ridge ? Is the diaphragm issue you mentioned because there is no blocking at the edge of the decking ?

      I read through Title 24 last night. Not all 363 pages, just the sections that mention insulation. Clear as mud. It ticks me off that I probably will need to pay for a Title 24 consultant. Plus, apparently a lot of other consultants, since it seems like no permits will be issued without compliance certificates of every detail, and those certificates have to be done using approved software and filed electronically.

      They SAY "owner builder" is still accepted, but these regulations sure are discouraging. Architects and designers always say you should build to suit your own needs, not some future owner, but the State sees it reversed. I'd like to save a bunch of money with homemade materials and making material choices at the last minute as costs fluctuate, but they won't issue a permit unless all materials are specified ahead of time, manufactured and approved and tested per their standards. You'd think it was THEIR money and money is no object to them.

  13. user-6184358 | | #31

    A title 24 report on a single family home is a $2-300.00. The custom fire vents will need to be checked with your building inspector. The Wildland code has a test method for approval. The county of San Diego is a good website & resource for Wildland construction drawings and info.
    The roof blocking is usually installed at the ridge so the roof diaphragm can be continuously edge nailed at the change in roof pitch & wall lines. It will depend on your engineer as to how it needs to be done. The 1.5" drop blocking is not a standard practice in California for seismic issues.
    As for you changing materials & design changes, that ended decades ago in California, except for Mendocino County, who has an owner builder warning that is attached to the house title, it allows owners to build what they want.
    Ask your insurance company how they want the house built. They are one of the drivers of better built houses.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |