GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter X 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

Old Victorian with no sheathing – Insulating walls but code requires fire blocking

oaklandcavictorian | Posted in Energy Efficiency and Durability on

Hi all.  I have read Martin Holladay’s article (, and related newer articles here ( and here (

One thing all of these articles have in common is that they assume the stud bay can extend vertically from floor to ceiling, or in the case of my grand old 1890 Victorian home (Oakland, California) with balloon framing, from basement to attic.  Unfortunately, when I pull out the lathe and plaster walls to “gut retrofit” my wall assemblies and insulate them, my local code will be requiring the addition of fire blocking every 8 ft high and at the floor and ceilings of every floor.

What are your suggestions on achieving good vapor permeability and dry-out of stud bays that have substantial horizontal fire block interruptions in each stud bay cavity?  I have some ideas, but I want to hear what folks much more experienced than I have to say about it!  Thank you.

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. aaron_p | | #1

    Per the IRC, fire blocking doesn't have to be wood blocking - in 2015 version it is section R302.11.1 Fireblocking Materials. There are a bunch of wood options and then there are options for Mineral or Glass Fiber - it appears these generally need to be batts or blankets. It doesn't appear loose-fill is allowed unless that insulation is specifically tested for that.

    I believe this should alleviate the expense and concerns with traditional lumber fire blocking.

  2. Mike_J | | #2

    R302.11.1 in the IRC allows the use of mineral wool for fire blocking, which is a common way of handling areas that are hard to block with 2x material. The code does not state a minimum thickness for mineral wool but I have found installation easier with 5-1/2" batts. If they are cut a little large and compressed into place they meet the "securely retained in place" requirement without any extra work.


  3. Deleted | | #3


  4. Expert Member


    If your code does require wood fire-blocking, I'm not sure it makes any difference to the permeability or drying of the walls. It basically just transforms your framing from balloon to platform. Walls don't dry vertically, and wood is permeable. I don't see it causing any issues.

    1. abbyk | | #12

      Hi! Can you explain your statement that "walls don't dry vertically?"

      I am new to my "old" home and am currently researching how to handle a relatively complicated roof assembly, and have a potential retrofit solution, but have concerns about potential repercussions of air sealing where the top plate meets the roof assembly/attic, in case the walls DID ventilate that way. I'm in a hot/humid climate and trying to solve for comfort/air quality (without inviting mold into the walls/attic, of course) and if air sealing here is okay, I think I may be in business! As stated I'm new to it and have poured into roof assembly research, but have hardly scratched the surface on wall assemblies. I ask pros where I can, but most of the contractors I've had access to for 2 years only seem to understand new construction "this is how we do it" and not really the "why's" or how it works in order to help me troubleshoot.


      1. Expert Member
        MALCOLM TAYLOR | | #13


        There is a fundamental difference between the way old houses with lots of air-leaks function, and what happens once you decide to add insulation. The walls in old houses rely on lots of warm air moving through them to stay dry - and it is generally quite effective, except that there is a huge energy and comfort penalty to pay.

        Once you add insulation you want to design your walls to dry by diffusion, either to the outside, interior, or both. You don't want any air movement in your walls, or to use air-movement as a mechanism to dry them. The only exceptions to this are purpose build ventilation channels on the exterior (rain-screens on walls, ventilation spaces on roofs). What you do want is an effective WRB and cladding on the outside to limit water intruding that way, and an effective air-barrier and interior vapor-retarder to stop moist inside air making it's way into the walls.

  5. Expert Member
    BILL WICHERS | | #5

    I don't see any issue with fire blocking either, especially if you have some amount of inward drying potential. If you're worried, mineral wool would allow for vapor diffusion while still acting as a fire block material. Note that some local codes require mineral wool to be covered with fire putty pads too when used as a fire block, and that putty is NOT vapor permeable. If you want to use mineral wool here, I recommend you check with your local building department first to see if they will allow it uncovered.

    Personally, I'd just put in wood fire blocks in the usual way and not worry about it. Unless you really need an open vent channel, similar to how a rain screen works, the blocking shouldn't really change much in terms of drying inside the wall.


  6. Expert Member
    Akos | | #6

    You want solid blocking, not insulation. To have any hope of air sealing a balloon frame house you have the stop the giant chimeny from the basement to the attic. Wood blocking cut to size and spray foamed in place is the best. Make sure to also install blocking at the end of each floor joist bay, this is also big leak path.

    Lot of times blocking in the attic is completely missing both on interior and exterior walls. This needs to be added in as well.

    Insulating houses with no sheathing is challenging, without proper detailing you are asking for mold in your walls, I would read through here first:

  7. oaklandcavictorian | | #7

    Thanks for all of the helpful replies. My secret idea was indeed to use mineral wool as the horizontal fire blocking member. I am glad I am not crazy!

    Note that our home is on the local historic register and we must not mess with the exterior. I should have also mentioned that we are currently considering two insulation wall assemblies. One is an homage to Kiley ( with the outboard to inboard assembly being {exterior siding, 1/2" air gap, 1/2" vapor permeable rigid polyiso, 2.5" dense-pack cellulose (15% or higher borate) blown to 3.5 pcf density, intello plus smart vapor retarder and air barrier as inboard netting for the cellulose}. But that 1/2" air gap would get interrupted every 8' vertically by a full depth mineral wool fire block.
    The other assembly, again listed outboard to inboard, would be: {1/2" drainage mat or rainscreen, high perm drainable housewrap stapled to the studs and wrapped along the full face of each stud bay, 3" cellulose same as above, intello plus netting same as above}. The 1/2" rain screen would be interrupted every 8' vertically by the full depth mineral wool fireblock, and wrapping the housewrap would become very difficult in practice, so we are leaning toward the former assembly even with the higher embodied CO2e of the polyiso foam.

    I have added some photos of the now gutted walls (i.e. with lathe + plaster removed). You can see the vertical 2x4 studs and the exterior shiplap "clapboard" siding... which is the only thing between exterior and interior. There are areas of "daylight" showing and signs of moisture weeping in at the seams during these recent winter rainstorms. No evidence (so far) of water pouring down the inboard face of the siding, but it is clear that even if we close any current gaps in the shiplap, we should expect substantial winter moisture. Likewise, we are installing an air source heat pump system which will contribute to moisture the reverse way during summer months.

    We desire to be very cautious about mold buildup since this will be our forever home and we have two little girls under 3!

    Ok, so with all of that detail, here is my ongoing concern. @MALCOLM TAYLOR says "walls don't dry vertically"... my concern is that adding horizontal fire block material, even mineral wool, will reduce the convective currents that create "good airflow" inside the 1/2 air gap cavity. This house's balloon framing was horrendous for draftiness and energy efficiency, but the airflow against the inboard face of the shiplap siding must have been phenomenal for the past 130 years. Won't we lose that airflow with much smaller stud bay cavities that are horizontally closed off from each other every 8 ft vertically? And without the airflow, won't that moisture take far longer to flow into the house for drying?

    1. Expert Member
      MALCOLM TAYLOR | | #8


      I don't think having smaller inboard "rain-screen" cavities won't be a problem for two reasons.

      - The need for drying will be greatly reduced once you have air-sealed the cavities, so you won't have the heavy load of moist interior air moving through the walls, and the cladding and sheathing will dry to the outside as they should.
      - The main benefit of the gap is as a capillary break, which doesn't depend on convective currents or vertical air movement. The exterior rain-screens we use here in BC are rarely vented at the top, and function extremely well in the absence of any appreciable air-movement beyond pressure equalization.

      1. oaklandcavictorian | | #9

        Malcolm, thanks for the reply. I saw your comment about capillary break in the Kiley comments section as well.

        Wouldn’t the horizontal fire block provide a capillary bridge between the inboard face of the siding and the insulation? I could see moisture pooling or collecting on a horizontal wood 2x4 and migrating inboard, or if I opt for mineral wool as horizontal fire block then the mineral wool would “wick” and absorb the water that lands on it and it would transfer further inboard through the mineral wool.

        I appreciate the anecdote about BC though, that certainly inspires confidence for an Oakland, CA location!

        1. Expert Member
          MALCOLM TAYLOR | | #10


          The walls already have frequent capillary bridges in the form of the studs, and it's worth considering that the advice in the two articles assumes various horizontal ones at the plates and rim-joists. You won't be adding anything that isn't usually there.

          If you particularly concerned, you could use larger stock for the blocking and angle cut the two ends so that it sloped slightly to the exterior. You could also add strips of peel & stick membrane to the top and outer edge.

          However: Like head-flashing over windows, rain-screens gaps do typically provide a drainage path, but I think that if you have sufficient moisture accumulating in the cavity that it needs to actually drain (as opposed to dissipating through drying) your cladding and sheathing are simply too ineffective to be kept.

          My own feeling is that using the methods you propose, with the stud bays divided up vertically by the blocking, your walls will perform well.

          1. oaklandcavictorian | | #11

            Thanks for the further intuition.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |