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

Hybrid insulation: rockwool + foam board (and canned foam for airtightness and extra R-value)

ChrisInIllinois | Posted in Energy Efficiency and Durability on

This my first home, very hand have done remodels and stuff before. I wanted to use spray foam but it is out of the question cost wise. I have a 1900 house with plaster. I will be going room by room removing plaster to wire, insulate, drywall and replace windows. I have not found a lot of info on making a home air tight yet. So here is 2 plans not sure how they work work out:

Note 1 The plaster wall should give me 4-4.5 inch of stud bay depth.

Note 2 will be pulling down vinyl siding to put up 1 inch r5 foam board outside, then hopefully reusing siding.

Plan 1:

air tight stud bays some how caulking / can foam etc..?? not sure need to look this up.
put in r15 rockwool.
vapor retarder (not barrier)

plan 2:
cut foam board to stud bay size, can spray foam around it. The extra depth should allow for 0.5 to 1inch of foam board + rockwool insulation.
put in r15 rockwool
vapor retarder or not (not sure as the foam board could be a barrier)

My issues are with the dew point / moisture mold problems. Will this work if not, What is the best way to air tight the stud bays before the rockwool.

I will be trying to make house air tight, handle dew point issues, thermal bridging, and air quality. The wall and ceiling have to be fixed first.

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
    Dana Dorsett | | #1

    The location/climate matters. R5 is not sufficient for dew point control at the sheathing in colder climates, but would be fine in US climate zones 1-4. What works in Akron doesn't necessarily work in Anchorage, so where are you?

    Cut'n'cobbled foam leaks air, even when can-foamed in place. That's fine if there is only vinyl siding on the exterior- the sheathing can still dry to the exterior, but if there is a vapor-impermeable layer (say, foil faced foam) on the exterior of the sheathing there could be some moisture risks.

    Circa 1900 framing doesn't fit batt insulation either on width or depth. Blown insulation is usually a better choice with non-standard stud spacing & depths.

    The most important thing to get right is the window & door flashing details. Most buildings in 1900 had none, but without wall cavity insulation incidental bulk water incursions dried quickly. That won't happen after you insulate, so flashing the rough framing for the windows to direct bulk water to the exterior of your foam layer (or housewrap/#15 felt, if no exterior foam) is critical to the long term moisture handling performance.

    If the R5 foam is 1" XPS, from a design point of view it has to be derated to no more than R4.5, which is the most the manufacturer will warranty, and even that may be optimistic. A conservative derating would be R4.2. This is all due to the fact that the R5/inch of XPS is partly a function of the HFC blowing agents used (which are extremely powerful greenhouse gases). As the HFCs diffuse out over a few decades (doing their climate damage) performance drops toward that of EPS of similar density (ergo R4.2 at full depletion.)

    An inch of foil-faced polyisocyanurate is blown with much lower impact hydrocarbons, and is usually labeled R6. With the air space behind the vinyl siding it will add another ~R1 of performance, but in cold climates some grades of polyiso will drop in performance during cold weather, and need to be derated for temperature depending on climate. In US climate zone 4 or lower the foam + foil/air-gap is good for about R7. Dow alleges that their Thermax polyiso does not need to be derated for temperature (they found the magic formula blowing agent or something), so if it turns you NEED the full R6-R7 for dew point control, that would be a better choice than Brand-X.

    Foil facers are true vapor barriers, so it's important to have enough exterior R to be able to use standard latex paint as the interior vapor retarder, to provide a reasonable drying path for the structural wood.

  2. GBA Editor
    Martin Holladay | | #2

    First of all, what's your name? (I'm Martin.)

    If you plan to remove the siding, the best location for your air barrier is on the exterior. This could be plywood, OSB, or the rigid foam that you plan to install (as long as the foam seams are carefully sealed with a high-quality tape).

    You may want to read these articles:

    "Cut-and-Cobble Insulation"

    "Flash-and-Batt Insulation"

    "Questions and Answers About Air Barriers"

  3. irene3 | | #3

    I'm currently putting rockwool batts in my 1901 basement stud bays, which are all over the map in width and height (if you're wondering why the height would differ, some of them have diagonals in them, goodness knows why). It's a little bit of a pain but not very difficult. (It's taking me ages because I've only been doing a bit at a time, but I haven't put many hours into it total.) The air sealing I'm doing with silicone caulk (apart from one or two larger holes that I'll have to manage some other way). I would think exposed walls in daylight would be still easier to work with.

  4. Expert Member
    Dana Dorsett | | #4

    IRENE3: Is there anything between the basement stud bays and foundation, or are the stud bays only on the above grade portion of the walls?

    The diagonals were most likely for bracing against racking forces (wind,etc), fairly common in plank sheathed (or unsheathed) walls of the era.

    Martin: Most circa 1900 homes had plank sheathing, not plywood, and many had no sheathing, with the siding nailed directly to the studs. I suspect the vinyl siding on this one is place over the original siding, but there's a lot we don't yet know about the stackup of the wall, and the climate where it's located.

  5. GBA Editor
    Martin Holladay | | #5

    My reference to plywood or OSB assumed that these modern sheet materials might be added as an air barrier and to improve racking resistance. An expensive option, to be sure, but I've seen it done over old board sheathing.

  6. Expert Member
    Dana Dorsett | | #6

    Ah- got it (on the second read, anyway.) If the structural capacity against racking forces is already adequate, the additional thickness might be better filled with rigid foam board (seams taped, detailed as an air barrier) than plywood. A broadsheet WRB like housewrap between the plank sheathing & foam board can be detailed as a secondary air barrier for insurance. The long term air tightness of plywood is more assured than that approach, but foam board + housewrap can still be pretty good.

  7. irene3 | | #7

    Dana writes: "The diagonals were most likely for bracing against racking forces (wind, etc.), fairly common in plank sheathed (or unsheathed) walls of the era." That makes sense. Yes, the wood portion of the walls is above grade.

  8. ChrisInIllinois | | #8

    The name is chris, Thanks for all the good info its nice to have a name to the madness of the foam board can foam I was reading about. I think the house has the slats nailed under the siding not osb or anything else. The zip is 62812. lower IL.

  9. Expert Member
    Dana Dorsett | | #9

    You are in US climate zone 4A. In Zone 4A from a dew point control point of view it doesn't matter what the R-value of the exterior side foam, so if you want to use cut up foam board as the exterior side air barrier for the fiber insulation it really doesn't matter how thick it is- it won't cause enough moisture to collect at the foam/fiber boundary over the winter to become a problem.

    Flashing details still matter big time!

    It's not clear what is meant by "...the house has the slats nailed under the siding...". Does that mean it has a plank sheathing layer between the framing and siding?

  10. MThuman | | #10

    Personally here is my proposal.
    1. Air seal then insulate the rim board and mud plate with 2" XP cut loose and sprayfoamed all around to create a air seal and thermal barrier.
    2. Air seal and insulate the mud plate (horizontal board the covers the block or concrete wall) with the same method above.
    3. Prep the basement walls per insofast instructions.
    4. install insofast panels which are glued to the basement wall and foam is used were the block wall is exposed to seal and insulate. Further more seal the bottom of the wall but not the top. Then install drywall 1/2 or 5/8 and latex paint.

    This allows the basement wall to never condense because you are keeping the warm moist air inside the conditioned space. Then if any moisture comes in from the outside it can dry to the inside. No wood no vapor barriers and simple.
    Has anyone else used this product and what was your experience.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |