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Wanting to know if my extensive insulation/energy retrofit on an adobe house built in 1882 sounds right.

dtE7ZWgjbk | Posted in Green Building Techniques on

Hello all.

This is going to be long since we’re doing a complete energy retrofit of an adobe brick home built in 1882 in Salt Lake City, Utah (Zone 5b). The home is currently gutted and almost completely accessible. I’d like to know if what we have planned sounds good.

Here are the basics: The house has two floors with a full walkout basement (about 700’ down and 1200’ up). The main part of the main floor is essentially a square, 2-course-thick, dirt adobe brick structure with a “notch” where porch is. Added to the entire north and west sides are 8’ wide wood-framed sections (2×6 with ½” OSB sheathing), with 2×12” and 2×10” cathedral ceilings. Those sides will have exterior 3” polyiso rigid foam insulation with 1x furring on the studs creating a drainage plane for the fibrous-cement siding. The windows are low-e, double-paned, installed at the OSB plane (innies) and flashed using flex-wrap and membrane flashing to the house wrap. The foam is on top of all that and will extend up inside the rafter ends on the outside of the rafter blocking – the spray foam will create a sandwich on the other side inside the rafter bays.

Only the east and south walls of the main floor adobe are exposed. Adobe experts I’ve spoken with in NM (where they have a code for adobe) have recommended 3” spray foam on all exposed adobe areas since un-insulated adobe in cold climates performs very poorly. The adobe walls are 11” thick total – no room to insulate between. Since it’s in a historical district, it has to be stucco on the old adobe areas like it used to be and no exterior elements can be altered; i.e. no addition or removal of windows, no increase in height of roof, etc.

–> Does this sound about right? I am thinking that we’ll use closed-cell foam, sprayed directly to the adobe. Window bucks have been built for inset mounted windows and all window and door protrusions extended to accommodate the added thickness. The windows will be installed after the foam is on, with 1” rigid foam inserts covering the nailing flanges.

Since the foam will have to be shaved, I assume it will lose its water-tightness at least in that first layer of micro bubbles. In NM, they install lath and stucco right over the foam (no WRB added) and the windows are membrane-flashed both under (before spraying), and over the foam (after spraying). I don’t know how you’d get a sticky membrane to stick to these dirt bricks though (In NM the bricks are new, stabilized, and less “dirty” than my 130-yr-old bricks).

–> My biggest questions here are, should I use closed-cell foam, which would force the bricks to dry to the inside (they’re happiest at about 60% humidity) but would be great for added lateral strength? Should I install a WRB over the foam and flash the windows to that after it’s sprayed, or just flash to the outside of the foam like they do in NM? If I do install a WRB over the foam, should it be a complete barrier (which would be 2 barriers due to the closed-cell foam), or do I use a water impervious/vapor permeable layer? I’m guessing if I use a WRB, I would need furring strips behind the stucco. Would it matter at that point if I used synthetic stucco (which gives me the heebie, jeebies) or traditional cement stucco? FYI – normally, you would ONLY use soft, hydraulic lime-based stucco direct to adobe, but with the foam, I don’t think it wouldn’t matter since the two materials would be isolated.

I’ve also read volumes about unvented cathedral ceilings, which every ceiling in this house will be. On the north and west sides (with the 10” and 12” cathedral framing), I am planning on using spray foam direct to the roof deck to achieve R-38, which (I think) is code here. I may fill the balance of the space with something for additional Rs. There are 4 existing, newer, 2’x4’ skylights on the north side.

In the old roof sections (over the adobe), it is framed with true 2x4s with OSB sheets over the old 1x plank roof deck, and then felt with asphalt shingles. I believe there is ice & water shield on the edges – the roof does not sag. We will be adding knee walls inside for support and installing 2” x 1.5” wood strips horizontally around the entire underside of the roof so that we can fill the whole space (now 6” deep) with closed-cell foam. The only thermal bridging will be at the intersections (16” OC) of the vertical 2x4s and the horizontal 2×1.5s. We will install NO can lights and there are no roof penetrations in this section other than two chimneys.

–> Do our unvented cathedral ceiling plans sound ok? How do we detail the insulation around the brick chimneys to keep condensation issues at bay in an unventilated system?

FYI – the foundation of the entire structure, including under the north and west wood-framed sections is 19” thick, river rock and sandstone, with moderate efflorescence and no way to access the exterior walls to waterproof them. The walls are 9’ high. The basement slab is newly poured with 4” polyiso rigid foam below, 1.5″ on the sides and plastic vapor barrier beneath that. It has radiant heat with in-slab pex below, an aluminum-heat-plate pex-sandwich application above, a high efficiency variable temp boiler, and various manifolds/zones allowing a low-temp system.

I am leaving one of the exterior basement stone foundation walls exposed. It does have efflorescence that I’ll have to deal with, but it’s a really beautiful sandstone wall.

–> So, would open or closed, spray or rigid, foam be best on the basement walls that will be framed? I cannot stop capillary action from creating some moistness in the basement since the foundation is 130 years old and I can’t access the outsides of the foundation walls. I’m concerned with the interaction of the efflorescence and the spray foam sprayed direct to the walls. What happens over time under the foam? BTW – All surfaces around the outside of the foundation are cement and the efflorescence isn’t all that bad.

It’s a lot to contemplate, but I’ve spent A LOT of time trying to understand all this and really want to do things the right way – this will be my home for the long haul.

I am to the point of analysis paralysis and would love some input. Thanks!!!


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  1. GBA Editor
    Martin Holladay | | #1

    It probably wasn't a great idea to use polyiso under your basement slab. Polyiso can absorb water, and it shouldn't be used in contact with the ground. Either XPS or high-density EPS would have been a better choice.

    Closed-cell spray polyurethane foam can be used as roofing, so I see no reason to doubt the New Mexico experts who install closed-cell foam on the exterior of adobe walls, followed by stucco. Pay close attention to your window flashing details, and you should be OK.

    I think your cathedral ceiling plan sounds fine.

    If you are worried about moisture coming through your basement walls, and if your budget can afford it, you can install an interior French drain around the perimeter of your basement, and you can install a dimple mat against the stone wall. You can then install rigid foam or spray polyurethane foam against the dimple mat.

  2. Expert Member
    ARMANDO COBO | | #2

    Old adobe walls in NM were never designed to be insulated. I learned from an adobe building class I participated in the pueblo of Taos, the wall thickness (12”-30”) was designed to absorb and release heat, and the thicknesses vary with the wall location in relation to the sun. Any moisture trapped in the wall will disintegrate the wall in time. That’s way the people of Taos Pueblo recover their houses on the regular bases.
    All these foams are a “new” thing, and I doubt anyone has really done some long term studies about its application on adobe walls. I would suggest you contact the State of NM building code office in Santa Fe and talk to them. I also know that couple of years a go, you were “allowed” to install up to 2” of CC foam w/o an engineer’s report; it had to do with the lath attachment and the stucco weight.

  3. dtE7ZWgjbk | | #3

    Thank so much for your replies!

    Martin - Thanks for the heads up on the polyiso. I thought that as a closed-cell foam it wouldn't absorb water. In any event, there is a fairly thick sheet of visqueen under the foam, against the ground, hopefully that will solve that problem and not create a new one with the concrete direct to the foam (?). This particular foam has a papery, fiberglass coating on both sides that could make it more water resistant - I'll have to check into that. I'll plan on the orange dimple mat against the interior basement walls as well. Thank you for that suggestion.

    Regarding the insulation on adobe. Adobe works great in hot climates to keep things cool since it is an excellent thermal mass for storage and slow release of heat - i.e. temperature regulation. Everything I have read - including information from Quentin Wilson at New Mexico's College of Adobe Building Sciences - indicates that in severe cold, an adobe structure will be miserably cold if it's not insulated on the outside. Apparently, if you insulate the interior walls it ruins all the great benefits of the adobe. We have a mixed climate in SLC; up to 105 in the summer and well below 0 in the winter at times, so it presents a unique problem.

    Regarding adobe's need to breathe - I had thought about open cell foam but I can't put more than 3" on without getting in trouble with the historic commission and 3" of open-cell wouldn't add enough insulation. Again, from what I can tell, if it can dry to the interior, we should be ok - I will be digging into ventilation sources next. Incidentally, there are fasteners rated to hold the weight of stucco through up to 6" of foam, but they were tested being screwed into wood. I'm still testing our adobe's ability to hold onto the screws and will likely double the recommended screw pattern.

    I'd also rather used closed cell since 130-year-old adobe has very little lateral strength and the foam could actually tighten things up that way.

    Could anyone comment on the chimney detail with an unvented cathedral ceiling system?

    Thanks again!


  4. dtE7ZWgjbk | | #4

    Some pictures ...

  5. Expert Member
    ARMANDO COBO | | #5

    Building Science 101: Just because the code allows an application, it doesn’t mean it’s the right thing to do; the Pueblo people have known that for a very long time. Having said that… here is what the NM code allows:
    NMAC Wall insulation.
    1. Insulating boards or foams not exceeding two (2) inches in thickness may be adhered to the exterior of the adobe wall. When insulation board is used, round-cap nails shall attach it to the adobe wall, with nails placed to avoid bed joints between courses. Cap nails shall have a maximum spacing of sixteen (16) inches from each other. Additionally, cap nails shall secure the rigid insulation boards around their perimeter edges, with nails spaced no less than twelve (12) inches apart. All cap nails shall penetrate a minimum of two (2) inches into the adobe wall. Insulating boards or foams shall not be used to form architectural shapes exceeding two (2) inches in thickness.
    2. Insulations exceeding two (2) inches in thickness may be used providing they do not form a vapor barrier. Their weight shall be supported by the stem wall below and contained within vertical furring strips, securely attached to the adobe wall. A sectional, scaled drawing for the proposed insulation scheme must be submitted for review by the building official.
    Two things: CCFoam is a vapor barrier, and walls do not breathe, they dry.

  6. GBA Editor
    Martin Holladay | | #6

    Thanks for the detailed code information. Very interesting -- I learned something.

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