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

Best insulation for brick and frame extertior walls?

GBA Editor | Posted in Green Building Techniques on

What type of insulation works best in an exterior framed wall with exterior brick cladding? I want to blow in foam …or fiberglass…into the old lath and plaster wall cavity. Is the loss of migrating heat going to cause freezing and spalling mortar as a result of this?

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. Robert Riversong | | #1

    Brick cladding should be installed with an air space between it and the wall sheathing for drainage, so it is somewhat decoupled from the heat flow through the wall. There should be little difference in exterior brick temperature and propensity to freezing.

    Expansive foams aren't generally blown into enclosed cavities. Fiberglass is poor insulation that won't stop air movement and is a known carcinogen. I would recommend blown cellulose, which is non-toxic, is largely recycled, has better insulating qualities, is relatively impermeable to air movement, is fire-resitant, insect-resistant, rodent-resistant and mold-resistant, and can help manage indoor relative humidity by absorption and release.

  2. Derek Vander Hoop | | #2

    Hi Susan,
    Do you intend to remove the old lath and plaster and either spray foam, dense pack cellulose, or a Blown-In-Blanket system, or do you want to drill holes and blow in insulation through the holes? If it is the former, my preference is always foam but it does cost more. There are now minimally expanding foams available for injecting through holes for retrofit projects, but this is relatively new and I don't have any data or feedback on how that product is performing.

    It is preferred to have a 1" airspace behind brick, with weeps that allow air in to dry the cavity behind the brick. Some older homes have weeps, many don't, and mortar net (which provides a conduit for moisture to travel to the weeps and a conduit for air to "wash" the cavity) wasn't available back then. There isn't much you can do about existing brick veneer, but if you were greatly concerned, you could spray siloxane or a similar product on the wall to prevent water infiltration. That is something that has to be repeated every two to three years.

  3. susan clellen | | #3

    As a builder for 25 years I am aware of the typical wall cross-section. What I am stymied about is the lack of hard technical solutions for "green" rehab. I follow a couple of trade journals which have until recently been resistant to "trading in the SUV" in construction. I have had to looked at Canadian Gov data to find some satisfaction...but still not enough is out there. We are all just guessing what might be the best solutions for reducing energy consumption with older homes. I do not trust a lot of "products" until time has tested the material and the "system" that it supports to be safe. Sealing the exterior surface of brick really scares me...I have witnessed spalling from brick walls that were painted. I have seen "Nansulate" paint pull multiple layers of paint right off the plaster from moisture trapped inside the wall. I think the dense pack , cellulose will at least allow some multi-directional migration of vapor and moisture. Hopefully the brick will have enough air circulation to prevent freeze damage.

  4. Robert Riversong | | #4


    Siloxane and silane are two masonry sealers that are extremely water-proof but highly permeable to moisture migration. I concur with Derek on this.

  5. Derek Vander Hoop | | #5

    I bought an 1898 farm house in Wisconsin in 1978. Only the north and west walls were insulated, with cellulose, by the prior owners. The siding, after 80 years, was in terrible shape, and being a purist of sorts, stripped the siding, blew in cellulose in the walls that were un-insulated, added additional insulation into the attic, and installed new cedar ½” x 6 cedar siding.
    So what did I find along the way? Well, I found building paper (something I’d describe as being something like rosin paper) in very poor condition and some rotten ¾” x 6” T&G sheathing here and there. Inside the formerly empty cavities, in portions of the house I gutted later during various remodeling projects over the years, I found a very heavy paper nailed to the studs next to the sheathing, something the original builder probably figured would cut down on air infiltration.
    So how does this apply to you? Your structure has probably relied on heat inside the building to drive moisture through the structure, drying/driving out moisture in the process. We all know that tightening up the building envelope is the first and most important task in making our structures more energy efficient. I believe that closed cell polyurethane insulation is the best insulation that can be used in our homes. It is impervious to moisture and vapor and absolutely takes air infiltration out of the equation. Add to that its superior R-Value (aged 6.2), and you end up with a much more energy efficient assembly. Knowing what I know now, if I could go back and do it all over again, I would approach almost everything differently (but of course, you can't do that.").
    But using closed cell foam insulation, cellulose insulation, or any insulation you choose doesn’t consider the condition of the building paper, rosin paper or tar paper applied over the sheathing of your structure which is supposed to function as a water and air barrier. And I know of no way that you can determine with absolute assurance that that this “barrier” is in good condition everywhere. If the barrier is compromised, like mine was, then there is a great likelihood that water is attacking the structure So, short of removing all the brick on the structure, it seems to me, the most prudent approach would be to do whatever I can to prevent water infiltration. Which brings us back to Siloxane.
    You may find this article helpful:

    Hope this helps.

  6. Derek Vander Hoop | | #6

    .......To save time, check out section 6A.......

  7. Robert Riversong | | #7


    Your experience with an old wood-clad house has little relevance to Susan's brick house. Brick, unlike wood cladding, is an air barrier (remember, the big bad wolf couldn't blow down the brick house) but it can also be a moisture reservoir.

    And your statement that "closed cell polyurethane insulation is the best insulation that can be used in our homes" is a blanket judgement that doesn't consider the particulars of a project or a specific application. Spray foam may have high R/in (5.9 is the industry standard), but it has its share of liabilities (moisture impermeability is not necessarily a plus, and its air-tightness depends on quality of installation and other factors), and it's certainly not a green product.

  8. Derek Vander Hoop | | #8


    I described my home in detail because it, too, was an old lath and plaster uninsulated structure which, aside from being clad in wood instead of brick, is similar to Susan's structure. Many of the issues I found in my own home may be shared by her project.

    In my opinion, the single most important component of Susan’s wall assembly is the wind/water barrier and it is the one component she can’t visually inspect. Having done a lot of remodeling over the years to my home and many others, I have frequently found the building paper to be in poor condition or improperly installed, and to me this is Susan's biggest concern. In a driving rain, water can appear on the back side of brick in as little as 30 seconds. If the building paper (tar paper, rosin paper, etc.) is compromised, with mortar droppings at the bottom of the cavity behind the brick, water running down the back side of the brick is certainly wetting either the building paper or the sheathing underneath. In the past, her building's heat could have acted as the drying agent. Insulating the structure with cellulose or foam will remove that drying potential. For that reason, I think we both agree that keeping as much water out of the brick as possible is smart.

    I didn't understand what you meant when you wrote that siloxane is highly permeable to moisture migration. Did you mean vapor and not moisture? If so, I didn't know that. Good to know.

    I think that moisture and vapor impermeability is almost always a good thing. In a wall assembly, the temperature is going to meet the dew point somewhere in the insulation, and condensation will form on the first condensing surface it can find, usually the sheathing in a heating climate or the visquine in a cooling climate. If moisture/vapor can't get into the insulation, then condensation is not going to occur.

    I know closed cell foam is not green, and that does concern me, but in the end I am first concerned with performance, efficiency and conservation. Her structure probably has 2x4 walls and, if she guts the interior, she can maximize her wall's R-Value, eliminate almost all air infiltration, and create an assembly that is pretty impervious to vapor drive.

    Susan’s specific question asked whether the elimination of migrating heat will cause greater brick spalling. I don’t think so. After all, many new construction projects with brick veneer have airtight, well insulated wall assemblies and spalling is not a concern with those projects, as long as they are built with a well made brick. Spalling is caused by repeated freeze thaw cycles working on the water contained within the brick surface. Water expands when it freezes, “popping” the surface of the brick. So in a sense, Siloxane is performing two functions, minimizing how much water penetrates the brick veneer, coming in contact with a possibly compromised water and air barrier, and helping to prevent spalling of the brick.

  9. Robert Riversong | | #9

    Derek: "I didn't understand what you meant when you wrote that siloxane is highly permeable to moisture migration. Did you mean vapor and not moisture?"

    I meant moisture as in water vapor diffusion, rather than bulk water movement. Siloxane, because it coats the pores of masonry rather than sealing them, doesn't change the permeability of the primary material but dramatically reduces the absorption coefficient. And, because it penetrates below the surface, it can last up to 10 years, unlike surface treatments.

    Derek: "the single most important component of Susan’s wall assembly is the wind/water barrier and it is the one component she can’t visually inspect... If the building paper (tar paper, rosin paper, etc.) is compromised... water running down the back side of the brick is certainly wetting either the building paper or the sheathing underneath."

    Agreed, and the primary entry vector for water through masonry walls is poor-quality or defective mortar joints and penetrations & flashings. Those need to be repaired before water sealing is performed.

    But the likelihood of some water penetration through any masonry wall is reason to avoid a vapor-impermeable insulation or other building material inboard of the cladding, like closed-cell foam, as that would eliminate any drying potential to the inside, and summer inward vapor drive is often more of a concern with reservoir claddings like brick.

    Derek: "I think that moisture and vapor impermeability is almost always a good thing...I know closed cell foam is not green, and that does concern me, but in the end I am first concerned with performance, efficiency and conservation."

    I am first concerned with a healthy and life-supporting shelter, which is the purpose of housing. A thermal envelope, like our clothing and our skin, needs to breathe in order to maintain conditions conducive to life. Mechanical life-support systems are necessary only in environments that are completely inhospitibale to life. Perhaps some day that's what our terrestrial environment will become (in some urban areas, it already is), but until then we should be building homes that are semi-permeable membranes. This also makes them forgiving to the occasional and inevitable wetting that will occur someting in the life of the building.

  10. susan clellen | | #10

    I can see some differences in philosophy here. I came to the "natural" building materials movement in the late 80's when I became (and remain) a staunch advocate for passive design as well as natural materials. I have also put in many years restoring century homes as well as remodeling work. What I have come to understand is that natural materials are predictable in how they respond to the forces of nature. Do we really know the life expectancy of TYVEC , elastomeric stucco, spray foams or for that matter these brick "treatments"? If I were to seal my walls from "both" sides as Derek suggests then the impact of "trapped" moisture under freezing conditions becomes even more dramatic.... even ice can eventually evaporate if there is air available. In the end there is no perfect solution to preserving the building envelope . When I ride my bike in the rain I wear Gortex ...I don't get drenched but I do get soaked! Thanks.

  11. adkjac upstateny | | #11

    Melissa... you asked and answered your own question.,,, and sound very experienced and knowledgeable.

    So... go ahead and use cellulose. Let us know what you end up doing.

  12. john | | #12

    i live in a brick home , built in the 70's. we have found no insulationin the walls. my landlord wants to blow insulation into the walls from the inside. is this ok to do???

  13. Riversong | | #13


    Start a new question. Piggybacking on an old one just makes everything confusing and you're less likely to get the answers you want.

  14. GA | | #14

    This question of insulating a wood frame brick sheathed home also interests me. I have a 1925 house in the Atlanta area - hot and humid in summer, wet and fairly cold in winter (we usually have 6-10 freeze thaw cycles in the winter). The house currently has no insulation. I am getting ready to add a living space to the attic area, so will be doing some considerable work, including insulation.

    The house framed with 2x4's (nominal 3 3/4"). On the interior side, there is lath attached to the 2x4, and plaster on the lath as the interior finish. On the exterior side, 1x6 are attached to the 2x4's, as sheathing, then there is an inconsistent "air" gap (sometimes with quite a bit of mortar, then the exterior bricks. Parts of the house have a short basement with no plaster & lath, so I can see the stud cavities, sheathing and a bit of the bricks from there. It appears there is no building paper at any location, including none between the 1x6 sheathing and brick. There are brick ties.

    I do NOT plan to removeeither the bricks or the interior plaster (which is in remarkably good shape). It is a 1 level bungalow, and the brick only goes to the top of the windows. There is a 1x12 that goes across the top of the windows and the brick. I plan to remove this 1x12, which will give a large access to the stud cavities. Spot renovation/repair work indicates there is no horizontal bridging between the 2x4's but it remains to be seen if that is true throughout the house. Access to the stud cavity under the window is from the inside by removing the decorative trim under the sill.

    I'm considering cellulose insulation, but the cons are the lack of an airtight seal, concern that the cellulose will hold moisture (and may cause mold), concern about settling, and concern that there are better options.

    I'm also considering minimally expanding spray foam (not sure if this is open or closed cell) as the potential to get into every crack and crevice to limit air infiltration is appealing. The cons and concerns are access is only from the top of the stud cavity, so I have to use a foam that fills the cavity rather than one that only puts on a 2" or so layer. I'm also concerned that even minimally expanding could expand into the gaps between the sheathing and block what there is of an airspace between the sheathing and the back of the brick. The issues mentioned above about trapping moisture, etc are also a concern.

    Another concern about the foam is off-gassing. My research indicates even the minimally expanding soy foam products are primarily petroleum based.

    All of the above leads me to post here. I would appreciate and look forward to any insight, information, comments, questions, etc.

  15. Riversong | | #15


    Start a new thread with the Ask a Question button at the top of the Q&A page.

    When you revive a more-than-year old thread, the original poster gets an email each time another post is made to it. If you start your own thread, you'll be automatically notified by email each time a response is made.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |