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Building Science

Insulation Retrofits on Old Masonry Buildings – Building Science Podcast

This week, Dr. Joe Lstiburek talks about retrofitting old masonry buildings with insulation and the water problems that it can cause.

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_This podcast series is excerpted from a two-day class called “Building Science Fundamentals” taught by Dr. Joe Lstiburek and Dr. John Straube, of Building Science Corporation._ _For information on attending a live class, go to_
_Last week
Dr. Joe talked about brick walls and water problems. This week, Dr. Joe talks about retrofitting old masonry buildings with insulation and the water problems that it can cause._ _______________________________________

How Do You Retrofit an Old Building to be Energy Efficient And Durable? The context here is very important. You don’t insulate the wall of a mass assembly until you’ve taken all of the other low-hanging fruit into account, right? What’s the biggest penalty in a big stone house? Well, the glazing system — followed by what? The roof, followed by… Air tightness, right? Then the efficiency of all of the other stuff. This stone wall is already an R-10 wall. The low-hanging fruit is not the wall; I already have an R-10 wall with an R-1 window. So I’m not going to deal with the wall until I’ve done the other things. Historic restrictions may get in the way Now sometimes I might not be allowed to do some of the other things. You know it’s very difficult sometimes to redo these windows because of the historic restrictions. There are ridiculous rules and these agencies are going to end up having to start recognizing that, look, we’ve got a planet to save and a republic to protect. Now there are all kinds of folks that have different views. There’s a pretty famous architect in town, who interprets the historic tax credit stuff differently than other people, and we should be able to rehabilitate and do something with these windows. But many times you’re going to be told that you can’t touch the windows, and if you can’t touch the windows, you’re going to run out of things to do pretty quickly, and so the walls are going to become a candidate.

When insulating old masonry buildings it’s important to make sure the water draining details are right.

Some masonry walls can hold more water than they get hit with Well, where’s the drainage plane in a masonry wall like this? You know, this is four layers of stone. Well, one percent of the water passes through the first layer, and then one percent of the one percent passes through the second layer, and then one percent of the one percent of the one percent passes through the third layer, and by the time we get to the fourth layer there isn’t any stuff passing through. So the water is leaking in, but it’s stored and dissipated. We have what we call a very large hygric buffer. Water enters, but it’s stored in the mass of the assembly. And the assembly is moisture-tolerant enough to deal with it, as long as we don’t put what on the inside? Vinyl wallpaper. And it would be really stupid in that wall to put steel studs on the inside, insulate the steel studs with fiberglass, and cover it with what? A plastic vapor barrier. That would be a stupid thing. Could you insulate it on the inside? Well, it would be real nice to insulate it with a couple of inches of foam. The high-density foam probably wouldn’t want to go more than three inches, because it’s three perms per inch and so it’s a vapor retarder rather than a vapor barrier. With the low-density foam you could go to ten or twenty inches. If that doesn’t make sense, you could go as much as you possibly wanted as long as you had the space. But you probably don’t want to put something on the inside that doesn’t breathe at all. Ugliness is not sustainable So what’s the likelihood of wrapping the outside of this beautiful stone building with a membrane, putting insulation on the outside, and adding vinyl siding? This is why we don’t do things just for energy conservation reasons, because that would make the building what? Really ugly! And ugly is bad. Beautiful is good. In fact, I wouldn’t let them touch the walls at all —until after they did the roof, until after they did the foundation, until after they did the windows, until after they did lighting, and the heating, and mechanical systems. And over my dead body, then, maybe then, would I let them do something with the walls. In fact the only thing that I would like them to do is maybe re-point the mortar and the joints. I know I’m not going to completely stop water, but I’m going to maybe reduce it. And later on, when we talk about retrofits and foundations, John will talk about how important it is to get the chemistry of the mortar that you’re going to use for re-pointing just right. Insulation can cause water problems Are there negative consequences that are associated with adding thermal resistance to the interior? Is that going to change other factors? (Remember we talked about drying potential.) Can I add too much insulation, or sufficient insulation that the water content in the masonry stays high enough, long enough, to hurt the masonry? And the answer is Yes. I can cause problems with freeze/thaw. I can also enhance problems with osmosis and efflorescence and other issues. So the question is, “Well, how much insulation can I add before I get into trouble?” And you’re going to hate this answer, or love this answer depending on whether you’re a client or a consultant. The consultant’s answer is, “It depends.” And you should pay me to tell you what it depends on because it depends a great deal on where you are, and what’s going on. The client hates that, because the client would like to know, well, what?! All right, well, what does is depend on? Well, freeze/thaw damage isn’t a problem if water doesn’t saturate, or wet your cladding system or your wall system. And so the best, most effective way of dealing with freeze/thaw is all of these wonderful water-shedding elements. You can insulate walls to a great extent if water isn’t concentrated on the surfaces. And so there’s a big difference between something with wonderful drainage mechanisms all over the façade, versus a very smooth, brick veneer, for example. And it’s one of those judgment things that I can look at a building and say, Wow, this is really well drained. I’m not going to have water concentrated, or running down one aspect of the façade. Now, usually an old building will tell you if it has those problems, because you walk around the building and you look at the stain marks. Right? So the first thing that you do before you do any insulation, is you install your drip edges, you deal with your flashings, you deal with the rain shedding things that will basically kick the water off of the surface. So if the water isn’t concentrated, you’re never going to have a freeze/ thaw problem. Location matters too What’s the next issue? Well, you don’t even have to do that if you’re in that part of the country where it doesn’t rain. There’s much less of a problem with freeze/ thaw in Edmonton than in Montreal. Both are cold, miserable, disgusting places, but one has more rain than the other, one has more likelihood of surfaces getting wet. So what’s the other factor? Well, we have rain, concentration of water on surfaces — you also need cold temperatures. It’s rare that we have to worry about freeze/thaw problems in places like New Jersey and New York City. I don’t think we have much to worry about in Boston. But I’d be real nervous in Burlington, Vermont, and I’m real nervous in Ottawa, and I’m kind of semi-nervous in Toronto. And so, in most of the United States there isn’t an issue with respect to insulating these mass assemblies on the interior. So this is more of a Canadian issue, and a Northern New England issue, and a Minnesota issue than it is a Chicago issue, a Boston issue, a New York City issue, a Philadelphia issue. OK, can I insulate a wall on the inside in Boston that’s a mass assembly? I’m going to say, pretty much yes, with a restriction in terms of R-value. Should I have a vapor barrier on the inside? No, you shouldn’t. And it would be nice to keep water from being concentrated on the surface of the building. When we start getting into, say, Portland, Maine, I’m going to say you probably shouldn’t insulate more than R-10. What’s that based on? A gut feeling. I have a really big gut, I’ve been around a long time, and it’s an intuitive, experience-based thing. I’ve looked a buildings a long time, I know what works and what doesn’t based on what I’ve seen out there. The secret lives of consulting engineers Now people always ask us to do a calculation, and I’m sort of saying, why? Why don’t you trust my judgment and intuition? Well, the sophisticated client will — because they know that if we generate a calculation, it’s going to be based on what? Intuition, judgment, and experience, right? You know, the unsophisticated client, we love them — especially if they have money — because we get to do what? Take it. And you know, basically we write up a report. We do a wonderful simulation, and it looks really slick. There’ s all kinds of numbers and equations in there, and at the end of the day we know it has the right answer — because we knew the right answer before we started. And the right answer came to us from what? Our observation and experience. We worry a lot about this stuff in Montreal and Ottawa, we’re nervous about it in Toronto, we’re not nervous about it in Boston at all, cause we know exactly what we have to do in Boston to never get in trouble: Control the flashings and the rain water, don’t put a vapor barrier on the inside. Knock yourself out, re-point your joints — you’re done. It’s so difficult in Ottawa and Montreal, and we generally don’t comment on it, because we don’t know what the answer is. We let other people take the risk and we watch them, right? Because how many times is a client going to hire you when they ask you a question, and you say, “Shoot, I don’t know. Let’s try this and see what happens.” That would be the honest thing to do, and when have you ever had a consultant tell you that? The honest thing? We did a retrofit of a bakery that in Newark, New Jersey, with two inches of high-density spray foam, and then an inch and a half metal stud with gypsum board. What a slick way of doing stuff. Now, are we doing that kind of stuff in Burlington, Vermont? The answer is, Well, yeah. But how come I’m not worried about it? Well, two or three buildings I’ve been involved with, the folks really went to a lot of effort to ensure that the parapets were properly flashed, that the windows were properly flashed, that we had drip edges, and so that we weren’t concentrating the water on the surface. When we did that, we unhesitatingly said, “OK, now you can insulate with the closed-cell high-density foam, because we don’t have the risk of water leading to this damage.” At the end of the day: no water, no problem.


  1. Chris Prelitz | | #1

    Lowest hanging Fruit
    Great article! I completely agree on not wasting money on calculations and modeling....with the caveat that the consultant has to have both experience and common sense ( a rare commodity today)

  2. Anonymous | | #2

    Ridiculous Rules
    The author writes pertaining to historic windows, "There are ridiculous rules and, you know, these agencies are gonna end up having to start recognizing that, look, we got a planet to save and a republic to protect"

    So it is apparently ridiculous to protect the windows that came from old growth trees. The author apparently would rather rip out these windows and discard them into a save energy? Don't forget the energy consumed to produce the "green" vinyl replacement windows. Problem solved.

  3. Mike O'Brien | | #3

    Sarcasm aside, there's no need to "rip out these windows" to upgrade their energy efficiency. For example, our City Hall has upgraded windows. The old sashes were taken out and modified to accomodate double glazing, and new glides were installed to reduce air leaks. They don't look any different unless you get up really close, in which case the security guards will want to know what you're up to before you really have a chance to scope the windows, and they actually can be opened now.

  4. Joe Lstiburek | | #4

    I never said that
    To Anonymous,

    You reached a conclusion not based on anything I said in the posting. If you are going to trash me do so with something I actually said or wrote.

    You missed the line: "we should be able to rehabilitate and do something with these windows." Many windows can't be touched according to arbitrary and capricious rules. Nowhere did I say trash wood windows and dump them in a land fill and replace them with vinyl. Your words, not mine. Old wood windows can be rehabilitated.

    But, I am often limited in what I can do with them. I love wood. I often can't beef up the frames and install double glazing, low E and gas fill even though the look does not change from the exterior. It
    gets even worse - the ridiculous rules that irritate me the most involve steel and iron windows. Can't touch them at all, or if I do I have to replace with identical units - even though I point out I
    can build in a thermal break with double glazing, low E and gas fill and no one will be able to tell. "Ah, but it is not authentic".

    To Mike O'Brien,

    You also missed the line "we should be able to rehabilitate and do something with these windows." You clearly are not quoting me "rip out these windows" because that is not what I said. Read my response to Anonymous. I like what your City Hall has done. Bravo. You folks apparently got a reasonable historical consultant.


  5. Greg Randall | | #5

    Thank You
    Not necessarily the proper venue for this, BUT Thank You Mr. Lstiburek. I've read your books pertaining to our climate and do everything I can to educate our clients and builders on the methods of construction you recommend. Again, I learn something new every time I read, on the web or in print, when I am researching an issue pertaining to a project I am working on. This discussion directly relates to an old house we are renovating with a brick foundation in Chicago. I was about to detail the refinishing of the interior walls at the basement level for occupancy and came upon this article. As I was about to note the installation of the insulation against the wall per your recommendations for new concrete foundations, I though I should go back and double check your information. I was surprised to read about being cautious with the insulation against the masonry wall. The tricky part of our foundation is that it is 3.5ft below grade and 5.5ft above grade (typical walk-up, brick foundation w/ dirt-sleepers-wood flooring, wood wall above from 1880s). So I was worried about insulating properly this now habitable space. So, is it smart to provide a 1" airspace between the masonry and the wood studs, then insulate the wood stud cavity? I know, cheap way to glean advice, but a dedicated student of yours (unfortunately you weren't at Univ of Michigan in the mid to late 80s).

  6. Ronald Crouch, AIA | | #6

    Hygric Buffers Under SPF Interior Insulatoin?
    Hi Dr. Joe,
    Thank you for sharing these helpful presentations.

    Your subject retrofit bakery project in Newark, New Jersey struck home. We also plan to spray 2 lb density SPF insulation on the back of an existing all-masonry wall. It’s a 1950’s library gut job (ASHRAE zone 4). It will return to life as a library. There is an empty air gap between an outer brick veneer and inner CMU wythe. Stud furring and gypsum will conceal the SPF insulation.

    We have read of concerns by masonry preservationists, that if moisture builds up against the back side of mass wall via closed cell SPF, the old soft mortars could deteriorate through multiple freeze- thaw cycles over the years.

    I have read of recommendations for hygric buffer mats (like Keene DRiWall HMB-025-1 MC) to be placed on the interior face of the masonry wall prior to the SPF insulation. I am told the buffers intent is to act as a ventilation/drainage plane to allow drying potential at the interface of the existing (old) masonry with the insulation.

    Worth having a "Hygric Buffer"?

    It was interesting to hear your concerns on freeze-thaw damage do not begin until very far north in the US. So you think it is a very unlikely freeze-thaw damage potential in Zones 3 and 4?



  7. GBA Editor
    Martin Holladay | | #7

    Response from Joseph Lstiburek
    [Joseph Lstiburek just sent this answer to me by e-mail and asked me to post it.]

    The best hygric buffer is the air space you have between the outer brick veneer and the inner CMU wythe. Nothing else is needed.

    Next, even if there was no air space you are not at risk for freeze-thaw issues in Zones 3 and 4.

    Finally, the wall has to be in really, really bad shape before water builds up at the interior interface of the masonry mass wall and the closed cell high density SPF. Bottom line, if you keep rain water out of the exterior of the wall you can pretty much do anything you want on the inside of the wall. The best way to keep rain water out the wall is to repoint the masonry.


  8. Marie | | #8

    cinder block home
    i bought a 1895 cinder block twin home. Not really well cared for. Previous owner did a lot of hack jobs, tying in electric, ripped out some plaster and installed poor drywall. It did not include any of the exterior walls. My connected neighbor did some work in there house and found only lath and plaster between the two of us. I can smell cigarete smoke from them in some areas of the house. All exterior walls are freezing. I want to put in insulation. I have gotten different advice from different conttractors. 1. rip out, insulate and drywall. 2. insulate and drywall over plaster. 3. drill and foam insulate. 4. drill and blow insulation.

    The only problems I have had with water are in the basement where the mortar has worn away and one contractor suggested sump pump. I disregard all his advice because past experience tells me dig on outside, grade properly and repoint all bricks. The other problem was around the chimeny which in attic and basement. Chimeny is detriorating and I will have it removed and side vent the appliances. Cheap replacement windows

    Best way to insulate the house is??? update replacement windows, add carpet to bedroom over outdoor porch and what to about the walls?

  9. Marie | | #9

    oh forgot to add
    I live in NJ in a essentially 5 store home. unfinished Basement, main floor, 2nd floor, walkup attic (no heat but finished), and an unfinished attic above walk up. I am guessing I need to start by adding insulation to attic. Front of home on 2nd floor has wooden siding but the rest except for add on enclosed porch is cinder block

  10. GBA Editor
    Martin Holladay | | #10

    Response to Marie
    Q. "Best way to insulate the house is???"

    A. The best way to proceed is (a) to pay for a thorough energy audit of your home, including a blower door test, and (b) to hire an experienced home-performance contractor to implement the suggestions that are made by your energy auditor.

    Q. "Update replacement windows?"

    A. No. The cost of installing replacement windows can rarely be recouped in anticipated energy savings.

    Q. "Add carpet to bedroom over outdoor porch?"

    A. No; it sounds like you need air sealing work and improved insulation there.

    Q. "What to about the walls?"

    A. Listen to your energy auditor.

  11. jgThXHagpg | | #11

    Historic Brick Maonsry Building
    Great Podcast. I'm working on the rehab of a historic brick masonry building in Pennsylvania. We're going to be furring out all of the interior walls to increase energy efficiency but the National Park Service is seriously limiting our options. They will not allow us to use spray foam because it is not reversible (should the client ever wish to remove the furring and go restore the original plaster finish), so we're using EPS rigid insulation. We're putting a 1/2" of EPS between the brick and studs (not adhered to the brick) and filling the 2 1/2" stud cavities with EPS. The National Park Service is also asking us to install a vapor barrier underneath the new drywall. We're very concerned about this approach, especially the vapor barrier. What's the best coarse of of action given these constraints? Can anyone recommend any good papers/articles that we could present to The Park Service to talk them out of a the vapor barrier?

  12. GBA Editor
    Martin Holladay | | #12

    Response to Eric H.
    Building codes have never required a vapor barrier for framed walls; only vapor retarders have been required (by some building codes).

    Vapor retarder paint is all you need; such paint fully complies with all building code requirements. If you use paint, you are far less likely to have problems with summertime condensation in your walls (a potential problem if you were to mistakenly used polyethylene).

  13. user-910182 | | #13

    What about cellulose
    Joe, you mentioned using spray foam on the interior of masonry buildings, but did not mention cellulose. It seems to me that given the desire to allow the assembly to dry to the inside, cellulose would be preferable to spray foam in all situations due to the high perm rating, and the fact that it absorbs and dissipates moisture much better than even open cell foam. Thoughts?

  14. YyjLRiifDk | | #14

    Rehab Original Wood Windows
    Mike O'Brien: Where is your City Hall?

    I've got a houseful of old wood windows that need to be rehabbed. Some are starting to rot on their exterior sills and all of their leaded paint has started flaking. Some have original (?) interchangeable wood storms/ screens and others have permanent aluminum storms with screens. They're in a masonry house, built in 1918, that is in otherwise pretty good condition. I'm in Maryland - EPA Climate Zone 4.

    I'd never heard of rehabbing the old frames! They each need to be taken down, scraped, reglazed, and repainted (or stained) anyway. I will probably do the work myself (yes, I'm aware of the EPA lead regulations). Where can I find suggestions/ guidance/ advice on this process as well as suppliers of high quality glazing, as described above? All of the windows are double or single hung, with one piece of glass in each half.

    I had started debating new wood windows - but I think this window rehab may be my solution.

  15. Ndoy | | #15

    Painted Brick??
    I have a 1940s house that has single brick with 2x4 walls. R12 fiber between the 2X4s and a vapor barrier inside the drywall (must have been renovated sometime in the recent past). As I understand your commentary above, this configuration -- painted brick on the outside and vapor barrier on the inside -- has the potential to trap moisture within the wall since the moisture has nowhere to go. I haven't noticed any bad thing happening, but then again, I haven't looked inside the walls. Comments?

  16. GBA Editor
    Martin Holladay | | #16

    Response to Dave Klassen
    If you are curious or worried -- then look inside your walls!

    The most likely orientation for damage would be the south side. If your home is air conditioned, the risk is much higher than if it isn't.

  17. mma948 | | #17

    Brick and Cinder Block Wall
    Hi Dr. Joe

    I own a home in Toronto that was built in 1965. The construction seems to be solid masonry with brick on the outside and cinder block on the inside. Much of what I've read regarding interior insulation of masonry walls seems to pertain to "double brick". Are the risks and insulation recommendations (i.e. 1-3 inches of closed cell high density spray foam) the same with brick + cinder block construction?

  18. GBA Editor
    Martin Holladay | | #18

    Response to Martin Ma
    Good question; I hope that Joe Lsitburek can answer. In the meantime, here are my thoughts: interior insulation will definitely make the bricks colder, which is a potential concern. If there are any embedded floor joists or beams, you also need to worry about moisture accumulation in their embedded ends.

    More information here: Insulating Old Brick Buildings.

  19. Joe Lstiburek | | #19

    Answer to Martin Ma
    I grew up in Toronto in a similar home so I am familiar with this type of construction. The concern is considerably less due to the hollow channels in in the cinder block. The complex air flow network associated with the cinder block provides a significantly greater moisture drying and redistribution capacity than a multi wythe brick wall.
    -- Joseph Lstiburek

  20. mma948 | | #20

    Follow-up question re: Brick and Cinder Block House
    Thanks Joe and Martin! That's a great relief!

    How should I insulate these walls then? Can I use batt insulation with a plastic vapor barrier? Or would you still recommend the use high density spray foam?

  21. GBA Editor
    Martin Holladay | | #21

    Response to Martin Ma
    I would use spray foam or rigid foam -- not fiberglass.

  22. Joe Lstiburek | | #22

    Stay away from the fiberglass
    Stay away from the fiberglass and poly.
    Spray foam works.
    So would taped and sealed rigid extruded polystyrene.

  23. mma948 | | #23

    Brick and Cinder Block Home
    Hi Martin and Joe.
    A few follow-up questions to my original post Sept 23rd. (My home is in Toronto; solid masonry construction with brick on outer wall and cinder block on the inner wall).

    1) From several articles, including "Insulating Old Brick Buildings", I read that the high density spray foam should be applied directly to the inner brick/ concrete block surface. However, my architect says that on a prior project a consultant from Building Science recommended leaving an airspace between the brick/CMU and the spray insulation... thought that this would help dry things out. Can you help me resolve this?

    2) How much high density foam should I use? I've read 1-3 inches? If 3 inches, assuming R=6 per inch, then the total R would be 18. However, this article suggests keeping R < 10... so should I only be putting in 1.5 inches of foam? Or should I put in more because of thermal bridging losses?

    3) I do have embedded wood beams. Since I have CMU's, I presume I don't need to worry about rot as much? What would you recommend I do... just go ahead and spray foam? use borate salts? build a wood wall to support the beam? Are there any other articles that you'd recommend regarding this?

    The more I read, the more I realize I don't understand... and the more I wish I'd just left the house as is. Thanks in advance!

    Martin Ma

  24. GBA Editor
    Martin Holladay | | #24

    Response to Martin Ma
    I suggest that you re-read this article: Insulating Old Brick Buildings.

    The article lists all of the many variables that affect insulation decisions on brick buildings. After you finish reading the article, it should be clear that most of your questions can't be answered over the Internet. Answers to these questions require a site visit by a qualified inspector or experienced consultant.

    Good luck.

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