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Community and Q&A

Rigid foam insulation in an old brick home

user-7032327 | Posted in General Questions on

Hi there I am hoping someone can help me with an insulation situation i am facing in my 1895 brick house. There is and has never been any insulation in the house and i just removed all of the plaster walls. I dont want to frame the walls with 2×4 framing to allow for the needed space to do spray foam insulation and I have actually heard that spray foam can be really bad for old brick walls. Plus this would force me to lose interior square footage in an already small house and ruin all my windows and door frames and be very expensive. The plaster and lathe i removed was all attached to 1 inch vertical furring strips which are all still intact on the brick walls spaced 16 inches apart. I was thinking the best solution would be to attach 1 inch Rmax polyiso boards with the foil facing to the furring strips, leaving the 1 inch gap between the poyiso boards and the brick and then drywalling over the polyiso boards. I would tape all the seams and do my best to seal the edges at the floors and ceilings as well. This would not give me the R21 code requirement i need for upstate NY but my building inspector tells me i dont need to be to code for insulation since my house is so old and has never had insulation and since achieving R21 using spray foam could damage the brick wall. Can anyone help me decide if my plan to use the 1 inch foil facing polyiso panels is a good idea? Would using the Rmax Durasheath panels be better than the foil faced panels? I would also insulate the attic to R38 with Batt insulation. Thank you! Any thoughts and info would be greatly appreciated!

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  1. GBA Editor
    Martin Holladay | | #1

    I suggest that you read this article: Insulating Old Brick Buildings.

  2. Expert Member
    Dana Dorsett | | #2

    Martin: USER-7032327 signed off with the name "Armen".

    Armen: The 1" continuous layer of foil faced foam mounted to the 1" furring would probably work just fine, but it may also be fine to strip the 1" furring and install 2" of polyiso, which would have substantially higher performance. But it's complicated, and a better understanding of the wall construction, roof overhangs & building height would be necessary to comment any further.

  3. GBA Editor
    Martin Holladay | | #3


  4. Tyler_LeClear_Vachta | | #4

    Agreed with Dana, the 1" gap is overkill on a brick wall retrofit. You're not going to have any new mortar squeezings to clog your drainage gap. You could get away with doing a skinnier drainage gap, a product like Gravity Cavity which would maintain a drainage gap and let you put in another inch of insulation, or use insulation board with grooves in it that maintains a gap. If you go this route make sure that at the bottom of the wall any moisture that accumulates has a clear path to drain out. Not sure what your flashing scenario currently looks like. Joe Lstiburek had an article on Rebuilding Houston this past fall that share some similarities with your situation with regards to insulation, drainage and flashing.

    Martin's article makes a good point - if your walls are vulnerable you may not want to insulate them.

  5. user-7032327 | | #5

    Thanks Martin and Dana. Martin your article is very informative. Dana, the roof is a low pitch hip roof with around 15 inches overhang. It’s a two story brick house roughly 25 feet high. The brick walls are triple wythe on the first floor and double on the second floor. I had contemplated ripping the 1 inch polyiso panels into 16 inch wide strips and attaching them directly to the bricks between the furring strips then putting another 1 inch full panel over that to get the two inches and higher r value but I have been told hat it is important to keep an air gap between the polyiso and the bricks to they can dry out and breathe. It is for this reason I am concerned about doing spray foam directly on the brick. I also hate the idea of having to frame out the rooms with 2x4s to accommodate the foam. I think the polyiso boards make the most sense from a cost standpoint and for the health of my bricks but curious to hear what you guys think? Thank you!

  6. GBA Editor
    Martin Holladay | | #6

    My article, Insulating Old Brick Buildings, summarizes the best advice available from the most experienced building scientists in North America. Briefly, you need to correct water-management details on the exterior of the building to minimize wetting; you need to evaluate the soundness of the bricks; and, if the bricks prove to be durable enough, consider installing a limited amount of closed-cell spray foam on the interior side of the bricks.

    But if you don't believe the article, it's hard to give you further advice.

    You wrote, "I have been told" -- by who? -- "that it is important to keep an air gap between the polyiso and the bricks to they can dry out and breathe. It is for this reason I am concerned about doing spray foam directly on the brick."

    It's up to you to evaluate the source of your information. If your source seems evidence-based, and seems more reputable to you than the advice of John Straube and Joe Lstiburek, then I guess you should seek advice from this other source.

    There are many problems associated with your idea of keeping an air gap between the insulation and the bricks. Here's one: hidden air pathways are common in older homes. If interior air is allowed to flow into a gap between the insulation and the bricks, condensation and frost will build up on the interior side of your brick wall. You don't want that.

  7. user-7032327 | | #7

    Thanks Martin - do you think I can have spray foam applied to the walls in between the existing 1 inch Furring strips? Or I suppose I could add another 1 inch furring strip to the existing ones to make two inches of furring strips and the spray foam could be applied to that getting me 2 inches of closed cell spray foam? Or do you think a one inch flash spray is better for an old brick house since you advise installing a “limited amount”?

  8. GBA Editor
    Martin Holladay | | #8

    I'm going to refer you again to my article, Insulating Old Brick Buildings. Read the article.

    Determining whether you can insulate the walls of an old brick building on the interior depends on the results of a risk assessment. Either you can conduct that risk assessment, if you feel qualified to do so, or you can hire an experienced consultant to conduct the risk assessment for you.

    Depending on the results of that risk assessment, you may be able to install interior insulation. The thickness of the insulation will depend in part on the qualities of your bricks, and the exposure of your bricks to wind-driven rain.

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