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Community and Q&A

Spray foam vs. parging rubble walls

Anthony Yoder | Posted in Energy Efficiency and Durability on

We live in a brick twin home with a rubble stone foundation with dampness issues (not flooding) and the parging in the basement is failing. I was planning to have it parged again but I am wondering if it would be a better choice to apply spray foam. It’s unlikely we will ever finish the space. Is 2” appropriate and should I consider open cell at all?

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  1. Anthony Yoder | | #1

    Also, how worried should I be about moisture issues this might create in the let in floor joists? Thanks!

  2. GBA Editor
    Martin Holladay | | #2

    In general, I think that basement walls should be insulated in the northern half of the U.S.

    Where do you live?

    Whether or not your rubble stone wall is a good candidate for spray foam depends on a variety of factors, including how damp the wall is and the integrity of the existing mortar. An experienced spray foam contractor should be able to assess your walls and give you advice.

    If you go ahead with your project, use closed-cell spray foam, not open-cell spray foam.

  3. Expert Member
    Dana Dorsett | | #3

    Repairing any of the mortar would come first and foremost.

    A parge protects the mortar from breaking down via moisture leaching only if there is moisture passing through the wall. With a couple inches of closed cell foam in the interior side the moisture flow stops, and so does the leaching.

    Just 2" of closed cell foam would not quite make current IRC code minimum R15 for basement walls in US climate zones 4C, 5, and higher, but 3" would. For zones 4A/B or lower 2" would be fine.

    Open cell foam applied below graded would not stop the moisture transfer, and would become partially saturated with liquid water at different times of the year, and thus not recommended.

    If closed cell foam it's greener to only those blown HFO blowing agents, which has very low environmental impact. The industry standard HFC245fa is a powerful greenhouse gas (about 1000x CO2 @ 100 years). The HFO blown foam is usually about R7/inch, so at 2" it would ALMOST meet code min at 2" for the colder parts of the US. HFC blown foam runs R6/inch, to R6.5/inch.

    Most types of stone used in rubble foundations don't wick moisture very strongly (unlike concrete, which wicks moisture VERY readily). So even if the bottom of the rubble wall is saturated it doesn't present much of a moisture risk for let-in floor joists, as long as the bottoms of the joists are at least several inches above grade. If any part of the let-in joists are below grade they are susceptible and it will need a bit more site analysis.

  4. GBA Editor
    Martin Holladay | | #4

    Q. "How worried should I be about moisture issues this might create in the let-in floor joists?"

    A. If your rubble wall has embedded joists, there is some risk that insulating the wall with closed-cell spray foam will increase the risk of joist end rot.

    First, assess the situation with an awl. Are the ends of the joists currently sound?

    Here is a list of the factors that increase the risk of joist end rot:

    1. The distance from the top of the foundation wall to the exterior grade is short (8 inches or less).

    2. On the exterior of the house, the above-grade portion of the foundation is shaded by bushes.

    3. The basement is noticeably damp.

    Here is a list of the factors that decrease the risk of joist end rot:

    1. The distance from the top of the foundation wall to the exterior grade is 9 inches or more.

    2. On the exterior of the house, the above-grade portion of the foundation gets plenty of sun and air.

    3. The basement feels dry.

    The classic solution to this problem is to support the ends of the joists with a new framed bearing wall or a beam supported by new posts, and then to cut off the embedded joist ends with a chainsaw or reciprocating saw.

  5. Anthony Yoder | | #5

    This site and the advice provided are amazing. We are in Philadelphia and the walls were actually packed with mud/dirt and mortar only applied to the exterior. End rot is of little concern based on the criteria you listed. An initial quote has foam at the same price as parging which surprised me. My only remaining question is whether I should consider a French drain prior to insulating if I’ve never had standing water and should be able to reduce dampness further with exterior detailing. Thanks for the great advice.

  6. GBA Editor
    Martin Holladay | | #6

    If there is no history of liquid water entry into your basement, I don't think there is any need for an interior French drain.

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