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What kind of rigid foam insulation should I install over existing plaster walls before installing new sheetrock?

paulsmith | Posted in General Questions on

I have an older house with very little insulation and lots of cracks in the plaster. I would like to insulate by putting a layer of foam board over the plaster, and then covering the foam board with sheetrock. Are there any problems with doing this? I want to use a foam board that is green with no off-gasing.

Thank you,


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  1. jklingel | | #1

    Your "greener" foam is the EPS, or white bead board. Be sure to air seal the foam w/ tape/goo if you do go this way. I am not a pro, but I see no reason why you can't do as proposed. Screwing the sheet rock may be interesting, but doable. Is removing the plaster and insulating the walls viable instead?

  2. GBA Editor
    Martin Holladay | | #2

    From an environmental perspective, polyisocyanurate is usually the best choice. (Read this article to find out why.) It also has a high R-value per inch.

    Your plan will work. You will need to purchase and install extenders for all of your electrical boxes, and you will need to remove all of your window and door casing, and build jamb extensions for your windows and doors before you re-install the casing.

  3. paulsmith | | #3

    Dear Martin,

    Thank you for your response. I have an additional question about vapor barriers. Is there a potential problem if I install Poli 1" foil faced insulation boards over existing plaster walls, and then cover the Poli board with sheetrock? Also, is there a fire hazard associated with installing the Poli boards?

    Thank you,


  4. GBA Editor
    Martin Holladay | | #4

    Q. "Is there a potential problem if I install Polyiso 1-inch foil faced insulation boards over existing plaster walls, and then cover the Polyiso board with sheetrock?"

    A. No.

    Q. "Is there a fire hazard associated with installing the Polyiso boards?"

    A. No, as long as the polyiso is covered with a layer of drywall.

  5. user-1037041 | | #5

    Maybe I'm missing something here, but I thought foil-faced Polyiso could be left exposed? Seems I've read on the Dow website somewhere that exposed foil-faced polyiso can be used as an ignition barrier...

  6. jklingel | | #6

    "THERMAX™ sheathing meets required fire performance and thermal insulation criteria.....THERMAX™ sheathing will not sustain fire without a separate flame source such as combustible building components. Fire tests have been conducted for acceptance of THERMAX™ sheathing in exposed applications." from

    But then "THERMAX™ products are combustible and when used in a building containing combustible materials, may contribute to the spread of fire."

    WTH? I'd stick w/ Martin's advice.

  7. GBA Editor
    Martin Holladay | | #7

    You are right that Thermax doesn't need an ignition barrier when installed in a crawl space or attic. But in most other cases, building codes require foam to be covered with a layer that is more stringent than an ignition barrier -- namely, a 15-minute thermal barrier.

    Most building codes, including the International Residential Code (IRC), require foam insulation (including foam installed on the interior of basement walls) to be covered with a 15-minute thermal barrier. However, if the spray foam is located in a crawlspace or an attic “where entry is made only for service of utilities,” the code permits the installation of a less stringent covering: an ignition barrier rather than a 15-minute thermal barrier.

    In sections R314.5.3 and R314.5.4, the IRC defines an ignition barrier as one of six permissible materials: 1 ½-inch-thick mineral fiber insulation; ¼-inch-thick wood structural panels (e.g., plywood); 3/8-inch particleboard; ¼-inch-thick hardboard; 3/8-inch-thick gypsum board; or corrosion-resistant steel having a base metal thickness of 0.016 inch. Presumably, code officials also permit the installation of thicker versions of any of the six listed materials.

  8. slimpickens | | #8

    Thanks Martin. As usual.
    And, yes, John, like many huge corporations, Dow's website (and communication in general) seems contradictory!

    PS- I like your ™

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