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Community and Q&A

1920 stone/stucco insulation? Closed cell spray foam in 2020?

Corby100 | Posted in Energy Efficiency and Durability on

1920 built home – 
Currently ready for insulation after memorial of plaster and Lathe.

1st floor 4 foot stone walls exposed.

2nd/3rd floor – 1 inch solid 100 + year old wood planks covered by at least 6- 8 inches of stucco.

Roof – Planks covered by new plywood and new asphalt 30 year shingles.- triple thickness.  Roof vent is placed on top.  The third floor is living conditioned space,

What type of insulation should I use for the roofline.  Closed cell spray foam or fiberglass?  I would like to go green – however the insulation companies all recommend spray foam.  I would rather under insulate in this 100 year old home (It has 8 inch beams) with extra space behind the knee walls, and under the peak.)

The 2nd/3rd floor stucco with wooden planks and framing.

1st floor solid stone wall with stone foundation – Wissahickon shist.  

Home located in Philadelphia Pennsylvania.

Thank you.

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  1. GBA Editor
    Brian Pontolilo | | #1

    Hi Corby,

    If you are insulating an unvented roof from inside only, the best approach is to spray enough closed cell foam against the roof deck for condensation control. Then, you can fill up the rafter bays with any insulation you would like. I suggest you start here: How to Build an Insulated Cathedral Ceiling.

    Going this route, the easiest approach would be to continue the insulation all the way to the top plate, so that the attic space behind the kneewall is withing the thermal envelope. More on that here: Insulating Behind Kneewalls

  2. Corby100 | | #2

    Hello Brian,

    Is it permissible to use closed cell spray foam insulation on 1 inch wooden plank siding with over laying 6 inch 100 year old stucco? There are gaps between the planks of about 2 inches. I am planning 2 coats if elastomeric paint from Sherwin- Williams to seal the stucco from moisture absorption. Is it his acceptable technique? I always hear of “wood rot” and mold issues in homes?

    Thank you!

    1. GBA Editor
      Brian Pontolilo | | #3

      Hi Corby,

      Any significant thickness of closed-cell foam as cavity insulation will be less than 1 perm, so you'd be significantly changing the vapor profile of this wall. And because I am not familiar with your heavy stucco assembly, I'd be reluctant to make any recommendations. I would suggest that you consult with a local expert on this. Closed cell foam is not easily removed should it cause some sort of wall failure.

  3. Expert Member
    Dana Dorsett | | #4

    Is the ultra-thick stucco bonded to the plank sheathing (like really FAT lath), or is there an air gap?

  4. Corby100 | | #5

    I could touch felt paper under the stucco- between the spaces of the plank. Should I try to see if there is a wire mesh or lathe underneath??
    Thank you

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