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Community and Q&A

Proper way to insulate an old attic?

justinjeffreylee | Posted in Energy Efficiency and Durability on

I am helping to insulate a friend’s attic in Milwaukee, Climate Zone 6. The house is 100 years old, balloon frame with stucco. Attic floor is lathe and plaster. We would like to remove the fiberglass insulation, clean the attic floor, seal holes/top plate, then spray 2″ closed cell spray foam to air seal the lathe and plaster. After that fill in blown cellulose to R-60.

My question, if we are sealing off the top plate of the exterior walls, are we in danger of causing a condensation problem in those walls. They may or may not have insulation we are unsure, but the house is very drafty. Moisture has been moving through the walls for 100 years, and with the top plate sealed there will be less air movement so should be less moisture infiltration. I can imagine a little condensation on the exterior side of the top plate, where the cold be coming from the side through the stucco and the attic insulation will not help keep that edge warm.

I really want to spray foam to help air-seal the attic, but I obviously don’t want to cause a moisture issue. I think it will be fine, just trying to find a second opinion.


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  1. GBA Editor
    Martin Holladay | | #1

    In older buildings with balloon framing, it is important to air seal the tops of the walls to prevent air movement at this location. It sounds like the exterior walls of this house have top plates -- some balloon-framed buildings don't have top plates. Whether there are top plates or not, air sealing the top of the stud bays is certainly a good idea.

  2. Expert Member
    Dana Dorsett | | #2

    Some 100 year old balloon framed stucco-clad antiques have no sheathing layer with the lath nailed directly to the studs, with no wall insulation. Blocking the tops of the stud bays on one of those or even filling the stud bays with cellulose could have significant consequences if that's the way it was built. The full stackup of the wall assembly needs to be better understood before moving forward on sealing the tops of the stud bays or adding wall insulation.

    Air sealing the attic floor is a good idea, but it doesn't take 2" of closed cell foam to get there, nor is that a very green air-sealing solution, due to the high global warming potential of the blowing agent used (almost universally HFC245fa, which has a GWP ~1000x CO2.) If it's easier to air-seal with foam rather than membranes, ~3" of open cell foam air seals slightly better (according to Building Science Corp testing on wall assemblies), is more flexible, and is blown with water, which has a very low GWP. Also, 3.5" of open cell foam costs substantially less than 2" of closed cell foam, delivers the same or slightly higher R-value and uses less than half the amount of chemical polymer.

  3. justinjeffreylee | | #3

    I am open to open cell foam, those are pretty good reasons to choose it over closed cell.

    I don't think there are any open walls to check the full layout, and we will not be adding wall insulation at this point. How would you handle the attic insulation is the stucco had been installed in the worst case scenario?

  4. GBA Editor
    Martin Holladay | | #4

    I disagree with Dana on this one. Dana asserts that a stucco-clad building without any wall sheathing might be at risk if the tops of the empty stud cavities are sealed with an air barrier. I disagree. Empty stud cavities dry readily, even when the tops of the cavities are sealed.

  5. justinjeffreylee | | #5


    So empty cavities are good, should I be concerned if there is insulation? Again, pretty sure there isn't but am going to drill a hole and check if needed.

  6. GBA Editor
    Martin Holladay | | #6

    If the bays are already packed with cellulose, you aren't getting much ventilation drying due to air movement through the stud bays -- so capping the tops of the walls with an air barrier isn't going to make things worse.

  7. justinjeffreylee | | #7


  8. Expert Member
    Dana Dorsett | | #8

    Empty studwall cavities would be able to dry even if blocked off at the top- Martin has it right.

    Empty studwall cavites are good only from a wall-longevity point of view- they are an energy disaster worth treating in a Milwaukee climate. An empty balloon-framed cavity is essentially a flue that sucks air 24/365. The total wall area on most homes is a multiple of the total attic floor area. Bumping the wall insulation from nada to even R5 has a bigger net improvement on energy use than taking an attic from R19 to R60.

    Find a closet or cabinet on an exterior wall where you can drill a half-inch hole and peek around with a boroscope and/or probe it with something. If you drill carefully you can probably drill it all the way through without cracking the stucco and get a real layer-by-layer sampling of what's there. What you find will determine what makes the most sense for treating the walls.

  9. justinjeffreylee | | #9

    Foam is nasty I'll agree, but I haven't seen another way to effectively seal a lathe and plaster attic ceiling.

  10. Richard Beyer | | #10

    Considering the medical community does not know who or when a person may become sensitized to spray foam chemicals, why would anyone recommend this product aside from it's insulating properties? Is your health worth risking for energy savings? If you have children living in the home are you prepared for the potential consequences to their health?

  11. Richard Beyer | | #11

    Blown in mineral wool or fiberglass. It's your safest choice and it's proven.

    Spray foam brings along with it some very serious proofing issues (extremely flammable during and after installation), odors, health toxicity and the probability of rendering the home uninhabitable when it's not installed by the book. Know what your buying before you leap or this could become the most expensive learning experience you have ever wished not to have experienced.

    Ask Dr. Energy Saver's founder Larry Janesky about his experience....Here's his personal home shortly after installation of spray foam.... .... There was no saving this mansion once it ignited.

    Read and understand before you leap. SPFI is nothing to play around with.

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