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New insulation for old house

ABakke | Posted in Energy Efficiency and Durability on

Hey team, I’ve got a 100 year old Tudor house with brick and stucco exterior in great shape.  Plumbing leakage has prompted remodeling of master bath.  I’m going down to the studs and wondering if I should spray foam the exterior wall.  I’m nervous about impeding the drying capacity of my “reservoir cladding”.  The current insulation seems to be a horse hair/shredded paper concoction.  I live in one of the coldest cities in the lower 48.  Perhaps another insulation would be wiser.  Thanks much,  Andy

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  1. GBA Editor
    Brian Pontolilo | | #1

    Hey Andy.

    Where are you located and how are the walls assembled? Is there are air space between the wall sheathing and brick and stucco exteriors?

  2. ABakke | | #2

    Brian, I’m in Fargo, ND. I have not been into the walls before. There are some areas in the basement that have allowed me to see partially what’s inside. Lath and plaster interior, solid wood sheathing. 1919 construction methods and materials.

    1. Expert Member
      Dana Dorsett | | #3

      I'm assuming it 's full-dimension 2x4s?

      Fargo ND is at the warm edge of US climate zone 7A using the IECC 2004 map, but might already be in zone 6A if more recent weather data were used. In TABLE R702.7.1 the IRC prescribes R10 on the exterior for dew point control on a 2x4/R13 wall, or (10/23=) 43% of the total R for zone 7, but only R7.5 or (7.5/20.5= ) 37% for zone 6, which is practically spitting distance away from Fargo.

      In the 4" depth of a full depth 2x4 installing a 1" flash of HFO blown closed cell foam (~R7, and ~1 perm) and 3" of cellulose or compressed R13 batt (R11-R11.5) you'd be at about 38% of the total-R as foam, which should be good enough if a "smart" vapor retarder such as an asphalted kraft facer or Certainteed MemBrain is installed between the fiber insulation and wallboard. The sheathing's drying capacity will be limited by the 1-perm foam, but the fiber insulation's drying rate will be only limited by the interior finish paint, which would run about 3-5 perms for a few coats of interior latex. Cellulose has enough moisture buffering capacity that the interior side varpor retarder could be skipped, but "contractor roll" kraft faced R13s are ubiquitous and cheap, and a kraft facer is good enough, as long as there is at least 1" of closed cell foam and a fiber depth no more than 3".

      Installing any more closed cell foam than the minimum needed for dew point control would be a waste of environmentally & financially expensive foam:

      By 2050 Fargo is likely to be fully inside of zone 6A (maybe even the warm edge of zone 6) due to global warming/climate change. Even Winnipeg is projected to have a zone 6A climate by then:

      (zone 7 starts at 5000 annual HDD Celcius, 9000 HDD Fahrenheit)

      Given that Fargo is on the warm edge of zone 7 using last-century's weather data, you may already be in zone 6 now.

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