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Community and Q&A

Insulating an Unvented Cape Attic

Cincinnati1905 | Posted in Energy Efficiency and Durability on

Hi!  I’m looking for some advice on insulating a 3 story home built in 1905.  it has a finished 3rd floor with a cape cod, unvented attic and an original tile roof.  We’re in southern Ohio.  The unfinished space on the 3rd floor has flooring (no open beams) and knob and tube under the floors.  there is no knob and tube in the knee walls or the space above the 3rd floor.

My question is how should I insulate?  I have a quote to blow-in across the unfinished floors (but then I lose storage), batt the knee walls, and across the 3rd floor ceiling.  I have a separate quote to use 2″ of closed-cell foam to vapor seal the attic rafters and then across the 3rd floor ceiling, with batts/blow-in added for r-value.  I’m told since it’s unvented and it’s closed cell, this will not create issues for my roof decking.  The quotes are surprisingly not that different in price.

Any recommendations on which way to go?  My priority is clearly to save energy/comfort inside, though I also do not want to compromise an original tile roof by rotting the roof deck (though closed cell seems ok in this setting).

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  1. walta100 | | #1

    Sell the place while the market is hot.

    Sadly, the house you are describing is an energy pig and will be next to impossible to make efficient short of a gut rehab with a deep energy refit and upgrading the wiring. Unless you have some emotional attachment to this house the smart financial move is to sell without spending a ton of money on stuff the buyers can see and will not value.

    Since it sems unlikely you will sell. You can’t really insulate until you have gotten rid of the knob and tube because it is inevitable that it will be replace because it is inadequate given today’s life style and future seems likely to put higher demands on your outdated and unsafe old wiring. My opinion is any system without a grounding is unsafe.

    I hate spray foam but your house one of the few times it is the best option. Do a blower door test before and after it is installed if you are very lucky you may get down to 3 ACH 50 do your best to get there.

    All this work is risky in that the old windows are unlikely to have been flashed as well as we require today. It simply was not necessary as if a little water happened to get into the walls, it quickly evaporated because so much heat was passing thru the walls and there was lots of air moving thru the wall to carrier away the moisture. If you stop the heat and air flows walls that survived a hundred years may suddenly begin to rot.


  2. Expert Member
    PETER G ENGLE PE | | #2

    Insulating over and around K&T wiring is not allowed, as it is a fire hazard. If you are going to insulate those spaces, you must renovate that wiring first. Beyond that, as Walter says, it is very hard to effectively insulate and air seal a house like that without a full gut & rehab. A full gut & rehab often approaches the cost of new construction.

  3. brendanalbano | | #3

    The more difficult (i.e. expensive) it is to do a good job improving the envelope, the better solar panels start to look. This doesn't do anything for comfort issues, but is worth thinking about.

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