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What to do with energy audit advice?

web_3412 | Posted in General Questions on


Last fall, I purchased a 1940’s cape style house located in New England, and my first winter was tough with high heating bills and ice dams. My energy audit detected wool batting in the walls, fiberglass batts with the paper upside over wool insulation in the attic. I have a roof ridge vent, gable vents, no soffits, and plaster & steel lathe in the knee walls and gable end walls.

My energy auditor has recommended a energy plan that differs from the insulation contractors, and I am confused by this, and would greatly appreciate your feedback.

The energy auditor recommends in the knee wall areas, steel lathe & plaster and wool insulation removed, and the area insulated with cellulose and polyiso rigid foam board. In the attic slope, wool and homasote or paneling boards and wool batts removed and replaced with fiber batting and polyiso.

The contractors recommend leaving everthing in place, punching holes in the plaster / lathe or homosote and filling with cellulose on/over the wool insulation.

The auditor suggested removing the plaster and lathe because it is a large area to fill with cellulose, but I can’t image anyone fitting into the knee wall areas to work on this. (knee walls are 20” x 16” x 26”. ) However, I am concerned that without ventilation, there could be a mold issue or other issues with the cellulose added on the rock wool. Please advise. Thank you!

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  1. GBA Editor
    Martin Holladay | | #1

    The volume behind this type of kneewall is sometimes referred to by weatherization contractors as the "devil's triangle." I discussed options for addressing the devil's triangle in this article:
    Two ways to insulate attic kneewalls.

    In that article, I wrote: “If you have to insulate a triangular attic that is too small to crawl into, it may be necessary to fill the space with dense-packed cellulose. Because this technique doesn’t allow for roof ventilation, it is somewhat controversial. Nevertheless, this approach has been used successfully for years by weatherization contractors in New England. According to Bill Hulstrunk, technical manager at National Fiber, 'We do it only if the area is so small that we can’t get a body in there. In other words, the kneewall has to be 18-in. high or less. The first time the tube is inserted, it sits on the bottom of the floor. Then after a while, you pull out the tube and reinsert it higher up to top everything off.'”

    The article includes more information on options to address these areas. If you can afford the work, the best solution is to install new rigid foam insulation on top of your roof sheathing. Of course, this approach requires new roofing, so it isn't cheap.

  2. web_3412 | | #2


    Thank you for your reply. I read the article and found it very informative. I wish I could install the rigid foam insulation, but it is not possible at this time.

    Do you think there could be an issue if the knee walls contain the wool insulation and cellulose or should the contractors remove the wool batts ?


  3. GBA Editor
    Martin Holladay | | #3

    The main problem with leaving the existing insulation in place is that it is probably in the way of necessary air sealing work.

    This type of home is a nightmare to air seal. How much air sealing work you perform depends on access. Getting good access means ripping out existing insulation and finishes, so the work is expensive.

    There is no easy way to do this work. If you aim for a high standard, the work isn't cheap. Most homeowners end up compromising. Good luck.

  4. user-1072251 | | #4

    You don't mention either the energy auditor or the contractor mentioning "air sealing". Air leakage, or "infiltration" is typically the major energy problem in older homes. Insulation of any thickness works very poorly when moving air is passing through it. So the first step is to find a contractor knowledgeable about air sealing the house, starting at the basement and addressing issues in the walls and roof areas. This is typically done as a separate phase from insulation, although some insulations have sealing capabilities. Beware of the "one material/insulation does all" approach, since air leakage may be at framing intersections and not inside insulation cavities.

    You must have a "blower door" test result, so you will want to test with a blower door after the air sealing is complete and prior to the insulation being installed, and again after completion.

    Do not use batts! Dense packed cellulose is a better insulation in most ways and will conform to any shape cavity.

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