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Community and Q&A

Insulating and air sealing lath and plaster celing with ventilated attic

David Argilla | Posted in General Questions on

Hi, I want to seal and insulate my ceiling this summer. Old house with lathe and plaster, 2×4 joists, ventilated attic, and 3-4 inches of what looks like mineral wool insulation. Plan was to remove old insulation, seal any holes and cracks, and seal over interior wall partitions. Then add as much blown cellulose as possible though not sure yet if 2×4 joists will limit how much cellulose I can add weight wise.

In below JLC forum thread it was stated that you can’t use lathe and plaster as an air barrier.
“Air seal any obvious holes/penetrations, but lathe and plaster, unlike sheetrock, can’t be part of a good air barrier.”
Is this due to crack issues in the plaster? If cracks are painted over will this block air flow sufficiently for the paint to act as an air barrier (plaster has multiple layers of paint without obvious cracks, but I am sure that some of the plaster is cracked underneath) If cracks through paint are apparent, would sealing with dry wall compound and repainting be sufficient to seal the cracks.

Or is it advisable to try and seal the attic side of the ceiling (not the roof) using drywall or sheathing of some sort (homosote/plywood/…?)

At the eaves there is little room for adding cellulose, is it recommended to use a material with greater R value ie spray foam or fitted solid foam? There full length vent at peak of roof, but there are no eave vents. Some air flows through the attic siding, as there is no sheathing under the siding.
House is in Seattle.

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  1. GBA Editor
    Martin Holladay | | #1

    If you don't have much depth near the eaves to install adequate insulation, your best bet is to install closed-cell spray foam at the perimeter of your attic.

    People install cellulose over lath-and-plaster ceilings all the time. While such a ceiling may not reach Passivhaus levels of airtightness, it's possible to improve the airtightness of a ceiling enough to make a big difference in your home's energy bills. Start with the big leaks -- all of which are best addressed from the attic side, before you insulate: pipe penetrations, wire penetrations, electrical boxes, fan housings, duct penetrations, flues, chimneys, chases, and partition top plates.

    Then build a new attic access hatch and seal it with high-quality weatherstripping.

    1. Bryce Nesbitt | | #6

      Perhaps better to buy a new attic stair, pre-insulated and flashed. Weatherstripping an old attic hatch is hard....

  2. Jim Bannon | | #2

    We are presently doing an attic in an older home in southern NH with lathe and plaster ceiling and 2x8 joists. We have sucked out all the old insulation which was a combination of chopped glass and cellulose (plus lots of rodent and bat droppings). The client wants to use the attic for storage so we are adding a second set of 2x8 joists, attaching them to the rafter tails about 12" above the existing joists. To air seal, we are spraying the eaves and the top of the entire ceiling with about 2" of foam. We'll then top that off with about 16 or so inches of cellulose, shooting for about R-60. We've built up the sides of the attic hatch to accomodate the additional thickness of insulation, and adding foam gaskets around the hatch cover which is 4" thick rigid foam.

    The only ventilation is gable vents, but with our level of effort on air sealing, attic ventilation for moisture management is a vert minor concern. We may add a gable vent fan just to help dump hot air in the summer.

    We'll do new 3/4" Advantech flooring after all the cellulose is blown in.

    I'll admit, it's a cadillac approach.

  3. Jim Bannon | | #3

    To clarify my previous post, the tops of the new joists are ~12" above the tops of the original joists. There is about 4" gap between the top of the original joist and the bottom of the new joists.

  4. David Argilla | | #4

    Ok thanks. For the perimeter, any opinions on the spray foam kits that are sold for DIY (tiger foam or others)?
    I like the idea of adding joists for a floor above the insulation, storage is an issue in our house. Have to think about that.

  5. GBA Editor
    Martin Holladay | | #5

    I like your approach -- but please don't install the attic fan.

    You've created an excellent insulation layer, so you don't need to attempt to lower the temperature of your attic air. All the fan will do is raise the homeowner's energy bills -- an ironic undermining of your energy-saving efforts.

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