The roof of a single story room (it looks like an addition but was built with the house in 1951) on a two-story house will be sealed, insulated, and re-shingled (northern Zone 5a, almost 6). Three sides slope for about three feet until the slopes meet the flat part that makes up the majority of the roof...



The ceiling is plaster, penetrations will be sealed, the joints between the high-R polyiso boards used near the perimeter to compensate for the lower height will be staggered and sealed, air baffles allowing 2 - 3" of air flow beneath the slopes will be installed, we'll leave 2 - 4" of space under the flat roof for ventilation but the rest (about 24" under the ventilation space under the flat part + whatever will fit over the polyiso) will be filled with cellulose (please see the following drawing).  The flat part of the roof will be re-built at a very slight angle so water doesn't pool and Ice & Water Shield will be installed.  I'm aiming for a pseudo Passive House structure that is durable...



     Air sealing seems very important, but I've read it's impossible to seal old homes up super-tight.  Fine Home Building published an article saying "When blown-in insulation is installed in a closed framing cavity, it’s possible to achieve higher insulation densities than can be achieved on an attic floor...  In a closed cavity, cellulose should be installed by the dense-pack method—that is, to a density of at least 3-1/2 lb. per cu. ft."  Dense pack provides a partial air barrier too.

     With this short attic (29" height max), maybe it makes sense to consider it a closed cavity and use dense packed cellulose above and to the interior of the polyiso - packing up to the air baffles on the sides and leaving 2 - 4" of ventilation space below the flat roof.  This would increase R some, AND will impart the partial air barrier qualities of dense packed cellulose to a house that needs all the air-sealing help it can get.

     Would you expect problems with using dense-packed cellulose where "cellulose" is seen in diagram above (the squigly hand-drawn lines) with respect to moisture, insulation value, convection, etc.???  NEW Passive Houses use this much or more vertical loose-packed cellulose, but I've only seen dense-packed in wall structures in Passive Houses and normal construction.  Maybe 24 vertical inches of dense packed cellulose over a 12' x 13' ceiling is too much to allow some necessary process buildings undergo?  AND Passive Houses have well insulated walls, half-height attics, and the like abutting other similarly insulated structures.  Maybe having 24" deep dense packed cellulose abutting the minimally insulated 2cd floor brick wall and extending 12 feet out will cause some problem?  Please let me know if you think considering this a closed cavity and dense packing it as described above is safe or I should stick with normal cellulose (the R will still be great, and normal tape and caulk sealing will still be done, but the partial air sealing dense-pack provides wouldn't be part of the roof).

     Thank you very much in advance.

Gordon

There is no particular advantage to dense packing in this situation, and makes the installation more complicated. The loose-fill at ~1.3-1.5lbs density is sufficiently air retardent that it doesn't lose performance to convection at the cool weather extremes, and has the same or higher R/inch of dense pack.

Don't believe everything you read about retrofit air sealing, particularly when you're focusing on just one small area like that. (A deep energy retrofit on a 3-story 1890 antique I helped a friend with test at 464 cfm/50, and that was BEFORE he fixed the leaking weatherstripping on the basement door!) I assumin

In most brick veneer framed houses of your era the cavity between the wood sheathing and the masonry vents into the attic right at the eaves. If that's the case here, take care not to seal up the masonry cavity or you'll put the wall assembly at some risk. If there are no top vents to that cavity, it's worth opening up the vertical mortar at the top course of brick ever 3rd line or so to let the cavity convection-purge. (Some prefer to face-drill the brick for round screened plug-vents, but that's more complicated.)

Going all the way to R75 for the few square feet in the middle has very little benefit when you have only ~R30-35 at the edges with the stacked polyiso, since every square foot of perimeter is conducting 2x the heat of a square foot of area in the middle. It also adds a lot of weight- at dense-pack density you'd be looking at 6-7lbs per square foot dead-weight out in the middle, loose fill about 3lbs per square foot. Stacking the 5" or so with polyiso about a foot in from the eaves is worth it, and filling the rest with 14-15" of loose fill cellulose would deliver pretty good R50-ish performance in the middle, or a bit over 1.5x that of the 1' perimeter. (If you were going to go all PassiveHouse on this you'd be doing something different anyway, and insulate over the roof deck to guarantee R75-R100 everywhere.)