Good Morning All - I'm working on a renovation on the first floor of the home my parents purchased and have lived in for over 30 years ago. It's an all masonry rowhouse with 2 floors and a finished basement. There will be a series of ongoing questions I'll have as I go on, but to start here is some background leading to my first question. I'm tearing out the hand-nailed 1/4" solid wood parquet (most of it is in bad shape) and the damaged underlying T&G 3/4" plywood planks so I can lay two new layers of 3/4" exterior grade plywood. My joists are old lumber -- just about 2.5" wide and 7 3/4" deep and run on 16" centers then set into the wythe brick pockets. The joists are firecut too, which is surprising since the house was built in 1928. The house has settled a bit and once I have the flooring I can see where my high & low points are and calculate the difference. The cross-bracing is done at 10ft of the joist span, which means the other 10ft runs to the party wall which adjoins the neighbors property. Fortunately, this house is on the corner, so only one party wall exists. The house runs on a 1-pipe steam system with no plans on replacing it. That being said my questions are these: I'm torn between insulating between floor joists. In my specific example, between the finished basement and the first floor and between the first floor and the second floor. The house is not exactly air tight, since it's two layers of wythe brick. One train of thought suggests the house has been able to "breathe" without ever developing any condensation so why would I introduce an insulation (e.g roxul safe-n-sound) product that could complicate the houses inherent stack affect? I'd love to hear the feedback from the community on whether it would be a good idea to insulate between floor joists knowing the buildings structure/config? Will it potentially cause condensation in in the joist bays ? Since the house was plaster and lath, the studs were "strapped" to the brick with cut nails making only about a 3/4" depth which was, years ago by my parents, insulated using blue foam boards. I'm now moving the radiators a bit so i can properly frame using 2x4 -- is the minimum Rvalue for a 1st floor R20? I honestly can't afford any of the spray foam products and will be going with batts. Any recommendation on which batt product? Also - what type of product (silkaflex caulking?) should i use at areas of the brick where air is entering the building? Or should i let the batts block that? Thanks all https://www.dropbox.com/s/5ta2ijdxko...ll%29.jpg?dl=0

Paragraph breaks would make this more readable... (If you used line & paragraph breaks but they just don't show up, use a different web-browser such as FireFox.)

You'll never have to worry about condensation from air movement between floors, since they're all about the same temperature. While it's still good to make the floor-by-floor transitions reasonably air tight, with steam systems it'll never be perfectly air tight, since some amount of clearance between the steam piping and wooden floor/subfloor is required to keep it from charring over time.

Fiber insulation is not air tight. While blown insulation can enhance air-tightness if installed at a sufficiently high density, batt insulation has almost zero effect on controlling stack effect infiltration.

Putting insulation between the basement and first floor thermally isolates the boiler from the first floor, which will often be REDUCE system efficiency, since the boiler's jacket losses and distribution losses then accrue primarily to the basement. (If the boiler room is the warmest place in the house, you absolutely DON'T want to put insulation between the boiler and conditioned spaces!)

Insulating the exterior walls is right thing to do, but how much and how you go about it depends on your climate and construction, including details of how joists are supported by the structural double-wythe brick, and how much roof overhang you have, etc. In your case the joists are hung into masonry pockets, which depending on the amount of roof overhang/rain-wetting and the climate, you may need to leave a bit of clearance between the joist pocket any any insulation layer to keep the joist-ends warm & dry enough to avoid rot, or, you may be able to insulate around them at a somewhat lower-R, or something else entirely. Climate-counts- what's your location?

Since the studwalls are not structural, only designed to hold the insulation and interior finish wall you can get better thermal performance than an R20 / 2x6 standard studwall by using 2x2 stud plates set with the interior face 5" from the brick, with 2x4 studs turned sideways flush with the interior side, and installing R15 rock wool or fiberglass butted tight together for a continuous layer of R15 not thermally bridged by studs. You can then split some batts carefully trimmed to fill the 1.5" deep gaps between the studs for a compression fit. Whether this solution is suitable for your construction & location depends on your climate & construction details. An interior side air-barrier is a must (using air-tight wallboard techniques works), but whether or not a vapor retarder makes sense is also climate dependent.

Thanks for the reply Dana.

I apologize for the lack of line breaks ; I anticipate this post being easier on the eyes.

To answer some of your questions:

The house is in the Northeast. Specifically, Queens, NY.

Dana- when you mention using an air-tight wallboard, are you suggesting something along the lines of "comfortboard" ramset or tapcon'd into the brick and then a layer of rockwool (R15) filled between stud cavities? I'm probably going traditional 2x4 timber studs on the walls.

The entire structure is square for the most part. There is a flat roof, that was recently re-done(without any insulation on the roof side) and two layers of wythe brick that carry from the foundation to the parapet walls.