I have searched high and low and cannot seem to find a solid answer to my question. My ‘79 quad level has 2x10 cantilevers over a brick facade separating the lower level from the upper level. Having just removed the stucco style interior ceiling, I pulled out a mix of r11 paper faced batts and some bays additionally had cellulose blown in at some point. I’m caulking all interior cantilever bay seams with polyurethane caulk and need to figure out how to best insulate the area. The main issue is how to address moisture barrier in the cavity given the “floor” of each bay has an open brick facade with air gap. See photo. I’d like to use Mineral Wool, but the 2x10 height creates a size issue that can only be filled by stacking R23 5.5” and R15 3.5”. Even if I do this, still not understanding where/how to place a moisture barrier.
Cantilever insulation again
- Thread starter rextang
- Start date
-
- Tags
- cantilever
Users who are viewing this thread
WorthFlorida
Clinical Trail on a Cancer Drug Started 1/31/24. ☹
Spray foam insulation.
Sponsor
Paid Advertisement
It's unfortunate that they vented the brick veneer into the cantilever rather than to the outdoors. On the outside there should be a set of weep holes in the mortar every third brick or so. To vent it to the exterior, make a corresponding set of holes in the mortar to the exterior, at which point it's safe to air-seal between the brick cavity and the cantilevered joist bay. That air barrier could be done with any number of materials, such as cut'n'cobbled foam board can-foam sealed at the edges.
The only "moisture barriers" needed are between the cantilevered cavity and indoors. Odds are high that the top side of the cavity needs nothing, assuming it's plywood and any electrical penetrations are robustly sealed. The one you have to worry about is making the path to the joist bay in the basement air tight and vapor retardent, while leaving a vapor-drying path out the bottom of the cantilever.
I'm going to throw this picture up, but then point out what's potentially wrong with it:
The foam board shown that is tight to the subfloor in most cases is NOT needed. A 3/4" plywood subfloor has a very low vapor permeance, well under 1 perm. In IRC code terms it's a "Class-II vapor retarder", which is more than adequate as an interior side vapor retarder in heating-dominated climates, and meets the Canadian NBC code definition of a "vapour barrier". As long as it's air-tight, nothing more is needed on the top side of the cavity for moisture control.
The vertical joist over the insulated studwall shown in the picture is a critical air barrier for a cantilever over a foundation wall, and in your case it's absent. If filling the stud bays with tightly fitted rock wool an air barrier at the of the wall could be half-inch CDX or OSB (about 0.5-1 perm when dry) , or cut'n'cobbled half-inch foil-faced polyiso foam board (less than 0.1 perms.), taking care to robustly air seal the perimeter.
The foam board on the bottom is potentially an actively BAD idea, since it creates a moisture trap. If your wall-side barrier ever leaks it will collect moisture all winter creating mold conditions. Even though most of the time stack effect infiltration from the outdoors prevents that, counting on a leaky exterior to safe the day is bad practice. Ideally the bottom sheathing on the exterior would be something reasonably air tight but moisture tolerant and vapor permeable, such as asphalted fiberboard (Menards may carry it, in your location, most box stores don't), or MDF, or exterior grade gypsum board such as GP DensGlass. Less than ideal but still OK would be CDX or OSB, as long as you don't paint it with anything more vapor-tight than latex paint.
So if foam board is used to block the brick veneer cavity, be sure it doesn't extend any further out than the exterior of the brick, leaving the bottom sheathing uncovered by the low-permeance foam.
This stackup would be preferable to the one above, with or without the subfloor foam, but DEFINITELY the vapor retardent air barrier where it crosses the wall (whether air tight wood or air-tight foam board):
The only "moisture barriers" needed are between the cantilevered cavity and indoors. Odds are high that the top side of the cavity needs nothing, assuming it's plywood and any electrical penetrations are robustly sealed. The one you have to worry about is making the path to the joist bay in the basement air tight and vapor retardent, while leaving a vapor-drying path out the bottom of the cantilever.
I'm going to throw this picture up, but then point out what's potentially wrong with it:
The foam board shown that is tight to the subfloor in most cases is NOT needed. A 3/4" plywood subfloor has a very low vapor permeance, well under 1 perm. In IRC code terms it's a "Class-II vapor retarder", which is more than adequate as an interior side vapor retarder in heating-dominated climates, and meets the Canadian NBC code definition of a "vapour barrier". As long as it's air-tight, nothing more is needed on the top side of the cavity for moisture control.
The vertical joist over the insulated studwall shown in the picture is a critical air barrier for a cantilever over a foundation wall, and in your case it's absent. If filling the stud bays with tightly fitted rock wool an air barrier at the of the wall could be half-inch CDX or OSB (about 0.5-1 perm when dry) , or cut'n'cobbled half-inch foil-faced polyiso foam board (less than 0.1 perms.), taking care to robustly air seal the perimeter.
The foam board on the bottom is potentially an actively BAD idea, since it creates a moisture trap. If your wall-side barrier ever leaks it will collect moisture all winter creating mold conditions. Even though most of the time stack effect infiltration from the outdoors prevents that, counting on a leaky exterior to safe the day is bad practice. Ideally the bottom sheathing on the exterior would be something reasonably air tight but moisture tolerant and vapor permeable, such as asphalted fiberboard (Menards may carry it, in your location, most box stores don't), or MDF, or exterior grade gypsum board such as GP DensGlass. Less than ideal but still OK would be CDX or OSB, as long as you don't paint it with anything more vapor-tight than latex paint.
So if foam board is used to block the brick veneer cavity, be sure it doesn't extend any further out than the exterior of the brick, leaving the bottom sheathing uncovered by the low-permeance foam.
This stackup would be preferable to the one above, with or without the subfloor foam, but DEFINITELY the vapor retardent air barrier where it crosses the wall (whether air tight wood or air-tight foam board):
thanks Dana for the thorough follow-up. If I were to leave the air gap open, tightly pack the cavity with Roxul and install the foil faced polyiso, will the cavity sufficiently allow moisture to do whatever it needs to do while blocking as much air movement (cold intrusion) as possible?
What air gap? Do NOT leave any air gaps!
Air gaps are both potential thermal bypasses and paths for convective transfer of moisture via air. Even tightly packed fiber insulation is highly air & water vapor permeable. The indoor air is the major source of moisture that can get into the cantelivered cavity and create mold/rot conditions at cantilevered joists & bottom side sheathing.
Making it as air tight as possible on all sides, and leaving the bottom side sheathing at least semi-permeable to water vapor (not air) is the safest approach.
If you want a thermal break on the joists, installing 1-2" of foil faced polyiso on the stud edges (deepening the available space for fiber insulation and through-screwing/nailing the bottom sheathing can work without impairing the drying ability. A 2x10 is 9.25" deep, a pair of R23 rock wool batts is 11" deep, so with 1.5" polyiso (R9) on the joist edges there would be 1/4" of compression, for a total R-value of about R45 on the rock wool. And with R9 on the joist edges the total R-value of the framing will be nearly doubled to about R20, nearly eliminating any temperature striping issues on the floor (more of an issue in colder locations such as the U.P. than it is in Kalamazoo.) It's pretty easy to cut up 1.5" foil faced polyiso cleanly into 1.5" wide strips using a 4-5" steel wallboard taping knife that has been sharpened on the edges.
Code minimum is R-30 between joists, so it will be substantially better than code even without insulating edge strips on the joists and R45-ish rock wool.
Ok, I was following until the math on the polyiso on joist edges and 1/4” batt compression. I picked up the asphalt fiber board sheathing and foil faced polyiso at Menards, so that will go in tomorrow. R23 Rockwool is all special order everywhere in southeast Michigan.
R21 fiberglass is fine, and would also meet/beat code (with conviction!)
A pair of R23 (or R21s) has a manufactured loft of 5.5" per layer, 11.0" total. The nominal depth of a 2x10 is 9.25". Adding 1.5" of depth with edge strips brings the total depth to 10.75", not 11". So it will be a modest compression of 0.25", which reduces the R-value of the batts by a bit less than R1.
So a stacked pair of R23s would deliver R45-ish performance, a pair of R21s would be R41-ish, either of which blows away the R30 code minimum by a decent amount.
And that is even before you factor in the performance boost of taking the thermally bridging (9.25" x R1.2/inch=) R11.1 joists beinging improved by another R9 with the 1.5" polyiso. The half-inch fiberboard is also insulating, adding more than R0.5 to what the assembly would have been if using half-inch CDX or OSB.
